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aims: we report on the first major temporal morphological changes observed on the surface of the nucleus of comet 67p/churyumov-gerasimenko in the smooth terrains of the imhotep region.methods: we used images of the osiris cameras onboard rosetta to follow the temporal changes from 24 may 2015 to 11 july 2015.results: the morphological changes observed on the surface are visible in the form of roundish features that are growing in size from a given location in a preferential direction at a rate of 5.6-8.1 × 10-5 m s-1 during the observational period. the location where the changes started and the contours of the expanding features are bluer than the surroundings, which suggests that ices (h2o and/or co2) are exposed on the surface. however, sublimation of ices alone is not sufficient to explain the observed expanding features. no significant variations in the dust activity pattern are observed during the period of changes. | temporal morphological changes in the imhotep region of comet 67p/churyumov-gerasimenko |
the rosetta mission shall accompany comet 67p/churyumov-gerasimenko from a heliocentric distance of >3.6 astronomical units through perihelion passage at 1.25 astronomical units, spanning low and maximum activity levels. initially, the solar wind permeates the thin comet atmosphere formed from sublimation, until the size and plasma pressure of the ionized atmosphere define its boundaries: a magnetosphere is born. using the rosetta plasma consortium ion composition analyzer, we trace the evolution from the first detection of water ions to when the atmosphere begins repelling the solar wind (~3.3 astronomical units), and we report the spatial structure of this early interaction. the near-comet water population comprises accelerated ions (<800 electron volts), produced upstream of rosetta, and lower energy locally produced ions; we estimate the fluxes of both ion species and energetic neutral atoms. | birth of a comet magnetosphere: a spring of water ions |
context. the cometary secondary ion mass analyzer (cosima) on board rosetta is dedicated to the collection and compositional analysis of the dust particles in the coma of 67p/churyumov-gerasimenko (67p).aims: investigation of the physical properties of the dust particles collected along the comet trajectory around the sun starting at a heliocentric distance of 3.5 au.methods: the flux, size distribution, and morphology of the dust particles collected in the vicinity of the nucleus of comet 67p were measured with a daily to weekly time resolution.results: the particles collected by cosima can be classified according to their morphology into two main types: compact particles and porous aggregates. in low-resolution images, the porous material appears similar to the chondritic-porous interplanetary dust particles collected in earth's stratosphere in terms of texture. we show that this porous material represents 75% in volume and 50% in number of the large dust particles collected by cosima. compact particles have typical sizes from a few tens of microns to a few hundreds of microns, while porous aggregates can be as large as a millimeter. the particles are not collected as a continuous flow but appear in bursts. this could be due to limited time resolution and/or fragmentation either in the collection funnel or few meters away from the spacecraft. the average collection rate of dust particles as a function of nucleo-centric distance shows that, at high phase angle, the dust flux follows a 1/d2comet law, excluding fragmentation of the dust particles along their journey to the spacecraft. at low phase angle, the dust flux is much more dispersed compared to the 1/d2comet law but cannot be explained by fragmentation of the particles along their trajectory since their velocity, indirectly deduced from the cosima data, does not support such a phenomenon. the cumulative size distribution of particles larger than 150 μm follows a power law close to r- 0.8 ± 0.1, confirming measurements made by another rosetta dust instrument grain impact analyser and dust accumulator (giada). the cumulative size distribution of particles between 30 μm and 150 μm has a power index of -1.9 ± 0.3. the excess of dust in the 10-100 μm range in comparison to the 100 μm-1 mm range together with no evidence for fragmentation in the inner coma, implies that these particles could have been released or fragmented at the nucleus right after lift-off of larger particles. below 30 μm, particles exhibit a flat size distribution. we interprete this knee in the size distribution at small sizes as the consequence of strong binding forces between the sub-constitutents. for aggregates smaller than 30 μm, forces stronger than van-der-waals forces would be needed to break them apart. | dust particle flux and size distribution in the coma of 67p/churyumov-gerasimenko measured in situ by the cosima instrument on board rosetta |
the size and velocity distribution of cosmic dust particles entering the earth's atmosphere is uncertain. here we show that the relative concentrations of metal atoms in the upper mesosphere, and the surface accretion rate of cosmic spherules, provide sensitive probes of this distribution. three cosmic dust models are selected as case studies: two are astronomical models, the first constrained by infrared observations of the zodiacal dust cloud and the second by radar observations of meteor head echoes; the third model is based on measurements made with a spaceborne dust detector. for each model, a monte carlo sampling method combined with a chemical ablation model is used to predict the ablation rates of na, k, fe, mg, and ca above 60 km and cosmic spherule production rate. it appears that a significant fraction of the cosmic dust consists of small (<5 µg) and slow (<15 km s-1) particles. | on the size and velocity distribution of cosmic dust particles entering the atmosphere |
we report the detection of the pre-biotic molecule ch3nco in a solar-type protostar, iras16293-2422 b. a significant abundance of this species on the surface of the comet 67p/churyumov-gerasimenko has been proposed, and it has recently been detected in hot cores around high-mass protostars. we observed iras16293-2422 b with the atacama large millimeter array in the 90 to 265 ghz range, and detected eight unblended transitions of ch3nco. from our local thermodynamic equilibrium analysis, we derived an excitation temperature of 110 ± 19 k and a column density of (4.0 ± 0.3) × 1015 cm-2, which results in an abundance of ≤(1.4 ± 0.1) × 10-10 with respect to molecular hydrogen. this implies a ch3nco/hnco and ch3nco/nh2cho column density ratios of ∼0.08. our modelling of the chemistry of ch3nco suggests that both ice surface and gas phase formation reactions of this molecule are needed to explain the observations. | detection of methyl isocyanate (ch3nco) in a solar-type protostar |
comets in the oort cloud evolve under the influence of internal and external perturbations, such as giant planets, stellar passages, and the galactic gravitational tidal field. we aim to study the dynamical evolution of the comets in the oort cloud, accounting for the perturbation of the galactic tidal field and passing stars. we base our study on three main approaches; analytic, observational, and numerical. we first construct an analytical model of stellar encounters. we find that individual perturbations do not modify the dynamics of the comets in the cloud unless very close (<0.5 pc) encounters occur. using proper motions, parallaxes, and radial velocities from gaia dr2 and combining them with the radial velocities from other surveys, we then construct an astrometric catalogue of the 14 659 stars that are within 50 pc of the sun. for all these stars we calculate the time and distance of closest approach to the sun. we find that the cumulative effect of relatively distant (≤1 pc) passing stars can perturb the comets in the oort cloud. finally, we study the dynamical evolution of the comets in the oort cloud under the influence of multiple stellar encounters from stars that pass within 2.5 pc of the sun and the galactic tidal field over ±10 myr. we use the astrophysical multipurpose software environment (amuse), and the gpu-accelerated direct n-body code abie. we considered two models for the oort cloud, compact (a ≤ 0.25 pc) and extended (a ≤ 0.5 pc). we find that the cumulative effect of stellar encounters is the major perturber of the oort cloud for a compact configuration while for the extended configuration the galactic tidal field is the major perturber. in both cases the cumulative effect of distant stellar encounters together with the galactic tidal field raises the semi-major axis of 1.1% of the comets at the edge of the oort cloud up to interstellar regions (a > 0.5 pc) over the 20 myr period considered. this leads to the creation of transitional interstellar comets (tics), which might become interstellar objects due to external perturbations. this raises the question of the formation, evolution, and current status of the oort cloud as well as the existence of a "cloud" of objects in the interstellar space that might overlap with our oort cloud, when considering that other planetary systems should undergo similar processes leading to the ejection of comets. the movie associated to fig. 11 is available at http://www.aanda.orgthe full table 3 is also available at the cds via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/cat/j/a+a/629/a139 | galactic tide and local stellar perturbations on the oort cloud: creation of interstellar comets |
we investigate the delivery of regular and deuterated forms of water from prestellar cores to circumstellar disks. we adopt a semi-analytical, axisymmetric, two-dimensional collapsing core model with post-processing gas-ice astrochemical simulations, in which a layered ice structure is considered. the physical and chemical evolutions are followed until the end of the main accretion phase. in our models, when mass averaged over the whole disk, a forming disk has a similar h2o abundance and hdo/h2o abundance ratio (within a factor of 2) as the precollapse values of these quantities, regardless of time. consistent with previous studies, our models suggest that interstellar water ice is delivered to forming disks without significant alteration. on the other hand, the local vertically averaged h2o ice abundance and hdo/h2o ice ratio can differ more, by up to a factor of several, depending on time and distance from a central star. key parameters for the local variations are the fluence of stellar uv photons en route into the disk and the ice layered structure, the latter of which is mostly established in the prestellar stages. we also find that even if interstellar water ice is destroyed by stellar uv and (partly) reformed prior to disk entry, the hdo/h2o ratio in reformed water ice is similar to the original value. this finding indicates that some caution is needed in discussions on the prestellar inheritance of h2o based on comparisons between the observationally derived hdo/h2o ratio in clouds/cores and that in disks/comets. alternatively, we propose that the ratio of d2o/hdo to hdo/h2o better probes the prestellar inheritance of h2o. it is also found that in forming disks icy organics are more enriched in deuterium than water ice. the differential deuterium fractionation in water and organics is inherited from prestellar stages. | water delivery from cores to disks: deuteration as a probe of the prestellar inheritance of h2o |
context. our galaxy is composed of different stellar populations with varying chemical abundances, which are thought to imprint the composition of planet building blocks (pbbs). as such, the properties of stars should affect the properties of planets and small bodies formed in their systems. in this context, high-resolution spectroscopic surveys open a window into the chemical links between and their host stars.aims: we aim to determine the pbb composition trends for various stellar populations across the galaxy by comparing the two large spectroscopic surveys apogee and galah. we assess the reliability of the pbb composition as determined with these surveys with a propagation error study.methods: stellar spectroscopic abundances from the large surveys galah-dr3 and apogee-dr17 were used as input with a stoichiometric condensation model. we classified stars into different galactic components and we quantified the pbb composition trends as a function of [fe/h]. we also analysed the distribution composition patterns in the [α/fe]-[fe/h] diagram.results: our propagation error study suggests that the overall trends with [fe/h] and [α/fe] are robust, which is supported by the double study of both apogee and galah. we therefore confirm the existence of a bimodal pbb composition separating the thin disc stars from the thick disc stars. furthermore, we confirm that the stoichiometric water pbb content is anti-correlated with [fe/h].conclusions: our results imply that metal-poor stars both in the thin and thick disks are suitable hosts for water-rich pbbs and for ice-rich small bodies. however, for metal-poor stars ([fe/h]<0), the pbbs around thick disc stars should have a higher water content than that around thin disc stars because of the α-content dependence of the water mass fraction. given the importance of the initial water abundance of the pbbs in recent planet formation simulations, we expect that the star origin influences the exoplanet population properties across the galaxy. | how the origin of stars in the galaxy impacts the composition of planetary building blocks |
in-situ study of comet 1p/halley during its 1986 apparition revealed a surprising abundance of organic coma species. it remained unclear, whether or not these species originated from polymeric matter. now, high-resolution mass-spectrometric data collected at comet 67p/churyumov-gerasimenko by esa's rosetta mission unveil the chemical structure of complex cometary organics. here, we identify an ensemble of individual molecules with masses up to 140 da while demonstrating inconsistency of the data with relevant amounts of polymeric matter. the ensemble has an average composition of c1h1.56o0.134n0.046s0.017, identical to meteoritic soluble organic matter, and includes a plethora of chain-based, cyclic, and aromatic hydrocarbons at an approximate ratio of 6:3:1. its compositional and structural properties, except for the h/c ratio, resemble those of other solar system reservoirs of organics—from organic material in the saturnian ring rain to meteoritic soluble and insoluble organic matter -, which is compatible with a shared prestellar history. | identification and characterization of a new ensemble of cometary organic molecules |
the effect of dust ion collisional frequency on dust ion acoustic waves (diaws) is investigated in a magnetized collisional dusty plasma with isothermal electrons considering an external periodic perturbation. employing the reductive perturbation technique, the damped kadomtsev-petviashili equation is derived to describe the dynamic oscillations of diaws in the presence of collisions between dusts and ions. it is perceived that the dust ion collisional frequency significantly changes the dynamics of the diaws and plays a crucial role in the transition from quasiperiodic motion to limit cycle oscillation. this study may be helpful to understand the effect of dust ion collisional frequency on the features of diaws in planetary rings, interstellar clouds, and comet tails. | effect of dust ion collisional frequency on transition of dust ion acoustic waves from quasiperiodic motion to limit cycle oscillation in a magnetized dusty plasma |
we identify a sample of 27 long-period comets for which both nongravitational accelerations and lyα-based gas production rates are available. seven of the 27 comets (i.e., ~25%) did not survive perihelion because of nucleus fragmentation or complete disintegration. empirically, the latter nuclei have the smallest gas production rates and the largest nongravitational accelerations, which are both indicators of small size. specifically, the disintegrating nuclei have a median radius of only 0.41 km, one-quarter of the 1.60 km median radius of those surviving perihelion. the disintegrating comets also have a smaller median perihelion distance (0.48 au) than do the survivors (0.99 au). we compare the order-of-magnitude timescale for outgassing torques to change the nucleus spin, τs , with the time spent by each comet in strong sublimation, δt, finding that the disrupted comets are those with τs< δt. the destruction of near-sun long-period comets is thus naturally explained as a consequence of rotational breakup. we discuss this process as a contributor to oort's long mysterious "fading parameter." | destruction of long-period comets |
recently, a rapid increase in radiocarbon (14c) was observed in japanese tree rings at ad 774/775. various explanations for the anomaly have been offered, such as a supernova, a γ-ray burst, a cometary impact, or an exceptionally large solar particle event (spe). however, evidence of the origin and exact timing of the event remains incomplete. in particular, a key issue of latitudinal dependence of the 14c intensity has not been addressed yet. here, we show that the event was most likely caused by the sun and occurred during the spring of ad 774. particularly, the event intensities from various locations show a strong correlation with the latitude, demonstrating a particle-induced 14c poleward increase, in accord with the solar origin of the event. furthermore, both annual 14c data and carbon cycle modelling, and separate earlywood and latewood 14c measurements, confine the photosynthetic carbon fixation to around the midsummer. | solar superstorm of ad 774 recorded subannually by arctic tree rings |
the osiris (optical, spectroscopic and infrared remote imaging system) instrument on board the esa rosetta spacecraft collected data of 67p/churyumov-gerasimenko for over 2 yr. osiris consists of two cameras, a narrow angle camera and a wide angle camera. for specific imaging sequences related to the observation of dust aggregates in 67p's coma, the two cameras were operating simultaneously. the two cameras are mounted 0.7 m apart from each other, as a result this baseline yields a parallax shift of the apparent particle trails on the analysed images directly proportional to their distance. thanks to such shifts, the distance between observed dust aggregates and the spacecraft was determined. this method works for particles closer than 6000 m to the spacecraft and requires very few assumptions. we found over 250 particles in a suitable distance range with sizes of some centimetres, masses in the range of 10-6-102 kg and a mean velocity of about 2.4 m s-1 relative to the nucleus. furthermore, the spectral slope was analysed showing a decrease in the median spectral slope of the particles with time. the further a particle is from the spacecraft the fainter is its signal. for this reason, this was counterbalanced by a debiasing. moreover, the dust mass-loss rate of the nucleus could be computed as well as the afρ of the comet around perihelion. the summed-up dust mass-loss rate for the mass bins 10-4-102 kg is almost 8300 kg s-1. | dust mass distribution around comet 67p/churyumov-gerasimenko determined via parallax measurements using rosetta's osiris cameras |
we present observations of decimetre-sized, likely ice-containing aggregates ejected from a confined region on the surface of comet 67p/churyumov-gerasimenko. the images were obtained with the narrow angle camera of the optical, spectroscopic, and infrared remote imaging system on board the rosetta spacecraft in 2016 january when the comet was at 2 au from the sun outbound from perihelion. we measure the acceleration of individual aggregates through a 2 h image series. approximately 50 per cent of the aggregates are accelerated away from the nucleus, and 50 per cent towards it, and likewise towards either horizontal direction. the accelerations are up to one order of magnitude stronger than local gravity, and are most simply explained by the combined effect of gas drag accelerating all aggregates upwards, and the recoil force from asymmetric outgassing, either from rotating aggregates with randomly oriented spin axes and sufficient thermal inertia to shift the temperature maximum away from an aggregate's subsolar region, or from aggregates with variable ice content. at least 10 per cent of the aggregates will escape the gravity field of the nucleus and feed the comet's debris trail, while others may fall back to the surface and contribute to the deposits covering parts of the northern hemisphere. the rocket force plays a crucial role in pushing these aggregates back towards the surface. our observations show the future back fall material in the process of ejection, and provide the first direct measurement of the acceleration of aggregates in the innermost coma (<2 km) of a comet, where gas drag is still significant. | acceleration of individual, decimetre-sized aggregates in the lower coma of comet 67p/churyumov-gerasimenko |
we present a photometric detection of the first brightness dips of the unique variable star kic 8462852 since the end of the kepler space mission in 2013 may. our regular photometric surveillance started in 2015 october, and a sequence of dipping began in 2017 may continuing on through the end of 2017, when the star was no longer visible from earth. we distinguish four main 1%-2.5% dips, named “elsie,” “celeste,” “skara brae,” and “angkor,” which persist on timescales from several days to weeks. our main results so far are as follows: (i) there are no apparent changes of the stellar spectrum or polarization during the dips and (ii) the multiband photometry of the dips shows differential reddening favoring non-gray extinction. therefore, our data are inconsistent with dip models that invoke optically thick material, but rather they are in-line with predictions for an occulter consisting primarily of ordinary dust, where much of the material must be optically thin with a size scale ≪1 μm, and may also be consistent with models invoking variations intrinsic to the stellar photosphere. notably, our data do not place constraints on the color of the longer-term “secular” dimming, which may be caused by independent processes, or probe different regimes of a single process. | the first post-kepler brightness dips of kic 8462852 |
a consensus view on the formation of planetesimals is now exposed to a threat, since recent numerical studies on the mechanical properties of dust aggregates tend to dispute the conceptual picture that submicrometer-sized grains conglomerate into planetesimals in protoplanetary discs. with the advent of precise laboratory experiments and extensive computer simulations on the interaction between elastic spheres comprising dust aggregates, we revisit a model for the tensile strength of dust aggregates consisting of small elastic grains. in the framework of contact mechanics and fracture mechanics, we examine outcomes of computer simulations and laboratory experiments on the tensile strength of dust aggregates. we provide a novel analytical formula that explicitly incorporates the volume effect on the tensile strength, namely, the dependence of tensile strength on the volume of dust aggregates. we find that our model for the tensile strength of dust aggregates well reproduces results of computer simulations and laboratory experiments, if appropriate values are adopted for the elastic parameters used in the model. moreover, the model with dust aggregates of submicrometer-sized grains is in good harmony with the tensile strength of cometary dust and meteoroids derived from astronomical observations. therefore, we reaffirm the commonly believed idea that the formation of planetesimals begins with conglomeration of submicrometer-sized grains condensed in protoplanetary discs. | the tensile strength of dust aggregates consisting of small elastic grains: constraints on the size of condensates in protoplanetary discs |
this paper considers the dynamics of the scattering of planetesimals or planetary embryos by a planet on a circumstellar orbit. we classify six regions in the planet's mass versus semimajor axis parameter space according to the dominant outcome for scattered objects: ejected, accreted, remaining, escaping, oort cloud, and depleted oort cloud. we use these outcomes to consider which planetary system architectures maximize the observability of specific signatures, given that signatures should be detected first around systems with optimal architectures (if such systems exist in nature). giant impact debris is most readily detectable for 0.1-10 m⊕ planets at 1-5 au, depending on the detection method and spectral type. while a stars have putative giant impact debris at 4-6 au consistent with this sweet spot, that of fgk stars is typically ≪1 au contrary to expectations; an absence of 1-3 au giant impact debris could indicate a low frequency of terrestrial planets there. three principles maximize the cometary influx from exo-kuiper belts: a chain of closely separated planets interior to the belt, none of which is a jupiter-like ejector; planet masses not increasing strongly with distance (for a net inward torque on comets); and ongoing replenishment of comets, possibly by embedded low-mass planets. a high oort cloud comet influx requires no ejectors and architectures that maximize the oort cloud population. cold debris discs are usually considered classical kuiper belt analogues. here we consider the possibility of detecting scattered disc analogues, which could be betrayed by a broad radial profile and lack of small grains, as well as spherical 100-1000 au mini-oort clouds. some implications for escaping planets around young stars, detached planets akin to sedna, and the formation of super-earths are also discussed. | how to design a planetary system for different scattering outcomes: giant impact sweet spot, maximizing exocomets, scattered discs |
molecular oxygen has been detected in the coma of comet 67p/churyumov-gerasimenko with abundances in the 1%-10% range by the rosetta orbiter spectrometer for ion and neutral analysis-double focusing mass spectrometer instrument on board the rosetta spacecraft. here we find that the radiolysis of icy grains in low-density environments such as the presolar cloud may induce the production of large amounts of molecular oxygen. we also show that molecular oxygen can be efficiently trapped in clathrates formed in the protosolar nebula (psn), and that its incorporation as crystalline ice is highly implausible, because this would imply much larger abundances of ar and n2 than those observed in the coma. assuming that radiolysis has been the only o2 production mechanism at work, we conclude that the formation of comet 67p/churyumov-gerasimenko is possible in a dense and early psn in the framework of two extreme scenarios: (1) agglomeration from pristine amorphous icy grains/particles formed in ism and (2) agglomeration from clathrates that formed during the disk’s cooling. the former scenario is found consistent with the strong correlation between o2 and h2o observed in comet 67p/churyumov-gerasimenko’s coma while the latter scenario requires that clathrates formed from ism icy grains that crystallized when entering the psn. | origin of molecular oxygen in comet 67p/churyumov-gerasimenko |
nitrogen fractionation is commonly used to assess the thermal history of solar system volatiles. with alma it is for the first time possible to directly measure {}14{{n}}/{}15{{n}} ratios in common molecules during the assembly of planetary systems. we present alma observations of the {{{h}}}13{cn} and {{hc}}15{{n}} j=3-2 lines at 0.″5 angular resolution, toward a sample of six protoplanetary disks, selected to span a range of stellar and disk structure properties. adopting a typical {}12{{c}}/{}13{{c}} ratio of 70, we find comet-like {}14{{n}}/{}15{{n}} ratios of 80-160 in five of the disks (3 t tauri and 2 herbig ae disks) and lack constraints for one of the t tauri disks (im lup). there are no systematic differences between t tauri and herbig ae disks, or between full and transition disks within the sample. in addition, no correlation is observed between disk-averaged d/h and {}14{{n}}/{}15{{n}} ratios in the sample. one of the disks, v4046 sgr, presents unusually bright hcn isotopologue emission, enabling us to model the radial profiles of {{{h}}}13{cn} and {{hc}}15{{n}}. we find tentative evidence of an increasing {}14{{n}}/{}15{{n}} ratio with radius, indicating that selective photodissociation in the inner disk is important in setting the {}14{{n}}/{}15{{n}} ratio during planet formation. | nitrogen fractionation in protoplanetary disks from the h13cn/hc15n ratio |
context. certain types of supernova remnants (snrs) in our galaxy are assumed to be pevatrons, capable of accelerating cosmic rays (crs) to ~ pev energies. however, conclusive observational evidence for this has not yet been found. the snr g106.3+2.7, detected at 1-100 tev energies by different γ-ray facilities, is one of the most promising pevatron candidates. this snr has a cometary shape, which can be divided into a head and a tail region with different physical conditions. however, in which region the 100 tev emission is produced has not yet been identified because of the limited position accuracy and/or angular resolution of existing observational data. additionally, it remains unclear as to whether the origin of the γ-ray emission is leptonic or hadronic.aims: with the better angular resolution provided by new magic data compared to earlier γ-ray datasets, we aim to reveal the acceleration site of pev particles and the emission mechanism by resolving the snr g106.3+2.7 with 0.1° resolution at tev energies.methods: we observed the snr g106.3+2.7 using the magic telescopes for 121.7 h in total - after quality cuts - between may 2017 and august 2019. the analysis energy threshold is ~0.2 tev, and the angular resolution is 0.07−0.1°. we examined the γ-ray spectra of different parts of the emission, whilst benefitting from the unprecedented statistics and angular resolution at these energies provided by our new data. we also used measurements at other wavelengths such as radio, x-rays, gev γ-rays, and 10 tev γ-rays to model the emission mechanism precisely.results: we detect extended γ-ray emission spatially coincident with the radio continuum emission at the head and tail of snr g106.3+2.7. the fact that we detect a significant γ-ray emission with energies above 6.0 tev from only the tail region suggests that the emissions above 10 tev detected with air shower experiments (milagro, hawc, tibet asγ and lhaaso) are emitted only from the snr tail. under this assumption, the multi-wavelength spectrum of the head region can be explained with either hadronic or leptonic models, while the leptonic model for the tail region is in contradiction with the emission above 10 tev and x-rays. in contrast, the hadronic model could reproduce the observed spectrum at the tail by assuming a proton spectrum with a cutoff energy of ~1 pev for that region. such high-energy emission in this middle-aged snr (4−10 kyr) can be explained by considering a scenario where protons escaping from the snr in the past interact with surrounding dense gases at present.conclusions: the γ-ray emission region detected with the magic telescopes in the snr g106.3+2.7 is extended and spatially coincident with the radio continuum morphology. the multi-wavelength spectrum of the emission from the tail region suggests proton acceleration up to ~pev, while the emission mechanism of the head region could either be hadronic or leptonic. | magic observations provide compelling evidence of hadronic multi-tev emission from the putative pevatron snr g106.3+2.7 |
the polar regions of the moon and mercury both have permanently shadowed environments, potentially capable of harboring ice (cold traps). while cold traps are likely to have been stable for nearly 4 gyr on mercury, this has not been the case for the moon. roughly 3 ± 1 gya, when the moon is believed to have resided at approximately half of its current semimajor axis, lunar obliquities have been calculated to have reached as high as 77°. at this time, lunar polar temperatures were much warmer and cold traps did not exist. since that era, lunar obliquity has secularly decreased, creating environments over approximately the last 1-2 gyr where ice could be stable (assuming near current recession rates). we argue that the paucity of ice in the present lunar cold traps is evidence that no cometary impact has occurred in the past billion years that is similar to the one(s) which are thought to have delivered volatiles to mercury's poles. however, the present ice distribution may be compatible with a cometary impact if it occurred not in today's lunar thermal environment, but in a past one. if ice were delivered during a past epoch, the distribution of ground ice would be dictated not by present day temperatures, but rather by these ancient, warmer, temperatures. in this paper, we attempt to recreate the thermal environments for past lunar orbital configurations to characterize the history of lunar environments capable of harboring ice. we will develop models of ice stability and mobility to examine likely fossil remains of past ice delivery (e.g. a comet impact) that could be observed on the present moon. we attempt to quantify when in the moon's outward evolution areas first became stable for ice deposition and when ice mobility would have ceased. | evolution of lunar polar ice stability |
broadband high reflectance in nature is often the result of randomly, three-dimensionally structured materials. this study explores unique optical properties associated with one-dimensional nanostructures discovered in silk cocoon fibers of the comet moth, argema mittrei. the fibers are populated with a high density of air voids randomly distributed across the fiber cross-section but are invariant along the fiber. these filamentary air voids strongly scatter light in the solar spectrum. a single silk fiber measuring ~50 μm thick can reflect 66% of incoming solar radiation, and this, together with the fibers' high emissivity of 0.88 in the mid-infrared range, allows the cocoon to act as an efficient radiative-cooling device. drawing inspiration from these natural radiative-cooling fibers, biomimetic nanostructured fibers based on both regenerated silk fibroin and polyvinylidene difluoride are fabricated through wet spinning. optical characterization shows that these fibers exhibit exceptional optical properties for radiative-cooling applications: nanostructured regenerated silk fibers provide a solar reflectivity of 0.73 and a thermal emissivity of 0.90, and nanostructured polyvinylidene difluoride fibers provide a solar reflectivity of 0.93 and a thermal emissivity of 0.91. the filamentary air voids lead to highly directional scattering, giving the fibers a highly reflective sheen, but more interestingly, they enable guided optical modes to propagate along the fibers through transverse anderson localization. this discovery opens up the possibility of using wild silkmoth fibers as a biocompatible and bioresorbable material for optical signal and image transport. | nanostructured fibers as a versatile photonic platform: radiative cooling and waveguiding through transverse anderson localization |
the microwave instrument on the rosetta orbiter (miro) has been observing the coma of comet 67p/churyumov-gerasimenko almost continuously since june 2014 at wavelengths near 0.53 mm. we present here a map of the water column density in the inner coma (within 3 km from nucleus center) when the comet was at 3.4 au from the sun. based on the analysis of the ho and ho (110-101) lines, we find that the column density can vary by two orders of magnitude in this region. the highest column density is observed in a narrow region on the dayside, close to the neck and north pole rotation axis of the nucleus, while the lowest column density is seen against the nightside of the nucleus where outgassing seems to be very low. we estimate that the outgassing pattern can be represented by a gaussian distribution in a solid angle with fwhm ≈ 80°. | distribution of water around the nucleus of comet 67p/churyumov-gerasimenko at 3.4 au from the sun as seen by the miro instrument on rosetta |
methanol is a key species in astrochemistry because it is the most abundant organic molecule in the interstellar medium and is thought to be the mother molecule of many complex organic species. estimating the deuteration of methanol around young protostars is of crucial importance because it highly depends on its formation mechanisms and the physical conditions during its moment of formation. we analyse several dozen transitions from deuterated methanol isotopologues coming from various existing observational datasets obtained with the iram-pdbi and alma sub-millimeter interferometers to estimate the methanol deuteration surrounding three low-mass protostars on solar system scales. a population diagram analysis allows us to derive a [ch2doh]/[ch3oh] abundance ratio of 3-6% and a [ch3od]/[ch3oh] ratio of 0.4-1.6% in the warm inner (≤100-200 au) protostellar regions. these values are typically ten times lower than those derived with previous single-dish observations towards these sources, but they are one to two orders of magnitude higher than the methanol deuteration measured in massive hot cores. dust temperature maps obtained from herschel and planck observations show that massive hot cores are located in warmer molecular clouds than low-mass sources, with temperature differences of 10 k. the comparison of our measured values with the predictions of the gas-grain astrochemical model grainoble shows that such a temperature difference is sufficient to explain the different deuteration observed in low- to high-mass sources. this suggests that the physical conditions of the molecular cloud at the origin of the protostars mostly govern the present-day observed deuteration of methanol and therefore of more complex organic molecules. finally, the methanol deuteration measured towards young solar-type protostars on solar system scales seems to be higher by a factor of 5 than the upper limit in methanol deuteration estimated in comet hale-bopp. if this result is confirmed by subsequent observations of other comets, it would imply that an important reprocessing of the organic material likely occurred in the solar nebula during the formation of the solar system. the reduced datacubes are only available at the cds via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/cat/j/a+a/632/a19 | interferometric observations of warm deuterated methanol in the inner regions of low-mass protostars |
we report the first in situ detection of halogens in a cometary coma, that of 67p/churyumov-gerasimenko. neutral gas mass spectra collected by the european space agency's rosetta spacecraft during four periods of interest from the first comet encounter up to perihelion indicate that the main halogen-bearing compounds are hf, hcl and hbr. the bulk elemental abundances relative to oxygen are ∼8.9 × 10-5 for f/o, ∼1.2 × 10-4 for cl/o and ∼2.5 × 10-6 for br/o, for the volatile fraction of the comet. the cometary isotopic ratios for 37cl/35cl and 81br/79br match the solar system values within the error margins. the observations point to an origin of the hydrogen halides in molecular cloud chemistry, with frozen hydrogen halides on dust grains, and a subsequent incorporation into comets as the cloud condensed and the solar system formed. | halogens as tracers of protosolar nebula material in comet 67p/churyumov-gerasimenko |
the 163 comets observed during the wise/neowise prime mission represent the largest infrared survey to date of comets, providing constraints on dust, nucleus size, and co + co2 production. we present detailed analyses of the wise/neowise comet discoveries, and discuss observations of the active comets showing 4.6 μm band excess. we find a possible relation between dust and co + co2 production, as well as possible differences in the sizes of long and short period comet nuclei. | the neowise-discovered comet population and the co + co2 production rates |
earth is deficient in carbon and nitrogen by up to ~4 orders of magnitude compared with the sun. destruction of (carbon- and nitrogen-rich) refractory organics in the high-temperature planet-forming regions could explain this deficiency. assuming a refractory cometary composition for these grains, their destruction enhances nitrogen-containing, oxygen-poor molecules in the hot gas (≳300 k) after the initial formation and sublimation of these molecules from oxygen-rich ices in the warm gas (~150 k). using observations of 37 high-mass protostars with the atacama large millimeter/submillimeter array, we find that oxygen-containing molecules (ch3oh and hnco) systematically show no enhancement in their hot component. in contrast, nitrogen-containing, oxygen-poor molecules (ch3cn and c2h3cn) systematically show an enhancement of a factor ~5 in their hot component, pointing to additional production of these molecules in the hot gas. assuming only thermal excitation conditions, we interpret these results as a signature of destruction of refractory organics, consistent with the cometary composition. this destruction implies a higher c/o and n/o in the hot gas than the warm gas, while the exact values of these ratios depend on the fraction of grains that are effectively destroyed. this fraction can be found by future chemical models that constrain c/o and n/o from the abundances of minor carbon, nitrogen, and oxygen carriers presented here. | evidence for ubiquitous carbon grain destruction in hot protostellar envelopes |
in recent years, phosphorus monoxide (po), an important molecule for prebiotic chemistry, has been detected in star-forming regions and in the comet 67p/churyumov-gerasimenko. these studies have revealed that, in the interstellar medium (ism), po is systematically the most abundant p-bearing species, with abundances that are about one to three times greater than those derived for phosphorus nitride (pn), the second-most abundant p-containing molecule. the reason why po is more abundant than pn remains still unclear. experimental studies with phosphorus in the gas phase are not available, probably because of the difficulties in dealing with its compounds. therefore, the reactivity of atomic phosphorus needs to be investigated using reliable computational tools. to this end, state-of-the-art quantum-chemical computations have been employed to evaluate accurate reaction rates and branching ratios for the p + oh → po + h and p + h2o → po + h2 reactions in the framework of a master equation approach based on ab initio transition state theory. the hypothesis that oh and h2o can be potential oxidizing agents of atomic phosphorus is based on the ubiquitous presence of h2o in the ism. its destruction then produces oh, which is another very abundant species. while the reaction of atomic phosphorus in its ground state with water is not a relevant source of po because of emerged energy barriers, the p + oh reaction represents an important formation route of po in the ism. our kinetic results show that this reaction follows an arrhenius-kooij behavior, and thus its rate coefficients (α = 2.28 × 10-10 cm3 molecule-1 s-1, β = 0.16 and γ = 0.37 k) increase by increasing the temperature. | formation of phosphorus monoxide (po) in the interstellar medium: insights from quantum-chemical and kinetic calculations |
context. di-deuterated molecules are observed in the earliest stages of star formation at abundances of a few percent relative to their nondeuterated isotopologs, which is unexpected considering the scarcity of deuterium in the interstellar medium. with sensitive observations leading to the detection of a steadily increasing number of di-deuterated species, it is becoming possible to explore successive deuteration chains.aims: the accurate quantification of the column density of di-deuterated methanol is a key piece of the puzzle that is missing in the otherwise thoroughly constrained family of d-bearing methanol in the deeply embedded low-mass protostellar system and astrochemical template source iras 16293-2422. a spectroscopic dataset for astrophysical purposes was built for chd2oh and made publicly available to facilitate the accurate characterization of this species in astrochemical surveys.methods: the newly computed line list and partition function were used to search for chd2oh toward iras 16293-2422 a and b in data from the atacama large millimeter/submillimeter array (alma) protostellar interferometric line survey (pils). only nonblended, optically thin lines of chd2oh were used for the synthetic spectral fitting.results: the constructed spectroscopic database contains line frequencies and strengths for 7417 transitions in the 0-500 ghz frequency range. alma-pils observations in the 329-363 ghz range were used to identify 105 unique, nonblended, optically thin line frequencies of chd2oh for synthetic spectral fitting. the derived excitation temperatures and column densities yield high d/h ratios of chd2oh in iras 16293-2422 a and b of 7.5 ± 1.1% and 7.7 ± 1.2%, respectively.conclusions: deuteration in iras 16293-2422 is not higher than in other low-mass star-forming regions (l483, svs13-a, ngc 1333-iras2a, -iras4a, and -iras4b). di-deuterated molecules consistently have higher d/h ratios than their mono-deuterated counterparts in all low-mass protostars, which may be a natural consequence of h-d substitution reactions as seen in laboratory experiments. the solar system's natal cloud, as traced by comet 67p/churyumov-gerasimenko, may have had a lower initial abundance of d, been warmer than the cloud of iras 16293-2422, or been partially reprocessed. in combination with accurate spectroscopy, a careful spectral analysis, and the consideration of the underlying assumptions, successive deuteration is a robust window on the physicochemical provenance of star-forming systems. the constructed spectroscopic database for chd2oh containing line frequencies and strengths for 7417 transitions in the 0-500 ghz frequency range are only available at the cds via anonymous ftp to cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/cat/j/a+a/659/a69 | successive deuteration in low-mass star-forming regions: the case of d2-methanol (chd2oh) in iras 16293-2422 |
a major point of interest in cometary plasma physics has been the diamagnetic cavity, an unmagnetized region in the innermost part of the coma. here we combine langmuir and mutual impedance probe measurements to investigate ion velocities and electron temperatures in the diamagnetic cavity of comet 67p, probed by the rosetta spacecraft. we find ion velocities generally in the range 2-4 km/s, significantly above the expected neutral velocity ≲1 km/s, showing that the ions are (partially) decoupled from the neutrals, indicating that ion-neutral drag was not responsible for balancing the outside magnetic pressure. observations of clear wake effects on one of the langmuir probes showed that the ion flow was close to radial and supersonic, at least with respect to the perpendicular temperature, inside the cavity and possibly in the surrounding region as well. we observed spacecraft potentials ≲-5 v throughout the cavity, showing that a population of warm (∼5 ev) electrons was present throughout the parts of the cavity reached by rosetta. also, a population of cold (≲0.1 ev) electrons was consistently observed throughout the cavity, but less consistently in the surrounding region, suggesting that while rosetta never entered a region of collisionally coupled electrons, such a region was possibly not far away during the cavity crossings. | ion velocity and electron temperature inside and around the diamagnetic cavity of comet 67p |
context. there is an active debate about whether the properties of comets as observed today are primordial or, alternatively, if they are a result of collisional evolution or other processes.aims: we investigate the effects of collisions on a comet with a structure like 67p/churyumov-gerasimenko (67p). we develop scaling laws for the critical specific impact energies qreshape required for a significant shape alteration. these are then used in simulations of the combined dynamical and collisional evolution of comets in order to study the survival probability of a primordially formed object with a shape like 67p. although the focus of this work is on a structure of this kind, the analysis is also performed for more generic bi-lobe shapes, for which we define the critical specific energy qbil. the simulation outcomes are also analyzed in terms of impact heating and the evolution of the porosity.methods: the effects of impacts on comet 67p are studied using a state-of-the-art smooth particle hydrodynamics shock physics code. in the 3d simulations, a publicly available shape model of 67p is applied and a range of impact conditions and material properties are investigated. the resulting critical specific impact energy qreshape (as well as qbil for generic bi-lobe shapes) defines a minimal projectile size which is used to compute the number of shape-changing collisions in a set of dynamical simulations. these simulations follow the dispersion of the trans-neptunian disk during the giant planet instability, the formation of a scattered disk, and produce 87 objects that penetrate into the inner solar system with orbits consistent with the observed jfc population. the collisional evolution before the giant planet instability is not considered here. hence, our study is conservative in its estimation of the number of collisions.results: we find that in any scenario considered here, comet 67p would have experienced a significant number of shape-changing collisions, if it formed primordially. this is also the case for generic bi-lobe shapes. our study also shows that impact heating is very localized and that collisionally processed bodies can still have a high porosity.conclusions: our study indicates that the observed bi-lobe structure of comet 67p may not be primordial, but might have originated in a rather recent event, possibly within the last 1 gy. this may be the case for any kilometer-sized two-component cometary nuclei. | how primordial is the structure of comet 67p?. combined collisional and dynamical models suggest a late formation |
we have studied the dependence of azimuthal anisotropy v2 for inclusive and identified charged hadrons in au +au and cu +cu collisions on collision energy, species, and centrality. the values of v2 as a function of transverse momentum pt and centrality in au +au collisions at √{s nn}=200 and 62.4 gev are the same within uncertainties. however, in cu +cu collisions we observe a decrease in v2 values as the collision energy is reduced from 200 to 62.4 gev. the decrease is larger in the more peripheral collisions. by examining both au +au and cu +cu collisions we find that v2 depends both on eccentricity and the number of participants, npart. we observe that v2 divided by eccentricity (ɛ ) monotonically increases with npart and scales as npart1 /3. the cu +cu data at 62.4 gev falls below the other scaled v2 data. for identified hadrons, v2 divided by the number of constituent quarks nq is independent of hadron species as a function of transverse kinetic energy k et=mt-m between 0.1 <k et/nq<1 gev. combining all of the above scaling and normalizations, we observe a near-universal scaling, with the exception of the cu +cu data at 62.4 gev, of v2/(nq.ɛ .npart1 /3) vs k et/nq for all measured particles. | systematic study of azimuthal anisotropy in cu + cu and au + au collisions at √{snn}=62.4 and 200 gev |
the first interstellar object, 1i/2017 u1 ('oumuamua), exhibited several unique properties, including an extreme aspect ratio, a lack of typical cometary volatiles, and a deviation from a keplerian trajectory. several authors have hypothesized that the non-gravitational acceleration was caused by either cometary outgassing or radiation pressure. here, we investigate the spin dynamics of 'oumuamua under the action of high-surface-area fractional activity and radiation pressure. we demonstrate that a series of transient jets that migrate across the illuminated surface will not produce a secular increase in the spin rate. we produce 3d tumbling simulations that approximate the dynamics of a surface-covering jet and show that the resulting synthetic light curve and periodogram are reasonably consistent with the observations. moreover, we demonstrate that radiation pressure also produces a steady spin state. while carbon monoxide (co) has been dismissed as a possible accelerant because of its non-detection in emission by spitzer, we show that outgassing from a surface characterized by a modest covering fraction of co ice can satisfy the non-ballistic dynamics for a plausible range of assumed bulk densities and surface albedos. spitzer upper limits on co emission are, however, inconsistent with the co production necessary to provide the acceleration. nonetheless, an ad hoc but physically plausible explanation is that the activity level varied greatly during the time that the trajectory was monitored. we reproduce the astrometric analysis presented in micheli et al., and verify that the non-gravitational acceleration was consistent with stochastic changes in outgassing. | on the spin dynamics of elongated minor bodies with applications to a possible solar system analogue composition for 'oumuamua |
we examine the dispersion of the degree of linear polarization p in comets at phase angle 90° where the maximum amplitude of positive polarization pmax occurs. the range of polarization observed in comets is from 7% up to more than 30%, and this cannot be explained through depolarization by gaseous emissions. instead, we suggest that the observed dispersion of p results from different properties in cometary dust. we simulate the spectral polarimetric observations of comets using model agglomerated debris particles. the vast majority of observations can be reproduced with a mixture of weakly absorbing and highly absorbing agglomerated debris particles, which obey the same power-law size distribution. within this extremely simple approach, polarization at side-scattering angles in a given comet is governed by the relative abundance of weakly and strongly absorbing particles. we find that in comets with the highest polarization, the weakly absorbing particles appear in proportions of only 14-23% by volume; whereas, in comets with the lowest polarization pmax, their abundance is much greater, 82-95%. we conclude that the polarization at side-scattering angles unambiguously measures the relative abundance of mg-rich silicates and refractory organics or amorphous carbon in comets. we put forth a hypothesis that low pmax could be an indicator for presence of a well-developed refractory surface layer covering cometary nucleus. | the positive-polarization of cometary comae |
nonlinear damped modified gardner equation is derived from the dust hydrodynamic model in dusty plasma, which is collisional, unmagnetized, and has two temperature ions. an extended version of the reductive perturbation technique (rpt) is used to derive the said equations. approximate analytical solitary wave solution is obtained for compositional parameters near the critical composition. it is found that various nonlinear structures like super periodic, superhomoclinic, and special types of nonlinear structures are emerging while an investigation is done through the system of first-order differential equations. the effects of different plasma parameters like collisions and dust charge variables on the wave structures have been noticed. in the circumstances of nebulas, interstellar clouds, planetary magnetospheres, and cometary environments recognized by astrophysicists, voyager spacecraft observation on the fine structure of saturn's rings, semiconductor devices, it has been investigated that the dust charge variable modifies the super nonlinear structures significantly. | variable dust charge generates a special type of nonlinear structures through modified gardner equations |
aims: we provide a detailed morphological analysis of the aswan site on comet 67p/churyumov-gerasimenko (67p). we derive the size-frequency distribution of boulders ≥2 m and correlate this distribution with the gravitational slopes for the first time on a comet. we perform the spectral analysis of this region to understand if possible surface variegation is related to thedifferent surface textures observable on the different units.methods: we used two osiris narrow angle camera (nac) image data sets acquired on september 19 and 22, 2014, with a scale of 0.5 m/px. gravitational slopes derived from the 3d shape model of 67p were used to identify and interpret the different units of the site. by means of the high-resolution nac data sets, boulders ≥2.0 m can be unambiguously identified and extracted using the software arcgis. coregistered and photometrically corrected color cubes were used to perform the spectral analyses, and we retrieved the spectral properties of the aswan units.results: the high-resolution morphological map of the aswan site (0.68 km2) shows that this site is characterized by four different units: fine-particle deposits located on layered terrains, gravitational accumulation deposits, taluses, and the outcropping layered terrain. multiple lineaments are identified on the aswan cliff, such as fractures, exposed layered outcrops, niches, and terraces. close to the terrace margin, several arched features observed in plan view suggest that the margin progressively retreats as a result of erosion. the size-frequency of boulders ≥2 m in the entire study area has a power-law index of -3.9 +0.2/-0.3 (1499 boulders ≥2 m/km2), suggesting that the aswan site is mainly dominated by gravitational events triggered by sublimation and/or thermal insolation weathering causing regressive erosion. the boulder size-frequency distribution versus gravitational slopes indicates that when higher gravitational slope terrains are considered, only boulders ≤10 m are identified, as well as steeper power-slope indices. in addition, no boulders ≥2 m are observed on slopes ≥50°. this may indicate that larger blocks detached from a sublimating cliff cannot rest at these slopes and consequently fall down. the spectral analysis performed on the site shows that despite different morphologic units, no spectral differences appear in the multiple textures. this may confirm a redistribution of particles across the nucleus as a consequence of airfall, whether coming from hapi or from the southern hemisphere when it is active during perihelion. | aswan site on comet 67p/churyumov-gerasimenko: morphology, boulder evolution, and spectrophotometry |
several lines of observational evidence suggest that white dwarfs receive small birth kicks due to anisotropic mass-loss. if other stars possess extrasolar analogues to the solar oort cloud, the orbits of comets in such clouds will be scrambled by white dwarf natal kicks. although most comets will be unbound, some will be placed on low angular momentum orbits vulnerable to sublimation or tidal disruption. the dusty debris from these comets will manifest itself as an ir excess temporarily visible around newborn white dwarfs; examples of such discs may already have been seen in the helix nebula, and around several other young white dwarfs. future observations with the james webb space telescope may distinguish this hypothesis from alternatives such as a dynamically excited kuiper belt analogue. although competing hypotheses exist, the observation that ≳15 per cent of young white dwarfs possess such discs, if interpreted as indeed being cometary in origin, provides indirect evidence that low-mass gas giants (thought necessary to produce an oort cloud) are common in the outer regions of extrasolar planetary systems. hydrogen abundances in the atmospheres of older white dwarfs can, if sufficiently low, also be used to place constraints on the joint parameter space of natal kicks and exo-oort cloud models. | evaporation and accretion of extrasolar comets following white dwarf kicks |
the ampte (active magnetospheric particle tracer explorers) mission provided in situ measurements of collisionless momentum and energy exchange between an artificial, photo-ionized barium plasma cloud and the streaming, magnetized hydrogen plasma of the solar wind . one of its most significant findings was the unanticipated displacement of the barium ion `comet head’ (and an oppositely directed deflection of the streaming hydrogen ions) transverse to both the solar wind flow and the interplanetary magnetic field, defying the conventional expectation that the barium ions would simply move downwind. while subsequent theoretical and computational efforts to understand the cause of the transverse motion reached differing conclusions, several authors attributed the observations to larmor coupling, a collisionless momentum exchange mechanism believed to occur in various astrophysical and space-plasma environments and to participate in cosmic magnetized collisionless shock formation. here we present the detection of larmor coupling in a reproducible laboratory experiment that combines an explosive laser-produced plasma cloud with preformed, magnetized ambient plasma in a parameter regime relevant to the ampte barium releases. in our experiment, time-resolved doppler spectroscopy reveals ambient ion acceleration transverse to both the laser-produced plasma flow and the background magnetic field. utilizing a detailed numerical simulation, we demonstrate that the ambient ion velocity distribution corresponding to the measured doppler-shifted spectrum is qualitatively and quantitatively consistent with larmor coupling. | collisionless momentum transfer in space and astrophysical explosions |
on 2016 july 03, several instruments onboard esa's rosetta spacecraft detected signs of an outburst event on comet 67p, at a heliocentric distance of 3.32 au from the sun, outbound from perihelion. we here report on the inferred properties of the ejected dust and the surface change at the site of the outburst. the activity coincided with the local sunrise and continued over a time interval of 14-68 min. it left a 10-m-sized icy patch on the surface. the ejected material comprised refractory grains of several hundred microns in size, and sub-micron-sized water ice grains. the high dust mass production rate is incompatible with the free sublimation of crystalline water ice under solar illumination as the only acceleration process. additional energy stored near the surface must have increased the gas density. we suggest a pressurized sub-surface gas reservoir, or the crystallization of amorphous water ice as possible causes. | evidence of sub-surface energy storage in comet 67p from the outburst of 2016 july 03 |
rotating star clusters near supermassive black holes are studied using touma-tremaine thermodynamics of gravitationally interacting orbital ellipses. a simple numerical procedure for calculating thermodynamic equilibrium states for an arbitrary distribution of stars over masses and semimajor axes is described. spontaneous symmetry breaking and breakdown of thermodynamics at low positive temperatures are rigorously proven for nonrotating clusters. rotation is introduced through a second temperature-like parameter. both axially symmetric and lopsided rotational equilibria are found; the lopsided equilibria precess with the angular velocity that is given by the ratio of the two temperatures. the eccentric stellar disk in the nucleus of the andromeda galaxy may be an example of a lopsided thermodynamic equilibrium of a rotating black hole star cluster. stellar-mass black holes occupy highly eccentric orbits in broken-symmetry star clusters, and form flattened disklike configurations in rotating star clusters. they are attracted to orbits that are stationary in the frame of reference rotating with the angular velocity of the cluster. in spherical clusters, stellar-mass black holes' orbits are significantly more eccentric than those of the lighter stars if the temperature is negative and more circular if the temperature is positive. finally, we note that planets, comets, dark matter particles, and other light bodies tend to form a spherically symmetric nonrotating subcluster with maximum-entropy eccentricity distribution ${s}_{\mathrm{cr}}p(e)=2e$ , even if their host cluster is rotating and lopsided. | the thermodynamics of rotating black hole star clusters |
we present the results of a molecular survey of comet 46p/wirtanen undertaken with the iram 30-m and noema radio telescopes in december 2018. observations at iram 30-m during the 12-18 december period comprise a 2 mm spectral survey covering 25 ghz and a 1 mm survey covering 62 ghz. the gas outflow velocity and kinetic temperature have been accurately constrained by the observations. we derive abundances of 11 molecules, some being identified remotely for the first time in a jupiter-family comet, including complex organic molecules such as formamide, ethylene glycol, acetaldehyde, or ethanol. sensitive upper limits on the abundances of 24 other molecules are obtained. the comet is found to be relatively rich in methanol (3.4% relative to water), but relatively depleted in co, cs, hnc, hnco, and hcooh. the radio spectra are only available at the cds via anonymous ftp to cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/cat/j/a+a/648/a49 based on observations carried out with the iram 30-m and noema telescopes. iram is supported by insu/cnrs (france), mpg (germany), and ign (spain). | molecular composition of comet 46p/wirtanen from millimetre-wave spectroscopy |
acetaldehyde is of interest to astrochemists for its relevance to both interstellar and cometary chemistry, but little infrared (ir) spectral data have been published for the solid phases of this compound. here we present ir spectra of three forms of solid acetaldehyde, with spectra for one form being published for the first time. direct measurements of band strengths and absorption coefficients also are reported for the first time for amorphous aldehyde, the form of greatest interest for astrochemical work. an acetaldehyde band strength at ∼1350 cm-1 that has been used as a reference for about 20 yr is seen to be in error by about 80 per cent when compared to the direct measurements presented here. spectra and peak positions also are presented for h13c(o)13ch3, and then used for the first identification of ketene as a radiation product of solid acetaldehyde. | quantifying acetaldehyde in astronomical ices and laboratory analogues: ir spectra, intensities, 13c shifts, and radiation chemistry |
we review the different scenarios for the origin of light noble gases (he, ne, and ar) on earth. several sources could have contributed to the earth's noble gas budget: implanted solar wind, solar nebula gas, chondrites, and comets. although there is evidence for "solar-like" neon in the earth's mantle, questions remain as to its origin. a new compilation of noble gas data in lunar soils, interplanetary dust particles, micrometeorites, and solar wind allows examination of the implanted solar wind composition, which is key to understanding the "solar-like" mantle neon isotope composition. we show that lunar soils that reflect this solar-wind-implanted signature have a 20ne/22ne ratio very close to that of ocean island basalts. new data and calculations illustrate that the measured plume source 20ne/22ne ratio is close to the primitive mantle ratio, when taking into account mixing with the upper mantle (that has lower 20ne/22ne ratio). this favors early solar wind implantation to account for the origin of light volatiles (he, ne, and possibly h) in the earth's mantle: they were incorporated by solar wind irradiation into the earth's precursor grains during the first few myr of the solar system's formation. these grains must have partially survived accretion processes (only a few percent are needed to satisfy the earth's budget of light volatiles). as for the atmosphere, the neon isotope composition can be explained by mixing 36% of mantle gases having this solar-wind-implanted signature and 64% of chondritic gases delivered in a late veneer phase. | origin of light noble gases (he, ne, and ar) on earth: a review |
aims: 67p/churyumov-gerasimenko is the target comet of the esa's rosetta mission. after commissioning at the end of march 2014, the optical, spectroscopic, and infrared remote imaging system (osiris) onboard rosetta, started imaging the comet and its dust environment to investigate how they change and evolve while approaching the sun.methods: we focused our work on narrow angle camera (nac) orange images and wide angle camera (wac) red and visible-610 images acquired between 2014 march 23 and june 24 when the nucleus of 67p was unresolved and moving from approximately 4.3 au to 3.8 au inbound. during this period the 67p - rosetta distance decreased from 5 million to 120 thousand km.results: through aperture photometry, we investigated how the comet brightness varies with heliocentric distance. 67p was likely already weakly active at the end of march 2014, with excess flux above that expected for the nucleus. the comet's brightness was mostly constant during the three months of approach observations, apart from one outburst that occurred around april 30 and a second increase in flux after june 20. coma was resolved in the profiles from mid-april. analysis of the coma morphology suggests that most of the activity comes from a source towards the celestial north pole of the comet, but the outburst that occurred on april 30 released material in a different direction. | 67p/churyumov-gerasimenko: activity between march and june 2014 as observed from rosetta/osiris |
the anomalously low ortho-to-para ratios (oprs) exhibited by gaseous water in space have been used to determine the formation temperature (<50 kelvin) of ice on cold interstellar dust. this approach assumes that the opr of water desorbed from ice is related to the ice formation temperature on the dust. however, we report that water desorbed from ice at 10 kelvin shows a statistical high-temperature opr of 3, even when the ice is produced in situ by hydrogenation of o2, a known formation process of interstellar water. this invalidates the assumed relation between opr and temperature. the necessary reinterpretation of the low oprs will help elucidate the chemical history of interstellar water from molecular clouds and processes in the early solar system, including comet formation. | statistical ortho-to-para ratio of water desorbed from ice at 10 kelvin |
we measured direct photons for pt<5 gev /c in minimum bias and 0%-40% most-central events at midrapidity for cu +cu collisions at √{snn}=200 gev . the e+e- contribution from quasireal direct virtual photons has been determined as an excess over the known hadronic contributions in the e+e- mass distribution. a clear enhancement of photons over the binary scaled p +p fit is observed for pt<4 gev /c in cu +cu data. the pt spectra are consistent with the au +au data covering a similar number of participants. the inverse slopes of the exponential fits to the excess after subtraction of the p +p baseline are 285 ±53 (stat )±57 (syst )mev /c and 333 ±72 (stat )±45 (syst )mev /c for minimum bias and 0%-40% most-central events, respectively. the rapidity density, d n /d y , of photons demonstrates the same power law as a function of d nch/d η observed in au +au at the same collision energy. | low-momentum direct-photon measurement in cu + cu collisions at √{sn n}=200 gev |
aims: using data from the rosetta mission to comet 67p/churyumov-gerasimenko, we evaluate the physical properties of the surface and subsurface of the nucleus and derive estimates for the thermal inertia (ti) and roughness in several regions on the largest lobe of the nucleus.methods: we have developed a thermal model to compute the temperature on the surface and in the uppermost subsurface layers of the nucleus. the model takes heat conduction, self-heating, and shadowing effects into account. to reproduce the brightness temperatures measured by the miro instrument, the thermal model is coupled to a radiative transfer model to derive the ti. to reproduce the spatially resolved infrared measurements of the virtis instrument, the thermal model is coupled to a radiance model to derive the ti and surface roughness. these methods are applied to rosetta data from september 2014.results: the resulting ti values from both instruments are broadly consistent with each other. from the millimetre channel on miro, we determine the ti in the subsurface to be <80 jk-1 m-2 s-0.5 for the seth, ash, and aten regions. the submillimetre channel implies similar results but also suggests that higher values could be possible. a low ti is consistent with other miro measurements and in situ data from the mupus instrument at the final landing site of philae. the virtis results give a best-fitting value of 80 jk-1 m-2 s-0.5 and values in the range 40-160 jk-1 m-2 s-0.5 in the same areas. these observations also allow the subpixel scale surface roughness to be estimated and compared to images from the osiris camera. the virtis data imply that there is significant roughness on the infrared scale below the resolution of the available shape model and that, counter-intuitively, visually smooth terrain (centimetre scale) can be rough at small (micrometre-millimetre) scales, and visually rough terrain can be smooth at small scales. | thermal inertia and roughness of the nucleus of comet 67p/churyumov-gerasimenko from miro and virtis observations |
two basic routes for planetesimal formation have been proposed over the last decades. one is a classical `slow-growth' scenario. another one is particle concentration models, in which small pebbles are concentrated locally and then collapse gravitationally to form planetesimals. both types of models make certain predictions for the size spectrum and internal structure of newly born planetesimals. we use these predictions as input to simulate collisional evolution of debris discs left after the gas dispersal. the debris disc emission as a function of a system's age computed in these simulations is compared with several spitzer and herschel debris disc surveys around a-type stars. we confirm that the observed brightness evolution for the majority of discs can be reproduced by classical models. further, we find that it is equally consistent with the size distribution of planetesimals predicted by particle concentration models - provided the objects are loosely bound `pebble piles' as these models also predict. regardless of the assumed planetesimal formation mechanism, explaining the brightest debris discs in the samples uncovers a `disc mass problem'. to reproduce such discs by collisional simulations, a total mass of planetesimals of up to ∼1000 earth masses is required, which exceeds the total mass of solids available in the protoplanetary progenitors of debris discs. this may indicate that stirring was delayed in some of the bright discs, that giant impacts occurred recently in some of them, that some systems may be younger than previously thought or that non-collisional processes contribute significantly to the dust production. | debris disc constraints on planetesimal formation |
we report on imaging and spectroscopic observations of comet c/2016 r2 (pan-starrs) obtained with the 0.8 m and 2.7 m telescopes of mcdonald observatory in 2017 november and december, respectively. the comet was at a heliocentric distance greater than 3 au during both sets of observations. the images showed a well-developed tail with properties that suggested it was an ion tail. the spectra confirmed that we were observing well-developed bands of co+ and n2+. the n2+ detection was unequivocally cometary and was one of the strongest bands of n2+ detected in a comet spectrum. we derived the ratio of these two ions and from that we were able to derive that n2/co = 0.15. this is the highest such ratio reported for a comet. this letter includes data taken at the mcdonald observatory of the university of texas at austin. | strong co+ and n2+ emission in comet c/2016 r2 (pan-starrs) |
we present the first high-resolution, submillimeter-wavelength polarimetric observations of—and thus direct observations of the magnetic field morphology within—the dense gas of the pillars of creation in m16. these 850 μm observations, taken as part of the b-fields in star-forming region observations survey (bistro) using the pol-2 polarimeter on the submillimeter common-user bolometer array 2 (scuba-2) camera on the james clerk maxwell telescope (jcmt), show that the magnetic field runs along the length of the pillars, perpendicular to and decoupled from the field in the surrounding photoionized cloud. using the chandrasekhar-fermi method we estimate a plane-of-sky magnetic field strength of 170-320 μg in the pillars, consistent with their having been formed through the compression of gas with initially weak magnetization. the observed magnetic field strength and morphology suggests that the magnetic field may be slowing the pillars’ evolution into cometary globules. we thus hypothesize that the evolution and lifetime of the pillars may be strongly influenced by the strength of the coupling of their magnetic field to that of their parent photoionized cloud—i.e., that the pillars’ longevity results from magnetic support. | first observations of the magnetic field inside the pillars of creation: results from the bistro survey |
context. dust deposits or dust cover are a prevalent morphology in the northern hemi-nucleus of comet 67p/churyumov-gerasimenko (67p). the evolution of the dust deposits was captured by the osiris camera system onboard the rosetta spacecraft having escorted the comet for over two years. the observations shed light on the fundamental role of cometary activity in shaping and transforming the surface morphology.aims: we aim to present osiris observations of surface changes over the dust deposits before and after perihelion. the distribution of changes and a timeline of their occurrence are provided. we perform a data analysis to quantify the surface changes and investigate their correlation to water activity from the dust deposits. we further discuss how the results of our investigation are related to other findings from the rosetta mission.methods: surface changes were detected via systematic comparison of images, and quantified using shape-from-shading technique. thermal models were applied to estimate the erosion of water ice in response to the increasing insolation over the areas where surface changes occurred. modeling results were used for the interpretation of the observed surface changes.results: surface changes discussed here were concentrated at mid-latitudes, between about 20°n and 40°n, marking a global transition from the dust-covered to rugged terrains. the changes were distributed in open areas exposed to ample solar illumination and likely subject to enhanced surface erosion before perihelion. the occurrence of changes followed the southward migration of the sub-solar point across the latitudes of their distribution. the erosion at locations of most changes was at least about 0.5 m, but most likely did not exceed several meters. the erosive features before perihelion had given way to a fresh, smooth cover of dust deposits after perihelion, suggesting that the dust deposits had been globally restored by at least about 1 m with ejecta from the intensely illuminated southern hemi-nucleus around perihelion, when the north was inactive during polar night.conclusions: the erosion and restoration of the northern dust deposits are morphological expressions of seasonality on 67p. based on observations and thermal modeling results, it is inferred that the dust deposits contained a few percent of water ice in mass on average. local inhomogeneity in water abundance at spatial scales below tens of meters is likely. we suspect that dust ejected from the deposits may not have escaped the comet in bulk. that is, at least half of the ejected mass was afloat in the inner-coma or/and redeposited over other areas of the nucleus. | seasonal erosion and restoration of the dust cover on comet 67p/churyumov-gerasimenko as observed by osiris onboard rosetta |
the complete sample of large-perihelion nearly-parabolic comets discovered during the period 1901-2010 is studied, starting with their orbit determination. next, an orbital evolution that includes three perihelion passages (previous-observed-next) is investigated in which a full model of galactic perturbations and perturbations from passing stars is incorporated. we show that the distribution of planetary perturbations suffered by actual large-perihelion comets during their passage through the solar system has a deep, unexpected minimum around zero, which indicates a lack of 'almost unperturbed' comets. using a series of simulations we show that this deep well is moderately resistant to some diffusion of the orbital elements of the analysed comets. it seems reasonable to assert that the observed stream of these large-perihelion comets experienced a series of specific planetary configurations when passing through the planetary zone. an analysis of the past dynamics of these comets clearly shows that dynamically new comets can appear only when their original semimajor axes are greater than 20 000 au. on the other hand, dynamically old comets are completely absent for semimajor axes longer than 40 000 au. we demonstrate that the observed 1/aori-distribution exhibits a local minimum separating dynamically new from dynamically old comets. long-term dynamical studies reveal a wide variety of orbital behaviour. several interesting examples of the action of passing stars are also described, in particular the impact of gliese 710, which will pass close to the sun in the future. however, none of the obtained stellar perturbations is sufficient to change the dynamical status of the analysed comets. | oort spike comets with large perihelion distances |
context. comets are planetesimals left over from the formation of planets in the solar system. with a growing number of observed molecular abundances in many comets, and an improved understanding of chemical evolution in protoplanetary disk midplanes, comparisons can be made between models and observations that could potentially constrain the formation histories of comets.aims: our aim is to carry out the first statistical comparison between cometary volatile ice abundances and modelled evolving abundances in a protoplanetary disk midplane.methods: a χ2-method was used to determine maximum likelihood surfaces for 14 different comets that formed at a given time (up to 8 myr) and place (out to beyond the co iceline) in the pre-solar nebula midplane. this was done using observed volatile abundances for the 14 comets and the evolution of volatile abundances from chemical modelling of disk midplanes. two assumptions for the chemical modelling starting conditions (cloud inheritance or chemical reset), as well as two different sets of cometary molecules (parent species, with or without sulphur species) were investigated.results: considering all parent species (ten molecules) in the reset scenario, χ2 likelihood surfaces show a characteristic trail in the parameter space with high likelihood of formation around 30 au at early times and 12 au at later times for ten comets. this trail roughly traces the vicinity of the co iceline in time.conclusions: a statistical comparison between observed and modelled chemical abundances in comets and comet-forming regions could be a powerful tool for constraining cometary formation histories. the formation histories for all comets were constrained to the vicinity of the co iceline, assuming that the chemistry was partially reset early in the pre-solar nebula. this is found, both when considering carbon-, oxygen-, and sulphur-bearing molecules (ten in total), and when only considering carbon- and oxygen-bearing molecules (seven in total). since these 14 comets did not previously fall into the same taxonomical categories together, this chemical constraint may be proposed as an alternative taxonomy for comets. based on the most likely time for each of these comets to have formed during the disk chemical evolution, a formation time classification for the 14 comets is suggested. | cometary compositions compared with protoplanetary disk midplane chemical evolution. an emerging chemical evolution taxonomy for comets |
2i/borisov is the first-ever observed interstellar comet (and the second detected interstellar object (iso)). it was discovered on 2019 august 30 and has a heliocentric orbital eccentricity of ∼3.35, corresponding to a hyperbolic orbit that is unbound to the sun. given that it is an iso, it is of interest to compare its properties—such as composition and activity—with the comets in our solar system. this study reports low-resolution optical spectra of 2i/borisov. the spectra were obtained by the mdm observatory hiltner 2.4 m telescope/ohio state multi-object spectrograph (on 2019 october 31.5 and november 4.5, ut). the wavelength coverage spanned from 3700 to 9200 å. the dust continuum reflectance spectra of 2i/borisov show that the spectral slope is steeper in the blue end of the spectrum (compared to the red). the spectra of 2i/borisov clearly show cn emission at 3880 å, as well as c2 emission at both 4750 and 5150 å. using a haser model to covert the observed fluxes into estimates for the molecular production rates, we find q(cn) = 2.4 ± 0.2 × 1024 s-1, and q(c2) = (5.5 ± 0.4) × 1023 s-1 at the heliocentric distance of 2.145 au. our q(cn) estimate is consistent with contemporaneous observations, and the q(c2) estimate is generally below the upper limits of previous studies. we derived the ratio q(c2)/q(cn) = 0.2 ± 0.1, which indicates that 2i/borisov is depleted in carbon-chain species, but is not empty. this feature is not rare for the comets in our solar system, especially in the class of jupiter-family comets. | detection of diatomic carbon in 2i/borisov |
as discovery of complex molecules and ions in our solar system and the interstellar medium has proliferated, several groups have turned to laboratory experiments in an effort to simulate and understand these chemical processes. so far only infrared (ir) and ultraviolet (uv) spectroscopy has been able to directly probe these reactions in ices in their native, low-temperature states. here we report for the first time results using a complementary technique that harnesses two-step two-color laser ablation and ionization to measure mass spectra of energetically processed astrophysical and cometary ice analogs directly without warming the ices—a method for hands-off in situ ice analysis. electron bombardment and uv irradiation of h2o, ch3oh, and nh3 ices at 5 k and 70 k led to complex irradiation products, including hco, ch3co, formamide, acetamide, methyl formate, and hcn. many of these species, whose assignment was also strengthened by isotope labeling studies and correlate with ir-based spectroscopic studies of similar irradiated ices, are important ingredients for the building blocks of life. some of them have been detected previously via astronomical observations in the interstellar medium and in cometary comae. other species such as ch3co (acetyl) are yet to be detected in astrophysical ices or interstellar medium. our studies suggest that electron and uv photon processing of astrophysical ice analogs leads to extensive chemistry even in the coldest reaches of space, and lend support to the theory of comet-impact-induced delivery of complex organics to the inner solar system. | direct detection of complex organic products in ultraviolet (lyα) and electron-irradiated astrophysical and cometary ice analogs using two-step laser ablation and ionization mass spectrometry |
compressed ultrafast photography (cup), a computational imaging technique, is synchronized with short-pulsed laser illumination to enable dynamic three-dimensional (3d) imaging. by leveraging the time-of-flight (tof) information of pulsed light backscattered by the object, tof-cup can reconstruct a volumetric image from a single camera snapshot. in addition, the approach unites the encryption of depth data with the compressed acquisition of 3d data in a single snapshot measurement, thereby allowing efficient and secure data storage and transmission. we demonstrated high-speed 3d videography of moving objects at up to 75 volumes per second. the tof-cup camera was applied to track the 3d position of a live comet goldfish. we have also imaged a moving object obscured by a scattering medium. | encrypted three-dimensional dynamic imaging using snapshot time-of-flight compressed ultrafast photography |
the study of dust, the most abundant material in cometary nuclei, is pivotal in understanding the original materials forming the solar system. measuring the coma phase function provides a tool to investigate the nature of cometary dust. rosetta/osiris sampled the coma phase function of comet 67p/churyumov-gerasimenko, covering a large phase angle range in a small amount of time. twelve series were acquired in the period from 2015 march to 2016 february for this scientific purpose. these data allowed, after stray light removal, measuring the phase function shape, its reddening, and phase reddening while varying heliocentric and nucleocentric distances. despite small dissimilarities within different series, we found a constant overall shape. the reflectance has a u-shape with minimum at intermediate phase angles, reaching similar values at the smallest and largest phase angle sampled. the comparison with cometary phase functions in literature indicates osiris curves being consistent with the ones found in many other single comets. the dust has a negligible phase reddening at α < 90°, indicating a coma dominated by single scattering. we measured a reddening of [11-14] %/100 nm between 376 and 744 nm. no trend with heliocentric or nucleocentric distance was found, indicating the coma doesn't change its spectrum with time. these results are consistent with single coma grains and close-nucleus coma photometric results. comparison with nucleus photometry indicates a different backscattering phase function shape and similar reddening values only at α < 30°. at larger phase angles, the nucleus becomes significantly redder than the coma. | the scattering phase function of comet 67p/churyumov-gerasimenko coma as seen from the rosetta/osiris instrument |
nanotechnology has rapidly entered into human society, revolutionized many areas, including technology, medicine and cosmetics. this progress is due to the many valuable and unique properties that nanomaterials possess. in turn, these properties might become an issue of concern when considering potentially uncontrolled release to the environment. the rapid development of new nanomaterials thus raises questions about their impact on the environment and human health. this review focuses on the potential of nanomaterials to cause genotoxicity and summarizes recent genotoxicity studies on metal oxide/silica nanomaterials. though the number of genotoxicity studies on metal oxide/silica nanomaterials is still limited, this endpoint has recently received more attention for nanomaterials, and the number of related publications has increased. an analysis of these peer reviewed publications over nearly two decades shows that the test most employed to evaluate the genotoxicity of these nanomaterials is the comet assay, followed by micronucleus, ames and chromosome aberration tests. based on the data studied, we concluded that in the majority of the publications analysed in this review, the metal oxide (or silica) nanoparticles of the same core chemical composition did not show different genotoxicity study calls (i.e. positive or negative) in the same test, although some results are inconsistent and need to be confirmed by additional experiments. where the results are conflicting, it may be due to the following reasons: (1) variation in size of the nanoparticles; (2) variations in size distribution; (3) various purities of nanomaterials; (4) variation in surface areas for nanomaterials with the same average size; (5) differences in coatings; (6) differences in crystal structures of the same types of nanomaterials; (7) differences in size of aggregates in solution/media; (8) differences in assays; (9) different concentrations of nanomaterials in assay tests. indeed, due to the observed inconsistencies in the recent literature and the lack of adherence to appropriate, standardized test methods, reliable genotoxicity assessment of nanomaterials is still challenging. | genotoxicity of metal oxide nanomaterials: review of recent data and discussion of possible mechanisms |
context. this paper describes the initial modelling of gas and dust data acquired in august and september 2014 from the european space agency's rosetta spacecraft when it was in close proximity to the nucleus of comet 67p/churyumov-gerasimenko.aims: this work is an attempt to provide a self-consistent model of the innermost gas and dust coma of the comet, as constrained by the rosetta orbiter spectrometer for ion and neutral analysis (rosina) data set for the gas and by the optical, spectroscopic, and infrared remote imaging system (osiris) data set for the dust.methods: the model uses a previously developed shape model for the nucleus, and from this the water sublimation rate and gas temperatures at the surface are computed with a simple thermal model. the gas expansion is modelled with a 3d parallel implementation of a direct simulation monte carlo algorithm. a dust drag algorithm is then used to produce dust densities in the coma, which are then converted to brightnesses using mie theory and a line-of-sight integration.results: we show that a purely insolation-driven model for surface outgassing does not produce a reasonable fit to rosina/cops data. a stronger source in the "neck" region of the nucleus (region hapi) is needed to match the observed modulation of the gas density in detail. this agrees with osiris data, which shows that the dust emission from the "neck" was dominant in the august-september 2014 time frame. the current model matches this observation reasonably if a power index of 2-3 for the dust size distribution is used. a better match to the osiris data is seen by using a single large particle size for the coma.conclusions: we have shown possible solutions to the gas and dust distributions in the inner coma, which are consistent with rosina and osiris data. | modelling observations of the inner gas and dust coma of comet 67p/churyumov-gerasimenko using rosina/cops and osiris data: first results |
this paper describes the simulation and analysis work on rosetta lander philae's touchdown dynamics which was done to support its landing site selection process. the simulation part consists of a numerical multi-body simulation to describe philae's touchdown dynamics. suitable performance metrics in conjunction with monte carlo trajectory data from the flight dynamics analysis yields landing area specific landing gear performance and safety figures. these were then incorporated into the site selection process with regard to landing system performance margins and touchdown safety. while philae finally made a nearly successful landing the actual flight data were used to review and discuss the applicability of the presented simulation and analysis scheme. | rosetta lander philae - landing performance and touchdown safety assessment |
gas detection around main-sequence stars is becoming more common with around 20 systems showing the presence of co. however, more detections are needed, especially around later spectral type stars to better understand the origin of this gas and refine our models. to do so, we carried out a survey of 10 stars with predicted high likelihoods of secondary co detection using alma in band 6. we looked for continuum emission of mm-dust as well as gas emission (co and cn transitions). the continuum emission was detected in 9/10 systems for which we derived the discs' dust masses and geometrical properties, providing the first mm-wave detection of the disc around hd 106906, the first mm-wave radius for hd 114082, 117214, hd 15745, hd 191089, and the first radius at all for hd 121191. a crucial finding of our paper is that we detect co for the first time around the young 10-16 myr old g1v star hd 129590, similar to our early sun. the gas seems colocated with its planetesimal belt and its total mass is likely in the range of (2-10) × 10-5 m⊕. this first gas detection around a g-type main-sequence star raises questions as to whether gas may have been released in the solar system as well in its youth, which could potentially have affected planet formation. we also detected co gas around hd 121191 at a higher signal-to-noise ratio than previously and find that the co lies much closer-in than the planetesimals in the system, which could be evidence for the previously suspected co viscous spreading owing to shielding preventing its photodissociation. finally, we make estimates for the co content in planetesimals and the hcn/co outgassing rate (from cn upper limits), which we find are below the level seen in solar system comets in some systems. | survey of planetesimal belts with alma: gas detected around the sun-like star hd 129590 |
the upper atmospheres of unmagnetized solar system bodies interact more directly with their local plasma environment than their counterparts on magnetized bodies such as earth. one consequence of this interaction is that atmospheric particles can gain energy from the flowing plasma, as well as solar photons, and escape to space. escape proceeds through a number of different mechanisms that can remove neutral particles (jeans escape, photochemical escape, and sputtering) and mechanisms that can remove ions (ion pickup, magnetic shear and tension-related escape, and pressure gradients). here we discuss the plasma interactions and escape processes and rates from five solar system objects spanning 3 orders of magnitude in size: comets, pluto, titan, mars, and venus. we describe similarities and differences in escape for the different objects and provide four open questions that should be addressed in the coming years. | atmospheric escape from unmagnetized bodies |
comets are thought to have information about the formation process of our solar system. recently, detailed information about comet 67p/churyumov-gerasimenko was found by rosetta. it is remarkable that its tensile strength was estimated. in this paper, we measure and formulate the tensile strength of porous dust aggregates using numerical simulations, motivated by a porous dust aggregation model of planetesimal formation. we perform three-dimensional numerical simulations using a monomer interaction model with a periodic boundary condition. we stretch out a dust aggregate with a various initial volume filling factor between 10-2 and 0.5. we find that the tensile stress takes the maximum value at the time when the volume filling factor decreases to about half of the initial value. the maximum stress is defined to be the tensile strength. we take an average of the results with 10 different initial shapes to smooth out the effects of initial shapes of aggregates. finally, we numerically obtain the relation between the tensile strength and the initial volume filling factor of dust aggregates. we also use a simple semi-analytical model and successfully reproduce the numerical results, which enables us to apply a wide parameter range and different materials. the obtained relation is consistent with previous experiments and numerical simulations about silicate dust aggregates. we estimate that the monomer radius of comet 67p has to be about 3.3-220 μm to reproduce its tensile strength using our model. | tensile strength of porous dust aggregates |
we present individual dynamical masses for the nearby m9.5+t5.5 binary wise j072003.20-084651.2ab, a.k.a. scholz’s star. combining high-precision canada-france-hawaii telescope/wircam photocenter astrometry and keck adaptive optics resolved imaging, we measure the first high-quality parallactic distance ({6.80}-0.06+0.05 pc) and orbit ({8.06}-0.25+0.24yr period) for this system composed of a low-mass star and brown dwarf. we find a moderately eccentric orbit (e={0.240}-0.010+0.009), incompatible with previous work based on less data, and dynamical masses of 99 ± 6 {m}jup} and 66 ± 4 {m}jup} for the two components. the primary mass is marginally inconsistent (2.1σ) with the empirical mass-magnitude-metallicity relation and models of main-sequence stars. the relatively high mass of the cold ({t}eff}=1250+/- 40 k) brown dwarf companion indicates an age older than a few gigayears, in accord with age estimates for the primary star, and is consistent with our recent estimate of ≈70 {m}jup} for the stellar/substellar boundary among the field population. our improved parallax and proper motion, as well as an orbit-corrected system velocity, improve the accuracy of the system’s close encounter with the solar system by an order of magnitude. wise j0720-0846ab passed within 68.7 ± 2.0 kau of the sun 80.5 ± 0.7 kyr ago, passing through the outer oort cloud where comets can have stable orbits. | wise j072003.20-084651.2b is a massive t dwarf |
context. early observations of comet c/2016 r2 (panstarrs) have shown that the composition of this comet is very peculiar. radio observations have revealed a co-rich and hcn-poor comet and an optical coma dominated by strong emission bands of co+ and, more surprisingly, n2+.aims: the strong detection of n2+ in the coma of c/2016 r2 provided an ideal opportunity to measure the 14n/15n isotopic ratio directly from n2+ for the first time, and to estimate the n2/co ratio, which is an important diagnostic to constrain formation models of planetesimals, in addition to the more general study of coma composition.methods: we obtained high resolution spectra of the comet in february 2018 when it was at 2.8 au from the sun. we used the uves spectrograph of the european southern observatory very large telescope, complemented with narrowband images obtained with the trappist telescopes.results: we detect strong emissions from the n2+ and co+ ions, but also co2+, emission lines from the ch radical, and much fainter emissions of the cn, c2, and c3 radicals that were not detected in previous observations of this comet. we do not detect oh or h2o+, and we derive an upper limit of the h2o+/co+ ratio of 0.4, implying that the comet has a low water abundance. we measure a n2+/co+ ratio of 0.06 ± 0.01. the non-detection of nh2 indicates that most of the nitrogen content of the comet is in n2. together with the high n2+/co+ ratio, this could indicate a low formation temperature of the comet or that the comet is a fragment of a large differentiated kuiper belt object. the co2+/co+ ratio is 1.1 ± 0.3. we do not detect 14n15n+ lines and can only put a lower limit on the 14n/15n ratio (measured from n2+) of about 100, which is compatible with measurements of the same isotopic ratio for nh2 and cn in other comets. finally, in addition to the [oi] and [ci] forbidden lines, we detect for the first time the forbidden nitrogen lines [ni] doublet at 519.79 and 520.03 nm in the coma of a comet. | high resolution optical spectroscopy of the n2-rich comet c/2016 r2 (panstarrs) |
based on the occurrence rates implied by the discoveries of 1i/'oumuamua and 2i/borisov, the forthcoming rubin observatory legacy survey of space and time (lsst) should detect ≥one interstellar object every year. we advocate for future measurements of the production rates of h2o, co2, and co in these objects to estimate their carbon-to-oxygen ratios, which trace formation locations within their original protoplanetary disks. we review similar measurements for solar system comets, which indicate formation interior to the co snow line. by quantifying the relative processing in the interstellar medium and solar system, we estimate that production rates will not be representative of primordial compositions for the majority of interstellar comets. preferential desorption of co and co2 relative to h2o in the interstellar medium implies that measured c/o ratios represent lower limits on the primordial ratios. specifically, production rate ratios of q(co)/q(h2o) < 0.2 and q(co)/q(h2o) > 1 likely indicate formation interior and exterior to the co snow line, respectively. the high c/o ratio of 2i/borisov implies that it formed exterior to the co snow line. we provide an overview of the currently operational facilities capable of obtaining these measurements that will constrain the fraction of ejected comets that formed exterior to the co snow line. this fraction will provide key insights into the efficiency of and mechanisms for cometary ejection in exoplanetary systems. | the volatile carbon-to-oxygen ratio as a tracer for the formation locations of interstellar comets |
aims: using spectroscopic and continuum data measured by the miro instrument on board rosetta of comet 67p/churyumov-gerasimenko, it is possible to derive and track the change in the water production rate, to learn how the outgassing evolves with heliocentric distance. the miro data are well suited to investigate the evolution of 67p, in unprecedented spatial and temporal detail.methods: to obtain estimates of the local effective haser production rates we developed an efficient and reliable retrieval approach with precalculated lookup tables. we employed line area ratios (h216o/h218o) from pure nadir observations as the key variable, along with the doppler shift velocity, and continuum temperature. this method was applied to the miro data from august 2014 until april 2016. perihelion occurred on august 13, 2015 when the comet was 1.24 au from the sun.results: during the perihelion approach, the water production rates increased by an order of magnitude, and from the observations, the derived maximum for a single observation on august 29, 2015 is (1.42 ± 0.51) ×1028. modeling the data indicates that there is an offset in the peak outgassing, occurring 34 ± 10 days after perihelion. during the pre-perihelion phase, the production rate changes with heliocentric distance as rh-3.8±0.2; during post-perihelion, the dependence is rh-4.3±0.2. the comet is calculated to have lost 0.12 ± 0.06 % of its mass during the perihelion passage, considering only water ice sublimation. additionally, this method provides well sampled data to determine the spatial distribution of outgassing versus heliocentric distance. the time evolution is definitely not uniform across the surface. pre- and post-perihelion, the surface temperature on the southern hemisphere changes rapidly, as does the sublimation rate with an exponent of -6. there is a strong latitudinal dependence on the rh exponent with significant variation between northern and southern hemispheres, and so the average over the comet surface may only be of limited importance. we present more detailed regional variation in the outgassing, demonstrating that the highest derived production rates originate from the wosret, neith and bes regions during perihelion. the dataset used to make fig. 4 is only available at the cds via anonymous ftp to http://cdsarc.u-strasbg.fr (http://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?j/a+a/603/a87 | spatially resolved evolution of the local h2o production rates of comet 67p/churyumov-gerasimenko from the miro instrument on rosetta |
context. the rosetta spacecraft arrived at the comet 67p/churyumov-gerasimenko on august 6, 2014, which has made it possible to perform the first study of the solar wind interacting with the coma of a weakly outgassing comet.aims: it is shown that the solar wind experiences large deflections (>45°) in the weak coma. the average ion velocity slows from the mass loading of newborn cometary ions, which also slows the interplanetary magnetic field (imf) relative to the solar wind ions and subsequently creates a lorentz force in the frame of the solar wind. the lorentz force in the solar wind frame accelerates ions in the opposite direction of cometary pickup ion flow, and is necessary to conserve momentum.methods: data from the ion and electron sensor are studied over several intervals of interest when significant solar wind deflection was observed. the deflections for protons and for he++ were compared with the flow of cometary pickup ions using the instrument's frame of reference. we then fit the data with a three-dimensional maxwellian, and rotated the flow vectors into the comet sun equatorial coordinate system, and compared the flow to the spacecraft's position and to the local imf conditions.results: our observations show that the solar wind may be deflected in excess of 45° from the anti-sunward direction. furthermore, the deflections change direction on a variable timescale. solar wind protons are consistently more deflected than the he++. the deflections are not ordered by the spacecraft's position relative to the comet, but large changes in deflection are related to changes in the orthogonal imf components. | rosetta observations of solar wind interaction with the comet 67p/churyumov-gerasimenko |
in this paper we suggest penning ionization as an important route of formation for ionic species in upper planetary atmospheres. our goal is to provide relevant tools to researchers working on kinetic models of atmospheric interest, in order to include penning ionizations in their calculations as fast processes promoting reactions that cannot be neglected. ions are extremely important for the transmission of radio and satellite signals, and they govern the chemistry of planetary ionospheres. molecular ions have also been detected in comet tails. in this paper recent experimental results concerning production of simple ionic species of atmospheric interest are presented and discussed. such results concern the formation of free ions in collisional ionization of h2o, h2s, and nh3 induced by highly excited species (penning ionization) as metastable noble gas atoms. the effect of penning ionization still has not been considered in the modeling of terrestrial and extraterrestrial objects so far, even, though metastable helium is formed by radiative recombination of he+ ions with electrons. because helium is the second most abundant element of the universe, penning ionization of atomic or molecular species by he*(23s1) is plausibly an active route of ionization in relatively dense environments exposed to cosmic rays. | the possible role of penning ionization processes in planetary atmospheres |
recent observations of the long-period comet c/2016 r2 (panstarrs; hereafter r2) indicate an unusually high n2/co abundance ratio, typically larger than ∼0.05, and at least 2-3 times higher than the one measured in 67p/churyumov-gerasimenko. another striking compositional feature of this comet is its heavy depletion in h2o (h2o/co ∼ 0.32%), compared to other comets. here we investigate the formation circumstances of a generic comet whose composition reproduces these two key features. we first envisage the possibility that this comet agglomerated from clathrates, but we find that such a scenario does not explain the observed low water abundance. we then alternatively investigate the possibility that the building blocks of r2 agglomerated from grains and pebbles made of pure condensates via the use of a disk model describing the radial transport of volatiles. we show that n2/co ratios reproducing the value estimated in this comet can be found in grains condensed in the vicinity of the co and n2 ice lines. moreover, high co/h2o ratios (>100 times the initial gas-phase value) can be found in grains condensed in the vicinity of the co ice line. if the building blocks of a comet assembled from such grains, they should present n2/co and co/h2o ratios consistent with the measurements made in r2's coma. our scenario indicates that r2 formed in a colder environment than the other comets that share more usual compositions. our model also explains the unusual composition of the interstellar comet 2l/borisov. | cold traps of hypervolatiles in the protosolar nebula at the origin of the peculiar composition of comet c/2016 r2 (panstarrs) |
evidence for cometary activity beyond jupiter's and saturn's orbits-such as that observed for centaurs and long-period comets-suggests that the thermal processing of comet nuclei starts long before they enter the inner solar system, where they are typically observed and monitored. such observations raise questions as to the depth of unprocessed material and whether the activity of jupiter-family comets (jfcs) can be representative of any primitive material. here we model the coupled thermal and dynamical evolution of jfcs, from the moment they leave their outer solar system reservoirs until their ejection into interstellar space. we apply a thermal evolution model to a sample of simulated jfcs obtained from dynamical simulations that successfully reproduce the orbital distribution of observed jfcs. we show that due to the stochastic nature of comet trajectories toward the inner solar system, all simulated jfcs undergo multiple heating episodes resulting in significant modifications of their initial volatile contents. a statistical analysis constrains the extent of such processing. we suggest that primordial condensed hypervolatile ices should be entirely lost from the layers that contribute to cometary activity observed today. our results demonstrate that understanding the orbital (and thus, heating) history of jfcs is essential when putting observations in a broader context. | thermal processing of jupiter-family comets during their chaotic orbital evolution |
the trappist-1 system is unique in that it has a chain of seven terrestrial earth-like planets located close to or in its habitable zone. in this paper, we study the effect of potential cometary impacts on the trappist-1 planets and how they would affect the primordial atmospheres of these planets. we consider both atmospheric mass loss and volatile delivery with a view to assessing whether any sort of life has a chance to develop. we ran n-body simulations to investigate the orbital evolution of potential impacting comets, to determine which planets are more likely to be impacted and the distributions of impact velocities. we consider three scenarios that could potentially throw comets into the inner region (i.e. within 0.1 au where the seven planets are located) from an (as yet undetected) outer belt similar to the kuiper belt or an oort cloud: planet scattering, the kozai-lidov mechanism, and galactic tides. for the different scenarios, we quantify, for each planet, how much atmospheric mass is lost and what mass of volatiles can be delivered over the age of the system depending on the mass scattered out of the outer belt. we find that the resulting high-velocity impacts can easily destroy the primordial atmospheres of all seven planets, even if the mass scattered from the outer belt is as low as that of the kuiper belt. however, we find that the atmospheres of the outermost planets f, g, and h can also easily be replenished with cometary volatiles (e.g. ∼ an earth ocean mass of water could be delivered). these scenarios would thus imply that the atmospheres of these outermost planets could be more massive than those of the innermost planets, and have volatiles-enriched composition. | cometary impactors on the trappist-1 planets can destroy all planetary atmospheres and rebuild secondary atmospheres on planets f, g, and h |
we show that the scattering phase functions of the coma and the nucleus of the comet 67p/churyumov-gerasimenko measured by the rosetta/optical, spectroscopic, and infrared remote imaging system (osiris) instrument can be reproduced by a particle model involving clustered, densely packed submicrometer-sized grains composed of organic material and larger micrometer-sized silicate grains. the simulated and measured coma phase functions suggest that near the nucleus scattering is dominated by large particles, and the size distribution of dust particles varies with time and/or local coma environment. further, we show that the measured nucleus phase function is consistent with the coma phase function by modeling a nucleus-sized object consisting of the same particles that explain the coma phase functions. | interpretation of the phase functions measured by the osiris instrument for comet 67p/churyumov-gerasimenko |
we directly measured twenty overhanging cliffs on the surface of comet 67p/churyumov-gerasimenko extracted from the latest shape model and estimated the minimum tensile strengths needed to support them against collapse under the comet's gravity. we find extremely low strengths of around 1 pa or less (1 to 5 pa, when scaled to a metre length). the presence of eroded material at the base of most overhangs, as well as the observed collapse of two features andthe implied previous collapse of another, suggests that they are prone to failure and that the true material strengths are close to these lower limits (although we only consider static stresses and not dynamic stress from, for example, cometary activity). thus, a tensile strength of a few pascals is a good approximation for the tensile strength of the 67p nucleus material, which is in agreement with previous work. we find no particular trends in overhang properties either with size over the 10-100 m range studied here or location on the nucleus. there are no obvious differences, in terms of strength, height or evidence of collapse, between the populations of overhangs on the two cometary lobes, suggesting that 67p is relatively homogenous in terms of tensile strength. low material strengths are supportive of cometary formation as a primordial rubble pile or by collisional fragmentation of a small body (tens of km). | tensile strength of 67p/churyumov-gerasimenko nucleus material from overhangs |
the exocometary origin of co gas has been confirmed in several extrasolar kuiper belts, with co ice abundances consistent with solar system comets. we here present a molecular survey of the β pictoris belt with the submillimeter array (sma), reporting upper limits for cn, hcn, hco+, n2h+, and h2co, as well as for h2s, ch3oh, sio, and dcn from archival alma data. nondetections can be attributed to rapid molecular photodissociation due to the a-star’s strong uv flux. cn is the longest lasting and most easily detectable molecule after co in this environment. we update our nonlocal thermodynamic equilibrium excitation model to include uv fluorescence, finding it plays a key role in co and cn excitation, and we use it to turn the sma cn/co flux ratio constraint into an upper limit of < 2.5% on the hcn/(co+co2) ratio of outgassing rates. this value is consistent with, but at the low end of, the broad range observed in solar system comets. if sublimation dominates outgassing, then this low value may be caused by decreased outgassing for the less volatile molecule hcn compared to co. if instead uv photodesorption or collisional vaporization of unbound grains dominates outgassing, then this low ratio of rates would imply a low ice abundance ratio, which would in turn indicate a variation in cometary cyanide abundances across planetary systems. to conclude, we make predictions for future molecular surveys and show that cn and hcn should be readily detectable with alma around β pictoris for solar-system-like exocometary compositions. | molecular reconnaissance of the β pictoris gas disk with the sma: a low hcn/(co+co2) outgassing ratio and predictions for future surveys |
silver nanoparticles (17.78 ± 12.12 nm) were synthesized by the reduction of 0.5 m silver nitrate using formaldehyde as reducing and triethylamine as promoting and stabilizing agent. the particles were grain like agglomerates with spherical, centered-face cubic and crystalline in nature. the sample was highly pure with amine (nh) as associated and capping molecules. further, the genotoxicity and oxidative stress of these particles were evaluated using labeo rohita ( l. rohita) as genetic model exposed (10-55 mg l-1 dose) through aquatic medium for 28 days. the cells were produced with micronuclei, fragmented, lobed and buds nuclei in dose dependent manner. the highest incidence of comet was recoded (27.34 ± 5.68) at 55 mg l-1 ag-nps and 14 days treatment. then frequency was decreased to 22.65 ± 6.66% after 28 days due to complex repair mechanism. moreover, the treatment also produces the oxidative stress and disturbs the level of gst in gill and liver tissue. there was a sharp decline in the activities of gst and this decrease of activity increase the mda content. further, the elevated level of gsh represents that the liver has started defensive mechanism against oxyraidcals. this study concluded, ag-nps are genotoxic in nature and produce micronuclei, comet cells and also induces oxidative stress in aquatic organisms. | assessment of toxicity in fresh water fish labeo rohita treated with silver nanoparticles |
observations of the coma near the nucleus of comet 67p/churyumov-gerasimenko (67p) made by the ies (ion and electron sensor) instrument onboard the rosetta orbiter during late 2014 showed that electron fluxes greatly exceeded solar wind electron fluxes. the ies is part of the rosetta plasma consortium. this paper reports on electron energy spectra measured by ies near the nucleus as well as approximate densities and average energies for the suprathermal electrons when the comet was at a heliocentric distance of about 3 au. comparisons are made with electron densities measured by other instruments. the high electron densities observed (e.g., ne ≈ 10-100 cm-3) must be associated with the cometary ion density enhancement created mainly by the photoionization of cometary gas by solar radiation; there are other processes that also contribute. quasineutrality requires that the electron and ion densities be the same, and under certain conditions an ambipolar electric field is required to achieve quasi-neutrality. we present the results of a test particle model of cometary ion pickup by the solar wind and a two-stream electron transport code and use these results to interpret the ies data. we also estimate the effects on the electron spectrum of a compression of the electron fluid parcel. the electrons detected by ies can have energies as high as about 100-200 ev near the comet on some occasions, in which case the hot electrons can significantly enhance ionization rates of neutrals via impact ionization. | suprathermal electrons near the nucleus of comet 67p/churyumov-gerasimenko at 3 au: model comparisons with rosetta data |
in the first paper of this series, we examined existing methods of optical meteor trajectory estimation and developed a novel method which simultaneously uses both the geometry and the dynamics of meteors to constrain their trajectories. we also developed a simulator which uses an ablation model to generate realistic synthetic meteor trajectories which we use to test meteor trajectory solvers. in this second paper, we perform simulation validation to estimate radiant and velocity accuracy, which may be achieved by various meteor observation systems as applied to several meteor showers. for low-resolution all-sky systems, where the meteor deceleration is generally not measurable, the multi-parameter fit method assuming a constant velocity better reproduces the radiant and speed of synthetic meteors. for moderate field of view systems, our novel method performs the best at all convergence angles, while multi-parameter fit methods generally produce larger speed errors. for high-resolution, narrow field of view systems, we find our new method of trajectory estimation reproduces radiant and speed more accurately than all other methods tested. the ablation properties of meteoroids are commonly found to be the limiting factor in velocity accuracy. we show that the true radiant dispersion of meteor showers can be reliably measured with moderate field of view (or more precise) systems provided appropriate methods of meteor trajectory estimation are employed. finally, we compare estimated and real angular radiant uncertainty and show that for the solvers tested the real radiant error is on average underestimated by a factor of two. | estimating trajectories of meteors: an observational monte carlo approach - ii. results |
context. one of the main aims of the esa rosetta mission is to study the origin of the solar system by exploring comet 67p/churyumov-gerasimenko at close range.aims: in this paper we discuss the origin and evolution of comet 67p/churyumov-gerasimenko in relation to that of comets in general and in the framework of current solar system formation models.methods: we use data from the osiris scientific cameras as basic constraints. in particular, we discuss the overall bi-lobate shape and the presence of key geological features, such as layers and fractures. we also treat the problem of collisional evolution of comet nuclei by a particle-in-a-box calculation for an estimate of the probability of survival for 67p/churyumov-gerasimenko during the early epochs of the solar system.results: we argue that the two lobes of the 67p/churyumov-gerasimenko nucleus are derived from two distinct objects that have formed a contact binary via a gentle merger. the lobes are separate bodies, though sufficiently similar to have formed in the same environment. an estimate of the collisional rate in the primordial, trans-planetary disk shows that most comets of similar size to 67p/churyumov-gerasimenko are likely collisional fragments, although survival of primordial planetesimals cannot be excluded.conclusions: a collisional origin of the contact binary is suggested, and the low bulk density of the aggregate and abundance of volatile species show that a very gentle merger must have occurred. we thus consider two main scenarios: the primordial accretion of planetesimals, and the re-accretion of fragments after an energetic impact onto a larger parent body. we point to the primordial signatures exhibited by 67p/churyumov-gerasimenko and other comet nuclei as critical tests of the collisional evolution. | comet 67p/churyumov-gerasimenko: constraints on its origin from osiris observations |
mercury's calcium exosphere varies in a periodic way with that planet's true anomaly. we show that this pattern can be explained by impact vaporization from interplanetary dust with variations being due to mercury's radial and vertical excursions through an interplanetary dust disk having an inclination within 5 degrees of the plane of mercury's orbit. both a highly inclined dust disk and a two-disk model (where the two disks have a mutual inclination) fail to reproduce the observed variation in calcium exospheric abundance with mercury true anomaly angle. however, an additional source of impacting dust beyond the nominal dust disk is required near mercury's true anomaly (ν) 25° ± 5°. this is close to but not coincident with mercury's true anomaly (ν = 45°) when it crosses comet 2p/encke's present day orbital plane. interestingly, the taurid meteor storms at earth, which are also due to comet encke, are observed to occur when earth's true anomaly is ±20 or so degrees before and after the position where earth and encke orbital planes cross. the lack of exact correspondence with the present day orbit of encke may indicate the width of the potential stream along mercury's orbit or a previous cometary orbit. the extreme energy of the escaping calcium, estimated to have a temperature >50,000 k if the source is thermal, cannot be due to the impact process itself but must be imparted by an additional mechanism such as dissociation of a calcium-bearing molecule or ionization followed by recombination. | impact vaporization as a possible source of mercury's calcium exosphere |
we will briefly recapitulate the beginning of modern cometary physic. then we will assess the results of the cometary flyby missions previous to esa's rosetta rendezvous with comet 67p/churyumov-gerasimenko. emphasis is given to the physical properties of cometary nuclei. we will relate the results of the rosetta mission to those of the flybys. a major conclusion is that the visited cometary nuclei seem to be alike but represent different stages of evolution. coma composition and appearance are not only controlled by the composition of the nucleus but also strongly influenced by the shape and rotation axis orientation of the nucleus and resulting seasons that generate varying surface coverage by back fall material. rosetta showed that the coma composition is not only varying spatially but also strongly with time during the perihelion passage. hence past interpretations of cometary coma observations have to be re-considered. finally, we will try to assess the impact of the cornerstone mission leading to a critical evaluation of the mission results. lessons learned from rosetta are discussed; major progress and open points in cometary research are reviewed. | cometary nuclei—from giotto to rosetta |
observations of protoplanetary disks have revealed them to be complex and dynamic, with vertical and radial transport of gas and dust occurring simultaneously with chemistry and planet formation. previous models of protoplanetary disks focused primarily on chemical evolution of gas and dust in a static disk, or dynamical evolution of solids in a chemically passive disk. in this paper, we present a new 1d method for modeling pebble growth and chemistry simultaneously. gas and small dust particles are allowed to diffuse vertically, connecting chemistry at all elevations of the disk. pebbles are assumed to form from the dust present around the midplane, inheriting the composition of ices at this location. we present the results of this model after 1 myr of disk evolution around a 1m ⊙ star at various locations both inside and outside the co snowline. we find that for a turbulent disk (α = 10-3), co is depleted from the surface layers of the disk by roughly 1-2 orders of magnitude, consistent with observations of protoplanetary disks. this is achieved by a combination of ice sequestration and decreasing uv opacity, both driven by pebble growth. further, we find the selective removal of ice species via pebble growth and sequestration can increase gas phase c/o ratios to values of approximately unity. however, our model is unable to produce c/o values of ~1.5-2.0 inferred from protoplanetary disk observations, implying selective sequestration of ice is not sufficient to explain c/o ratios >1. | chemical feedback of pebble growth: impacts on co depletion and c/o ratios |
context. the esa/rosetta mission made it possible to monitor the plasma environment of a comet, from near aphelion to perihelion conditions. to understand the complex dynamics and plasma structures found at the comet, a modelling effort must be carried out in parallel.aims: firstly, we present a 3d hybrid model of the cometary plasma environment including photoionisation, solar wind charge exchange, and electron ionisation reactions; this model is used in stationary and dynamic conditions (mimicking the solar wind variations), and is thus especially adapted to a weakly outgassing comet such as 67p/churyumov-gerasimenko, the target of the esa/rosetta mission. secondly, we use the model to study the respective effects of ionisation processes on the formation of the dayside macroscopic magnetic and density boundaries upstream of comet 67p in perihelion conditions at 1.3 au. thirdly, we explore and discuss the effects of these processes on the magnetic field line draping, ionisation rates, and composition in the context of the rosetta mission.methods: we used a new quasi-neutral hybrid model, originally designed for weakly magnetised planetary bodies, such as venus, mars, and titan, and adapted here to comets. ionisation processes were monitored individually and together following a probabilistic interaction scheme. three-dimensional paraboloid fits of the bow shock surface, identified for a magnetosonic mach number equal to 2, and of the cometopause surface, were performed for a more quantitative analysis.results: we show that charge exchange and electron ionisation play a major role in the formation of a bow shock-like structure far upstream, while photoionisation is the main driver at and below the cometopause boundary, within 1000 km cometocentric distance. charge exchange contributes to 42% of the total production rate in the simulation box, whereas production rates from electron ionisation and photoionisation reach 33% and 25%, respectively. we also discuss implications for rosetta's observations, regarding the detection of the bow shock and the cometopause. | hybrid modelling of cometary plasma environments. i. impact of photoionisation, charge exchange, and electron ionisation on bow shock and cometopause at 67p/churyumov-gerasimenko |
we report the detection of intense emission from magnesium and iron in mars' atmosphere caused by a meteor shower following comet siding spring's close encounter with mars. the observations were made with the imaging ultraviolet spectrograph, a remote sensing instrument on the mars atmosphere and volatile evolution spacecraft orbiting mars. ionized magnesium caused the brightest emission from the planet's atmosphere for many hours, resulting from resonant scattering of solar ultraviolet light. modeling suggests a substantial fluence of low-density dust particles 1-100 µm in size, with the large amount and small size contrary to predictions. the event created a temporary planet-wide ionospheric layer below mars' main dayside ionosphere. the dramatic meteor shower response at mars is starkly different from the case at earth, where a steady state metal layer is always observable but perturbations caused by even the strongest meteor showers are challenging to detect. | maven iuvs observations of the aftermath of the comet siding spring meteor shower on mars |
we construct a 3d map of the spatial density of ob stars within 500 pc from the sun using the hipparcos catalogue and find three large-scale stream-like structures that allow a new view on the solar neighbourhood. the spatial coherence of these blue streams and the monotonic age sequence over hundreds of parsecs suggest that they are made of young stars, similar to the young streams that are conspicuous in nearby spiral galaxies. the three streams are 1) the scorpius to canis majoris stream, covering 350 pc and 65 myr of star formation history; 2) the vela stream, encompassing at least 150 pc and 25 myr of star formation history; and 3) the orion stream, including not only the well-known orion ob1abcd associations, but also a large previously unreported foreground stellar group lying only 200 pc from the sun. the map also reveals a remarkable and previously unknown nearby ob association, between the orion stream and the taurus molecular clouds, which might be responsible for the observed structure and star formation activity in this cloud complex. this new association also appears to be the birthplace of betelgeuse, as indicated by the proximity and velocity of the red giant. if this is confirmed, it would solve the long-standing puzzle of the origin of betelgeuse. the well-known nearby star-forming low-mass clouds, including the nearby t and r associations lupus, cha, oph, cra, taurus, vela r1, and various low-mass cometary clouds in vela and orion, appear in this new view of the local neighbourhood to be secondary star formation episodes that most likely were triggered by the feedback from the massive stars in the streams. we also recover well-known star clusters of various ages that are currently cruising through the solar neighbourhood. finally, we find no evidence of an elliptical structure such as the gould belt, a structure we suggest is a 2d projection effect, and not a physical ring. table 3 is available in elctronic form at http://www.aanda.org | cosmography of ob stars in the solar neighbourhood |
the previously defined regions on the nucleus of comet 67p/churyumov-gerasimenko have been mapped back onto the 3d shap7 model of the nucleus (preusker et al., 2017). the resulting regional definition is therefore self-consistent with boundaries that are well defined in 3 dimensions. the facets belonging to each region are provided as supplementary material. the shape model has then been used to assess inhomogeneity of nucleus surface morphology within individual regions. several regions show diverse morphology. we propose sub-division of these regions into clearly identifiable units (sub-regions) and a comprehensive table is provided. the surface areas of each sub-region have been computed and statistics based on grouping of unit types are provided. the roughness of each region is also provided in a quantitative manner using a technique derived from computer graphics applications. the quantitative method supports the sub-region definition by showing that differences between sub-regions can be numerically justified. | regional unit definition for the nucleus of comet 67p/churyumov-gerasimenko on the shap7 model |
this contributed monograph is the first work to present the latest results and findings on the new topic and hot field of planetary exploration and sciences, e.g., lunar surface iron content and mare orientale basalts, earth's gravity field, martian radar exploration, crater recognition, ionosphere and astrobiology, comet ionosphere, exoplanetary atmospheres and planet formation in binaries. by providing detailed theory and examples, this book helps readers to quickly familiarize themselves with the field. in addition, it offers a special section on next-generation planetary exploration, which opens a new landscape for future exploration plans and missions. prof. shuanggen jin works at the shanghai astronomical observatory, chinese academy of sciences, china. dr. nader haghighipour works at the university of hawaii-manoa, usa. prof. wing-huen ip works at the national central university, taiwan. | planetary exploration and science: recent results and advances |
context. the rosetta spacecraft is currently escorting comet 67p/churyumov-gerasimenko until its perihelion approach at 1.2 au. this mission has provided unprecedented views into the interaction of the solar wind and the comet as a function of heliocentric distance.aims: we study the interaction of the solar wind and comet at large heliocentric distances (>2 au) using data from the rosetta plasma consortium ion and electron sensor (rpc-ies). from this we gain insight into the suprathermal electron distribution, which plays an important role in electron-neutral chemistry and dust grain charging.methods: electron velocity distribution functions observed by ies fit to functions used to previously characterize the suprathermal electrons at comets and interplanetary shocks. we used the fitting results and searched for trends as a function of cometocentric and heliocentric distance.results: we find that interaction of the solar wind with this comet is highly turbulent and stronger than expected based on historical studies, especially for this weakly outgassing comet. the presence of highly dynamical suprathermal electrons is consistent with observations of comets (e.g., giacobinni-zinner, grigg-skjellerup) near 1 au with higher outgassing rates. however, comet 67p/churyumov-gerasimenko is much farther from the sun and appears to lack an upstream bow shock.conclusions: the mass loading process, which likely is the cause of these processes, plays a stronger role at large distances from the sun than previously expected. we discuss the possible mechanisms that most likely are responsible for this acceleration: heating by waves generated by the pick-up ion instability, and the admixture of cometary photoelectrons. | suprathermal electron environment of comet 67p/churyumov-gerasimenko: observations from the rosetta ion and electron sensor |
comet c/2017 k2 (panstarrs) was discovered by the pan-starrs1 (ps1) survey on 2017 may 21 at a distance of 16.09 au from the sun, the second most distant discovery of an active comet. pre-discovery images in the ps1 archive back to 2014 and additional deep cfht images between 2013 may 10-13 showed the comet to be active at 23.75 au. we derive an upper limit to the nucleus radius of rn= 80 km, assuming a 4% albedo. the spectral reflectivity of the comet surface is similar to “fresh” regions seen on comet 67p/churyumov-gerasimenko using the rosetta osiris camera. pre-discovery photometry combined with new data obtained with megacam on the cfht show that the activity is consistent with co-ice sublimation and inconsistent with co2-ice sublimation. the ice sublimation models were run out to perihelion in 2022 at 1.8 au to predict the co production rates, assuming that the outgassing area does not change. assuming a canonical 4% active surface area for water-ice sublimation, we present production rate ratios, {q}{co}/{q}{{h}2{{o}}}, for a range of nucleus sizes. comparing these results with other co-rich comets, we derive a lower limit to the nucleus radius of ∼14 km. we present predictions for {q}{co} at a range of distances that will be useful for planning observations with the james webb space telescope and large ground-based facilities. | co-driven activity in comet c/2017 k2 (panstarrs) |
conditions in the protosolar nebula have left their mark in the composition of cometary volatiles, thought to be some of the most pristine material in the solar system. cometary compositions represent the end point of processing that began in the parent molecular cloud core and continued through the collapse of that core to form the protosun and the solar nebula, and finally during the evolution of the solar nebula itself as the cometary bodies were accreting. disentangling the effects of the various epochs on the final composition of a comet is complicated. but comets are not the only source of information about the solar nebula. protostellar disks around young stars similar to the protosun provide a way of investigating the evolution of disks similar to the solar nebula while they are in the process of evolving to form their own solar systems. in this way we can learn about the physical and chemical conditions under which comets formed, and about the types of dynamical processing that shaped the solar system we see today. | the composition of the protosolar disk and the formation conditions for comets |
passing stars can perturb the oort cloud, triggering comet showers and potentially extinction events on earth. we combine velocity measurements for the recently discovered, nearby, low-mass binary system wise j072003.20-084651.2 (“scholz’s star”) to calculate its past trajectory. integrating the galactic orbits of this ∼0.15 m⊙ binary system and the sun, we find that the binary passed within only 52+23-14 kau (0.25+0.11-0.07 pc) of the sun 70+15-10 kya (1σ uncertainties), i.e., within the outer oort cloud. this is the closest known encounter of a star to our solar system with a well-constrained distance and velocity. previous work suggests that flybys within 0.25 pc occur infrequently (∼0.1 myr-1). we show that given the low mass and high velocity of the binary system, the encounter was dynamically weak. using the best available astrometry, our simulations suggest that the probability that the star penetrated the outer oort cloud is ∼98%, but the probability of penetrating the dynamically active inner oort cloud (<20 kau) is ∼10-4. while the flyby of this system likely caused negligible impact on the flux of long-period comets, the recent discovery of this binary highlights that dynamically important oort cloud perturbers may be lurking among nearby stars. | the closest known flyby of a star to the solar system |
by using the imagery acquired by the optical, spectroscopic, and infrared remote imaging system wide-angle camera (osiris wac), we prepare a high-resolution morphological map of the rosetta sais final landing site, characterized by an outcropping consolidated terrain unit, a coarse boulder deposit and a fine particle deposit. thanks to the 0.014 m resolution images, we derive the pebbles/boulders size-frequency distribution (sfd) of the area in the size range of 0.07-0.70 m. sais' sfd is best fitted with a two-segment differential power law: the first segment is in the range 0.07-0.26 m, with an index of -1.7 ± 0.1, while the second is in the range 0.26-0.50 m, with an index of -4.2 +0.4/-0.8. the `knee' of the sfd, located at 0.26 m, is evident both in the coarse and fine deposits. when compared to the agilkia rosetta lander imaging system images, sais surface is almost entirely free of the ubiquitous, cm-sized debris blanket observed by philae. none the less, a similar sfd behaviour of agilkia, with a steeper distribution above ∼0.3 m, and a flatter trend below that, is observed. the activity evolution of 67p along its orbit provides a coherent scenario of how these deposits were formed. indeed, different lift pressure values occurring on the two locations and at different heliocentric distances explain the presence of the cm-sized debris blanket on agilkia observed at 3.0 au inbound. contrarily, sais activity after 2.1 au outbound has almost completely eroded the fine deposits fallen during perihelion, resulting in an almost dust-free surface observed at 3.8 au. | the pebbles/boulders size distributions on sais: rosetta's final landing site on comet 67p/churyumov-gerasimenko |
in the context of an increasing number of complex multiparametric dust coma models it was found convenient to construct an elementary model with a minimum number of parameters selected to represent the key processes acting on the dust. the models outputs can be used as a reference evaluation of these processes with rough estimates of the resulting dust properties e.g. velocity. the present work introduces three, universal, dimensionless parameters which characterize the dust motion in an expanding flow, and computes as a function of these parameters the dust terminal velocity, the time it takes to acquire it, and the distance at which it is acquired. the motion of dust grains is presented as a system of dimensionless ordinary differential equations the solution of which depends upon the above mentioned three parameters. the numerical integration of this system was performed over a wide range of parameter space covering the whole range of physically possible conditions. precomputed results of dust terminal velocity, time and distance where it is reached are presented in dimensionless form. to obtain dimensional values for a particular case it is sufficient to perform algebraic operations. | asymptotics for spherical particle motion in a spherically expanding flow |
in comets, iron and nickel are found in refractory dust particles or in metallic and sulfide grains1. so far, no iron- or nickel-bearing molecules have been observed in the gaseous coma of comets2. iron and a few other heavy atoms, such as copper and cobalt, have been observed only in two exceptional objects: the great comet of 18823 and, almost a century later, c/1965 s1 (ikeya-seki)4-9. these sungrazing comets approached the sun so closely that refractory materials sublimated, and their relative abundance of nickel to iron was similar to that of the sun and meteorites7. more recently, the presence of iron vapour was inferred from the properties of a faint tail in comet c/2006 p1 (mcnaught) at perihelion10, but neither iron nor nickel was reported in the gaseous coma of comet 67p/churyumov-gerasimenko by the in situ rosetta mission11. here we report that neutral fe i and ni i emission lines are ubiquitous in cometary atmospheres, even far from the sun, as revealed by high-resolution ultraviolet-optical spectra of a large sample of comets of various compositions and dynamical origins. the abundances of both species appear to be of the same order of magnitude, contrasting the typical solar system abundance ratio. | iron and nickel atoms in cometary atmospheres even far from the sun |
isotopic ratios provide a powerful tool for understanding the origins of materials, including the volatile and refractory matter within solar system bodies. recent high sensitivity observations of molecular isotopologues, in particular with alma, have brought us new information on isotopic ratios of hydrogen, carbon, nitrogen and oxygen in star and planet forming regions as well as the solar system objects. solar system exploration missions, such as rosetta and cassini, have given us further new insights. meanwhile, the recent development of sophisticated models for isotope chemistry including detailed gas-phase and grain surface reaction network has made it possible to discuss how isotope fractionation in star and planet forming regions is imprinted into the icy mantles of dust grains, preserving a record of the initial isotopic state of solar system materials. this chapter reviews recent progress in observations of molecular isotopologues in extra-solar planet forming regions, prestellar/protostellar cores and protoplanetary disks, as well as objects in our solar system -- comets, meteorites, and planetary/satellite atmospheres -- and discusses their connection by means of isotope chemical models. | the isotopic links from planet forming regions to the solar system |
spacecraft charging is problematic for low-energy plasma measurements. the charged particles are attracted to or repelled from the charged spacecraft, affecting both the energy and direction of travel of the particles. the ion composition analyzer (rpc-ica) on board the rosetta spacecraft is suffering from this effect. rpc-ica was measuring positive ions in the vicinity of comet 67p/churyumov-gerasimenko, covering an energy range of a few ev/q to 40 kev/q. the low-energy part of the data is, however, heavily distorted by the negatively charged spacecraft. in this study we use the spacecraft plasma interaction software to model the influence of the spacecraft potential on the ion trajectories and the corresponding distortion of the field of view (fov) of the instrument. the results show that the measurements are not significantly distorted when the ion energy corresponds to at least twice the spacecraft potential. below this energy the fov is often heavily distorted, but the distortion differs between different viewing directions. generally, ions entering the instrument close to the aperture plane are less affected than those entering with extreme elevation angles. | the influence of spacecraft charging on low-energy ion measurements made by rpc-ica on rosetta |
in this investigation, the head-on collision of dust-acoustic (da) multi-solitons in a magnetized space dusty plasma consisting of negative dust, maxwellian electrons and q-nonextensively distributed ions under the influence of polarization force is studied. the presence of the q-nonextensive ions yields eloquent alteration in the polarization force. an increase in the nonextensive parameter (via q) lead to escalate the polarization parameter. two-sided kdv equations are obtained by adopting poincaré-lighthill-kuo (plk) method. further, the direct hirota method is employed to carry out multi-solitons solutions of kdv equations. the q-nonextensive polarization force has a great impact on the phase shifts after the interaction of single, double, and triple-(da) solitons. the combined effects of various physical parameters such as polarization force parameter (via r), nonextensivity (via q) and magnetic field strength (via ω) on the phase shifts of da multi-solitons have also been illustrated. it is found that the magnetic field alters the polarization effect which leads to modify the phase shifts. the findings of our investigation may be helpful to explore the interaction of multi-solitons in a magnetized space dusty plasma such as planetary rings and comet tails where nonextensively distributed ions, negative dust and maxwellian electrons are prevalent. | head-on collision of multi-solitons in a magnetized space dusty plasma in the presence of nonextensive polarization force |
we use observations from the ion and electron sensor (ies) on board the rosetta spacecraft to study the relationship between the cometary suprathermal electrons and the drivers that affect their density and temperature. we fit the ies electron observations with the summation of two kappa distributions, which we characterize as a dense and warm population (∼10 cm-3 and ∼16 ev) and a rarefied and hot population (∼0.01 cm-3 and ∼43 ev). the parameters of our fitting technique determine the populations' density, temperature, and invariant kappa index. we focus our analysis on the warm population to determine its origin by comparing the density and temperature with the neutral density and magnetic field strength. we find that the warm electron population is actually two separate sub-populations: electron distributions with temperatures above 8.6 ev and electron distributions with temperatures below 8.6 ev. the two sub-populations have different relationships between their density and temperature. moreover, the two sub-populations are affected by different drivers. the hotter sub-population temperature is strongly correlated with neutral density, while the cooler sub-population is unaffected by neutral density and is only weakly correlated with magnetic field strength. we suggest that the population with temperatures above 8.6 ev is being heated by lower hybrid waves driven by counterstreaming solar wind protons and newly formed, cometary ions created in localized, dense neutral streams. to the best of our knowledge, this represents the first observations of cometary electrons heated through wave-particle interactions. | statistical analysis of suprathermal electron drivers at 67p/churyumov-gerasimenko |
context. within just two years, two interstellar objects (isos) - 1i/`oumuamua and 2i/borisov - have been discovered, the first of their kind. large quantities of planetesimals form as a by-product of planet formation. therefore, it seems likely that isos are former planetesimals that became somehow unbound from their parent star. the discoveries raise the question of the dominant iso formation process.aims: here, we concentrate on planetesimals released during another star's close flybys. such close flybys happen most frequently during the first 10 myr of a star's life. here, we quantify the amount of planetesimals released during close stellar flybys, their ejection velocity and likely composition.methods: we numerically study the dependence of the effect of parabolic flybys on the mass ratio between the perturber and parent star, the periastron distance, inclination, and angle of periastron.results: as expected, close prograde flybys of high-mass stars produce the most considerable amount of isos. especially flybys of stars with m > 5 m⊙ on trajectories closer than 250 au can lead to more planetesimals turning into isos than remaining bound to the parent star. even strongly inclined orbits do not significantly reduce the iso production; only retrograde flybys lead to a significantly lower iso production. for perturbers slightly more massive than the parent star, there is a competition between iso production and planetesimals being captured by the perturber. whenever isos are produced, they leave their parent system typically with velocities in the range of 0.5-2 km s−1. this ejection velocity is distinctly different to that of isos produced by planet scattering (~4-8 km s−1) and those shed during the stellar post-main-sequence phase (~0.1-0.2 km s−1). using the typical disc truncation radius in various cluster environments, we find that clusters like the orion nebula cluster are likely to produce the equivalent of 0.85 earth-masses of isos per star. in contrast, compact clusters like ngc 3603 could produce up to 50 earth-masses of isos per star. our solar-system probably produced the equivalent of 2-3 earth masses of isos, which left our solar system at a mean ejection velocity of 0.7 km s−1.conclusions: most isos produced by flybys should be comet-like, similar to borisov and unlike `oumuamua. isos originating from compact long-lived clusters would often show a deficiency in co. as soon as a statistically significant sample of isos is discovered, the combined information of their observed velocities and composition might help in constraining the dominant production process. | significant interstellar object production by close stellar flybys |
aims: far away from the sun, at around 3 au, the activity of comet 67p/churyumov-gerasimenko is low and changes with local time (solar insolation), with location (chemical heterogeneity of the surface), and with season. when the activity is very low because the total cross section of the comet against the sun is small, the solar wind has access to the surface of the comet and causes ion-induced sputtering of surface material, which we wish to observe.methods: we used the double focussing mass spectrometer (dfms) of the rosina experiment on esa's rosetta mission to search for mass spectrometric evidence of sputtered refractory species. in high-resolution mode, dfms can separate some of the mass peaks of refractory elements from the many volatile species present in the coma.results: at present, the locations of solar wind surface access are in the southern hemisphere of the comet (the local winter). of particular interest is sputtering of dust grains on the surface. we observe global averages over the winter hemisphere of the refractory elements na, k, si, and ca, presumably sputtered from grains residing on the surface. compared to carbonaceous chondrites, the comet has the same na abundance, is depleted in ca, and has an excess of k. in addition, for si the signal strength is strong enough to compile a coarse compositional map of the southern hemisphere. most, perhaps all, of the observed variation can be explained by the solar wind being affected by the atmosphere of the comet. | solar wind sputtering of dust on the surface of 67p/churyumov-gerasimenko |
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