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context: co is an abundant species in comets, creating co$^+$ ion with emission lines that can be observed in the optical spectral range. a good modeling of its fluorescence spectrum is important for a better measurement of the co$^+$ abundance. such a species, if abundant enough, can also be used to measure the $^{12}$c/$^{13}$c isotopic ratio. aims: this study uses the opportunity of a high co content observed in the comet c/2016 r2 (panstarrs), that created bright co$^{+}$ emission lines in the optical range, to build and test a new fluorescence model of this species and to measure for the first time the $^{12}$c/$^{13}$c isotopic ratio in this chemical species with ground-based observations. methods: thanks to laboratory data and theoretical works available in the scientific literature we developed a new fluorescence model both for $^{12}$co$^+$ and $^{13}$co$^+$ ions. the $^{13}$co$^+$ model can be used for coadding faint emission lines and obtain a sufficient signal-to-noise ratio to detect this isotopologue. results: our fluorescence model provides a good modeling of the $^{12}$co$^+$ emission lines, allowing to publish revised fluorescence efficiencies. based on similar transition probabilities for $^{12}$co$^+$ and $^{13}$co$^+$ we derive a $^{12}$c/$^{13}$c isotopic ratio of 73$\pm$20 for co$^+$ in comet c/2016 r2. this value is in agreement with the solar system ratio of 89$\pm$2 within the error bars, making the possibility that this comet was an interstellar object unlikely. | comets $^{12}$co$^+$ and $^{13}$co$^+$ fluorescence models for measuring the $^{12}$c/$^{13}$c isotopic ratio in co$^+$ |
context. comets are remnants of the icy planetesimals that formed beyond the ice line in the solar nebula. growing from μm-sized dust and ice particles to km-sized objects is, however, difficult because of growth barriers and time scale constraints. the gravitational collapse of pebble clouds that formed through the streaming instability may provide a suitable mechanism for comet formation.aims: we study the collisional compression of silica, ice, and silica/ice-mixed pebbles during gravitational collapse of pebble clouds. using the initial volume-filling factor and the dust-to-ice ratio of the pebbles as free parameters, we constrain the dust-to-ice mass ratio of the formed comet and the resulting volume-filling factor of the pebbles, depending on the cloud mass.methods: we use the representative particle approach, which is a monte carlo method, to follow cloud collapse and collisional evolution of an ensemble of ice, silica, and silica/ice-mixed pebbles. therefore, we developed a collision model which takes the various collision properties of dust and ice into account. we study pebbles with a compact size of 1 cm and vary the initial volume-filling factors, φ0, ranging from 0.001 to 0.4. we consider mixed pebbles as having dust-to-ice ratios between 0.5 and 10. we investigate four typical cloud masses, m, between 2.6 × 1014 (very low) and 2.6 × 1023 g (high).results: except for the very low-mass cloud (m = 2.6 × 1014 g), silica pebbles are always compressed during the collapse and attain volume-filling factors in the range from ⟨ φ ⟩ v ≈ 0.22 to 0.43, regardless of φ0. ice pebbles experience no significant compression in very low-mass clouds. they are compressed to values in the range ⟨ φ ⟩ v ≈ 0.11 to 0.17 in low- and intermediate-mass clouds (m = 2.6 × 1017-2.6 × 1020 g); in high-mass clouds (m = 2.6 × 1023 g), ice pebbles end up with ⟨ φ ⟩ v ≈ 0.23. mixed pebbles obtain filling factors in between the values for pure ice and pure silica. we find that the observed cometary density of ~0.5 g cm-3 can only be explained by either intermediate- or high-mass clouds, regardless of φ0, and also by either very low- or low-mass clouds for initially compact pebbles. in any case, the dust-to-ice ratio must be in the range of between 3 ≲ ξ ≲ 9 to match the observed bulk properties of comet nuclei. | comet formation in collapsing pebble clouds. what cometary bulk density implies for the cloud mass and dust-to-ice ratio |
galaxy clusters are known to harbour magnetic fields, the nature of which remains unresolved. intra-cluster magnetic fields can be observed at the density contact discontinuity formed by cool and dense plasma running into hot ambient plasma1,2, and the discontinuity exists3 near the second-brightest galaxy4, mrc 0600-399, in the merging galaxy cluster abell 3376 (redshift 0.0461). elongated x-ray emission in the east-west direction shows a comet-like structure that reaches the megaparsec scale5. previous radio observations6,7 detected the bent jets from mrc 0600-399, moving in same direction as the sub-cluster, against ram pressure. here we report radio8,9 observations of mrc 0600-399 that have 3.4 and 11 times higher resolution and sensitivity, respectively, than the previous results6. in contrast to typical jets10,11, mrc 0600-399 shows a 90-degree bend at the contact discontinuity, and the collimated jets extend over 100 kiloparsecs from the point of the bend. we see diffuse, elongated emission that we name `double-scythe' structures. the spectral index flattens downstream of the bend point, indicating cosmic-ray reacceleration. high-resolution numerical simulations reveal that the ordered magnetic field along the discontinuity has an important role in the change of jet direction. the morphology of the double-scythe jets is consistent with the simulations. our results provide insights into the effect of magnetic fields on the evolution of the member galaxies and intra-cluster medium of galaxy clusters. | jets from mrc 0600-399 bent by magnetic fields in the cluster abell 3376 |
previous studies of cometary impacts in the outer solar system used the spatial distribution of ecliptic comets (ecs) from dynamical models that assumed ecs began on low-inclination orbits (≲5°) in the kuiper belt. in reality, the source population of ecs-the trans-neptunian scattered disk-has orbital inclinations reaching up to ~30°. in nesvorný et al., we developed a new dynamical model of ecs by following comets as they evolved from the scattered disk to the inner solar system. the model was absolutely calibrated from the population of centaurs and active ecs. here we use our ec model to determine the steady-state impact flux of cometary/centaur impactors on jupiter, saturn, uranus, and their moons. relative to previous work, we find slightly higher impact probabilities on the outer moons and lower impact probabilities on the inner moons. the impact probabilities are smaller when comet disruption is accounted for. the results provide a modern framework for the interpretation of the cratering record in the outer solar system. | impact rates in the outer solar system |
the astrochemistry of co2 ice analogues has been a topic of intensive investigation due to the prevalence of co2 throughout the interstellar medium and the solar system, as well as the possibility of it acting as a carbon feedstock for the synthesis of larger, more complex organic molecules. in order to accurately discern the physico-chemical processes in which co2 plays a role, it is necessary to have laboratory-generated spectra to compare against observational data acquired by ground- and space-based telescopes. a key factor which is known to influence the appearance of such spectra is temperature, especially when the spectra are acquired in the infrared and ultraviolet. in this present study, we describe the results of a systematic investigation looking into: (i) the influence of thermal annealing on the mid-ir and vuv absorption spectra of pure, unirradiated co2 astrophysical ice analogues prepared at various temperatures, and (ii) the influence of temperature on the chemical products of electron irradiation of similar ices. our results indicate that both mid-ir and vuv spectra of pure co2 ices are sensitive to the structural and chemical changes induced by thermal annealing. furthermore, using mid-ir spectroscopy, we have successfully identified the production of radiolytic daughter molecules as a result of 1 kev electron irradiation and the influence of temperature over this chemistry. such results are directly applicable to studies on the chemistry of interstellar ices, comets, and icy lunar objects and may also be useful as reference data for forthcoming observational missions. | mid-ir and vuv spectroscopic characterisation of thermally processed and electron irradiated co2 astrophysical ice analogues |
scientists use the earth as a tool for astrobiology by analyzing planetary field analogues (i.e. terrestrial samples and field sites that resemble planetary bodies in our solar system). in addition, they expose the selected planetary field analogues in simulation chambers to conditions that mimic the ones of planets, moons and low earth orbit (leo) space conditions, as well as the chemistry occurring in interstellar and cometary ices. this paper reviews the ways the earth is used by astrobiologists: (i) by conducting planetary field analogue studies to investigate extant life from extreme environments, its metabolisms, adaptation strategies and modern biosignatures; (ii) by conducting planetary field analogue studies to investigate extinct life from the oldest rocks on our planet and its biosignatures; (iii) by exposing terrestrial samples to simulated space or planetary environments and producing a sample analogue to investigate changes in minerals, biosignatures and microorganisms. the european space agency (esa) created a topical team in 2011 to investigate recent activities using the earth as a tool for astrobiology and to formulate recommendations and scientific needs to improve ground-based astrobiological research. space is an important tool for astrobiology (see horneck et al. in astrobiology, 16:201-243, 2016; cottin et al., 2017), but access to space is limited. complementing research on earth provides fast access, more replications and higher sample throughput. the major conclusions of the topical team and suggestions for the future include more scientifically qualified calls for field campaigns with planetary analogy, and a centralized point of contact at esa or the eu for the organization of a survey of such expeditions. an improvement of the coordinated logistics, infrastructures and funding system supporting the combination of field work with planetary simulation investigations, as well as an optimization of the scientific return and data processing, data storage and data distribution is also needed. finally, a coordinated eu or esa education and outreach program would improve the participation of the public in the astrobiological activities. | earth as a tool for astrobiology—a european perspective |
we present a study of interstellar comet 2i/2019 q4 (borisov) using both preperihelion and postperihelion observations spanning late september 2019 through late january 2020. the intrinsic brightness of the comet was observed to continuously decline throughout the timespan, likely due to the decreasing effective scattering cross-section as a result of volatile sublimation with a slope of -0.43 ± 0.02 km2 d-1. we witnessed no significant change in the slightly reddish color of the comet, with mean values of $\left\langle g-r\right\rangle =0.68\pm 0.04$ , $\left\langle r-i\right\rangle =0.23\pm 0.03$ , and the normalized reflectivity gradient across the g and i bands $\overline{s^{\prime} }\left(g,i\right)=\left(10.6\pm 1.4\right)$ % per 103 å, all unremarkable in the context of solar system comets. using the available astrometric observations, we confidently detect the nongravitational acceleration of the comet following a shallow heliocentric distance dependence of ${r}_{{\rm{h}}}^{-1\pm 1}$ . accordingly, we estimate that the nucleus is most likely $\lesssim 0.4$ km in radius, and that a fraction of ≳0.2% of the total mass of the nucleus has been eroded due to the sublimation activity since the earliest observation of the comet in 2018 december by the time of perihelion. our morphology simulation suggests that the dust ejection speed increased from ∼4 m s-1 in 2019 september to ∼7 m s-1 around perihelion for the optically dominant dust grains with $\beta \sim 0.01$ , and that the observable dust grains are no smaller than micron size. | physical characterization of interstellar comet 2i/2019 q4 (borisov) |
the spectrum of dicarbon (c2) is important in astrophysics and for spectroscopic studies of plasmas and flames. the c2 spectrum is characterized by many band systems with new ones still being actively identified; astronomical observations involve eight of these bands. recently, furtenbacher et al. presented a set of 5699 empirical energy levels for 12c2, distributed among 11 electronic states and 98 vibronic bands, derived from 42 experimental studies and obtained using the marvel (measured active rotational-vibrational energy levels) procedure. | an update to the marvel data set and exomol line list for 12c2 |
remote observations of comets, especially using high spectral resolution millimeter spectroscopy, have enabled the detection of over 25 molecules in comets for the last twenty years. among the molecules identified at radio wavelengths, complex organic molecules (coms) such as acetaldehyde, ethylene-glycol, formamide, methyl-formate or ethanol have been observed in several comets and their abundances relative to water and methanol precisely determined. significant upper limits on the abundance of several other coms have been determined and put constraints on the dominant isomer for three of them. the abundances measured in comets are generally of comparable order of magnitude as those measured in star-forming regions, suggesting that comets contain preserved material from the presolar cloud from which the solar system was born. | complex organic molecules in comets from remote-sensing observations at millimeter wavelengths |
measurements of midrapidity charged particle multiplicity distributions, $dn_{\rm ch}/d\eta$, and midrapidity transverse-energy distributions, $de_t/d\eta$, are presented for a variety of collision systems and energies. included are distributions for au$+$au collisions at $\sqrt{s_{_{nn}}}=200$, 130, 62.4, 39, 27, 19.6, 14.5, and 7.7 gev, cu$+$cu collisions at $\sqrt{s_{_{nn}}}=200$ and 62.4 gev, cu$+$au collisions at $\sqrt{s_{_{nn}}}=200$ gev, u$+$u collisions at $\sqrt{s_{_{nn}}}=193$ gev, $d$$+$au collisions at $\sqrt{s_{_{nn}}}=200$ gev, $^{3}$he$+$au collisions at $\sqrt{s_{_{nn}}}=200$ gev, and $p$$+$$p$ collisions at $\sqrt{s_{_{nn}}}=200$ gev. centrality-dependent distributions at midrapidity are presented in terms of the number of nucleon participants, $n_{\rm part}$, and the number of constituent quark participants, $n_{q{\rm p}}$. for all $a$$+$$a$ collisions down to $\sqrt{s_{_{nn}}}=7.7$ gev, it is observed that the midrapidity data are better described by scaling with $n_{q{\rm p}}$ than scaling with $n_{\rm part}$. also presented are estimates of the bjorken energy density, $\varepsilon_{\rm bj}$, and the ratio of $de_t/d\eta$ to $dn_{\rm ch}/d\eta$, the latter of which is seen to be constant as a function of centrality for all systems. | transverse energy production and charged-particle multiplicity at midrapidity in various systems from $\\sqrt{s_{nn}}=7.7$ to 200 gev |
recent field experiments conducted in the near-wake (up to 0.5 rotor diameters downwind of the rotor) of a 2.5 mw wind turbine using snow-based super-large-scale particle image velocimetery (slpiv) (hong et al., nature comm., vol. 5, 2014, no. 4216) were successful in visualizing tip vortex cores as areas devoid of snowflakes. the so-visualized snow voids, however, suggested tip vortex cores of complex shape consisting of circular cores with distinct elongated comet-like tails. we employ large-eddy simulation (les) to elucidate the structure and dynamics of the complex tip vortices identified experimentally. the les is shown to reproduce vortex cores in good qualitative agreement with the slpiv results, essentially capturing all vortex core patterns observed in the field in the tip shear layer. we show that the visualized vortex patterns are the result of energetic coherent dynamics in the rotor tip shear layer driven by interactions between the tip vortices and a second set of counter-rotating spiral vortices intertwined with the tip vortices. we further show that the mean flow within the region where such rich coherent dynamics occur satisfies the instability criterion proposed by leibovich and stewartson (j. fluid mech., vol. 126, 1983, pp. 335--356), indicating that the instability uncovered by the slpiv and the les is of centrifugal type. this study highlights the feasibility of employing snow voids to visualize tip vortices and demonstrates the enormous potential of integrating slpiv with les as a powerful tool for gaining novel insights into the wakes of utility scale wind turbines. | coherent dynamics in the rotor tip shear layer of utility-scale wind turbines |
we use neowise data from the four-band and three-band cryogenic phases of the wide-field infrared survey explorer mission to constrain size distributions of the comet populations and debias measurements of the short- and long-period comet (lpc) populations. we find that the fit to the debiased lpc population yields a cumulative size-frequency distribution (sfd) power-law slope (β) of -1.0 ± 0.1, while the debiased jupiter-family comet (jfc) sfd has a steeper slope with β = -2.3 ± 0.2. the jfcs in our debiased sample yielded a mean nucleus size of 1.3 km in diameter, while the lpcs’ mean size is roughly twice as large, 2.1 km, yielding mean size ratios (< {d}{lpc}> /< {d}{jfc}> ) that differ by a factor of 1.6. over the course of the 8 months of the survey, our results indicate that the number of lpcs passing within 1.5 au are a factor of several higher than previous estimates, while jfcs are within the previous range of estimates of a few thousand down to sizes near 1.3 km in diameter. finally, we also observe evidence for structure in the orbital distribution of lpcs, with an overdensity of comets clustered near 110° inclination and perihelion near 2.9 au that is not attributable to observational bias. | debiasing the neowise cryogenic mission comet populations |
hubble space telescope observations of interstellar comet 2i/borisov near perihelion show the ejection of large ( $\gtrsim 100$ μm) particles at $\lesssim 9$ m s-1 speeds, with estimated mass-loss rates of ∼35 kg s-1. the total mass loss from comet borisov corresponds to loss of a surface shell on the nucleus only ∼0.4 m thick. this shell is thin enough to be susceptible to past chemical processing in the interstellar medium by cosmic rays, meaning that the ejected materials cannot necessarily be considered as pristine. our high-resolution images reveal persistent asymmetry in the dust coma, best explained by a thermal lag on the rotating nucleus causing peak mass loss to occur in the comet nucleus afternoon. in this interpretation, the nucleus rotates with an obliquity of $30^\circ $ (pole direction r.a. = $205^\circ $ and decl. = $52^\circ $ ). the subsolar latitude varied from $-35^\circ $ (southern solstice) at the time of discovery to $0^\circ $ (equinox) in 2020 january, suggesting the importance of seasonal effects. subsequent activity likely results from regions freshly activated as the northern hemisphere is illuminated for the first time. | coma anisotropy and the rotation pole of interstellar comet 2i/borisov |
interstellar complex organic molecules are thought to be the building blocks of more complex prebiotic compounds. in particular, formamide (or methanimide, nh2cho), is presented as a multifunctional prebiotic precursor, the starting point of both pre-genetic and pre-metabolic species. nh2cho is widely observed in different astrophysical media as well as in comets that may have had a crucial role in the delivery of exogenous material to earth. in star-forming regions, gas phase synthesis of formamide is possible, even if it is still debated. in this paper we present laboratory experiments demonstrating formamide formation in interstellar ice analogues at astronomically relevant temperatures via simultaneous hydrogenation of no and h2co, two abundant molecules in star-forming regions. inclusion of the experimental results in an astrochemical gas-grain model confirms the importance of the investigated solid-state reaction channel leading a high yield of formamide in dark interstellar clouds, and adds a valuable perspective about the way this refractory molecule may have been part of the prebiotic molecular building blocks delivered to the young earth. | efficient formation route of the prebiotic molecule formamide on interstellar dust grains |
meteorites, fe and si-rich microspherules, positive ir and pt anomalies, and burned charcoal-rich hopewell habitation surfaces demonstrate that a cosmic airburst event occurred over the ohio river valley during the late holocene. a comet-shaped earthwork was constructed near the airburst epicenter. twenty-nine radiocarbon ages establish that the event occurred between 252 and 383 ce, a time when 69 near-earth comets were documented. while hopewell people survived the catastrophic event, it likely contributed to their cultural decline. the hopewell airburst event expands our understanding of the frequency and impact of cataclysmic cosmic events on complex human societies. | retracted article: the hopewell airburst event, 1699-1567 years ago (252-383 ce) |
the polar regions of the moon host some of the most extreme low temperatures in the inner solar system due to its low obliquity, lack of atmosphere, and topographic relief. some of these regions are already confirmed to host water ice. proposed sources of water and other volatiles include lunar volcanic outgassing, solar wind, and comet impacts. each of these possible sources would carry a potentially identifiable compositional signature beyond water. determining the dominant sources of lunar volatiles, therefore, requires assessing the long-term thermal stability of an array of compounds. we present results of mapping the surface thermal stability locations of multiple key volatiles, including water, from the diviner lunar radiometer data from 60° to 90° latitude in both hemispheres. we find the annual maximum temperature for each pixel of interest in the map (~300 m) to determine which volatiles of interest would be stable there. we report on the thermal stability area of each volatile, as well as the geologic context in some cases. we find that while the thermal stability area for volatiles is larger in the south pole generally, both the north pole and south pole host areas where potential tracer volatiles from lunar volcanism, solar wind, and cometary impacts would be thermally stable for billions of years if such volatiles were ever delivered. we find several areas equatorward of ~80° on the lunar nearside that could host water ice, where future missions could potentially access volatile deposits in order to place constraints on water delivery to the moon. | spatial distribution and thermal diversity of surface volatile cold traps at the lunar poles |
methane (ch4) is the second most important anthropogenic greenhouse gas, whose atmospheric concentration is modulated by human-induced activities, and it has a larger global warming potential than carbon dioxide (co2). because of its short atmospheric lifetime relative to that of co2, the reduction of the atmospheric abundance of ch4 is an attractive target for short-term climate mitigation strategies. however, reducing the atmospheric ch4 concentration requires a reduction of its emissions and, therefore, knowledge of its sources. for this reason, the co2 and methane (comet) campaign in may and june 2018 assessed emissions of one of the largest ch4 emission hot spots in europe, the upper silesian coal basin (uscb) in southern poland, using top-down approaches and inventory data. in this study, we will focus on ch4 column anomalies retrieved from spectral radiance observations, which were acquired by the 1d nadir-looking passive remote sensing methane airborne mapper (mamap) instrument, using the weighting-function-modified differential optical absorption spectroscopy (wfm-doas) method. the column anomalies, combined with wind lidar measurements, are inverted to cross-sectional fluxes using a mass balance approach. with the help of these fluxes, reported emissions of small clusters of coal mine ventilation shafts are then assessed. the mamap ch4 column observations enable an accurate assignment of observed fluxes to small clusters of ventilation shafts. ch4 fluxes are estimated for four clusters with a total of 23 ventilation shafts, which are responsible for about 40 % of the total ch4 mining emissions in the target area. the observations were made during several overflights on different days. the final average ch4 fluxes for the single clusters (or sub-clusters) range from about 1 to 9 t ch4 h-1 at the time of the campaign. the fluxes observed at one cluster during different overflights vary by as much as 50 % of the average value. associated errors (1σ) are usually between 15 % and 59 % of the average flux, depending mainly on the prevailing wind conditions, the number of flight tracks, and the magnitude of the flux itself. comparison to known hourly emissions, where available, shows good agreement within the uncertainties. if only emissions reported annually are available for comparison with the observations, caution is advised due to possible fluctuations in emissions during a year or even within hours. to measure emissions even more precisely and to break them down further for allocation to individual shafts in a complex source region such as the uscb, imaging remote sensing instruments are recommended. | quantification of ch4 coal mining emissions in upper silesia by passive airborne remote sensing observations with the methane airborne mapper (mamap) instrument during the co2 and methane (comet) campaign |
we present the first alma dust polarization observations toward the high-mass star-forming regions w51 e2, e8, and w51 north in band 6 (230 ghz) with a resolution of about 0\buildrel{\prime\prime}\over{.} 26 (∼5 mpc). polarized emission in all three sources is clearly detected and resolved. measured relative polarization levels are between 0.1% and 10%. while the absolute polarization shows complicated structures, the relative polarization displays the typical anticorrelation with stokes i, although with a large scatter. inferred magnetic (b) field morphologies are organized and connected. detailed substructures are resolved, revealing new features such as comet-shaped b-field morphologies in satellite cores, symmetrically converging b-field zones, and possibly streamlined morphologies. the local b-field dispersion shows some anticorrelation with the relative polarization. moreover, the lowest polarization percentages together with largest dispersions coincide with b-field convergence zones. we put forward \sin ω , where ω is the measurable angle between a local b-field orientation and local gravity, as a measure of how effectively the b field can oppose gravity. maps of \sin ω for all three sources show organized structures that suggest a locally varying role of the b field, with some regions where gravity can largely act unaffectedly, possibly in a network of narrow magnetic channels, and other regions where the b field can work maximally against gravity. | polarization properties and magnetic field structures in the high-mass star-forming region w51 observed with alma |
nitrogen-bearing molecules in cold molecular clouds exhibit a range of isotopic fractionation ratios and these molecules may be the precursors of 15n enrichments found in comets and meteorites. chemical model calculations indicate that atom-molecular ion and ion-molecule reactions could account for most of the fractionation patterns observed. however, recent quantum-chemical computations demonstrate that several of the key processes are unlikely to occur in dense clouds. related model calculations of dense cloud chemistry show that the revised 15n enrichments fail to match observed values. we have investigated the effects of these reaction rate modifications on the chemical model of wirström et al. (2012) for which there are significant physical and chemical differences with respect to other models. we have included 15n fractionation of cn in neutral-neutral reactions and also updated rate coefficients for key reactions in the nitrogen chemistry. we find that the revised fractionation rates have the effect of suppressing 15n enrichment in ammonia at all times, while the depletion is even more pronounced, reaching 14n/15n ratios of >2000. taking the updated nitrogen chemistry into account, no significant enrichment occurs in hcn or hnc, contrary to observational evidence in dark clouds and comets, although the 14n/15n ratio can still be below 100 in cn itself. however, such low cn abundances are predicted that the updated model falls short of explaining the bulk 15n enhancements observed in primitive materials. it is clear that alternative fractionating reactions are necessary to reproduce observations, so further laboratory and theoretical studies are urgently needed. | revised models of interstellar nitrogen isotopic fractionation |
we present observations showing inbound long-period comet c/2017 k2 (panstarrs) to be active at a record heliocentric distance. nucleus temperatures are too low (60-70 k) either for water ice to sublimate or for amorphous ice to crystallize, requiring another source for the observed activity. using the hubble space telescope we find a sharply bounded, circularly symmetric dust coma 105 km in radius, with a total scattering cross-section of ∼105 km2. the coma has a logarithmic surface brightness gradient -1 over much of its surface, indicating sustained, steady-state dust production. a lack of clear evidence for the action of solar radiation pressure suggests that the dust particles are large, with a mean size ≳0.1 mm. using a coma convolution model, we find a limit to the apparent magnitude of the nucleus v> 25.2 (absolute magnitude h> 12.9). with assumed geometric albedo pv= 0.04, the limit to the nucleus circular equivalent radius is <9 km. prediscovery observations from 2013 show that the comet was also active at 23.7 au heliocentric distance. while neither water ice sublimation nor exothermic crystallization can account for the observed distant activity, the measured properties are consistent with activity driven by sublimating supervolatile ices such as co2, co, o2, and n2. survival of supervolatiles at the nucleus surface is likely a result of the comet’s recent arrival from the frigid oort cloud. | a comet active beyond the crystallization zone |
galaxies can grow through their mutual gravitational attraction and subsequent union. while orbiting a regular high-surface-brightness galaxy, the body of a low-mass galaxy can be stripped away. however, the stellar heart of the infalling galaxy, if represented by a tightly bound nuclear star cluster, is more resilient. from archival hubble space telescope images, we have discovered a red, tidally stretched star cluster positioned ~5″ (~400 pc in projection) from, and pointing toward the center of, the post-merger spiral galaxy ngc 4424. the star cluster, which we refer to as "nikhuli," has a near-infrared luminosity of (6.88 ± 1.85) × 106 l ⊙,f160wand likely represents the nucleus of a captured/wedded galaxy. moreover, from our chandra x-ray observatory image, nikhuli is seen to contain a high-energy x-ray point source, with ${l}_{0.5-8\,\mathrm{kev}}={6.31}_{-3.77}^{+7.50}\times {10}^{38}$ erg s-1 (90% confidence). we argue that this is more likely to be an active massive black hole than an x-ray binary. lacking an outward-pointing comet-like appearance, the stellar structure of nikhuli favors infall rather than the ejection from a gravitational-wave recoil event. a minor merger with a low-mass early-type galaxy may have sown a massive black hole, aided an x-shaped pseudobulge, and be sewing a small bulge. the stellar mass and the velocity dispersion of ngc 4424 predict a central black hole of (0.6-1.0) × 105 m ⊙, similar to the expected intermediate-mass black hole in nikhuli, and suggestive of a black hole supply mechanism for bulgeless late-type galaxies. we may potentially be witnessing black hole seeding by capture and sinking, with a nuclear star cluster the delivery vehicle. | potential black hole seeding of the spiral galaxy ngc 4424 via an infalling star cluster |
the interstellar traveller, 2i/borisov, is the first clearly active extrasolar comet ever detected in our solar system. we obtained high-resolution interferometric observations of 2i/borisov with the atacama large millimeter/submillimeter array (alma) and multi-colour optical observations with the very large telescope (vlt) to gain a comprehensive understanding of the dust properties of this comet. we found that the dust coma of 2i/borisov consists of compact `pebbles' of radii exceeding ~1 mm, suggesting that the dust particles have experienced compaction through mutual impacts during the bouncing collision phase in the protoplanetary disk. we derived a dust mass-loss rate of ≳200 kg s−1 and a dust-to-gas ratio ≳3. our long-term monitoring of 2i/borisov with the vlt indicates a steady dust mass-loss with no significant dust fragmentation and/or sublimation occurring in the coma. we also detected emissions from carbon monoxide (co) gas with alma and derived the gas production rate of q(co) = (3.3 ± 0.8) × 1026 s−1. we found that the co/h2o mixing ratio of 2i/borisov changed drastically before and after perihelion, indicating the heterogeneity of the cometary nucleus, with components formed at different locations beyond the volatile snow-line with different chemical abundances. our observations suggest that 2i/borisov's home system, much like our own system, experienced efficient radial mixing from the innermost parts of its protoplanetary disk to beyond the frost line of co. | compact pebbles and the evolution of volatiles in the interstellar comet 2i/borisov |
in the context of the recently proposed contact-binary model (sekanina 2021), i investigate the circumstances of the first perihelion passage of the kreutz sungrazers in orbits with barycentric periods near 735 yr, following the initial near-aphelion splitting of the presumed progenitor, aristotle's comet of 372 bc. given favorable conditions at this breakup and at episodes of secondary fragmentation in its aftermath, the fragments should have arrived at their first perihelion nearly simultaneously, reminiscent of the anticipated outcome for the two-superfragment model's perihelion return of ad 356 (sekanina & chodas 2004). the relevant case of a swarm of kreutz sungrazers is examined to appraise possible scientific ramifications of the brief remark by ammianus marcellinus, a roman historian, that "in broad daylight comets were seen" in late ad 363, only seven years later. the tested scenario, which does not contradict ammianus' narrative and is consistent with the contact-binary model, involves a set of ten sungrazers visible in the daytime, all reaching perihelion over a period of 4.6 days. as part of this work, i comment on the role of the rapidly developing, brilliant post-perihelion tail; revise the apparent magnitude typical for the first and last naked-eye sightings; compare the visibility conditions in full daylight, in twilight, and at night; and, for the first time, present circumstantial evidence that favors comet x/1106 c1 as the parent to c/1843 d1 rather than to c/1882 r1 and c/1965 s1. | unprecedented daylight display of kreutz sungrazers in ad 363? |
giada (grain impact analyzer and dust accumulator) on-board the rosetta space probe is designed to measure the momentum, mass and speed of individual dust particles escaping the nucleus of comet 67p/churyumov-gerasimenko (hereafter 67p). from 2014 august to 2016 june, rosetta escorted comet 67p during its journey around the sun. here, we focus on giada data taken between 2015 january and 2016 february which included 67p's perihelion passage. to better understand cometary activity and more specifically the presence of dust structures in cometary comae, we mapped the spatial distribution of dust density in 67p's coma. in this manner, we could track the evolution of high-density regions of coma dust and their connections with nucleus illumination conditions, namely tracking 67p's seasons. we also studied the link between dust particle speeds and their masses with respect to heliocentric distance, i.e. the level of cometary activity. this allowed us to derive a global and a local correlation of the dust particles' speed distribution with respect to the h2o production rate. | 67p/c-g inner coma dust properties from 2.2 au inbound to 2.0 au outbound to the sun |
reflectance of cometary dust is a key parameter used in the characterization of comets. in the literature, the reflectance of single-scattering cometary dust particles is widely assumed to be the same as that of the cometary nucleus. we discuss this assumption and demonstrate its inconsistency with photometric observations of comets, laboratory optical measurements, and numerical simulation of light scattering from single-scattering dust particles and particulate surfaces composed of the same particles. we estimate the reflectance of cometary dust particles using a comprehensive physical model of polarization measured in comets over wide range of phase angle and at different wavelengths in the visible. the model predicts that the reflectance of dust in comets inversely correlates with their maximum of positive polarization pmax. we find that even the darkest dust particles appearing in comets with the highest pmax, reflect considerably more incident solar-radiation energy, up to 200%, compared to what is thought for cometary nuclei. we also find that the reflectance retrieved from polarimetry in the visible appears in good quantitative accordance with previous estimations from infrared observations of comets. our findings suggest that the dust production of comets is currently overestimated and may require revision. | on the reflectance of dust in comets |
the plasma environment has been measured for the first time near the surface of a comet. this unique data set has been acquired at 67p/churyumov-gerasimenko during esa/rosetta spacecraft's final descent on 2016 september 30. the heliocentric distance was 3.8 au and the comet was weakly outgassing. electron density was continuously measured with rosetta plasma consortium (rpc)-mutual impedance probe (mip) and rpc-langmuir probe (lap) during the descent from a cometocentric distance of 20 km down to the surface. data set from both instruments have been cross-calibrated for redundancy and accuracy. to analyse this data set, we have developed a model driven by rosetta orbiter spectrometer for ion and neutral analysis-cometary pressure sensor total neutral density. the two ionization sources considered are solar extreme ultraviolet radiation and energetic electrons. the latter are estimated from the rpc-ion and electron sensor (ies) and corrected for the spacecraft potential probed by rpc-lap. we have compared the results of the model to the electron densities measured by rpc-mip and rpc-lap at the location of the spacecraft. we find good agreement between observed and modelled electron densities. the energetic electrons have access to the surface of the nucleus and contribute as the main ionization source. as predicted, the measurements exhibit a peak in the ionospheric density close to the surface. the location and magnitude of the peak are estimated analytically. the measured ionospheric densities cannot be explained with a constant outflow velocity model. the use of a neutral model with an expanding outflow is critical to explain the plasma observations. | vertical structure of the near-surface expanding ionosphere of comet 67p probed by rosetta |
context. while exoplanets are now routinely detected, the detection of small bodies in extrasolar systems remains challenging. since the discovery of sporadic events, which are interpreted to be exocomets (falling evaporating bodies) around β pic in the early 1980s, only ∼20 stars have been reported to host exocomet-like events.aims: we aim to expand the sample of known exocomet-host stars, as well as to monitor the hot-gas environment around stars with previously known exocometary activity.methods: we have obtained high-resolution optical spectra of a heterogeneous sample of 117 main-sequence stars in the spectral type range from b8 to g8. the data were collected in 14 observing campaigns over the course of two years from both hemispheres. we analysed the ca ii k&h and na i d lines in order to search for non-photospheric absorptions that originated in the circumstellar environment and for variable events that could be caused by the outgassing of exocomet-like bodies.results: we detected non-photospheric absorptions towards 50% of the sample, thus attributing a circumstellar origin to half of the detections (i.e. 26% of the sample). hot circumstellar gas was detected in the metallic lines inspected via narrow stable absorptions and/or variable blue- and red-shifted absorption events. such variable events were found in 18 stars in the ca ii and/or na i lines; six of them are reported in the context of this work for the first time. in some cases, the variations we report in the ca ii k line are similar to those observed in β pic. while we do not find a significant trend in the age or location of the stars, we do find that the probability of finding cs gas in stars with larger v sin i is higher. we also find a weak trend with the presence of near-infrared excess and with anomalous (λ boo-like) abundances, but this would require confirmation by expanding the sample. table c.1 is 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/639/a11 | exocomets: a spectroscopic survey |
kic 8462852 is a completely ordinary f3 main-sequence star, except that the light curve from kepler shows episodes of unique and inexplicable day-long dips with up to 20% dimming. here, i provide a light curve of 1338 johnson b-band magnitudes from 1890 to 1989 taken from archival photographic plates at harvard. kic 8462852 displays a secular dimming at an average rate of 0.164 ± 0.013 mag per century. from the early-1890s to the late-1980s, kic 8462852 faded by 0.193 ± 0.030 mag. the decline is not an artifact because nearby check stars have closely flat light curves. this century-long dimming is unprecedented for any f-type main-sequence star. thus, the harvard light curve provides the first confirmation (past the several dips seen in the kepler light curve alone) that kic 8462852 has anything unusual. the century-long dimming and the day-long dips are both just extreme ends of a spectrum of timescales for unique dimming events. by ockham’s razor, two such unique and similar effects are very likely produced by one physical mechanism. this one mechanism does not appear as any isolated catastrophic event in the last century, but rather must be some ongoing process with continuous effects. within the context of dust-occultation models, the century-long dimming trend requires 104-107 times as much dust as for the deepest kepler dip. within the context of the comet-family idea, the century-long dimming trend requires an estimated 648,000 giant comets (each with 200 km diameter) all orchestrated to pass in front of the star within the last century. | kic 8462852 faded at an average rate of 0.164 ± 0.013 magnitudes per century from 1890 to 1989 |
motivated by recent visits from interstellar comets, along with continuing discoveries of minor bodies in orbit of the sun, this paper studies the capture of objects on initially hyperbolic orbits by our solar system. using an ensemble of ∼500 million numerical experiments, this work generalizes previous treatments by calculating the capture cross section as a function of asymptotic speed. the resulting velocity-dependent cross section can then be convolved with any distribution of relative speeds to determine the capture rate for incoming bodies. this convolution is carried out for the usual maxwellian distribution, as well as the velocity distribution expected for rocky debris ejected from planetary systems. we also construct an analytic description of the capture process that provides an explanation for the functional form of the capture cross section in both the high- and low-velocity limits. | on the capture of interstellar objects by our solar system |
glycolaldehyde is a key molecule in the formation of biologically relevant molecules such as ribose. we report its detection with the plateau de bure interferometer toward the class 0 young stellar object ngc 1333 iras2a, which is only the second solar-type protostar for which this prebiotic molecule is detected. local thermodynamic equilibrium analyses of glycolaldehyde, ethylene glycol (the reduced alcohol of glycolaldehyde), and methyl formate (the most abundant isomer of glycolaldehyde) were carried out. the relative abundance of ethylene glycol to glycolaldehyde is found to be ~5 - higher than in the class 0 source iras 16293-2422 (~1), but similar to the lower limits derived in comets (≥3-6). the different ethylene glycol-to-glycolaldehyde ratios in the two protostars might be related to different ch3oh:co compositions of the icy grain mantles. in particular, a more efficient hydrogenation on the grains in ngc 1333 iras2a would favor the formation of both methanol and ethylene glycol. in conclusion, it is possible that like ngc 1333 iras2a, other low-mass protostars show high ethylene glycol-to-glycolaldehyde abundance ratios. the cometary ratios might consequently be inherited from earlier stages of star formation if the young sun experienced conditions similar to ngc 1333 iras2a. based on observations carried out with the iram plateau de bure interferometer. iram is supported by insu/cnrs (france), mpg (germany) and ign (spain).figures 3-4 and table 1 are available in electronic form at http://www.aanda.org | detection of glycolaldehyde toward the solar-type protostar ngc 1333 iras2a |
spacecraft missions have resolved the nuclei of six periodic comets and revealed a set of geologically intriguing and active small bodies. the shapes of these cometary nuclei are dominantly bilobate reflecting their formation from smaller cometesimals. cometary surfaces include a diverse set of morphologies formed from a variety of mechanisms. sublimation of ices, driven by the variable insolation over the time since each nucleus was perturbed into the inner solar system, is a major process on comets and is likely responsible for quasi-circular depressions and ubiquitous layering. sublimation from near-vertical walls is also seen to lead to undercutting and mass wasting. fracturing has only been resolved on one comet but likely exists on all comets. there is also evidence for mass redistribution, where material lifted off the nucleus by subliming gases is deposited onto other surfaces. it is surprising that such sedimentary processes are significant in the microgravity environment of comets. there are many enigmatic features on cometary surfaces including tall spires, kilometer-scale flows, and various forms of depressions and pits. furthermore, even after accounting for the differences in resolution and coverage, significant diversity in landforms among cometary surfaces clearly exists. yet why certain landforms occur on some comets and not on others remains poorly understood. the exploration and understanding of geologic processes on comets is only beginning. these fascinating bodies will continue to provide a unique laboratory for examining common geologic processes under the uncommon conditions of very high porosity, very low strength, small particle sizes, and near-zero gravity. | evidence for geologic processes on comets |
a major upgrade to the nirspec instrument at the keck ii telescope was successfully completed in time for near-infrared spectroscopic observations of comet 46p/wirtanen during its exceptionally close flyby of earth in 2018 december. these studies determined the abundances of several volatiles, including c2h2, c2h6, ch3oh, nh3, hcn, h2co, and h2o. long-slit spatial distributions of gas rotational temperature and column density are diagnostic for the presence of icy grains in the coma and understanding if different volatiles are associated with common or distinct outgassing sources. these spatial distributions suggest that c2h2, c2h6, and hcn have a common outgassing source, whereas h2o and ch3oh have additional, more extended sources. the synergy of these findings with observations by space missions (rosetta and epoxi) motivates continuing studies to address whether or not c2h6, c2h2, and hcn have a common source of release (plausibly associated with co2) in a larger sample of comets and whether systematic differences exist in the release of these species compared to h2o and ch3oh. abundances of volatiles are reported relative to h2o, as traditionally done, as well as c2h6. while not unique, the choice of c2h6 demonstrates the value of extending the chemical taxonomy of parent volatiles in comets toward additional compositional "baselines" and, importantly, closer integration between coma abundances and the underlying volatile associations as revealed by spatial distributions. our findings on composition and sources of outgassing include information relevant to future evaluations of 46p/wirtanen as a prospective spacecraft target. | first comet observations with nirspec-2 at keck: outgassing sources of parent volatiles and abundances based on alternative taxonomic compositional baselines in 46p/wirtanen |
we present the results of our orbital computations in support of the recently proposed contact-binary model for the kreutz sungrazer system (sekanina 2021, 2022). we demonstrate that comet ikeya-seki (c/1965 s1) previously passed perihelion decades after the great comet of 1106 (x/1106 c1) and argue that, like the great september comet of 1882 (c/1882 r1), it evidently was a fragment of the comet recorded by the chinese in september 1138. the 1106 sungrazer appears instead to have been the previous appearance of the great march comet of 1843 (c/1843 d1). with no momentum exchange involved, fragments of a kreutz sungrazer breaking up tidally near perihelion are shown to end up in orbits with markedly different periods because their centers of mass are radially shifted by a few kilometers relative to the parent. the daylight comets of ad 363, recorded by a roman historian, are accommodated in our computations as the first appearance of the kreutz sungrazers after their bilobed progenitor's breakup. we link the 1843-1106-363 (lobe i) and 1882-1138-363 (lobe ii) returns to perihelion by single nongravitational orbits and gravitationally with minor center-of-mass shifts acquired in fragmentation events. we also successfully model the motion of aristotle's comet as the rotating progenitor that at aphelion split (at a few m/s) into the two lobes, the precursors of, respectively, the 1843 and 1882 sungrazers; and provide a 1963-1041-363 link for comet pereyra (c/1963 r1). material fatigue could contribute to sungrazers' fragmentation throughout the orbit, including aphelion. -- continuing problems with the nongravitational law in orbit software are noted. | the great comet of 1106, a chinese comet of 1138, and daylight comets in late 363 as key objects in computer simulated history of kreutz sungrazer system |
recent observational and experimental evidence for the presence of complex organics in space is reviewed. remote astronomical observations have detected ∼ 200 gas-phased molecules through their rotational and vibrational transitions. many classes of organic molecules are represented in this list, including some precursors to biological molecules. a number of unidentified spectral phenomena observed in the interstellar medium are likely to have originated from complex organics. the observations of these features in distant galaxies suggests that organic synthesis had already taken place during the early epochs of the universe. in the solar system, almost all biologically relevant molecules can be found in the soluble component of carbonaceous meteorites. complex organics of mixed aromatic and aliphatic structures are present in the insoluble component of meteorites. hydrocarbons cover much of the surface of the planetary satellite titan and complex organics are found in comets and interplanetary dust particles. the possibility that the early solar system, or even the early earth, have been enriched by interstellar organics is discussed. | complex organics in space from solar system to distant galaxies |
in order to explore the link between comet 81p/wild 2 and materials in primitive meteorites, seven particles 5 to 15 μm in diameter from comet 81p/wild 2 have been analyzed for their oxygen isotope ratios using a secondary ion mass spectrometer. most particles are single minerals consisting of olivine or pyroxene with mg# higher than 85, which are relatively minor in 81p/wild 2 particles (∼1/3 of the 16o-poor cluster). four particles extracted from track 149 are 16o-poor and show δ17o (= δ17o - 0.52 × δ18o) values from -2‰ to +1‰, similar to previous studies, while one enstatite (en99) particle shows lower δ17o value of - 7 ± 4 ‰ (2 σ). this compositional range has not been reported among 16o-poor particles in 81p/wild 2, but is commonly observed among chondrules in carbonaceous chondrites and in particular in cr chondrites. the distribution in δ17o indicates that 16o-poor 81p/wild 2 particles are most similar to chondrules (and their fragments) in the cr chondrites and tagish lake-like wis91600 chondrite chondrule silicate grains, which indicates that they likely come from a reservoir with similar dust/ice ratios as cr chondrites and wis91600. however, differences in the mg# distribution imply that the 81p/wild 2 reservoir was comparatively more oxidized, with a higher dust enrichment. two nearly pure enstatite grains from track 172 are significantly enriched in 16o, with δ18o values of - 51.2 ± 1.5 ‰ (2 σ) and - 43.0 ± 1.3 ‰ (2 σ), respectively, and δ17o values of - 22.3 ± 1.9 ‰ (2 σ) and - 21.3 ± 2.3 ‰ (2 σ), respectively. they are the first 16o-rich pyroxenes found among 81p/wild 2 particles, with similar δ17o values to those of 16o-rich low-iron, manganese-enriched (lime) olivine and cai (calcium and aluminum-rich inclusions) - like particles from 81p/wild 2. the major element and oxygen isotopic compositions of the pyroxenes are similar to those of enstatite in amoeboid olivine aggregates (aoas) in primitive chondrites, in which 16o-rich pyroxenes have previously been found, and thus suggest a condensation origin. | origin of crystalline silicates from comet 81p/wild 2: combined study on their oxygen isotopes and mineral chemistry |
discs of dusty debris around main-sequence stars indicate fragmentation of orbiting planetesimals, and for a few a-type stars, a gas component is also seen that may come from collisionally released volatiles. here we find the sixth example of a co-hosting disc, around the ∼30 myr-old a0-star hd 32997. two more of these co-hosting stars, hd 21997 and 49 cet, have also been imaged in dust with scuba-2 within the scuba-2 survey of nearby stars project. a census of 27 a-type debris hosts within 125 pc now shows 7/16 detections of carbon-bearing gas within the 5-50 myr epoch, with no detections in 11 older systems. such a prolonged period of high fragmentation rates corresponds quite well to the epoch when most of the earth was assembled from planetesimal collisions. recent models propose that collisional products can be spatially asymmetric if they originate at one location in the disc, with co particularly exhibiting this behaviour as it can photodissociate in less than an orbital period. of the six co-hosting systems, only β pic is in clear support of this hypothesis. however, radiative transfer modelling with the prodimo code shows that the co is also hard to explain in a proto-planetary disc context. | gas and dust around a-type stars at tens of myr: signatures of cometary breakup |
the european space agency spacecraft rosetta accompanied the jupiter-family comet 67p/churyumov-gerasimenko for over 2 yr along its trajectory through the inner solar system. between 2014 and 2016, it performed almost continuous in situ measurements of the comet's gaseous atmosphere in close proximity to its nucleus. in this study, the 16o/18o ratio of h2o in the coma of 67p/churyumov-gerasimenko, as measured by the rosina dfms mass spectrometer onboard rosetta, was determined from the ratio of h216o/h218o and 16oh/18oh. the value of 445 ± 35 represents an 11% enrichment of 18o compared with the terrestrial ratio of 498.7 ± 0.1. this cometary value is consistent with the comet containing primordial water, in accordance with leading self-shielding models. these models predict primordial water to be between 5 and 20% enriched in heavier oxygen isotopes compared to terrestrial water. | 16o/18o ratio in water in the coma of comet 67p/churyumov-gerasimenko measured with the rosetta/rosina double-focusing mass spectrometer |
this paper provides a brief overview of the journey of molecules through the cosmos, from local diffuse interstellar clouds and pdrs to distant galaxies, and from cold dark clouds to hot star-forming cores, protoplanetary disks, planetesimals and exoplanets. recent developments in each area are sketched and the importance of connecting astronomy with chemistry and other disciplines is emphasized. fourteen challenges for the field of astrochemistry in the coming decades are formulated. | astrochemistry: overview and challenges |
charge exchange (cx) has emerged in x-ray emission modeling as a significant process that must be considered in many astrophysical environments—particularly comets. comets host an interaction between solar wind ions and cometary neutrals to promote solar wind charge exchange (swcx). x-ray observatories provide astronomers and astrophysicists with data for many x-ray emitting comets that are impossible to accurately model without reliable cx data. here, we utilize a streamlined set of computer programs that incorporate the multi-channel landau-zener theory and a cascade model for x-ray emission to generate cross sections and x-ray line ratios for a variety of bare and non-bare ion single electron capture (sec) collisions. namely, we consider collisions between the solar wind constituent bare and h-like ions of c, n, o, ne, na, mg, al, and si and the cometary neutrals h2o, co, co2, oh, and o. to exemplify the application of this data, we model the x-ray emission of comet c/2000 wm1 (linear) using the cx package in spex and find excellent agreement with observations made with the xmm-newton rgs detector. our analyses show that the x-ray intensity is dominated by swcx with h, while h2o plays a secondary role. this is the first time, to our knowledge, that cx cross sections have been implemented into a x-ray spectral fitting package to determine the h to h2o ratio in cometary atmospheres. the cx data sets are incorporated into the modeling packages spex and kronos. | line ratios for solar wind charge exchange with comets |
the philae lander accomplished the first soft landing and the first scientific experiments of a human-made spacecraft on the surface of a comet. planned, expected and unexpected activities and events happened during the descent, the touch-downs, the hopping across and the stay and operations on the surface. the key results were obtained during 12-14 november 2014, at 3 au from the sun, during the 63 h long period of the descent and of the first science sequence on the surface. thereafter, philae went into hibernation, waking up again in late april 2015 with subsequent communication periods with earth (via the orbiter), too short to enable new scientific activities. the science return of the mission comes from eight of the 10 instruments on-board and focuses on morphological, thermal, mechanical and electrical properties of the surface as well as on the surface composition. it allows a first characterization of the local environment of the touch-down and landing sites. unique conclusions on the organics in the cometary material, the nucleus interior, the comet formation and evolution became available through measurements of the philae lander in the context of the rosetta mission. this article is part of the themed issue 'cometary science after rosetta'. | the philae lander mission and science overview |
the isotopic ratio of nitrogen measured in primitive solar system bodies shows a broad range of values, the origin of which remains unknown. one key question is whether these isotopic reservoirs of nitrogen predate the comet formation stage or are posterior to it. another central question is elucidating the processes that can produce the observed variations in the 14n/15n isotopic ratio. disks that orbit pre-main-sequence (t tauri) stars provide unique opportunities for observing the chemical content of analogs of the protosolar nebula and therefore for building a comprehensive scenario that can explain the origin of nitrogen in the solar system and in planet-forming disks. with alma, it has become possible to measure isotopic ratios of nitrogen-bearing species in such environments. we present spectrally and spatially resolved observations of the hyperfine structure of the 4-3 rotational transition of hcn and its main isotopologs h13cn and hc15n in the disk orbiting the 8 myr old t tauri star tw hya. the sensitivity allows directly measuring the hcn/h13cn and hcn/hc15n abundance ratios with minimal assumptions. averaged spatially over the disks, the ratios are 86 ± 4 and 223 ± 21, respectively. the latter value is significantly lower than the cn/c15n ratio of 323 ± 30 in this disk and thus provides the first evidence that two isotopic reservoirs of nitrogen are present in a disk at the stage of giant planet and comet formation. furthermore, we find clear evidence for an increase in the ratio of hcn to hc15n with radius. the ratio in the outer disk, at 45 au, is 339 ± 28, in excellent agreement with direct measurements in the local interstellar medium, and with the bulk nitrogen isotopic ratio predicted from galactic evolution calculations. in the comet formation region at r = 20 au, the ratio is a factor ≈3 lower, 121 ± 11. this radial increase qualitatively agrees with the scenario in which selective photodissociation of n2 is the dominant fractionation process. however, our isotopic ratios and kinetic temperature of the hcn-emitting layers quantitatively disagree with models of nitrogen chemistry in disks. | multiple nitrogen reservoirs in a protoplanetary disk at the epoch of comet and giant planet formation |
charge exchange x-ray emission provides unique insight into the interactions between cold and hot astrophysical plasmas. besides its own profound science, this emission is also technically crucial to all observations in the x-ray band, since charge exchange with the solar wind often contributes a significant foreground component that contaminates the signal of interest. by approximating the cross sections resolved to n and l atomic subshells and carrying out complete radiative cascade calculation, we have created a new spectral code to evaluate the charge exchange emission in the x-ray band. compared to collisional thermal emission, charge exchange radiation exhibits enhanced lines from large-n shells to the ground, as well as large forbidden-to-resonance ratios of triplet transitions. our new model successfully reproduces an observed high-quality spectrum of comet c/2000 wm1 (linear), which emits purely by charge exchange between solar wind ions and cometary neutrals. it demonstrates that a proper charge exchange model will allow us to probe the ion properties remotely, including charge state, dynamics, and composition, at the interface between the cold and hot plasmas. | plasma code for astrophysical charge exchange emission at x-ray wavelengths |
organic matter and hydrous silicates are intimately mixed in the matrix of chondrites and in-situ determination of their individual d/h ratios is therefore challenging. nevertheless, the d/h ratio of each pure component in this mixture should yield a comprehensible signature of the origin and evolution of water and organic matter in our solar system. we measured hydrogen isotope ratios of organic and hydrous silicates in the matrices of two carbonaceous chondrites (orgueil ci1 and renazzo cr2) and one unequilibrated ordinary chondrite (semarkona, ll3.0). a novel protocol was adopted, involving nanosims imaging of h isotopes of monoatomatic (h-) and molecular (oh-) secondary ions collected at the same location. this allowed the most enriched component with respect to d to be identified in the mixture. using this protocol, we found that in carbonaceous chondrites the isotopically homogeneous hydrous silicates are mixed with d-rich organic matter. the opposite was observed in semarkona. hydrous silicates in semarkona display highly heterogeneous d/h ratios, ranging from 150 to 1800 ×10-6 (δdsmow = - 40 to 10 600‰). organic matter in semarkona does not show such large isotopic variations. this suggests limited isotopic exchange between the two phases during aqueous alteration. our study greatly expands the range of water isotopic values measured so far in solar system objects. this d-rich water reservoir was sampled by the ll ordinary chondrite parent body and an estimate (≤9%) of its relative contribution to the d/h ratio of water in oort cloud family comets is proposed. | micron-scale d/h heterogeneity in chondrite matrices: a signature of the pristine solar system water? |
we present a summary of the campaign of remote observations that supported the european space agency's rosetta mission. telescopes across the globe (and in space) followed comet 67p/churyumov-gerasimenko from before rosetta's arrival until nearly the end of the mission in september 2016. these provided essential data for mission planning, large-scale context information for the coma and tails beyond the spacecraft and a way to directly compare 67p with other comets. the observations revealed 67p to be a relatively `well-behaved' comet, typical of jupiter family comets and with activity patterns that repeat from orbit to orbit. comparison between this large collection of telescopic observations and the in situ results from rosetta will allow us to better understand comet coma chemistry and structure. this work is just beginning as the mission ends-in this paper, we present a summary of the ground-based observations and early results, and point to many questions that will be addressed in future studies. this article is part of the themed issue 'cometary science after rosetta'. | the 67p/churyumov-gerasimenko observation campaign in support of the rosetta mission |
explaining the unexpected presence of dune-like patterns at the surface of the comet 67p/churyumov-gerasimenko requires conceptual and quantitative advances in the understanding of surface and outgassing processes. we show here that vapor flow emitted by the comet around its perihelion spreads laterally in a surface layer, due to the strong pressure difference between zones illuminated by sunlight and those in shadow. for such thermal winds to be dense enough to transport grains - 10 times greater than previous estimates - outgassing must take place through a surface porous granular layer, and that layer must be composed of grains whose roughness lowers cohesion consistently with contact mechanics. the linear stability analysis of the problem, entirely tested against laboratory experiments, quantitatively predicts the emergence of bedforms in the observed wavelength range and their propagation at the scale of a comet revolution. although generated by a rarefied atmosphere, they are paradoxically analogous to ripples emerging on granular beds submitted to viscous shear flows. this quantitative agreement shows that our understanding of the coupling between hydrodynamics and sediment transport is able to account for bedform emergence in extreme conditions and provides a reliable tool to predict the erosion and accretion processes controlling the evolution of small solar system bodies. | giant ripples on comet 67p/churyumov-gerasimenko sculpted by sunset thermal wind |
we present and compare measurements of the spacecraft potential (vs/c) of the rosetta spacecraft throughout its stay in the inner coma of comet 67p/churyumov-gerasimenko, by the rosetta plasma consortium-langmuir probe (rpc-lap) and ion composition analyzer (rpc-ica) instruments. vs/c has mainly been negative, driven by the high temperature (∼5-10 ev) of the coma photoelectrons. the magnitude of the negative vs/c traces heliocentric, cometocentric, seasonal and diurnal variations in cometary outgassing, consistent with production at or inside the cometocentric distance of the spacecraft being the dominant source of the observed plasma. lap only picks up a portion of the full vs/c since the two probes, mounted on booms of 2.2 and 1.6 m length, respectively, are generally inside the potential field of the spacecraft. comparing with the minimum energy of positive ions collected by ica, we find numerous cases with strong correlation between the two instruments, from which the fraction of vs/c picked up by lap is found to vary between about 0.7 and 1. we also find an ica energy offset of 13.7 ev (95 per cent ci: [12.5, 15.0]). many cases of poor correlation between the instruments are also observed, predominantly when local ion production is weak and accelerated ions dominate the flux, or during quiet periods with low dynamic range in vs/c and consequently low signal-to-noise ratios. | measurements of the electrostatic potential of rosetta at comet 67p |
context. cyanopolyynes are chains of carbon atoms with an atom of hydrogen and a cn group on either side. they are detected almost everywhere in the interstellar medium (ism), as well as in comets. in the past, they have been used to constrain the age of some molecular clouds, since their abundance is predicted to be a strong function of time. finally, cyanopolyynes can potentially contain a large portion of molecular carbon.aims: we present an extensive study of the cyanopolyynes distribution in the solar-type protostar iras 16293-2422. the goals are (i) to obtain a census of the cyanopolyynes in this source and of their isotopologues; (ii) to derive how their abundance varies across the protostar envelope; and (iii) to obtain constraints on the history of iras 16293-2422 by comparing the observations with the predictions of a chemical model.methods: we analysed the data from the iram-30 m unbiased millimeter and submillimeter spectral survey towards iras 16293-2422 named timasss. the derived spectral line energy distribution (sled) of each detected cyanopolyyne was compared with the predictions from the radiative transfer code grenoble analysis of protostellar envelope spectra (grapes) to derive the cyanopolyyne abundances across the envelope of iras 16293-2422. finally, the derived abundances were compared with the predictions of the chemical model ucl_chem.results: we detect several lines from cyanoacetylene (hc3n) and cyanodiacetylene (hc5n), and report the first detection of deuterated cyanoacetylene, dc3n, in a solar-type protostar. we found that the hc3n abundance is roughly constant ( 1.3 × 10-11) in the outer cold envelope of iras 16293-2422, and it increases by about a factor 100 in the inner region where the dust temperature exceeds 80 k, namely when the volcano ice desorption is predicted to occur. the hc5n has an abundance similar to hc3n in the outer envelope and about a factor of ten lower in the inner region. the comparison with the chemical model predictions provides constraints on the oxygen and carbon gaseous abundance in the outer envelope and, most importantly, on the age of the source. the hc3n abundance derived in the inner region, and where the increase occurs, also provide strong constraints on the time taken for the dust to warm up to 80 k, which has to be shorter than 103-104 yr. finally, the cyanoacetylene deuteration is about 50% in the outer envelope and ≤5% in the warm inner region. the relatively low deuteration in the warm region suggests that we are witnessing a fossil of the hc3n abundantly formed in the tenuous phase of the pre-collapse and then frozen into the grain mantles at a later phase.conclusions: the accurate analysis of the cyanopolyynes in iras 16293-2422 unveils an important part of its past story. it tells us that iras 16293-2422 underwent a relatively fast (≤105 yr) collapse and a very fast (≤103-104 yr) warming up of the cold material to 80 k. | history of the solar-type protostar iras 16293-2422 as told by the cyanopolyynes |
mercury is embedded in a tenuous and highly anisotropic sodium exosphere, generated mainly by plasma-surface interactions. the absolute values of the sodium ion density are still under debate. observations by messenger's fast imaging plasma spectrometer (fips) instrument suggest the density of exospheric ions to be several orders of magnitude lower than the upstream solar wind density, indicating that the sodium exosphere has no substantial influence on the magnetospheric current systems. however, messenger magnetic field observations of field line resonances revealed sodium ion densities comparable to the upstream solar wind density. to investigate how a dense exosphere would affect the current systems within mercury's magnetosphere, we apply an established hybrid (kinetic ions, fluid electrons) model and conduct multiple model runs with gradually increasing exospheric density, ranging from no sodium ions at all to comet-like configurations. we demonstrate how a sufficiently dense exosphere leads to self-shielding of the sodium ion population from the ambient electric field and a significant inflation and symmetrization of mercury's magnetosphere, which is decreasingly affected by the dipole offset. once the sodium ion density is sufficiently high, region 2 field-aligned currents emerge close to the planet. the modeled region 2 currents are located below the orbit of messenger, thereby providing a possible explanation for the absence of these currents in observations. the sodium exosphere also closes a significant fraction of the region 1 currents through pedersen and hall currents before the "guiding" magnetic field lines even reach the planetary surface. the modeled sodium ion and solar wind densities agree well with observations. | influence of mercury's exosphere on the structure of the magnetosphere |
context. measurements of isotopic abundances in cometary ices are key to understanding and reconstructing the history and origin of material in the solar system. comets are considered the most pristine material in the solar system. isotopic fractionation (enrichment of an isotope in a molecule compared to the initial abundance) is sensitive to environmental conditions at the time of comet formation. therefore, measurements of cometary isotope ratios can provide information on the composition, density, temperature, and radiation during formation of the molecules, during the chemical evolution from the presolar cloud to the protosolar nebula, and the protoplanetary disk before accretion in solid bodies. most isotopic abundances of 12c/13c and 16o/18o in comets to date are in agreement with terrestrial abundances. prior to the rosetta mission, measurements of 12c/13c in comets were only available for hcn, cn, and c2 and for 16o/18o in h2o. measurements of 12c/13c in comets were only available from ground based observations and remote sensing, while 16o/18o in h2o had also been measured in-situ. to date, no measurements of the co2 isotopologues in comets were available.aims: this paper presents the first measurements of the co2 isotopologues in the coma of 67p/churyumov-gerasimenko (67p).methods: we analyzed measurements taken by the double focusing mass spectrometer (dfms) of the rosina experiment on board the esa spacecraft rosetta in the coma of 67p.results: the co2 isotopologues results for 67p are: 12c/13c = 84 ± 4, 16o/18o = 494 ± 8, and 13c16o2/12c18o16o = 5.87 ± 0.07. the oxygen isotopic ratio is within error bars compatible with terrestrial abundances but not with solar wind measurements.conclusions: the carbon isotopic ratio and the combined carbon and oxygen isotopic ratio are slightly (14%) enriched in 13c, within 1σ uncertainty, compared to solar wind abundances and solar abundances. the small fractionation of 12c/13c in co2 is probably compatible with an origin of the material in comets from the native cloud. | isotopic composition of co2 in the coma of 67p/churyumov-gerasimenko measured with rosina/dfms |
on the 12th of november 2014, the rosetta lander philae became the first spacecraft to softly land on a comet nucleus. due to the double failure of the cold gas hold-down thruster and the anchoring harpoons that should have fixed philae to the surface, it spent approximately two hours bouncing over the comet surface to finally come at rest one km away from its target site. nevertheless it was operated during the 57 h of its first science sequence. the fss, performed with the two batteries, should have been followed by the long term science sequence but philae was in a place not well illuminated and fell into hibernation. yet, thanks to reducing distance to the sun and to seasonal effect, it woke up at end of april and on 13th of june it contacted rosetta again. to achieve this successful landing, an intense preparation work had been carried out mainly between august and november 2014 to select the targeted landing site and define the final landing trajectory. after the landing, the data collected during on-comet operations have been used to assess the final position and orientation of philae, and to prepare the wake-up. this paper addresses the flight dynamics studies done in the scope of this landing preparation from lander side, in close cooperation with the team at esa, responsible for rosetta, as well as for the reconstruction of the bouncing trajectory and orientation of the lander after touchdown. | rosetta lander philae: flight dynamics analyses for landing site selection and post-landing operations |
the carbon dimer, the 12c2 molecule, is ubiquitous in astronomical environments. experimental-quality rovibronic energy levels are reported for 12c2, based on rovibronic transitions measured for and among its singlet, triplet, and quintet electronic states, reported in 42 publications. the determination utilizes the measured active rotational-vibrational energy levels (marvel) technique. the 23,343 transitions measured experimentally and validated within this study determine 5699 rovibronic energy levels, 1325, 4309, and 65 levels for the singlet, triplet, and quintet states investigated, respectively. the marvel analysis provides rovibronic energies for six singlet, six triplet, and two quintet electronic states. for example, the lowest measurable energy level of the {{a}}{}3{{{\pi }}}{{u}} state, corresponding to the j = 2 total angular momentum quantum number and the f 1 spin-multiplet component, is 603.817(5) cm-1. this well-determined energy difference should facilitate observations of singlet-triplet intercombination lines, which are thought to occur in the interstellar medium and comets. the large number of highly accurate and clearly labeled transitions that can be derived by combining marvel energy levels with computed temperature-dependent intensities should help a number of astrophysical observations as well as corresponding laboratory measurements. the experimental rovibronic energy levels, augmented, where needed, with ab initio variational ones based on empirically adjusted and spin-orbit coupled potential energy curves obtained using the duo code, are used to obtain a highly accurate partition function, and related thermodynamic data, for 12c2 up to 4000 k. | experimental energy levels and partition function of the 12c2 molecule |
recent in-situ studies of the environment of comet 67p/churyumov-gerasimenko by the dust instruments onboard the rosetta spacecraft have indicated a complex structure of cometary dust particles. the majority of those particles appeared to be large aggregates of hierarchical structure, i.e. aggregates of particles, which, in turn, were aggregates of smaller particles. this confirmed an earlier hypothesis that dust particles in protoplanetary disks grow under hierarchical growth process. thus, it is very likely that hierarchical aggregates are common type of natural dust particles. in this paper, we present results of computer simulations of light scattering by a variety of hierarchical aggregates to determine how their structure affects their brightness and polarization phase curves as well as photometric and polarimetric color, and albedo. the computations were done using multi-sphere t-matrix method. our results show that the type of hierarchical structure of aggregates, specified by the number of particles at each level of hierarchy, causes variations in their light-scattering characteristics, which noticeably exceed the variations caused by different configuration of monomers in the aggregates of the same hierarchical structure. although we could not find any regularities in the brightness and polarization dependence on the structure of the aggregates, our results clearly show that not only composition or size of aggregates, but also their specific structure should be carefully examined when light scattering by cometary or any other type of dust is modeled. specifically, we may need to reconsider modeling the cometary dust particles using simple ballistic particle-cluster and cluster-cluster aggregates. | light scattering by hierarchical aggregates |
vacuum-uv (vuv) photodesorption from water-rich ice mantles coating interstellar grains is known to play an important role in the gas-to-ice ratio in star- and planet-forming regions. quantitative photodesorption yields from water ice are crucial for astrochemical models. we aim to provide the first quantitative photon-energy dependent photodesorption yields from water ice in the vuv. this information is important to understand the photodesorption mechanisms and to account for the variation of the yields under interstellar irradiation conditions. experiments have been performed on the desirs beamline at the soleil synchrotron, delivering tunable vuv light, using the spices (surface processes and ices) set-up. compact amorphous solid water ice (h$_2$o and d$_2$o) has been irradiated from 7 to 13.5 ev. quantitative yields have been obtained by detection in the gas phase with mass-spectrometry for sample temperatures ranging from 15 k to 100 k. photodesorption spectra of h$_2$o (d$_2$o), oh (od), h$_2$ (d$_2$) and o$_2$ peak around 9-10 ev and decrease at higher energies. average photodesorption yields of intact water at 15 k are 5 $\times$ 10$^{-4}$ molecule/photon for h$_2$o and 5 $\times$ 10$^{-5}$ molecule/photon for d$_2$o over the 7-13.5 ev range. the strong isotopic effect can be explained by a differential chemical recombination between oh (od) and h (d) photofragments originating from lower kinetic energy available for the oh photofragments upon direct water photodissociation and/or possibly by an electronic relaxation process. it is expected to contribute to water fractionation during the building-up of the ice grain mantles in molecular clouds and to favor oh-poor chemical environment in comet-formation regions of protoplanetary disks. the yields of all the detected species except oh (od) are enhanced above (70 $\pm$10) k, suggesting an ice restructuration at this temperature. | vacuum-uv photodesorption from compact amorphous solid water: photon energy dependence, isotopic and temperature effects |
the surface and subsurface of comets preserve material from the formation of the solar system. the properties of cometary material thus provide insight into the physical and chemical conditions during their formation. we present mass spectra taken by the ptolemy instrument 20 minutes after the initial touchdown of the philae lander on the surface of comet 67p/churyumov-gerasimenko. regular mass distributions indicate the presence of a sequence of compounds with additional -ch2- and -o- groups (mass/charge ratios 14 and 16, respectively). similarities with the detected coma species of comet halley suggest the presence of a radiation-induced polymer at the surface. ptolemy measurements also indicate an apparent absence of aromatic compounds such as benzene, a lack of sulfur-bearing species, and very low concentrations of nitrogenous material. | cho-bearing organic compounds at the surface of 67p/churyumov-gerasimenko revealed by ptolemy |
we analytically discuss wave excitation in a homogeneous three component plasma consisting of solar wind protons, electrons and a beam of cometary water ions applied to the plasma environment of comet 67p/churyumov-gerasimenko. the resulting dispersion relations are studied in a solar wind rest frame, where a cometary current is solely generated by the water ion beam, and a cometary rest frame representing the rest frame of the rosetta spacecraft. a modified ion-weibel instability is excited by the cometary current and predominantly grows perpendicular to this current. the corresponding water ion mode is connected to a frequency of about 40&thinsp;mhz in agreement with wave measurements of rosetta's magnetometer in the cometary rest frame. furthermore, the superposition of the strongest growing waves result in a fan-like phase structure close to the comet. | modified ion-weibel instability as a possible source of wave activity at comet 67p/churyumov-gerasimenko |
water ice-rich patches have been detected on the surface of comet 67p/churyumov-gerasimenko by the virtis (visible infrared and thermal imaging spectrometer) hyperspectral imager on board the rosetta spacecraft, since the orbital insertion in late 2014 august. among these, three icy patches have been selected, and virtis data are used to analyse their properties and their temporal evolution while the comet was moving towards the sun. we performed an extensive analysis of the spectral parameters, and we applied the hapke radiative transfer model to retrieve the abundance and grain size of water ice, as well as the mixing modalities of water ice and dark terrains on the three selected water ice-rich areas. study of the spatial distribution of the spectral parameters within the ice-rich patches has revealed that water ice follows different patterns associated with a bimodal distribution of the grains: ∼50-μm sized and ∼2000-μm sized. in all three cases, after the first detections at about 3.5-au heliocentric distance, the spatial extension and intensity of the water ice spectral features increased, it reached a maximum after 60-100 d at about 3.0 au, and was followed by an approximately equally timed decrease and disappearance at about 2.2 au, before perihelion. the behaviour of the analysed patches can be assimilated to a seasonal cycle. in addition, we found evidence of short-term variability associated with a diurnal water cycle. the similar life cycle of the three icy regions indicates that water ice is uniformly distributed in the subsurface layers, and no large water ice reservoirs are present. | the temporal evolution of exposed water ice-rich areas on the surface of 67p/churyumov-gerasimenko: spectral analysis |
orbital evolution of binary systems in dense stellar clusters is important in a variety of contexts: origin of blue stragglers, progenitors of compact object mergers, millisecond pulsars, and so on. here we consider the general problem of secular evolution of the orbital elements of a binary system driven by the smooth tidal field of an axisymmetric stellar cluster (globular, nuclear, etc.) in which the binary orbits. we derive a secular hamiltonian (averaged over both the inner keplerian orbit of the binary and its outer orbit within the cluster) valid to quadrupole order for an arbitrary cluster potential and explore its characteristics. this doubly averaged `tidal' hamiltonian depends on just two parameters, which fully absorb the information about the background cluster potential and the binary's orbit within it: a dimensional parameter a setting the secular time-scale, and a dimensionless parameter γ which determines the phase portrait of the binary's inner orbital evolution. we examine the dependence of a and γ on cluster potential (both spherical and axisymmetric) and on the binary orbit within the cluster. our theory reproduces known secular results - such as lidov-kozai evolution and the effect of the galactic tide on oort cloud comets - in appropriate limits, but is more general. it provides a universal framework for understanding dynamical evolution of various types of binaries driven by the smooth tidal field of any axisymmetric potential. in a companion paper we provide a detailed exploration of the resulting orbital dynamics. | secular dynamics of binaries in stellar clusters - i. general formulation and dependence on cluster potential |
we present the first results from the hst archival legacy project "skysurf." as described in windhorst et al., skysurf utilizes the large hst archive to study the diffuse uv, optical, and near-ir backgrounds and foregrounds in detail. here, we utilize skysurf's first sky-surface-brightness measurements to constrain the level of near-ir diffuse extragalactic background light (ebl) in three near-ir filters (f125w, f140w, and f160w). this is done by comparing our preliminary sky measurements of >30,000 images to zodiacal light models, carefully selecting the darkest images to avoid contamination from stray light. our sky-surface-brightness measurements have been verified to an accuracy of better than 1%, which when combined with systematic errors associated with hst, results in sky-brightness uncertainties of ~ 2%-4% ≃ 0.005 mjy sr-1 in each image. when compared to the kelsall et al. zodiacal model, an isotropic diffuse background of ~30 nw m-2 sr-1 remains, whereas using the wright zodiacal model results in no discernible diffuse background. based primarily on uncertainties in the foreground model subtraction, we present limits on the amount of diffuse ebl of 29, 40, and 29 nw m-2 sr-1, for f125w, f140w, and f160w, respectively. while this light is generally isotropic, our modeling at this point does not distinguish between a cosmological origin or a solar system origin (such as a dim, diffuse, spherical cloud of cometary dust). | skysurf: constraints on zodiacal light and extragalactic background light through panchromatic hst all-sky surface-brightness measurements: ii. first limits on diffuse light at 1.25, 1.4, and 1.6 μm |
context. deuteration is a crucial tool for understanding the complexity of interstellar chemical processes, especially when they involve the interplay of gas-phase and grain-surface chemistry. in the case of multiple deuteration, comparing observation with the results of chemical modelling is particularly effective to study how molecules are inherited in the different stages within the process of star and planet formation.aims: we aim to study the d/h ratio in h2cs across the prototypical pre-stellar core l1544. this study allows us to test current gas-dust chemical models involving sulfur in dense cores.methods: here, we present single-dish observations of h2cs, hdcs and d2cs with the iram 30 m telescope. we analysed their column densities and distributions and compared these observations with gas-grain chemical models. the deuteration maps of h2cs in l1544 were compared with the deuteration maps of methanol, h2co, n2h+, and hco+ towards the same source. furthermore, we compared the single and double deuteration of h2cs towards the dust peak of l1544 with h2co and c-c3h2. the difference between the deuteration of these molecules in l1544 is discussed and compared with the prediction of chemical models.results: the maximum deuterium fractionation for the first deuteration of h2cs is n(hdcs)/n(h2cs) ~ 30% and is located towards the north-east at a distance of about 10000 au from the dust peak. while for c-c3h2 the first and second deuteration have a similar efficiency, for h2cs and h2co the second deuteration is more efficient, leading to d2cx/hdcx ~ 100% (with x = o or s).conclusions: our results imply that the large deuteration of h2co and h2cs observed in protostellar cores as well as in comets is likely inherited from the pre-stellar phase. however, comparison with state-of-the-art chemical models suggests that the reaction network for the formation of the doubly deuterated h2cs and h2co it is not complete yet. based on observations carried out with the iram 30m telescope. iram is supported by insu/cnrs (france), mpg (germany), and ign (spain). | h2cs deuteration maps towards the pre-stellar core l1544 |
this work presents numerical simulations of meteoroid streams released by comet 21p/giacobini-zinner over the period 1850-2030. the initial methodology, based on vaubaillon et al. (2005), has been updated and modified to account for the evolution of the comet's dust production along its orbit. the peak time, intensity, and duration of the shower were assessed using simulated activity profiles that are calibrated to match observations of historic draconid outbursts. the characteristics of all the main apparitions of the shower are reproduced, with a peak time accuracy of half an hour and an intensity estimate correct to within a factor of 2 (visual showers) or 3 (radio outbursts). our model also revealed the existence of a previously unreported strong radio outburst on october 9, 1999, that has since been confirmed by archival radar measurements. the first results of the model, presented in egal et al. (2018), provided one of the best predictions of the recent 2018 outburst. three future radio outbursts are predicted in the next decade, in 2019, 2025 and 2029. the strongest activity is expected in 2025 when the earth encounters the young 2012 trail. because of the dynamical uncertainties associated with comet 21p's orbital evolution between the 1959 and 1965 apparitions, observations of the 2019 radio outburst would be particularly helpful to improve the confidence of subsequent forecasts. | meteor shower modeling: past and future draconid outbursts |
on 29 november 2011 ut, 2060 chiron occulted a r = 14.9-mag star; data were successfully obtained at the 3-m nasa infrared telescope facility (irtf) on mauna kea and 2-m las cumbres observatory global telescope network (lcogt) faulkes telescope north (ftn) at haleakala. the moris lightcurve shows a solid-body detection of chiron's nucleus with a chord duration of 16.0 ± 1.4 s, corresponding to a chord length of 158 ± 14 km. symmetric, dual extinction features in the ftn light curve indicate the presence of optically thick material roughly 300 km from the body midpoint. the duration of the features indicates a 3 ± 2 km feature separated by 10-14 km from a second 7 ± 2 km feature. the symmetry, optical thickness, and narrow size of these features allow for the intriguing possibility of a near-circular arc or shell of material. | 29 november 2011 stellar occultation by 2060 chiron: symmetric jet-like features |
in this theoretical investigation, we have examined the combined effects of nonthermally modified polarization force on modulational instability of dust acoustic ($da$) waves and evolution of different kinds of rogue waves in space dusty plasma containing negatively charged dust as fluid, boltzmannian electrons and ions obeying cairns' nonthermal distribution. the nonthermality of ions has considerably altered the influence of polarization force. by employing the multiple-scale perturbation technique, the nonlinear schr\"{o}dinger equation ($nlse$) is derived to study the the modulational instability ($mi$) of the dust acoustic waves ($daws$). it is remarked that the impact of polarization force is to narrow the wavenumber domain. the rational solution of nlse illustrates the evolution of da breathers including, akhmediev breathers (ab), kuznetsov–ma (km) breathers and peregrine solitons (rogue waves). further, transformation of higher order rogue wave triplets to the super rogue waves due to the superposition is also discussed. it is noticed that amalgamation of polarization force and nonthermal ions have an explicit influence on the evolution of different kinds of da rogue waves. it is remarked that our present theoretical findings may shed the light on the generation of different kinds of $da$ rogue waves in laboratory experiments and space, especially in the planetary rings and comet tails. | breather structures and peregrine solitons in a polarized space dusty plasma |
cometary outgassing can produce torques that change the spin state of the cometary nucleus, which in turn influences the evolution and lifetime of the comet. if these torques increase the rate of rotation to the extent that centripetal forces exceed the material strength of the nucleus, the comet can fragment. torques that slow down the rotation can cause the spin state to become unstable, but if the torques persist the nucleus can eventually reorient itself and the rotation rate can increase again. simulations predict that most comets go through a short phase of rapid changes in spin state, after which changes occur gradually over longer times. here we report observations of comet 41p/tuttle-giacobini-kresák during its close approach to earth (0.142 astronomical units, approximately 21 million kilometres, on 1 april 2017) that reveal a rapid decrease in rotation rate. between march and may 2017, the apparent rotation period of the nucleus increased from 20 hours to more than 46 hours—a rate of change of more than an order of magnitude larger than has hitherto been measured. this phenomenon must have been caused by the gas emission from the comet aligning in such a way that it produced an anomalously strong torque that slowed the spin rate of the nucleus. the behaviour of comet 41p/tuttle-giacobini-kresák suggests that it is in a distinct evolutionary state and that its rotation may be approaching the point of instability. | a rapid decrease in the rotation rate of comet 41p/tuttle-giacobini-kresák |
2i/borisov is the first interstellar comet discovered on 2019 august 30, and it soon showed a coma and a dust tail. this study reports the results of images obtained at the telescopio nazionale galileo telescope, on la palma - canary islands, in 2019 november and december. the images have been obtained with the r filter in order to apply our dust tail model. the model has been applied to the comet 67p/churyumov-gerasimenko and compared to the rosetta dust measurements showing a very good agreement. it has been applied to the comet 2i/borisov, using almost the same parameters, obtaining a dust environment similar to that of 67p/churyumov-gerasimenko, suggesting that the activity may be very similar. the dust tail analysis provided a dust-loss rate qd ≈ 35 kg s-1 in 2019 november and qd ≈ 30 kg s-1 in 2019 december. | dust environment model of the interstellar comet 2i/borisov |
we present high spatial resolution images of the binary transneptunian object gǃkúnǁ'hòmdímà (229762 2007 uk126) obtained with the hubble space telescope and with the keck observatory on mauna kea to determine the orbit of gǃò'é ǃhúgǃò'é ǃhú, the much smaller and redder satellite. gǃò'é ǃhú orbits in a prograde sense, on a circular or near-circular orbit with a period of 11.3 days and a semimajor axis of 6000 km. tidal evolution is expected to be slow, so it is likely that the system formed already in a low-eccentricity configuration, and possibly also with the orbit plane of the satellite in or close to the plane of gǃkúnǁ'hòmdímà's equator. from the orbital parameters we can compute the system mass to be 1.4 × 1020 kg. combined with estimates of the size of gǃkúnǁ'hòmdímà from thermal observations and stellar occultations, we can estimate the bulk density as about 1 g cm-3. this low density is indicative of an ice-rich composition, unless there is substantial internal porosity. we consider the hypothesis that the composition is not unusually ice-rich compared with larger tnos and comet nuclei, and instead the porosity is high, suggesting that mid-sized objects in the 400 to 1000 km diameter range mark the transition between small, porous objects and larger objects that have collapsed their internal void space as a result of their much higher internal pressures and temperatures. | the mutual orbit, mass, and density of transneptunian binary gǃkúnǁ'hòmdímà (229762 2007 uk126) |
centaurs are minor planets thought to have originated in the outer solar system region known as the kuiper belt. active centaurs enigmatically display comet-like features (e.g., tails, comae) even though they orbit in the gas giant region where it is too cold for water to readily sublimate. only 18 active centaurs have been identified since 1927 and, consequently, the underlying activity mechanism(s) have remained largely unknown up to this point. here we report the discovery of activity emanating from centaur 2014 og392, based on archival images we uncovered plus our own new observational evidence acquired with the dark energy camera (cerro tololo inter-american observatory blanco 4 m telescope), the inamori-magellan areal camera & spectrograph (las campanas observatory 6.5 m walter baade telescope), and the large monolithic imager (lowell observatory 4.3 m discovery channel telescope). we detect a coma as far as 400,000 km from 2014 og392, and our novel analysis of sublimation processes and dynamical lifetime suggest carbon dioxide and/or ammonia are the most likely candidates for causing activity on this and other active centaurs. we find 2014 og392 is optically red, but co2 and nh3 are spectrally neutral in this wavelength regime so the reddening agent is as yet unidentified. | cometary activity discovered on a distant centaur: a nonaqueous sublimation mechanism |
the kepler-field star kic 8462852, an otherwise apparently ordinary f3 main-sequence star, showed several highly unusual dimming events of variable depth and duration. adding to the mystery was the discovery that kic 8462852 faded by 14 per cent from 1890 to 1989, as well as by another 3 per cent over the 4 yr kepler mission. following an initial suggestion by wright & sigurdsson, we propose that the secular dimming behaviour is the result of the inspiral of a planetary body or bodies into kic 8462852, which took place ∼10-104 yr ago (depending on the planet mass). gravitational energy released as the body inspirals into the outer layers of the star caused a temporary and unobserved brightening, from which the stellar flux is now returning to the quiescent state. the transient dimming events could then be due to obscuration by planetary debris from an earlier partial disruption of the same inspiralling bodies, or due to evaporation and outgassing from a tidally detached moon system. alternatively, the dimming events could arise from a large number of comet- or planetesimal-mass bodies placed on to high-eccentricity orbits by the same mechanism (e.g. lidov-kozai oscillations due to the outer m-dwarf companion) responsible for driving the more massive planets into kic 8462852. the required high occurrence rate of kic 8462852-like systems that have undergone recent major planet inspiral event(s) is the greatest challenge to the model, placing large lower limits on the mass of planetary systems surrounding f stars and/or requiring an unlikely probability to catch kic 8462852 in its current state. | secular dimming of kic 8462852 following its consumption of a planet |
the origin of the broad emission line region (belr) in quasars and active galactic nuclei is still unclear. i propose that condensations form in the warm, radiation-pressure-driven, accretion disk wind of quasars creating the bel clouds and uniting them with the other two manifestations of cool (∼104 k) gas in quasars, the low ionization phase of the warm absorbers (was) and the clouds causing x-ray eclipses. the cool clouds will condense quickly (days to years), before the wa outflows reach escape velocity (which takes months to centuries). cool clouds form in equilibrium with the warm phase of the wind because the rapidly varying x-ray quasar continuum changes the force multiplier, causing pressure waves to move gas into stable locations in pressure-temperature space. the narrow range of two-phase equilibrium densities may explain the (luminosity){}1/2 scaling of the belr size, while the scaling of cloud formation timescales could produce the baldwin effect. these dense clouds have force multipliers of order unity and so cannot be accelerated to escape velocity. they fall back on a dynamical timescale (months to centuries), producing an inflow that rains down toward the central black hole. as they soon move at mach ∼10-100 with respect to the wa outflow, these “raindrops” will be rapidly destroyed within months. this rain of clouds may produce the elliptical belr orbits implied by velocity-resolved reverberation mapping in some objects and can explain the opening angle and destruction timescale of the narrow “cometary” tails of the clouds seen in x-ray eclipse observations. some consequences and challenges of this “quasar rain” model are presented, along with several avenues for theoretical investigation. | quasar rain: the broad emission line region as condensations in the warm accretion disk wind |
in this paper we discuss in a thermodynamic, geologically empirical way the long-term nature of the stable majority ices that could be present in kuiper belt object (kbo) 2014 mu69 (also called arrokoth; hereafter "mu69") after its 4.6 gyr residence in the edgeworth-kuiper belt (ekb) as a cold classical object. we compare the upper bounds for the gas production rate (~1024 molecules/s) measured by the new horizons (nh) spacecraft flyby on 01 jan 2019 to estimates for the outgassing flux rates from a suite of common cometary and kbo ices at the average ~ 40 k sunlit surface temperature of mu69, but do not find the upper limit very constraining except for the most volatile of species (e.g. co, n2, ch4). more constraining is the stability versus sublimation into vacuum requirement over myr to gyr, and from this we find only 3 common ices that are truly refractory: hcn, ch3oh, and h2o (in order of increasing stability), while nh3 and h2co ices are marginally stable and may be removed by any positive temperature excursions in the ekb, as produced every 108-109 years by nearby supernovae and passing o/b stars. to date the nh team has reported the presence of abundant ch3oh and h2o on mu69's surface (stern et al., 2019; grundy et al., 2020). nh3 has been searched for, but not found. we predict that future absorption feature detections, if any are ever derived from higher signal-to-noise ratio spectra, will be due to an hcn or poly-h2co based species. consideration of the conditions present in the ekb region during the formation era of mu69 lead us to state that it is highly likely that it "formed in the dark", in an optically thick mid-plane, unable to see the nascent, variable, highly luminous young stellar object (yso)/ttauri sun, and that kbos contain hcn and ch3oh ice phases in addition to the h2o ice phases found in their short period (sp) comet descendants. finally, when we apply our ice thermal stability analysis to bodies/populations related to mu69, we find that methanol ice is likely ubiquitous in the outer solar system; that if pluto isn't a fully differentiated body, then it must have gained its hypervolatile ices from proto-planetary disk (ppd) sources in the first few myr of the solar system's existence; and that hypervolatile rich, highly primordial comet c/2016 r2 was placed onto an oort cloud orbit on a similar few myr timescale. | on the origin & thermal stability of arrokoth's and pluto's ices |
the distant long-period comet c/2017 k2 (panstarrs) has been outside the planetary region of the solar system for ∼3 myr, negating the possibility that heat retained from the previous perihelion could be responsible for its activity. this inbound comet is also too cold for water ice to sublimate and too cold for amorphous water ice, if present, to crystallize. c/2017 k2 thus presents an ideal target in which to investigate the mechanisms responsible for activity in distant comets. we have used the hubble space telescope to study the comet in the pre-perihelion heliocentric distance range 13.8 ≤ rh≤ 15.9 au. in this range, the coma maintains a logarithmic surface brightness gradient m = -1.010 ± 0.004, consistent with mass loss proceeding in steady state. the absence of a radiation pressure swept tail indicates that the effective particle size is large (radius ≳0.1 mm) and the mass-loss rate is ∼200 kg s-1, remarkable for a comet still beyond the orbit of saturn. extrapolation of the photometry indicates that activity began in 2012.1 ± 0.5, at rh= 25.9 ± 0.9 au, where the isothermal blackbody temperature is only tbb= 55 k. this large distance and low temperature suggest that cometary activity is driven by the sublimation of a super-volatile ice (e.g., co), presumably preserved by k2's long-term residence in the oort cloud. the mass-loss rate can be sustained by co sublimation from an area ≲2 km2, if located near the hot subsolar point on the nucleus. however, while the drag force from sublimated co is sufficient to lift millimeter-sized particles against the gravity of the cometary nucleus, it is 102-103 times too small to eject these particles against interparticle cohesion. our observations thus require either a new understanding of the physics of interparticle cohesion or the introduction of another mechanism to drive distant cometary mass loss. we suggest thermal fracture and electrostatic supercharging in this context. | distant comet c/2017 k2 and the cohesion bottleneck |
we here study the transfer process of material from one hemisphere to the other (deposition of airfall material) on an active comet nucleus, specifically 67p/churyumov-gerasimenko. our goals are to: 1) quantify the thickness of the airfall debris layers and how it depends on the location of the target area, 2) determine the amount of h2o and co2 ice that are lost from icy dust assemblages of different sizes during transfer through the coma, and 3) estimate the relative amount of vapor loss in airfall material after deposition in order to understand what locations are expected to be more active than others on the following perihelion approach. we use various numerical simulations, that include orbit dynamics, thermophysics of the nucleus and of individual coma aggregates, coma gas kinetics and hydrodynamics, as well as dust dynamics due to gas drag, to address these questions. we find that the thickness of accumulated airfall material varies substantially with location, and typically is of the order 0.1-1 m. the airfall material preserves substantial amounts of water ice even in relatively small (cm-sized) coma aggregates after a rather long (12 h) residence in the coma. however, co2 is lost within a couple of hours even in relatively large (dm-sized) aggregates, and is not expected to be an important component in airfall deposits. we introduce reachability and survivability indices to measure the relative capacity of different regions to simultaneously collect airfall and to preserve its water ice until the next perihelion passage, thereby grading their potential of contributing to comet activity during the next perihelion passage. | airfall on comet 67p/churyumov-gerasimenko |
here, we show that the detection of borisov implies that interstellar objects outnumber solar system objects in the oort cloud, whereas the reverse is true near the sun due to the stronger gravitational focusing of bound objects. this hypothesis can be tested with stellar occultation surveys of the oort cloud. furthermore, we demonstrate that $\sim 1 {{\ \rm per\ cent}}$ of carbon and oxygen in the milky way galaxy may be locked in interstellar objects, implying a heavy element budget for interstellar objects comparable to the heavy element budget of the minimum mass solar nebula model. there is still considerable uncertainty regarding the size distribution of the interstellar objects. | interstellar objects outnumber solar system objects in the oort cloud |
context. the nitrogen reservoir in planetary systems is a long-standing problem. some of the n-bearing molecules are probably incorporated into the ice bulk during the cold phases of the stellar evolution, and may be gradually released into the gas phase when the ice is heated, for example in active comets. the chemical nature of the n-reservoir should greatly influence how, when, and in what form n returns to the gas phase, or is incorporated into the refractory material forming planetary bodies.aims: we present the study of the thermal desorption of two ammonium salts, ammonium formate and ammonium acetate, from a gold surface and from a water ice substrate.methods: temperature-programmed desorption experiments and fourier transform infrared reflection spectroscopy were conducted to investigate the desorption behavior of ammonium salts.results: ammonium salts are semi-volatile species releasing neutral species as major components upon desorption, namely ammonia and the corresponding organic acid (hcooh and ch3cooh), at temperatures higher than the temperature of thermal desorption of water ice. their desorption follows a first-order wigner-polanyi law. we find the first-order kinetic parameters a = 7.7 ± 0.6 × 1015 s−1 and ebind = 68.9 ± 0.1 kj mol−1 for ammonium formate and a = 3.0 ± 0.4 × 1020 s−1 and ebind = 83.0 ± 0.2 kj mol−1 for ammonium acetate. the presence of a water ice substrate does not influence the desorption kinetics. ammonia molecules locked in salts desorb as neutral molecules at temperatures much higher than previously expected, and that are usually attributed to refractory materials.conclusions: the ammonia snow line has a smaller radius than the water snow line. as a result, the nh3/h2o ratio content in solar system bodies can be a hint to where they formed and subsequently migrated. | ammonia snow lines and ammonium salts desorption |
the bow shock is the first boundary the solar wind encounters as it approaches planets or comets. the rosetta spacecraft was able to observe the formation of a bow shock by following comet 67p/churyumov-gerasimenko toward the sun, through perihelion, and back outward again. the spacecraft crossed the newly formed bow shock several times during two periods a few months before and after perihelion; it observed an increase in magnetic field magnitude and oscillation amplitude, electron and proton heating at the shock, and the diminution of the solar wind further downstream. rosetta observed a cometary bow shock in its infancy, a stage in its development not previously accessible to in situ measurements at comets and planets. | the infant bow shock: a new frontier at a weak activity comet |
average flow patterns of ions around comet 67p detected by the rpc-ica instrument onboard rosetta are presented both as a time series and as a spatial distribution of the average flow in the plane perpendicular to the comet - sun direction (y-z plane in the coordinate systems used). cometary ions in the energy range up to 60 ev flow radially away from the nucleus in the y-z plane, irrespective of the direction of the magnetic field, throughout the mission. these ions may however be strongly affected by the spacecraft potential, the uncertainty due to this is briefly discussed. inside the solar wind ion cavity and in the periods just before and after, the cometary pick up ions moving antisunward are deflected against the inferred solar wind electric field direction. this is opposite to what is observed for lower levels of mass-loading. these pick up ions are behaving in a similar way to the solar wind ions and are deflected due to mass-loading. a spatial asymmetry can be seen in the observations of deflected pick up ions, with motion against the electric field primarily within a radius of 200 km of the nucleus and also in the negative electric field hemisphere. cometary ions observed by rpc-ica typically move in the antisunward direction throughout the mission. these are average patterns, full-resolution data show very much variability. | average cometary ion flow pattern in the vicinity of comet 67p from moment data |
context. the first long-term in-situ observation of the plasma environment in the vicinity of a comet, as provided by the european rosetta spacecraft.aims: here we offer characterisation of the solar wind flow near 67p/churyumov-gerasimenko (67p) and its long term evolution during low nucleus activity. we also aim to quantify and interpret the deflection and deceleration of the flow expected from ionization of neutral cometary particles within the undisturbed solar wind.methods: we have analysed in situ ion and magnetic field data and combined this with hybrid modeling of the interaction between the solar wind and the comet atmosphere.results: the solar wind deflection is increasing with decreasing heliocentric distances, and exhibits very little deceleration. this is seen both in observations and in modeled solar wind protons. according to our model, energy and momentum are transferred from the solar wind to the coma in a single region, centered on the nucleus, with a size in the order of 1000 km. this interaction affects, over larger scales, the downstream modeled solar wind flow. the energy gained by the cometary ions is a small fraction of the energy available in the solar wind.conclusions: the deflection of the solar wind is the strongest and clearest signature of the mass-loading for a small, low-activity comet, whereas there is little deceleration of the solar wind. | mass-loading of the solar wind at 67p/churyumov-gerasimenko. observations and modelling |
we report photometric observations of comet 29p/schwassmann-wachmann 1 made on august 12, 2016 with the broad-band b, v, r and i filters and the soar 4.1-meter telescope (chile). we find the comet active at that time. enhanced images obtained in all filters reveal three radial features in the 29p/schwassmann-wachmann 1 coma, regardless of the image-processing algorithm. using a high-resolution spectrum of comet 29p/schwassmann-wachmann 1 reported by ivanova et al. (2018) on the same date, we estimate the relative contribution of the gaseous emission and the continuum to the total response measured with our broadband b and v filters. the gaseous-emission contribution appears to be very small, 2.5%. we compute the dust production afρ in 29p/schwassmann-wachmann 1 for the four filters and find its growth with the wavelength, from 3,393 ± 93 cm in the b filter to 8,561 ± 236 cm in the i filter. we model the color slope of dust in comet 29p/schwassmann-wachmann 1 using agglomerated debris particles. simultaneous analysis of the color slope in the b-r and r-i pairs suggests a single dominant chemical species of 29p/schwassmann-wachmann 1 dust particles consisting of fe-mg silicates and obeying a power-law size distribution with index n ≈ 2.55. this conclusion is consistent with the previous thermal-emission study of 29p/schwassmann-wachmann 1. | comet 29p/schwassmann-wachmann 1 dust environment from photometric observation at the soar telescope |
the nature of the icy material accreted by comets during their formation in the outer regions of the protosolar nebula (psn) is a major open question in planetary science. some scenarios of comet formation predict that these bodies agglomerated from crystalline ices condensed in the psn. concurrently, alternative scenarios suggest that comets accreted amorphous ice originating from the interstellar cloud or from the very distant regions of the psn. on the basis of existing laboratory and modeling data, we find that the n2/co and ar/co ratios measured in the coma of the jupiter-family comet 67p/churyumov-gerasimenko by the rosetta orbiter spectrometer for ion and neutral analysis instrument on board the european space agency’s rosetta spacecraft match those predicted for gases trapped in clathrates. if these measurements are representative of the bulk n2/co and ar/co ratios in 67p/churyumov-gerasimenko, it implies that the ices accreted by the comet formed in the nebula and do not originate from the interstellar medium, supporting the idea that the building blocks of outer solar system bodies have been formed from clathrates and possibly from pure crystalline ices. moreover, because 67p/churyumov-gerasimenko is impoverished in ar and n2, the volatile enrichments observed in jupiter’s atmosphere cannot be explained solely via the accretion of building blocks with similar compositions and require an additional delivery source. a potential source may be the accretion of gas from the nebula that has been progressively enriched in heavy elements due to photoevaporation. | a protosolar nebula origin for the ices agglomerated by comet 67p/churyumov-gerasimenko |
recent observations of the long period comet c/2016 r2 (panstarrs) 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, 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 the comet c/2016 r2 (panstarrs) 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 icelines. moreover, high co/h2o ratios (>100 times the initial gas phase value) can be found in grains condensed in the vicinity of the co iceline. 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 comet c/2016 r2 (panstarrs)'s coma. our scenario indicates that comet c/2016 r2 (panstarrs) 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 comet c/2016 r2 (panstarrs)'s peculiar composition |
the comet database search software was initially released as an open source project in late 2012. prior to that, comet existed as the university of washington's academic version of the sequest database search tool. despite its availability and widespread use over the years, some details about its implementation have not been previously disseminated or are not well understood. we address a few of these details in depth and highlight new features available in the latest release. comet is freely available for download at http://comet-ms.sourceforge.net or it can be accessed as a component of a number of larger software projects into which it has been incorporated. | a deeper look into comet—implementation and features |
we present post-perihelion photometric and polarimetric observations of comet 67p/churyumov-gerasimenko performed at the 6-m telescope of the sao ras in the g-sdss (465/65 nm), r-sdss (620/60 nm) and r filters. observations in november and december 2015 and april 2016 covered the range of heliocentric distance 1.62-2.72 au and phase angle 33.2°-10.4°. the comet was very active. two persistent jets and long dust tail were observed during the whole observing period; one more jet was detected only in december. the radial profiles of surface brightness, colour and polarization significantly differed for the coma, jets and tail, and changed with increasing heliocentric distance. the dust production afρ decreased from 162 cm at r = 1.62 au to 51 cm at r = 2.72 au. the dust colour (g-r) gradually changed from 0.8 mag in the innermost coma to about 0.4 mag in the outer coma. the spectral slope was 8.2 ± 1.7 per cent/100 nm in the 465 to 620 nm wavelength domain. in november and december, the polarization in the near-nucleus area was about 8 per cent, dropped sharply to 2 per cent at the distance above 5000 km and then gradually increased with distance from the nucleus, reaching ∼8 per cent at 40 000 km. in april, at a phase angle 10.4°, the polarization varied between -0.6 per cent in the near-nucleus area and -4 per cent in the outer coma. circular polarization was not detected in the comet. the spatial variations of brightness, colour and polarization in different structural features suggest some evolution of particle properties, most likely decreasing the size of dust particles. | spatial variations of brightness, colour and polarization of dust in comet 67p/churyumov-gerasimenko |
modulation instability (mi) and characteristics of dust-acoustic rogue waves (darws) are theoretically investigated in electron-depleted dusty magnetoplasmas comprised of massive dust grains of opposite polarities and nonextensive ions by considering a three-dimensional geometry. for this purpose, the (3 + 1)-dimensional nonlinear schrödinger equation is derived using the derivative expansion method. also, the criteria of the mi are derived and discussed in detail. numerical analysis revealed that stable and unstable domains of the modulated waves are strongly sensitive to the magnetic field, ions nonextensivity, and some relevant dust grain characteristics (mass and density). it is worth to mention that the frequency range where the mi varies as in the one-dimensional (1d) case has been also identified. the obtained results are important to understand the darws in some plasma environments, including interstellar medium, jupiter's magnetosphere, upper mesosphere, comets, saturn's rings, and earth's atmosphere. | multidimensional dust-acoustic rogue waves in electron-depleted complex magnetoplasmas |
we present rosetta observations from comet 67p/churyumov-gerasimenko during the impact of a coronal mass ejection (cme). the cme impacted on 2015 oct 5-6, when rosetta was about 800 km from the comet nucleus, and 1.4 au from the sun. upon impact, the plasma environment is compressed to the level that solar wind ions, not seen a few days earlier when at 1500 km, now reach rosetta. in response to the compression, the flux of suprathermal electrons increases by a factor of 5-10 and the background magnetic field strength increases by a factor of ∼2.5. the plasma density increases by a factor of 10 and reaches 600 cm-3, due to increased particle impact ionization, charge exchange and the adiabatic compression of the plasma environment. we also observe unprecedentedly large magnetic field spikes at 800 km, reaching above 200 nt, which are interpreted as magnetic flux ropes. we suggest that these could possibly be formed by magnetic reconnection processes in the coma as the magnetic field across the cme changes polarity, or as a consequence of strong shears causing kelvin-helmholtz instabilities in the plasma flow. due to the limited orbit of rosetta, we are not able to observe if a tail disconnection occurs during the cme impact, which could be expected based on previous remote observations of other cme-comet interactions. | cme impact on comet 67p/churyumov-gerasimenko |
we observed comet c/2016 r2 (panstarrs) with the aro 10 m smt and report the first detection of co emission from this comet with amounts high enough to be the primary driver of activity. we obtained spectra and maps of the co j = 2-1 rotational line at 230 ghz between 2017 december and 2018 january. we calculated an average production rate of q(co) = (4.6 ± 0.4) × 1028 mol s-1 at r ∼ 2.9 au and δ ∼ 2.1 au. the co line is thin (δv fwhm ∼ 0.8 km s-1) with a slight blueshift (δv ∼ -0.1 km s-1) from the ephemeris velocity, and we derive a gas expansion velocity of v exp = 0.50 ± 0.15 km s-1. this comet produced approximately half the co that comet c/1995 o1 (hale-bopp) did at 3 au. if co production scales with nucleus surface area, then the radius need not exceed r r2 ∼ 15 km. the spectra and mapping data are consistent with co arising from a combination of a sunward-side active area and an isotropic source. for hcn, we calculated a 3σ upper limit production rate of q(hcn) < 8 × 1024 molecules s-1, which corresponds to an extraordinarily high abundance ratio limit of q(co)/q(hcn) > 5000. we inferred a production rate of molecular nitrogen of q(n2) ∼ 2.8 × 1027 molecules s-1 using our co data and the reported n2/co column density ratio. the comet does not show the typical nitrogen depletion seen in comets. the co-rich, n2-rich, and hcn-depleted values are consistent with formation in an environment of t ∼ 50 k that may have provided significant n2 shielding. | c/2016 r2 (panstarrs): a comet rich in co and depleted in hcn |
exocomets scattered by planets have been invoked to explain observations in multiple contexts, including the frequently found near- and mid-infrared excess around nearby stars arising from exozodiacal dust. here we investigate how the process of inward scattering of comets originating in an outer belt is affected by the architecture of a planetary system, to determine whether this could lead to observable exozodi levels or deliver volatiles to inner planets. using n-body simulations, we model systems with different planet mass and orbital spacing distributions in the 1-50 au region. we find that tightly packed (δap < 20rh, m) low-mass planets are the most efficient at delivering material to exozodi regions ( 5-7 per cent of scattered exocomets end up within 0.5 au at some point), although the exozodi levels do not vary by more than a factor of ∼7 for the architectures studied here. we suggest that emission from scattered dusty material in between the planets could provide a potential test for this delivery mechanism. we show that the surface density of scattered material can vary by two orders of magnitude (being highest for systems of low-mass planets with medium spacing), while the exozodi delivery rate stays roughly constant, and that future instruments such as jwst could detect it. in fact for η corvi, the current herschel upper limit rules out the scattering scenario by a chain of ≲30 m⊕ planets. finally, we show that exocomets could be efficient at delivering cometary material to inner planets ( 0.1-1 per cent of scattered comets are accreted per inner planet). overall, the best systems at delivering comets to inner planets are the ones that have low-mass outer planets and medium spacing (∼20rh, m). | scattering of exocomets by a planet chain: exozodi levels and the delivery of cometary material to inner planets |
we perform a thermo-physical analysis on water activity of comet 67p/churyumov-gerasimenko (67p). the sublimation of water is assumed to occur from beneath a global, desiccated dust mantle over the irregular-shaped nucleus. the concept of two thermal models, the recipe of model formulation and the strategy of application to comet 67p are described. for an accurate and efficient evaluation of energy input by insolation and self-heating over the nucleus, a landscape data base is devised based on polyhedral shape models of the nucleus. we apply the thermal models to investigate the impact of certain parameters of nucleus properties on water production. it is found that the measured water production of 67p can be overall attributed to sublimation of water ice with a mass abundance of a few to 10 per cent beneath a uniform dust mantle of several millimetres to one centimetre in thickness. insofar as 67p is concerned, we argue against the necessity to invoke assumptions on localized water activity, or on the distinction of active/dormant surface areas. | thermal modelling of water activity on comet 67p/churyumov-gerasimenko with global dust mantle and plural dust-to-ice ratio |
we use data on extreme radio scintillation to demonstrate that this phenomenon is associated with hot stars in the solar neighborhood. the ionized gas responsible for the scattering is found at distances up to 1.75 {pc} from the host star, and on average must comprise ∼105 distinct structures per star. we detect azimuthal velocities of the plasma, relative to the host star, up to 9.7 {km} {{{s}}}-1, consistent with warm gas expanding at the sound speed. the circumstellar plasma structures that we infer are similar in several respects to the cometary knots seen in the helix and in other planetary nebulae. there the ionized gas appears as a skin around tiny molecular clumps. our analysis suggests that molecular clumps are ubiquitous circumstellar features, unrelated to the evolutionary state of the star. the total mass in such clumps is comparable to the stellar mass. | extreme radio-wave scattering associated with hot stars |
carbonyl sulfide (ocs) is one of the sulfur-bearing molecules detected in different astronomical environments, including comets. the present-day sulfur chemistry in comets may reveal much about the origin of these ices and their subsequent processing history. cometary sulfur molecules such as h2s, h2cs, so2, so, cs, cs2, s2, and ns have been detected in many comets. however, ocs, the only sulfur-bearing species with fluorescence emission lines at infrared wavelengths, is under-represented in comet volatile studies, having been reported in only six comets so far. we targeted ocs with the nasa infrared telescope facility in comets 46p/wirtanen, 21p/giacobini-zinner, and c/2015 er61 (panstarrs) in 2017-2018 using the high-resolution ishell spectrograph, and in c/2002 t7 (linear) in 2004 using the heritage cshell spectrograph. in comet c/2015 er61, the ocs abundance was similar to those measured in bright comets such as comets c/2012 s1 (ison) and c/1996 b2 (hyakutake), whereas in c/2002 t7 it was relatively depleted. our ocs measurement in 21p/giacobini-zinner is the first definitive detection of this molecule in a jupiter-family comet from a ground-based facility and is close to the average ocs abundance determined in comet 67p/churyumov-gerasimenko by the rosetta mission. our 3σ upper limit for comet 46p/wirtanen is the lowest reported ocs abundance in any comet. we present production rates and mixing ratios (with respect to h2o) for these comets and place our results in the context of comets measured to date. | carbonyl sulfide (ocs): detections in comets c/2002 t7 (linear), c/2015 er61 (panstarrs), and 21p/giacobini-zinner and stringent upper limits in 46p/wirtanen |
we quantitatively investigate the contribution of large dust particles to the polarimetric response in comets using the light-scattering properties of model agglomerated debris particles. we demonstrate that large, supermicron-sized particles have a decreasing role on the degree of linear polarization at phase angle α ≤ 80°, and the effect of particles greater than 10 μm is minimal. at larger phase angles, they may only slightly increase the measured percent of polarization by up to 1%. omitting the effects of these particles in modeling the observations only slightly affects the retrievals of the microphysical properties of dust in comets and could lead to a small underestimation of the index in a power-law size distribution and population of weakly absorbing dust particles. | on the small contribution of supermicron dust particles to light scattering by comets |
comets can be regarded as active planetary bodies because they display evidence for nearly all fundamental geological processes, which include impact cratering, tectonism, and erosion. comets also display sublimation-driven outgassing, which is comparable to volcanism on larger planetary bodies in that it provides a conduit for delivering materials from the interior to the surface. however, in the domain of active geological bodies, comets occupy a special niche since their geologic activity is almost exclusively driven by externally supplied energy (i.e. solar energy) as opposed to an internal heat source, which makes them "seasonally-active" geological bodies. during their active phase approaching the sun, comets also develop a transient atmosphere that interacts with the surface and contributes to its evolution, particularly by transporting materials across the surface. variations in solar energy input on diurnal and seasonal scale cause buildup of thermal stresses within consolidated materials that lead to weathering through fracturing, and eventually mass-wasting. the commonly irregular shapes of comets also play a major role in their evolution by leading to (1) non-uniform gravitational forces that affect material movement across the surface, and (2) spatially heterogeneous outgassing patterns that affect the comet's orbital dynamics and lead to tidal stresses that can further fracture the nucleus. in this chapter, we review the surface morphology of comet 67p/churyumov-gerasimenko as well as its seasonal evolution as viewed by rosetta from august 2014 to september 2016, their link to various processes, and the forces that drive surface evolution. | surface morphology of comets and associated evolutionary processes: a review of rosetta's observations of 67p/churyumov-gerasimenko |
to date, at least three comets—2i/borisov, c/2016 r2 (panstarrs), and c/2009 p1 (garradd)—have been observed to have unusually high co concentrations compared to water. we attempt to explain these observations by modeling the effect of drifting solid (ice and dust) material on the ice compositions in protoplanetary disks. we find that, independent of the exact disk model parameters, we always obtain a region of enhanced ice-phase co/h2o that spreads out in radius over time. the inner edge of this feature coincides with the co snowline. almost every model achieves at least co/h2o of unity, and one model reaches a co/h2o ratio >10. after running our simulations for 1 myr, an average of 40% of the disk ice mass contains more co than h2o ice. in light of this, a population of co-ice-enhanced planetesimals are likely to generally form in the outer regions of disks, and we speculate that the aforementioned co-rich comets may be more common, both in our own solar system and in extrasolar systems, than previously expected. | ice-coated pebble drift as a possible explanation for peculiar cometary co/h2o ratios |
we used the new high spectral resolution cross-dispersed facility spectrograph, ishell, at the nasa infrared telescope facility on maunakea, hi, to observe jupiter-family comet (jfc) 45p/honda-mrkos-pajdušáková. we report water production rates, as well as production rates and abundance ratios relative to h2o, for eight trace parent molecules (native ices), co, ch4, h2co, ch3oh, hcn, nh3, c2h2, and c2h6, on 2 days spanning ut 2017 january 6/7 and 7/8, shortly following perihelion. trace species were measured simultaneously with h2o and/or oh prompt emission, a proxy for h2o production, thereby providing a robust and consistent means of establishing the native ice composition of 45p. its favorable geocentric radial velocity (approximately -35 km s-1) permitted sensitive measures of the "hypervolatiles" co and ch4, which are substantially undercharacterized in jfcs. our results represent the most precise ground-based measures of co and ch4 to date in a jfc, providing a foundation for building meaningful statistics regarding their abundances. the abundance ratio for ch4 in 45p (0.79% ± 0.06% relative to h2o) was consistent with its median value as measured among oort cloud comets, whereas co (0.60% ± 0.04%) was strongly depleted. compared with all measured comets, hcn (0.049% ± 0.012%) was strongly depleted, ch3oh (3.6% ± 0.3%) was enriched, and the remaining species were consistent with their respective median abundances. the volatile composition measured for 45p could indicate processing of ices prior to their incorporation into its nucleus. spatial analysis of emissions suggests enhanced release of more volatile species into the sunward-facing hemisphere of the coma. | hypervolatiles in a jupiter-family comet: observations of 45p/honda-mrkos-pajdušáková using ishell at the nasa-irtf |
the -ch-nh2 moiety represents the fundamental building block of all proteinogenic amino acids, with the cyclic amino acid proline being a special case (-ch-nh- in proline). exploiting a chemical retrosynthesis, we reveal that methylamine (ch3nh2) and/or ethylamine (ch3ch2nh2) are essential precursors in the formation of each proteinogenic amino acid. in the present study we elucidate the abiotic formation of methylamine and ethylamine from ammonia (nh3) and methane (ch4) ices exposed to secondary electrons generated by energetic cosmic radiation in cometary and interstellar model ices. our experiments show that methylamine and ethylamine are crucial reaction products in irradiated ices composed of ammonia and methane. using isotopic substitution studies we further obtain valuable information on the specific reaction pathways toward methylamine. the very recent identification of methylamine and ethylamine together with glycine in the coma of 67p/churyumov-gerasimenko underlines their potential to the extraterrestrial formation of amino acids. | formation of methylamine and ethylamine in extraterrestrial ices and their role as fundamental building blocks of proteinogenic α-amino acids |
we present a summary of the more than 3000 sungrazing and near-sun comets discovered in coronagraph images returned by the solar and heliospheric observatory (soho), since its launch in december 1995. we address each of the four main populations of objects observed by soho: kreutz (sungrazing) group, meyer group, marsden and kracht (96p-family) group and non-group comets. discussions for each group include basic properties, discovery statistics and morphological appearance. in addition to updating the community on the status of the discoveries by soho, we also show that the rate of discovery of kreutz sungrazers has probably remained static since approximately 2003 and we report on the first likely fragmentation pair observed within the meyer group. this article is part of the themed issue 'cometary science after rosetta'. | soho comets: 20 years and 3000 objects later |
understanding the cosmogonic record encoded in the parent volatiles stored in cometary nuclei requires investigating whether evolution (thermal or otherwise) has modified the composition of short-period comets during successive perihelion passages. as the most volatile molecules systematically observed in comets, the abundances of co, ch4, and c2h6 in short-period comets may serve to elucidate the interplay between natal conditions and post-formative evolution in setting present-day composition, yet secure measurements of co and ch4 in jupiter-family comets (jfcs) are especially sparse. the highly favorable 2018 apparition of jfc 21p/giacobini-zinner enabled a sensitive search for these “hypervolatiles” in a prototypical carbon-chain depleted comet. we observed 21p/giacobini-zinner with the ishell spectrograph at the nasa infrared telescope facility on four pre-perihelion dates, two dates near-perihelion, and one post-perihelion date. we obtained detections of co, ch4, and c2h6 simultaneously with h2o on multiple dates. we present rotational temperatures, production rates, and mixing ratios. combined with previous work, our results may indicate that the hypervolatile coma composition of 21p/giacobini-zinner was variable across apparitions as well as within a particular perihelion passage, yet the spread in these measurements is a relatively small fraction of the variation in each molecule from comet to comet. we discuss the implications of our measured hypervolatile content of 21p/giacobini-zinner for the evolution of jfcs, and place our results in the context of findings from the rosetta mission and ground-based studies of comets. | probing the evolutionary history of comets: an investigation of the hypervolatiles co, ch4, and c2h6 in the jupiter-family comet 21p/giacobini-zinner |
context. in june 2021, the discovery of an unusual comet c/2014 un271 (bernardinelli-bernstein) was announced. its cometary activity beyond the orbit of uranus has also refreshed interest in similar objects, including c/2017 k2 (panstarrs). another peculiarity of these objects is the long interval of positional data, taken at large heliocentric distances.aims: these two comets are suitable candidates for a detailed investigation of their long-term motion outside the planetary zone. using the carefully selected orbital solutions, we aim to estimate the orbital parameters of their orbits at the previous perihelion passage. this might allow us to discriminate between dynamically old and new comets.methods: to follow the dynamical evolution of long-period comets far outside the planetary zone, it is necessary to take into account both the perturbation caused by the overall galactic gravitational potential and the actions of individual stars appearing in the solar neighborhood. to this aim, we applied the recently published methods based on the ephemerides of stellar perturbers.results: for c/2014 un271, we obtained a precise orbital solution that can be propagated into the past and the future. for c/2017 k2, we have to limit ourselves to studying the past motion because some signs of nongravitational effects can be found in recent positional observations. therefore, we use a specially selected orbital solution suitable for past motion studies. using these starting orbits, we propagated both comets to their previous perihelia. we also investigated the future motion of c/2014 un271.conclusions: the orbital evolution of these two comets appears to be sensitive to perturbations from several stars that closely approach the sun. to the detriment of our analysis, the errors on the 6d data for some of these stars are too large to obtain definitive results for the studied comets; nevertheless, we deduce that both comets were probably outside the planetary zone in the previous perihelion. | the influence of individual stars on the long-term dynamics of comets c/2014 un271 and c/2017 k2 |
context. class i protostars are a bridge between class 0 protostars (≤105 yr old), and class ii (≥106 yr) protoplanetary disks. recent studies show gaps and rings in the dust distribution of disks younger than 1 myr, suggesting that planet formation may start already at the class i stage. to understand what chemistry planets will inherit, it is crucial to characterize the chemistry of class i sources and to investigate how chemical complexity evolves from class 0 protostars to protoplanetary disks.aims: there are two goals: (i) to perform a census of the molecular complexity in a sample of four class i protostars, and (ii) to compare the data with the chemical compositions of earlier and later phases of the sun-like star formation process.methods: we performed iram-30 m observations at 1.3 mm towards four class i objects (l1489-irs, b5-irs1, l1455-irs1, and l1551-irs5). the column densities of the detected species were derived assuming local thermodynamic equilibrium (lte) or large velocity gradients (lvgs).results: we detected 27 species: c-chains, n-bearing species, s-bearing species, si-bearing species, deuterated molecules, and interstellar complex organic molecules (icoms; ch3oh, ch3cn, ch3cho, and hcooch3). among the members of the observed sample, l1551-irs5 is the most chemically rich source. different spectral profiles are observed: (i) narrow lines (~1 km s−1) towards all the sources, (ii) broader lines (~4 km s−1) towards l1551-irs5, and (iii) line wings due to outflows (in b5-irs1, l1455-irs1, and l1551-irs5). narrow c-c3h2 emission originates from the envelope with temperatures of 5-25 k and sizes of ~2''-10''. the icoms in l1551-irs5 reveal the occurrence of hot corino chemistry, with ch3oh and ch3cn lines originating from a compact (~0.''15) and warm (t > 50 k) region. finally, ocs and h2s seem to probe the circumbinary disks in the l1455-irs1 and l1551-irs5 binary systems. the deuteration in terms of elemental d/h in the molecular envelopes is: ~10−70% (d2co/h2co), ~5−15% (hdcs/h2cs), and ~1−23% (ch2doh/ch3oh). for the l1551-irs5 hot corino we derive d/h ~2% (ch2doh/ch3oh).conclusions: carbon chain chemistry in extended envelopes is revealed towards all the sources. in addition, b5-irs1, l1455-irs1, and l1551-irs5 show a low-excitation methanol line that is narrow and centered at systemic velocity, suggesting an origin from an extended structure, plausibly uv-illuminated. the abundance ratios of ch3cn, ch3cho, and hcooch3 with respect to ch3oh measured towards the l1551-irs5 hot corino are comparable to that estimated at earlier stages (prestellar cores, class 0 protostars), and to that found in comets. the deuteration in our sample is also consistent with the values estimated for sources at earlier stages. these findings support the inheritance scenario from prestellar cores to the class i phase when planets start forming. a copy of the reduced spectra is 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/a67 | chemical survey of class i protostars with the iram-30 m |
earth occasionally crosses the debris streams produced by comets and other active bodies in our solar system. these manifest meteor showers that provide an opportunity to explore these bodies without a need to visit them in-situ. observations of meteor showers provide unique insights into the physical and dynamical properties of their parent bodies, as well as into the compositions and the structure of near-surface dust. in this chapter, we discuss the development and current state of affairs of meteor science, with a focus on its role as a tool to study comets, and review the established parent body -- meteor shower linkages. | comets and meteor showers |
plasma treatment constitutes an efficient method for chemical-free disinfection. a spray-based system for dispensing plasma-activated aerosols onto surfaces would facilitate disinfection of complex and/or hidden surfaces inaccessible to direct line-of-sight (for example, uv) methods. the complexity and size of current plasma generators (for example, plasma jet and cometary plasma systems)—which prohibit portable operation, together with the short plasma lifetimes, necessitate a miniaturized in situ technique in which a source can be simultaneously activated and administered on-demand onto surfaces. here, we demonstrate this possibility by combining two nanoscale technologies for plasma and aerosol generation into an integrated device that is sufficiently small and lightweight. plasma is generated on a carpet of zinc oxide nanorods comprising a nanoneedle ensemble, which when raised to a high electric potential, constitutes a massive point charge array with near-singular electric fields to effect atmospheric breakdown. the plasma is then used to activate water transported through an underlying capillary wick, that is subsequently aerosolized under mhz-order surface acoustic waves. we show that the system, besides being amenable to miniaturization and hence integration into a chipscale device, leads to a considerable improvement in plasma-activation over its macroscale cometary discharge predecessor, with up to 20% and 127% higher hydrogen peroxide and nitrite ion concentrations that are respectively generated in the plasma-activated aerosols. this, in turn, leads to a 67% reduction in the disinfection time to achieve 95% bacterial load reduction, therefore demonstrating the potential of the technology as an efficient portable platform for on-demand field-use surface disinfection. | nanoscale plasma-activated aerosol generation for in situ surface pathogen disinfection |
in situ images of the 67p/churyumov-gerasimenko nucleus acquired by the civa cameras on-board philae revealed a rough landscape dominated by consolidated materials. these data provide a unique view to constrain the past and present conditions prevailing at the surface of the comet. a quantitative analysis of microscopic structures (fractures and pebbles) is derived using a manual extraction from the images. fractures/cracks are rather ubiquitous at various spatial scales with network and size (from sub-cm to 10 cm) well correlated to the texture of the landscape. the pebble size distributions are derived and compared to the size distribution of other cometary materials. the nature of the landscape is then discussed in relation to endogenic and exogenic processes of surface modification. the block seen in civa no. 1 is interpreted to be close-ups of fractured boulder/cliff belonging to the boulder field identified from the orbit near abydos, this boulder field being itself the result of gravitational regressive erosion due to sublimation. the observed fractures are best explained by thermal insolation leading to thermal fatigue and/or to loss of volatile materials. this surficial fragmentation (up to >10 cm length) could generate macroscopic erosion that is also visible at larger scale from the orbit. there is at least an intriguing possibility that the pebbles are remnants of primordial accretion processes. we thus speculate that the abydos landscape could be in favour of pebble accretion model instead of runaway coagulation model with a formation location in the outer region of the solar system. | origin of the local structures at the philae landing site and possible implications on the formation and evolution of 67p/churyumov-gerasimenko |
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