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we present the results of 850 μm polarization and c18o (3 - 2) line observations toward the western hub-filament structure (w-hfs) of the dark streamer in ic 5146 using the james clerk maxwell telescope scuba-2/pol-2 and harp instruments. we aim to investigate how the relative importance of the magnetic field, gravity, and turbulence affects core formation in hfs by comparing the energy budget of this region. we identified four 850 μm cores and estimated the magnetic field strengths (b pos) of the cores and the hub and filament using the davis-chandrasekhar-fermi method. the estimated b pos is ~80 to 1200 μg. from wang et al., b pos of e-47, a core in the eastern hub (e-hub), and e-hub were reestimated to be 500 and 320 μg, respectively, with the same method. we measured the gravitational (e g), kinematic (e k), and magnetic energies (e b) in the filament and hubs and compared the relative importance among them. we found that an e b-dominant filament has aligned fragmentation type, while e g-dominant hubs show no and clustered fragmentation types. in the e g dominant hubs, it seems that the portion of e k determines whether the hub becomes to have clustered (the portion of e k ~ 20%) or no fragmentation type (~10%). we propose an evolutionary scenario for the e- and w-hfss, where the hfs forms first by the collision of turbulent flows, and then the hubs and filaments can go into various types of fragmentation depending on their energy balance of gravity, turbulence, and magnetic field. | evolution of the hub-filament structures in ic 5146 in the context of the energy balance of gravity, turbulence, and magnetic field |
we present the turbulence spectra of magnetic and electron density fluctuations in situ measured by voyager 1 in the local interstellar medium from 2012 to 2019. the magnetic spectrum shows a kolmogorov power law with a one-dimensional power-law index -5/3 at mk ≤ 10-8.8, where k is a wavenumber and m is the unit meter. a bulge of enhanced magnetic power is found at mk = 10-8.8-10-8.2. meanwhile, the electron density spectrum also shows a kolmogorov power law with a one-dimensional power-law index -5/3 in the inertial range. a bulge of enhanced power is found around the kinetic scales of mk ≈ 10-5-10-1. based on the observational data, the relationships between the outer scale of the turbulent system and the powers of electron density and magnetic fluctuations are obtained. we then calculate the spectra locally for six individual time periods, within which the electron density and magnetic fluctuations are simultaneously observed. it is found that the power of perpendicular magnetic fluctuations is usually higher than that of parallel magnetic fluctuations, which indicates the dominance of alfvén waves in turbulence spectrum. part of the observed turbulence spectra reveal that the normalized parallel magnetic power exhibits a much higher intensity than the normalized electron density power in the local interstellar medium of low to moderate plasma beta (β = 0.1-0.8). this dominance in the parallel magnetic power cannot be explained by the linear magnetohydrodynamic modes alone and may be associated with the arc/spherically polarized alfvén mode. | turbulence spectra of electron density and magnetic field fluctuations in the local interstellar medium |
the ordered magnetic field observed via polarised synchrotron emission in nearby disc galaxies can be explained by a mean-field dynamo operating in the diffuse interstellar medium (ism). additionally, vertical-flux initial conditions are potentially able to influence this dynamo via the occurrence of the magnetorotational instability (mri). we aim to study the influence of various initial field configurations on the saturated state of the mean-field dynamo. this is motivated by the observation that different saturation behaviour was previously obtained for different supernova rates. we perform direct numerical simulations (dns) of three-dimensional local boxes of the vertically stratified, turbulent interstellar medium, employing shearing-periodic boundary conditions horizontally. unlike in our previous work, we also impose a vertical seed magnetic field. we run the simulations until the growth of the magnetic energy becomes negligible. we furthermore perform simulations of equivalent 1d dynamo models, with an algebraic quenching mechanism for the dynamo coefficients. we compare the saturation of the magnetic field in the dns with the algebraic quenching of a mean-field dynamo. the final magnetic field strength found in the direct simulation is in excellent agreement with a quenched αω dynamo. for supernova rates representative of the milky way, field losses via a galactic wind are likely responsible for saturation. we conclude that the relative strength of the turbulent and regular magnetic fields in spiral galaxies may depend on the galaxy's star formation rate. we propose that a mean field approach with algebraic quenching may serve as a simple sub-grid scale model for galaxy evolution simulations including a prescribed feedback from magnetic fields. | dynamo saturation in direct simulations of the multi-phase turbulent interstellar medium |
we present high-precision optical polarimetric observations of the black hole x-ray binary cygnus x-1 that span several cycles of its 5.6-day orbital period. the week-long observations on two telescopes located in opposite hemispheres allowed us to track the evolution of the polarization within one orbital cycle with the highest temporal resolution to date. using the field stars, we determined the interstellar polarization in the source direction and subsequently its intrinsic polarization pint = 0.82%±0.15% with a polarization angle θint = 155° ±5°. the optical polarization angle is aligned with that in the x-rays recently obtained with the imaging x-ray polarimetry explorer. furthermore, it is consistent within the uncertainties with the position angle of the radio ejections. we show that the intrinsic polarization degree is variable with the orbital period with an amplitude of ∼0.2% and discuss various sites of its production. assuming that the polarization arises from a single thomson scattering of the primary star radiation by the matter that follows the black hole in its orbital motion, we constrained the inclination of the binary orbit i > 120° and its eccentricity e < 0.08. the asymmetric shape of the orbital profiles of the stokes parameters also implies the asymmetry of the scattering matter distribution in the orbital plane, which may arise from the tilted accretion disk. we compared our data to the polarimetric observations made in 1975-1987 and find good agrement within 1° between the intrinsic polarization angles. on the other hand, the polarization degree decreased by 0.4% over half a century, suggesting secular changes in the geometry of the accreting matter. | peering into the tilted heart of cyg x-1 with high-precision optical polarimetry |
our simultaneous three-colour (bvr) polarimetric observations of the low-mass black hole x-ray binary v404 cyg show a small but statistically significant change of polarization degree (δp ∼ 1 per cent) between the outburst in 2015 june and the quiescence. the polarization of v404 cyg in the quiescent state agrees within the errors with that of the visually close (1.4 arcsec) companion (pr = 7.3 ± 0.1 per cent), indicating that it is predominantly of interstellar origin. the polarization pattern of the surrounding field stars supports this conclusion. from the observed variable polarization during the outburst, we show that the polarization degree of the intrinsic component peaks in the v band, pv = 1.1 ± 0.1 per cent, at the polarization position angle of θv = -7° ± 2°, which is consistent in all three passbands. we detect significant variations in the position angle of the intrinsic polarization in the r band from -30° to ∼0° during the outburst peak. the observed wavelength dependence of the intrinsic polarization does not support non-thermal synchrotron emission from a jet as a plausible mechanism, but it is in better agreement with the combined effect of electron (thomson) scattering and absorption in a flattened plasma envelope or outflow surrounding the illuminating source. alternatively, the polarization signal can be produced by scattering of the disc radiation in a mildly relativistic polar outflow. the position angle of the intrinsic polarization, nearly parallel to the jet direction (i.e. perpendicular to the accretion disc plane), is in agreement with these interpretations. | high-precision optical polarimetry of the accreting black hole v404 cyg during the 2015 june outburst |
we describe the first complete polarimetric data set of the entire outburst of a low-mass black hole x-ray binary system and discuss the constraints for geometry and radiative mechanisms it imposes. during the decaying hard state, when the optical flux is dominated by the non-thermal component, the observed polarization is consistent with the interstellar values in all filters. during the soft state, the intrinsic polarization of the source is small, ∼0.15 per cent in b and v filters, and is likely produced in the irradiated disc. a much higher polarization, reaching ∼0.5 per cent in v and r filters, at a position angle of ∼25○ observed in the rising hard state coincides in time with the detection of winds in the system. this angle coincides with the position angle of the jet. the detected optical polarization is best explained by scattering of the non-thermal (hot flow or jet base) radiation in an equatorial wind. | disc and wind in black hole x-ray binary maxi j1820+070 observed through polarized light during its 2018 outburst |
fluctuation dynamos are thought to play an essential role in the evolution of magnetized galaxies, saturating within ∼0.01 gyr and thus potentially acting as seeds for large-scale mean-field dynamos. however, unambiguous observational confirmation of the fluctuation dynamo action in a galactic environment is still missing. this is because, in spiral galaxies, it is difficult to differentiate between small-scale magnetic fields generated by a fluctuation dynamo and those due to the tangling of the large-scale field. we propose that observations of magnetic fields in elliptical galaxies would directly probe the fluctuation dynamo action. this is motivated by the fact that in ellipticals, due to their lack of significant rotation, the conventional large-scale dynamo is absent and the fluctuation dynamo is responsible for controlling the strength and structure of the magnetic field. by considering turbulence injected by type ia supernova explosions and possible magnetic field amplification by cooling flows, we estimate expected magnetic field strengths of $0.2\mbox{--}1\,\mu {\rm{g}}$ in the centers of quiescent elliptical galaxies. we use a semianalytic model of galaxy formation to estimate the distribution and redshift evolution of field strengths, tentatively finding a decrease in magnetic field strength with decreasing redshift. we analyze a historical sample of radio sources that exhibit the laing-garrington effect (radio polarization asymmetry in jets) and infer magnetic field strengths between $0.14\,\mathrm{and}\,1.33\,\mu {\rm{g}}$ for a uniform thermal electron density and between $1.36\,\mathrm{and}\,6.21\,\mu {\rm{g}}$ for the thermal electron density following the king profile. we examine observational techniques for measuring the magnetic field saturation state in elliptical galaxies, focusing on faraday rotation measure grids, the laing-garrington effect, synchrotron emission, and gravitational lensing, and finding appealing prospects for future empirical analysis. | magnetic fields in elliptical galaxies: an observational probe of the fluctuation dynamo action |
context. observations of synchrotron emission at low radio frequencies reveal a labyrinth of polarised galactic structures. however, the explanation for the wealth of structures remains uncertain due to the complex interactions between the interstellar medium and the magnetic field. a multi-tracer approach to the analysis of large sky areas is needed.aims: this paper aims to use polarimetric images from the lofar two metre sky survey to produce the biggest mosaic of polarised emission in the northern sky at low radio frequencies (150 mhz) to date. the large area this mosaic covers allows for detailed morphological and statistical studies of polarised structures in the high-latitude outer galaxy, including the well-known loop iii region.methods: we produced a 3100 square degree faraday tomographic cube using a rotation measure synthesis tool. we calculated the statistical moments of faraday spectra and compared them with data sets at higher frequencies (1.4 ghz) and with a map of a rotation measure derived from extragalactic sources.results: the mosaic is dominated by polarised emission connected to loop iii. additionally, the mosaic reveals an abundance of other morphological structures, mainly narrow and extended depolarisation canals, which are found to be ubiquitous.conclusions: we find a correlation between the map of an extragalactic rotation measure and the lotss first faraday moment image. the ratio of the two deviates from a simple model of a burn slab (burn, b. j. 1966, mnras, 133, 67) along the line of sight, which highlights the high level of complexity in the magnetoionic medium that can be studied at these frequencies. the movie associated to appendix b is available at https://www.aanda.org and also at https://data.fulir.irb.hr/islandora/object/irb:106 | faraday tomography of lotss-dr2 data. i. faraday moments in the high-latitude outer galaxy and revealing loop iii in polarisation |
we solve the linearised vlasov-fokker-planck (vfp) equation to show that heat flow or an electrical current in a magnetized collisional plasma is unstable to the growth of a circularly polarised transverse perturbation to a zeroth order uniform magnetic field. the braginskii (1965) transport equations exhibit the same instability in the appropriate limit. this is relevant to laser-produced plasmas, inertial fusion energy (ife) and to dense cold interstellar plasmas. | instability in a magnetised collisional plasma driven by a heat flow or a current |
we investigate how observations of synchrotron intensity fluctuations can be used to probe the sonic and alfvénic mach numbers of interstellar turbulence, based on mock observations performed on simulations of magnetohydrodynamic turbulence. we find that the structure function slope and a diagnostic of anisotropy that we call the integrated quadrupole ratio modulus both depend on the alfvénic mach number. however, these statistics also depend on the orientation of the mean magnetic field in the synchrotron emitting region relative to our line of sight, and this creates a degeneracy that cannot be broken by observations of synchrotron intensity alone. we conclude that the polarization of synchrotron emission could be analyzed to break this degeneracy, and suggest that this will be possible with the square kilometre array. | radio synchrotron fluctuation statistics as a probe of magnetized interstellar turbulence |
understanding how the galactic magnetic field threads the multiphase interstellar medium (ism) remains a considerable challenge, as different magnetic field tracers probe dissimilar phases and field components. we search for evidence of a common magnetic field shared between the ionized and neutral ism by comparing 1.4 ghz radio continuum polarization and h i line emission from the galactic arecibo l-band feed array continuum transit survey (galfacts) and galactic arecibo l-band feed array h i (galfa-h i) survey, respectively. we compute the polarization gradient of the continuum emission and search for associations with diffuse/translucent h i structures. the polarization gradient is sensitive to changes in the integrated product of the thermal electron density and line-of-sight field strength (b ∥) in warm ionized gas, while narrow h i structures highlight the plane-of-sky field orientation in cold neutral gas. we identified one region in the high galactic latitude arecibo sky, g216+26 centered on (ℓ, b) ~ (216°, +26°), containing filaments in the polarization gradient that are aligned with narrow h i structures roughly parallel to the galactic plane. we present a comparison of multiphase observations and magnetic field tracers of this region, demonstrating that the warm ionized and cold neutral media are connected likely via a common magnetic field. we quantify the physical properties of a polarization gradient filament associated with hα emission, measuring a line-of-sight field strength b ∥ = 6 ± 4 μg and a plasma beta $\beta ={2.1}_{-2.1}^{+3.1}$ . we discuss the lack of widespread multiphase magnetic field alignments and consider whether this region is associated with a short-timescale or physically rare phenomenon. this work highlights the utility of multitracer analyses for understanding the magnetized ism. | a comparison of multiphase magnetic field tracers in a high galactic latitude region of the filamentary interstellar medium |
when the pulsar signal propagates in the interstellar medium (ism), the high frequency and low frequency components of the signal reach the radio telescope with a certain delay. therefore, the pulsar signal will appear energy dispersion, which will broaden the pulse profile, decrease the signal to noise ratio, and even lead to the disappearance of the pulse signal. in this paper, we analyze the sampling, polarization and arrangement of baseband data based on the coherent dedispersion algorithm for the problem of pulsar baseband data dedispersion. we systematically study the coherent dedispersion data processing procedure, and test the pulse profile changes under different fft block sizes. an optimal selection strategy of fft block sizes is proposed for reducing the operation time and obtaining a better pulse profile. we propose two methods, one is the generation of ism transfer function, the other is the pulsar period and phase prediction method at a certain time, and discuss integral and folding strategies. we test the algorithm based on the baseband data of caspsr and medusa terminals observed by the parkes 64 m radio telescope, and analyze the reading and processing methods of baseband data of different terminals. the experimental results show that the phase and amplitude information of the pulse profile processed by our algorithm is basically consistent with the results obtained by dspsr. | research on a coherent dedispersion algorithm for pulsar baseband data |
pulsars are rotating neutron stars emitting lighthouse-like beams. owing to their unique properties, pulsars are a unique astrophysical tool to test general relativity, inform on matter in extreme conditions, and probe galactic magnetic fields. understanding pulsar physics and emission mechanisms is critical to these applications. here we show that mechanical-optical rotation in the pulsar magnetosphere affects polarisation in a way which is indiscernible from faraday rotation in the interstellar medium for typical ghz observations frequency, but which can be distinguished in the sub-ghz band. besides being essential to correct for possible systematic errors in interstellar magnetic field estimates, this result offers a unique means to determine the rotation direction of pulsars, providing additional constraints on magnetospheric physics. with the ongoing development of sub-ghz observation capabilities, our finding promises discoveries, such as the spatial distribution of pulsars rotation directions, which could exhibit potentially interesting, but presently invisible, correlations or features. | determining the rotation direction in pulsars |
much of our present observational knowledge of the interstellar magnetic field of our galaxy comes from two different sources, both of which involve mechanisms operating at radio wavelengths: the first one is the faraday rotation of linearly-polarized radio waves propagating through the magneto-ionic interstellar medium, and the second one is the diffuse synchrotron emission from our galaxy. i will review what these two classical probes have taught us about the strength, the direction/orientation, and the spatial distribution of the interstellar magnetic field. i will then present a recent method, known as faraday tomography or rotation measure synthesis, which relies on a combination of faraday rotation and synchrotron emission and which makes it possible to probe the interstellar magnetic field in three dimensions. | faraday tomography: a new, three-dimensional probe of the interstellar magnetic field |
we present a comprehensive study of the rotational and emission properties of psr j0738-4042 using a combination of observations taken by the deep space network, hartebeesthoek, parkes (murriyang) and molonglo observatories between 1972 and 2023. our timing of the pulsar is motivated by previously reported profile/spin-down events that occurred in 2005 september and 2015 december, which result in an anomalously large braking index of n = 23 300 ± 1800. using a gaussian process regression framework, we develop continuous models for the evolution of the pulsar spin-down rate ($\dot{\nu }$) and profile shape. we find that the pulse profile variations are similar regardless of radio observing frequency and polarization. small-scale differences can be ascribed to changes in the interstellar medium along the line of sight and frequency-dependent changes in magnetospheric radio emission height. no new correlated spin-down or profile events were identified in our extended data set. however, we found that the disappearance of a bright emission component in the leading edge of archival profiles between 1981 and 1988 was not associated with a substantial change in $\dot{\nu }$. this marks a notable departure from the previous profile/spin-down events in this pulsar. we discuss the challenges these observations pose for physical models and conclude that interactions between the pulsar and in-falling asteroids or a form of magnetospheric state-switching with a long periodicity are plausible explanations. | rotational and radio emission properties of psr j0738-4042 over half a century |
we present simultaneous optical and near-infrared (nir) polarimetric results for the black hole binary v404 cyg spanning the duration of its seven-day-long optically brightest phase of its 2015 june outburst. the simultaneous r- and ks -band light curves showed almost the same temporal variation except for the isolated (∼30-minute duration) orphan ks -band flare observed at mjd 57193.54. we did not find any significant temporal variation of polarization degree (pd) and position angle (pa) in both r and ksbands throughout our observations, including the duration of the orphan nir flare. we show that the observed pd and pa are predominantly interstellar in origin by comparing the v404 cyg polarimetric results with those of the surrounding sources within the 7‧ × 7‧ field of view. the low intrinsic pd (less than a few percent) implies that the optical and nir emissions are dominated by either disk or optically thick synchrotron emission, or both. we also present the broadband spectra of v404 cyg during the orphan nir flare and a relatively faint and steady state by including quasi-simultaneous swift/xrt and integral fluxes. by adopting a single-zone synchrotron plus inverse-compton model as widely used in modeling of blazars, we constrained the parameters of a putative jet. because the jet synchrotron component cannot exceed the swift/xrt disk/corona flux, the cutoff lorentz factor in the electron energy distribution is constrained to be <102, suggesting that particle acceleration is less efficient in this microquasar jet outburst compared to active galactic nucleus jets. we also suggest that the loading of the baryon component inside the jet is inevitable based on energetic arguments. | no evidence of intrinsic optical/near-infrared linear polarization for v404 cygni during its bright outburst in 2015: broadband modeling and constraint on jet parameters |
wolf-rayet stars have strong, hot winds, with mass-loss rates at least a factor of 10 greater than their o-star progenitors, although their terminal wind speeds are similar. in this paper, we use the technique of multiband linear polarimetry to extract information on the global asymmetry of the wind in a sample of 47 bright galactic wr stars. our observations also include time-dependent observations of 17 stars in the sample. the path to our goal includes removing the dominating component of wavelength-dependent interstellar polarization (isp), which normally follows the well-known serkowski law. we include a wavelength-dependent isp position angle parameter in our isp law and find that 15 stars show significant results for this parameter. we detect a significant component of wavelength-independent polarization due to electron scattering in the wind for 10 cases, with most wr stars showing none at the ∼0.05% level precision of our data. the intrinsically polarized stars can be explained with binary interaction, large-scale wind structure, and clumping. we also found that 5 stars out of 19 observed with the strömgren b filter (probing the complex λ4600-4700 emission-line region) have significant residuals from the isp law and propose that this is due to wind asymmetries. we provide a useful catalog of isp for 47 bright galactic wr stars and upper limits on the possible level of intrinsic polarization. | a multiwavelength search for intrinsic linear polarization in wolf-rayet winds |
a critical challenge in the observation of the redshifted 21 cm line is its separation from bright galactic and extragalactic foregrounds. in particular, the instrumental leakage of polarized foregrounds, which undergo significant faraday rotation as they propagate through the interstellar medium, may harmfully contaminate the 21 cm power spectrum. we develop a formalism to describe the leakage due to instrumental widefield effects in visibility-based power spectra measured with redundant arrays, extending the delay-spectrum approach presented in parsons et al. we construct polarized sky models and propagate them through the instrument model to simulate realistic full-sky observations with the precision array to probe the epoch of reionization. we find that the leakage due to a population of polarized point sources is expected to be higher than diffuse galactic polarization at any k mode for a 30 m reference baseline. for the same reference baseline, a foreground-free window at k > 0.3 h mpc-1 can be defined in terms of leakage from diffuse galactic polarization even under the most pessimistic assumptions. if measurements of polarized foreground power spectra or a model of polarized foregrounds are given, our method is able to predict the polarization leakage in actual 21 cm observations, potentially enabling its statistical subtraction from the measured 21 cm power spectrum. | constraining polarized foregrounds for eor experiments. ii. polarization leakage simulations in the avoidance scheme |
recently we have demonstrated that high-precision polarization observations can detect the polarization resulting from the rotational distortion of a rapidly rotating b-type star. here, we investigate the extension of this approach to an a-type star. linear-polarization observations of α oph (a5iv) have been obtained over wavelengths from 400 to 750 nm. they show the wavelength dependence expected for a rapidly rotating star combined with a contribution from interstellar polarization. we model the observations by fitting rotating-star polarization models and adding additional constraints including a measured vesin i. however, we cannot fully separate the effects of rotation rate and inclination, leaving a range of possible solutions. we determine a rotation rate (ω = ω/ωc) between 0.83 and 0.98 and an axial inclination i > 60°. the rotation-axis position angle is found to be 142 ± 4°, differing by 16° from a value obtained by interferometry. this might be due to precession of the rotation axis due to interaction with the binary companion. other parameters resulting from the analysis include a polar temperature tp = 8725 ± 175 k, polar gravity log gp = 3.93 ± 0.08 (dex cgs), and polar radius rp = 2.52 ± 0.06 r⊙. comparison with rotating-star evolutionary models indicates that α oph is in the later half of its main-sequence evolution and must have had an initial ω of 0.8 or greater. the interstellar polarization has a maximum value at a wavelength (λmax) of 440 ± 110 nm, consistent with values found for other nearby stars. | the rotation of α oph investigated using polarimetry |
we report the detection of a curved magnetic field in the ring-like shell of the bubble n4, derived from near-infrared polarization of reddened diskless stars located behind this bubble. the magnetic field in the shell is curved and parallel to the ring-like shell, and its strength is estimated to be ∼ 120 μ {{g}} in the plane of the sky. the magnetic field strength in the shell is significantly enhanced compared to the local field strength. we calculate the mass-to-flux ratio for the submillimeter clumps in the shell and find that they are all magnetically subcritical. our results demonstrate that the magnetic field strengthens as the interstellar medium is compressed into a shell, and suggest that the magnetic field has the potential to hinder star formation triggered by h ii region expansion. | a curved magnetic field in the ring-like shell of bubble n4 |
aims: we report on the follow-up of the extremely bright long gamma-ray burst (grb) grb 210619b with optical polarimetry.methods: we conducted optopolarimetric observations of the optical afterglow of grb 210619b in the sdss-r band in the time window ∼5967 − 8245 s after the burst, using the robopol instrument at the skinakas observatory.results: we report a 5σ detection of polarization p = 1.5 ± 0.3 at polarization angle evpa = 8 ± 6°. we find that during our observations, the polarization is likely constant. these values have been corrected for polarization induced by the interstellar medium of the milky way and host-induced polarization is likely negligible. thus the polarization we quote is intrinsic to the grb afterglow. | grb 210619b optical afterglow polarization |
the detection of the redshifted cosmological 21-cm line signal requires the removal of the galactic and extragalactic foreground emission, which is orders of magnitude brighter anywhere in the sky. foreground cleaning methods currently used are efficient in removing spectrally smooth components. however, they struggle in the presence of not spectrally smooth contamination that is, therefore, potentially the most dangerous one. an example of this is the polarized synchrotron emission, which is faraday rotated by the interstellar medium and leaks into total intensity due to instrumental imperfections. in this work we present new full-sky simulations of this polarized synchrotron emission in the 50-200 mhz range, obtained from the observed properties of diffuse polarized emission at low frequencies. the simulated polarized maps are made publicly available, aiming to provide more realistic templates to simulate the effect of instrumental leakage and the effectiveness of foreground separation techniques. | simulations of galactic polarized synchrotron emission for epoch of reionization observations |
linearly polarized emission is described, in general, in terms of the stokes parameters q and u, from which the polarization intensity and polarization angle can be determined. although the polarization intensity and polarization angle provide an intuitive description of the polarization, they are affected by the limitations of interferometric data, such as missing single-dish data in the u-v plane, from which radio-frequency interferometric data is visualized. to negate the effects of these artifacts, it is desirable for polarization diagnostics to be rotationally and translationally invariant in the q-u plane. one rotationally and translationally invariant quantity, the polarization gradient, has been shown to provide a unique view of spatial variations in the turbulent interstellar medium when applied to diffuse radio-frequency synchrotron emission. in this paper, we develop a formalism to derive additional rotationally and translationally invariant quantities. we present new diagnostics that can be applied to diffuse or point-like polarized emission in any waveband, including a generalization of the polarization gradient, the polarization directional curvature, polarization wavelength derivative, and polarization wavelength curvature. in paper ii, we will apply these diagnostics to observed and simulated images of diffuse radio-frequency synchrotron emission. | advanced diagnostics for the study of linearly polarized emission. i. derivation |
we present 850 μm polarization and c18o (3-2) molecular line observations toward the x-shaped nebula in the california molecular cloud using james clerk maxwell telescope (jcmt)'s scuba-2/pol-2 and harp instruments. the 850 μm emission shows that the observed region includes two elongated filamentary structures (fil1 and fil2) having chains of regularly spaced cores. we measured the mass per unit length of the filaments and found that fil1 and fil2 are thermally super- and subcritical, respectively, but both are subcritical if nonthermal turbulence is considered. the mean projected spacings ( ${\rm{\delta }}\bar{s}$ ) of the cores in fil1 and fil2 are 0.13 and 0.16 pc, respectively. ${\rm{\delta }}\bar{s}$ is smaller than 4× the filament width expected in the classical cylinder fragmentation model. the large-scale magnetic field orientations shown by planck are perpendicular to the long axes of fil1 and fil2, while those in the filaments obtained from the high-resolution polarization data of jcmt are disturbed, but those in fil1 tend to have longitudinal orientations. using the modified davis-chandrasekhar-fermi method, we estimated the magnetic field strengths (b pos) of the filaments, which are 110 ± 80 and 90 ± 60 μg, respectively. we calculated the gravitational, kinematic, and magnetic energies of the filaments, and found that the fraction of magnetic energy is larger than 60% in both filaments. we propose that the dominant magnetic energy may lead the filament to be fragmented into aligned cores as suggested by tang et al., and the shorter core spacing can be due to a projection effect via the inclined geometry of the filaments or due to nonnegligible longitudinal magnetic fields in the case of fil1. | magnetic fields and fragmentation of filaments in the hub of california-x |
ammonia molecules have an important role in the radiation-induced chemistry that occurs on grains in the cold interstellar medium and leads to the formation of nitrogen containing molecules. such grains and surfaces are primarily covered by water ices; however, these conditions allow the growth of solid ammonia films as well. yet, solid ammonia know-how lags the vast volume of research that has been invested in the case of films of its "sibling" molecule water, which, in the porous amorphous phase, spontaneously form polar films and can cage coadsorbed molecules within their hydrogen-bonded matrix. here, we report on the effect of growth temperature on the spontaneous polarization of solid ammonia films (leading to internal electric fields of ∼105 v/m) within the range of 30 k-85 k on top of a ru(0001) substrate under ultra-high vacuum conditions. the effect of growth temperature on the films' depolarization upon annealing was recorded as well. by demonstrating the ability of ammonia to cage coadsorbed molecules, as water does, we show that temperature-programmed contact potential difference measurements performed by a kelvin probe and especially their temperature derivative can track film reorganization/reconstruction and crystallization at temperatures significantly lower than the film desorption. | spontaneous polarization of thick solid ammonia films |
diagnostics of polarized emission provide us with valuable information on the galactic magnetic field and the state of turbulence in the interstellar medium, which cannot be obtained from synchrotron intensity alone. in paper i, we derived polarization diagnostics that are rotationally and translationally invariant in the q-u plane, similar to the polarization gradient. in this paper, we apply these diagnostics to simulations of ideal magnetohydrodynamic turbulence that have a range of sonic and alfvénic mach numbers. we generate synthetic images of stokes q and u for these simulations for the cases where the turbulence is illuminated from behind by uniform polarized emission and where the polarized emission originates from within the turbulent volume. from these simulated images, we calculate the polarization diagnostics derived in paper i for different lines of sight relative to the mean magnetic field and for a range of frequencies. for all of our simulations, we find that the polarization gradient is very similar to the generalized polarization gradient and that both trace spatial variations in the magnetoionic medium for the case where emission originates within the turbulent volume, provided that the medium is not supersonic. we propose a method for distinguishing the cases of emission coming from behind or within a turbulent, faraday rotating medium and a method to partly map the rotation measure of the observed region. we also speculate on statistics of these diagnostics that may allow us to constrain the physical properties of an observed turbulent region. | advanced diagnostics for the study of linearly polarized emission. ii. application to diffuse interstellar radio synchrotron emission |
due to the inevitable accumulation of observational information in the direction of the line of sight, it is difficult to measure the local magnetic field of magnetohydrodynamic (mhd) turbulence. however, a correct understanding of the local magnetic field is a prerequisite for reconstructing the galactic 3d magnetic field. we study how to reveal the local magnetic field direction and the eddy anisotropy on the basis of the statistics of synchrotron polarization derivative with respect to the squared wavelength dp/dλ2. in the low-frequency and strong faraday rotation regime, we implement numerical simulations in the combination of multiple statistic techniques, such as structure function, quadrupole ratio modulus, spectral correlation function, correlation function anisotropy, and spatial gradient techniques. we find that (1) statistic analysis of dp/dλ2 indeed reveals the anisotropy of underlying mhd turbulence, the degree of which increases with the increase of the radiation frequency; and (2) the synergy of both correlation function anisotropy and gradient calculation of dp/dλ2 enables the measurement of the local magnetic field direction. | studying the local magnetic field and anisotropy of magnetic turbulence by synchrotron polarization derivative |
we present version x of the hammurabi package, the healpix-based numeric simulator for galactic polarized emission. improving on its earlier design, we have fully renewed the framework with modern c++ standards and features. multithreading support has been built in to meet the growing computational workload in future research. for the first time, we present precision profiles of the hammurabi line-of-sight integral kernel with multilayer healpix shells. in addition to fundamental improvements, this report focuses on simulating polarized synchrotron emission with gaussian random magnetic fields. two fast methods are proposed for realizing divergence-free random magnetic fields either on the galactic scale where field alignment and strength modulation are imposed, or on a local scale where more physically motivated models like a parameterized magnetohydrodynamic (mhd) turbulence can be applied. as an example application, we discuss the phenomenological implications of gaussian random magnetic fields for high galactic latitude synchrotron foregrounds. in this, we numerically find b/e polarization-mode ratios lower than unity based on gaussian realizations of either mhd turbulent spectra or in spatially aligned magnetic fields. | hammurabi x: simulating galactic synchrotron emission with random magnetic fields |
collisional energy transfer under cold conditions is of great importance from the fundamental and applicative point of view. here, we investigate low temperature collisions of the sh- anion with he. we have generated a three-dimensional potential energy surface (pes) for the sh-(x1σ+)-he(1s) van der waals complex. the ab initio multi-dimensional interaction pes was computed using the explicitly correlated coupled cluster approach with simple, double, and perturbative triple excitation in conjunction with the augmented-correlation consistent-polarized valence triple zeta gaussian basis set. the pes presents two minima located at linear geometries. then, the pes was averaged over the ground vibrational wave function of the sh- molecule and the resulting two-dimensional pes was incorporated into exact quantum mechanical close coupling calculations to study the collisional excitation of sh- by he. we have computed inelastic cross sections among the 11 first rotational levels of sh- for energies up to 2500 cm-1. (de-)excitation rate coefficients were deduced for temperatures ranging from 1 to 300 k by thermally averaging the cross sections. we also performed calculations using the new pes for a fixed internuclear sh- distance. both sets of results were found to be in reasonable agreement despite differences existing at low temperatures confirming that accurate predictions require the consideration of all internal degrees of freedom in the case of molecular hydrides. the rate coefficients presented here may be useful in interpreting future experimental work on the sh- negative ion colliding with he as those recently done for the oh--he collisional system as well as for possible astrophysical applications in case sh- would be detected in the interstellar medium. | cold collisions of sh- with he: potential energy surface and rate coefficients |
we observed seven epochs of spectropolarimetry in optical wavelengths for the type iib sn 2011hs, ranging from -3 to +40 d with respect to v-band maximum. a high degree of interstellar polarization was detected (up to ∼3 per cent), with a peak lying blueward of 4500 å. similar behaviours have been seen in some type ia supernovae (sne), but had never been observed in a type iib. we find that it is most likely the result of a relative enhancement of small silicate grains in the vicinity of the sn. significant intrinsic continuum polarization was recovered at -3 and +2 d (p = 0.55 ± 0.12 per cent and 0.75 ± 0.11 per cent, respectively). we discuss the change of the polarization angle across spectral lines and in the continuum as diagnostics for the 3d structure of the ejecta. we see a gradual rotation by about -50° in the continuum polarization angle between -2 and +18 d after v-band maximum. a similar rotation in he i λ5876, hα and the ca ii infrared triplet seems to indicate a strong influence of the global geometry on the line polarization features. the differences in the evolution of their respective loops on the stokes q - u plane suggest that line specific geometries are also being probed. possible interpretations are discussed and placed in the context of literature. we find that the spectropolarimetry of sn 2011hs is most similar to that of sn 2011dh, albeit with notable differences. | the 3d shape of type iib sn 2011hs |
on the basis of the modern understanding of mhd turbulence, we propose a new way of using synchrotron radiation, namely using synchrotron intensity gradients for tracing astrophysical magnetic fields. we successfully test the new technique using synthetic data obtained with the 3d mhd simulations and provide the demonstration of the practical utility of the technique by comparing the directions of magnetic field that are obtained with planck synchrotron intensity dats to the directions obtained with planck synchrotron polarization data. we demonstrate that the synchrotron intensity gradients (sigs) can reliably trace magnetic field in the presence of noise and can provide detailed maps of magnetic-field directions. we also show that the sigs are relatively robust for tracing magnetic fields while the low spacial frequencies of the synchrotron image are removed. this makes the sigs applicable to tracing of magnetic fields using interferometric data with single dish measurement absent. we discuss the synergy of using the sigs together with synchrotron polarization in order to find the actual direction of the magnetic field and quantify the effects of faraday rotation as well as with other ways of studying astrophysical magnetic fields. we test our method in the presence of noise and the resolution effects. we stress the complementary nature of the studies using the sig technique and those employing the recently-introduced velocity gradient techniques that traces the magnetic fields using spectroscopic data. | erratum: “synchrotron intensity gradients as tracers of interstellar magnetic fields” (2017, apj, 842, 1, 30) |
currently, about 100 carbon-containing molecules have been detected in interstellar and circumstellar environments, and this list includes isomeric substances. in most cases, the minimum energy principle is able to explain the ratio of abundances of the isomeric compounds but in some cases is not. trying to rationalize the questions unsolved within the energetic approach, we have theoretically studied the polarizability of isomeric and related compounds detected or proposed in interstellar conditions. as we found, in general both energy and polarizability provide the consistent estimates for the ratio of the isomers (e.g., for the isomers having generic formulae cho, chn, c3h, c3h2, chno, c2h3n, c2h6o, etc.). in the case of the c4h3n isomers, the most abundant isomer (cyanoallene) is not the most stable but the least polarizable that is in a good agreement with relevant experimental study. we assume that the efficiency of the use of polarizability is due to its relevance to the molecules' response to the external electric fields, i.e., more polarizable molecules are more responsive, more reactive, and, hence, less abundant. further, we have analyzed the polarizabilities of polycyclic aromatic hydrocarbons, fullerene hydrides (fulleranes), polyynes, and their derivatives with respect to their possible detection under interstellar conditions. | polarizability of isomeric and related interstellar compounds in the aspect of their abundance |
the radio arc is a system of organized nonthermal filaments (ntfs) located within the galactic center (gc) region of the milky way. recent observations of the radio arc ntfs revealed a magnetic field that alternates between being parallel and rotated with respect to the orientation of the filaments. this pattern is in stark contrast to the predominantly parallel magnetic field orientations observed in other gc ntfs. to help elucidate the origin of this pattern, we analyze spectro-polarimetric data of the radio arc ntfs using an australian telescope compact array data set covering the continuous frequency range from ~4 to 11 ghz at a spectral resolution of 2 mhz. we fit depolarization models to the spectral polarization data to characterize faraday effects along the line of sight. we assess whether structures local to the radio arc ntfs may contribute to the unusual magnetic field orientation. external faraday effects are identified as the most likely origin of the rotation observed for the radio arc ntfs; however, internal faraday effects are also found to be likely in regions of parallel magnetic field. the increased likelihood of internal faraday effects in parallel magnetic field regions may be attributed to the effects of structures local to the gc. one such structure could be the radio shell local to the radio arc ntfs. future studies are needed to determine whether this alternating magnetic field pattern is present in other multi-stranded ntfs, or is a unique property resulting from the complex interstellar region local to the radio arc ntfs. | analyzing the intrinsic magnetic field in the galactic center radio arc |
based on statistical analysis of synchrotron polarization intensity, we study the anisotropic properties of compressible magnetohydrodynamic (mhd) turbulence. the second-order normalized structure function, quadrupole ratio modulus, and anisotropic coefficient are synergistically used to characterize the anisotropy of the polarization intensity. on the basis of predecomposition data cubes, we first explore the anisotropy of the polarization intensity in different turbulence regimes and find that the most significant anisotropy occurs in the sub-alfvénic regime. using postdecomposition data cubes in this regime, we then study the anisotropy of the polarization intensity from alfvén, slow, and fast modes. the statistics of the polarization intensity from alfvén and slow modes demonstrate the significant anisotropy, while the statistics of the polarization intensity from fast modes show isotropic structures. this is consistent with earlier results provided in cho & lazarian. as a result, both quadrupole ratio modulus and anisotropic coefficient for polarization intensities can quantitatively recover the anisotropy of underlying compressible magnetohydrodynamic turbulence. the synergistic use of the two methods helps enhance the reliability of the magnetic field measurement. | studying the anisotropy of compressible magnetohydrodynamic turbulence by synchrotron polarization intensity |
observations of synchrotron polarization at multiple frequencies in the presence of faraday rotation can provide a way to reconstruct the 3d magnetic field distribution. in this paper we compare the well known faraday tomography (ft) technique to a new approach, synchrotron polarization gradients (spgs). we compare the strengths and limitations of the two techniques, and describe their synergy. in particular, we show that in situations when the ft technique fails, e.g., due to insufficient frequency coverage, the spg can still trace the 3d structure of a magnetic field. | a comparison between faraday tomography and synchrotron polarization gradients |
magnetic fields play important roles in many astrophysical processes. however, there is no universal diagnostic for the magnetic fields in the interstellar medium (ism) and each magnetic tracer has its limitation. any new detection method is thus valuable. theoretical studies have shown that submillimetre fine-structure lines are polarized due to atomic alignment by ultraviolet photon-excitation, which opens up a new avenue to probe interstellar magnetic fields. we will, for the first time, perform synthetic observations on the simulated three-dimensional ism to demonstrate the measurability of the polarization of submillimetre atomic lines. the maximum polarization for different absorption and emission lines expected from various sources, including star-forming regions are provided. our results demonstrate that the polarization of submillimetre atomic lines is a powerful magnetic tracer and add great value to the observational studies of the submilimetre astronomy. | polarization of submillimetre lines from interstellar medium |
bright-rimmed clouds (brcs) form on the edges of h ii regions affected by high-energy radiation from a central ionizing source. the uv radiation from the ionizing source results in compression and ionization, causing either cloud disruption or further star formation. in this work, we present r-band polarization measurements towards four brcs, namely ic 1396a, brc 37, brc 38 and brc 39, located in different directions in the h ii region, sh2-131, in order to map the magnetic field (b-field) in the plane of the sky. these brcs are illuminated by the o star hd 206267 and present a range of projected on-sky geometries. this provides an opportunity to gain an understanding of the magnetized evolution of brcs. the b-field geometries of the clouds deduced from the polarization data, after correction for foreground contamination by the interstellar medium, are seen to be connected to the ambient b-fields on large scales. they seem to play an important role in shaping the clouds ic 1396a and brc 37. brcs 38 and 39 show a broader and snubber head morphology, possibly because the b-fields are aligned with incoming radiation, as revealed in the simulations. a good general agreement is noted on comparing our observational results with the simulations, supporting the importance of b-fields in brc evolution. this work is the first step towards systematic mapping the b-fields morphology in multiple brcs in an expanding h ii region, extending our previous work. | magnetic fields in multiple bright-rimmed clouds in different directions of h ii region ic 1396 |
aims: our aim is to study the mass transfer, accretion environment, and wind outflows in the ss 433 system, concentrating on the so-called stationary lines.methods: we used archival high-resolution (x-shooter) and low-resolution (emmi) optical spectra, new optical multi-filter polarimetry, and low-resolution optical spectra (liverpool telescope), spanning an interval of a decade and a broad range of precessional and orbital phases, to derive the dynamical properties of the system.results: using optical interstellar absorption lines and h i 21 cm profiles, we derive e(b - v) = 0.86 ± 0.10, with an upper limit of e(b - v) = 1.8 ± 0.1 based on optical diffuse interstellar bands. we obtain revised values for the ultraviolet and u band polarizations and polarization angles (pa), based on a new calibrator star at nearly the same distance as ss 433 that corrects the published measurement and yields the same pa as the optical. the polarization wavelength dependence is consistent with optical-dominating electron scattering with a rayleigh component in u and the uv filters. no significant phase modulation was found for pa while there is significant variability in the polarization level. we fortuitously caught a flare event; no polarization changes were observed but we confirm the previously reported associated emission line variations. studying profile modulation of multiple lines of h i, he i, o i, na i, si ii, ca ii, fe ii with precessional and orbital phase, we derive properties for the accretion disk and present evidence for a strong disk wind, extending published results. using transition-dependent systemic velocities, we probe the velocity gradient of the wind, and demonstrate that it is also variable on timescales unrelated to the orbit. using the rotational velocity, around 140 ± 20 km s-1, a redetermined mass ratio q = 0.37 ± 0.04, and masses mx = 4.2 ± 0.4 m⊙, ma = 11.3 ± 0.6 m⊙, the radius of the a star fills - or slightly overfills - its roche surface. we devote particular attention to the o i 7772 å and 8446 å lines, finding that they show different but related orbital and precessional modulation and there is no evidence for a circumbinary component. the spectral line profile variability can, in general, be understood with an ionization stratified outflow predicted by thermal wind modeling, modulated by different lines of sight through the disk produced by its precession. the wind can also account for an extended equatorial structure detected at long wavelength. | an optical spectroscopic and polarimetric study of the microquasar binary system ss 433 |
we present our observational results of the 0.87 mm polarized dust emission in the class 0 protostar b335 obtained with the atacama large millimeter/submillimeter array (alma) at a 0"2 (20 au) resolution. we compared our data at 0.87 mm with those at 1.3 mm from the alma archive. the observed polarization orientations at the two wavelengths are consistent within the uncertainty, and the polarization percentages are systematically higher at 1.3 mm than 0.87 mm by a factor of ∼1.7, suggesting that the polarized emission originates from magnetically aligned dust grains. we inferred the magnetic field orientations from the observed polarization orientations. we found that the magnetic field changes from ordered and highly pinched to more complicated and asymmetric structures within the inner 100 au scale of b335, and the magnetic field connects to the center along the equatorial plane as well as along the directions that are ∼40°-60° from the equatorial plane. we performed nonideal mhd simulations of collapsing dense cores. we found that similar magnetic field structures appear in our simulations of dense cores with the magnetic field and rotational axis slightly misaligned by 15° but not in those with the aligned magnetic field and rotational axis. our results suggest that the midplane of the inner envelope within the inner 100 au scale of b335 could be warped because of the misaligned magnetic field and rotational axis, and the magnetic field could be dragged by the warped accretion flows. | transition from ordered pinched to warped magnetic field on a 100 au scale in the class 0 protostar b335 |
we made r-band polarization measurements of 234 stars towards the direction of the mbm 33-39 cloud complex. the distance of the mbm 33-39 complex was determined as 120 ± 10 pc using polarization results and near-infrared photometry from the 2mass survey. the magnetic field geometry of the individual clouds inferred from our polarimetric results reveals that the field lines are in general consistent with the global magnetic field geometry of the region obtained from previous studies. this implies that the clouds in the complex are permeated by the interstellar magnetic field. multi-wavelength polarization measurements of a few stars projected on to the complex suggest that the size of the dust grains in these clouds is similar to those found in the normal interstellar medium of the milky way. we studied a possible formation scenario of the mbm 33-39 complex by combining the polarization results from our study with those from the literature and by identifying the distribution of ionized, atomic and molecular (dust) components of material in the region. | polarization of seven mbm clouds at high galactic latitude |
cool evolved stars are known to be significant contributors to the enrichment of the interstellar medium through their dense and dusty stellar winds. high resolution observations of these outflows have shown them to possess high degrees of morphological complexity. we observed the asymptotic giant branch (agb) star ep aquarii with alma in band 6 and vlt/sphere/zimpol in four filters the visible. both instruments had an angular resolution of 0.025″. these are follow-up observations to the lower-resolution 2016 alma analysis of ep aquarii, which revealed that its wind possesses a nearly face-on, spiral-harbouring equatorial density enhancement, with a nearly pole-on bi-conical outflow. at the base of the spiral, the sio emission revealed a distinct emission void approximately 0.4″ to the west of the continuum brightness peak, which was proposed to be linked to the presence of a companion. the new alma data better resolve the inner wind and reveal that its morphology as observed in co is consistent with hydrodynamical companion-induced perturbations. assuming that photodissociation by the uv-field of the companion is responsible for the emission void in sio, we deduced the spectral properties of the tentative companion from the size of the hole. we conclude that the most probable companion candidate is a white dwarf with a mass between 0.65 and 0.8 m⊙, though a solar-like companion could not be definitively excluded. the radial sio emission shows periodic, low-amplitude perturbations. we tentatively propose that they could be the consequence of the interaction of the agb wind with another much closer low-mass companion. the polarised sphere/zimpol data show a circular signal surrounding the agb star with a radius of ∼0.1″. decreased signal along a pa of 138° suggests that the dust is confined to an inclined ring-like structure, consistent with the previously determined wind morphology. the reduced images and datacubes are also available at the cds via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/cat/j/a+a/642/a93 | a detailed view on the circumstellar environment of the m-type agb star ep aquarii. i. high-resolution alma and sphere observations |
we present a study of the relative orientation between the magnetic field and elongated cloud structures for the ρ oph a and ρ oph e regions in l1688 in the ophiuchus molecular cloud. combining inferred magnetic field orientation from hawc+ 154 μm observations of polarized thermal emission with column density maps created using herschel submillimeter observations, we find consistent perpendicular relative alignment at scales of 0.02 pc (33"6 at d ≈ 137 pc) using the histogram of relative orientations (hro) technique. this supports the conclusions of previous work using planck polarimetry and extends the results to higher column densities. combining this hawc+ hro analysis with a new planck hro analysis of l1688, the transition from parallel to perpendicular alignment in l1688 is observed to occur at a molecular hydrogen column density of approximately 1021.7 cm-2. this value for the alignment transition column density agrees well with values found for nearby clouds via previous studies using only planck observations. using existing turbulent, magnetohydrodynamic simulations of molecular clouds formed by colliding flows as a model for l1688, we conclude that the molecular hydrogen volume density associated with this transition is approximately ~104 cm-3. we discuss the limitations of our analysis, including incomplete sampling of the dense regions in l1688 by hawc+. | hawc+/sofia polarimetry in l1688: relative orientation of magnetic field and elongated cloud structure |
context. both simulations and observations of the interstellar medium show that the study of the relative orientations between filamentary structures and the magnetic field can bring new insight into the role played by magnetic fields in the formation and evolution of filaments and in the process of star formation.aims: we provide a first application of fildreams, the new method presented in the companion paper to detect and analyze filaments in a given image. the method relies on a template that has the shape of a rectangular bar with variable width. our goal is to investigate the relative orientations between the detected filaments and the magnetic field.methods: we apply fildreams to a small sample of four herschel fields (g210, g300, g82, g202) characterized by different galactic environments and different evolutionary stages. first, we look for the most prevalent bar widths, and we examine the networks formed by filaments of different bar widths as well as their hierarchical organization. second, we compare the filament orientations to the magnetic field orientation inferred from planck polarization data and, for the first time, we study the statistics of the relative orientation angle as functions of both spatial scale and h2 column density.results: we find preferential relative orientations in the four herschel fields: small filaments with low column densities tend to be slightly more parallel than perpendicular to the magnetic field; in contrast, large filaments, which all have higher column densities, are oriented nearly perpendicular (or, in the case of g202, more nearly parallel) to the magnetic field. in the two nearby fields (g210 and g300), we observe a transition from mostly parallel to mostly perpendicular relative orientations at an h2 column density ≃ 1.1 × 1021 cm−2 and 1.4 × 1021 cm−2, respectively, consistent with the results of previous studies.conclusions: our results confirm the existence of a coupling between magnetic fields at cloud scales and filaments at smaller scale. they also illustrate the potential of combining herschel and planck observations, and they call for further statistical analyses with our dedicated method. | fildreams. ii. application to the analysis of the relative orientations between filaments and the magnetic field in four herschel fields |
fast radio bursts (frbs) are energetic millisecond phenomena in the radio band. polarimetric studies of repeating frbs indicate that many of these sources occupy extreme and complex magnetoionized environments. recently, a frequency-dependent depolarization has been discovered in several repeating frbs. however, the temporal evolution of polarization properties is limited by the burst rate and observational cadence of telescopes. in this letter, the temporal evolution of depolarization in repeating frb 20201124a is explored. using the simultaneous variation of rotation measure and dispersion measure, we also measure the strength of a magnetic field parallel to the line of sight. the strength ranges from a few μg to 103 μg. in addition, we find that the evolution of depolarization and magnetic field traces the evolution of rotation measure. our result supports that the variation of depolarization, rotation measure, and the magnetic field are determined by the same complex magnetoionized screen surrounding the frb source. the derived properties of the screen are consistent with the wind and the decretion disk of a massive star. | temporal evolution of depolarization and magnetic field of fast radio burst 20201124a |
we present near-infrared polarimetric observations of the black hole x-ray binaries swift j1357.2-0933 and a0620-00. in both sources, recent studies have demonstrated the presence of variable infrared synchrotron emission in quiescence, most likely from weak compact jets. for swift j1357.2-0933, we find that the synchrotron emission is polarized at a level of 8.0 ± 2.5 per cent (a 3.2σ detection of intrinsic polarization). the mean magnitude and rms variability of the flux (fractional rms of 19-24 per cent in ks band) agree with previous observations. these properties imply a continuously launched (stable on long time-scales), highly variable (on short time-scales) jet in the swift j1357.2-0933 system in quiescence, which has a moderately tangled magnetic field close to the base of the jet. we find that for a0620-00, there are likely to be three components to the optical-infrared polarization; interstellar dust along the line of sight, scattering within the system, and an additional source that changes the polarization position angle in the reddest (h and ks) wavebands. we interpret this as a stronger contribution of synchrotron emission, and by subtracting the line-of-sight polarization, we measure an excess of ∼1.25 ± 0.28 per cent polarization and a position angle of the magnetic field vector that is consistent with being parallel with the axis of the resolved radio jet. these results imply that weak jets in low-luminosity accreting systems have magnetic fields which possess similarly tangled fields compared to the more luminous, hard state jets in x-ray binaries. | polarized synchrotron emission in quiescent black hole x-ray transients |
highly reddened type ia supernovae (sne ia) with low total-to-selective visual extinction ratio values, rv, also show peculiar linear polarization wavelength dependencies with peak polarizations at short wavelengths (λmax ≲ 0.4 μm). it is not clear why sightlines to sne ia display such different continuum polarization profiles from interstellar sightlines in the milky way with similar rv values. we investigate polarization profiles of a sample of galactic stars with low rv values, along anomalous extinction sightlines, with the aim to find similarities to the polarization profiles that we observe in sn ia sightlines. we undertook spectropolarimetry of 14 stars, used archival data for 3 additional stars, and ran dust extinction and polarization simulations (by adopting the picket-fence alignment model) to infer a simple dust model (size distribution, alignment) that can reproduce the observed extinction and polarization curves. our sample of galactic stars with low rv values and anomalous extinction sightlines displays normal polarization profiles with an average λmax 0.53 μm, and is consistent within 3σ to a larger coherent sample of galactic stars from the literature. despite the low rv values of dust toward the stars in our sample, the polarization curves do not show any similarity to the continuum polarization curves observed toward sne ia with low rv values. there is a correlation between the best-fit serkowski parameters k and λmax, but we did not find any significant correlation between rv and λmax. our simulations show that the k-λmax relationship is an intrinsic property of polarization. furthermore, we have shown that in order to reproduce polarization curves with normal λmax and low rv values, a population of large (a ≥ 0.1μm) interstellar silicate grains must be contained in the dust composition. the reduced spectra are only available at the cds via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?j/a+a/615/a42 | spectropolarimetry of galactic stars with anomalous extinction sightlines |
bow shocks and related density enhancements produced by the winds of massive stars moving through the interstellar medium provide important information regarding the motions of the stars, the properties of their stellar winds, and the characteristics of the local medium. since bow-shock nebulae are aspherical structures, light scattering within them produces a net polarization signal even if the region is spatially unresolved. scattering opacity arising from free electrons and dust leads to a distribution of polarized intensity across the bow-shock structure. that polarization encodes information about the shape, composition, opacity, density, and ionization state of the material within the structure. in this paper, we use the monte carlo radiative transfer code slip to investigate the polarization created when photons scatter in a bow-shock-shaped region of enhanced density surrounding a stellar source. we present results for electron scattering, and investigate the polarization behaviour as a function of optical depth, temperature, and source of photons for two different cases: pure scattering and scattering with absorption. in both regimes, we consider resolved and unresolved cases. we discuss the implications of these results as well as their possible use along with observational data to constrain the properties of observed bow-shock systems. in different situations and under certain assumptions, our simulations can constrain viewing angle, optical depth and temperature of the scattering region, and the relative luminosities of the star and shock. | polarization simulations of stellar wind bow-shock nebulae - i. the case of electron scattering |
the next generation balloon-borne large aperture submillimeter telescope (blast-tng) is a submillimeter mapping experiment planned for a 28 day long-duration balloon (ldb) flight from mcmurdo station, antarctica during the 2018-2019 season. blast-tng will detect submillimeter polarized interstellar dust emission, tracing magnetic fields in galactic molecular clouds. blast-tng will be the first polarimeter with the sensitivity and resolution to probe the 0.1 parsec-scale features that are critical to understanding the origin of structures in the interstellar medium. blast-tng features three detector arrays operating at wavelengths of 250, 350, and 500 m (1200, 857, and 600 ghz) comprised of 918, 469, and 272 dual-polarization pixels, respectively. each pixel is made up of two crossed microwave kinetic inductance detectors (mkids). these arrays are cooled to 275 mk in a cryogenic receiver. each mkid has a different resonant frequency, allowing hundreds of resonators to be read out on a single transmission line. this inherent ability to be frequency-domain multiplexed simplifies the cryogenic readout hardware, but requires careful optical testing to map out the physical location of each resonator on the focal plane. receiver-level optical testing was carried out using both a cryogenic source mounted to a movable xy-stage with a shutter, and a beam-filling, heated blackbody source able to provide a 10-50 c temperature chop. the focal plane array noise properties, responsivity, polarization efficiency, instrumental polarization were measured. we present the preflight characterization of the blast-tng cryogenic system and array-level optical testing of the mkid detector arrays in the flight receiver. | preflight characterization of the blast-tng receiver and detector arrays |
we present an analysis of archival multi-frequency very large array monitoring data of the two-image gravitational lens system class b1600+434, including the polarization properties at 8.5 ghz. from simulating radio light curves incorporating realistic external variability in image a, we find time delays consistent at 1σ for all frequencies and in total flux density and polarization. the delay with the smallest uncertainty (total flux density at 8.5 ghz) is $42.3^{+2.0}_{-1.8}$ (random) ±0.5 (systematic) d (equivalent to 42.3 ± 2.1 d) whereas combining all delay estimates gives a slightly higher value of 43.6 ± 1.2 d. both values are lower than the previously published radio result and inconsistent with that found in the optical. h0 determination is difficult due to the complicated lensing mass and the lack of constraints provided by only two images. however, analysis of archival very long baseline interferometry data reveals jets in this system for the first time, the orientations of which provide model constraints. in addition, extremely sensitive maps made from combining all the monitoring data reveal faint emission on one side of the lensing galaxy that we speculate might be the result of a naked-cusp lensing configuration. finally, we find clear evidence for external variability in image a on time-scales of days to years, the frequency dependence of which supports the previous conclusion that this is predominantly due to microlensing. external variability seems to be completely absent in image b and this does not appear to be a consequence of scatter-broadening in the interstellar medium of the lensing galaxy. | the time delay of class b1600+434 from vla multi-frequency and polarization monitoring |
afterglows of gamma-ray bursts (grbs) are emitted from expanding forward shocks, which are expected to have magnetic fields much stronger than the interstellar field, although the origin of the field is a long-standing problem. two field amplification mechanisms, plasma kinetic instabilities and magnetohydrodynamic instabilities, have been discussed so far. the coherence-length scales of the fields amplified by these two processes are different by 7-10 orders of magnitude, and the polarimetric observations may distinguish them. we construct a semi-analytic model of the forward-shock afterglow polarization under the assumption of hydrodynamic-scale turbulent magnetic field. we perform numerical calculations of synchrotron polarization for the isotropic turbulence and the zero viewing angle. we find that the polarization degrees are ~1% when the field coherence-length scale in the fluid co-moving frame is of the order of the thickness of the shocked regions. this range of polarization degree is comparable to that of the observed late-phase optical afterglows. our model also shows that the radio polarization degrees are comparable to the optical ones on average but can be higher than the optical ones at some time intervals. the polarization angles are shown to vary randomly and continuously. these polarimetric properties are clearly different from the case of plasma kinetic instability. simultaneous polarimetric observations of grb afterglows at the radio and optical bands have recently started, which will help us constrain the magnetic field amplification mechanism. | synchrotron polarization of gamma-ray burst afterglow shocks with hydrodynamic-scale turbulent magnetic field |
interstellar dust grains are often aligned. if the grain alignment direction varies along the line of sight, the thermal emission becomes circularly polarized. in the diffuse interstellar medium, the circular polarization at far-infrared and submillimeter wavelengths is predicted to be very small, and probably unmeasurable. however, circular polarization may reach detectable levels in photodissociation regions viewed through molecular clouds, in infrared dark clouds, and in protoplanetary disks. measurement of circular polarization could help constrain the structure of the magnetic field in infrared dark clouds, and may shed light on the mechanisms responsible for grain alignment in protoplanetary disks. | on far-infrared and submillimeter circular polarization |
we present a study of the continuum polarization over the 400-600 nm range of 19 sne ia obtained with fors at the vlt. we separate them into those that show na i d lines at the velocity of their hosts and those that do not. continuum polarization of the sodium sample near maximum light displays a broad range of values, from extremely polarized cases like sn 2006x to almost unpolarized ones like sn 2011ae. the non-sodium sample shows, typically, smaller polarization values. the continuum polarization of the sodium sample in the 400-600 nm range is linear with wavelength and can be characterized by the mean polarization ({p}{mean}). its values span a wide range and show a linear correlation with color, color excess, and extinction in the visual band. larger dispersion correlations were found with the equivalent width of the na i d and ca ii h and k lines, and also a noisy relation between {p}{mean} and rv , the ratio of total to selective extinction. redder sne show stronger continuum polarization, with larger color excesses and extinctions. we also confirm that high continuum polarization is associated with small values of rv . the correlation between extinction and polarization—and polarization angles—suggest that the dominant fraction of dust polarization is imprinted in interstellar regions of the host galaxies. we show that na i d lines from foreground matter in the sn host are usually associated with non-galactic ism, challenging the typical assumptions in foreground interstellar polarization models. based on observations made with eso telescopes at the paranal observatory under programs 068.d-0571(a), 069.d-0438(a), 070.d-0111(a), 076.d-0178(a), 079.d-0090(a), 080.d-0108(a), 081.d-0558(a), 085.d-0731(a), and 086.d-0262(a). also based on observations collected at the german-spanish astronomical center, calar alto (spain). | continuum foreground polarization and na i absorption in type ia sne |
synchrotron intensity and polarization gradients (sig and spg), proposed by lazarian et al. and lazarian & yuen, present a new way to recover the mean magnetic field direction in the plane of the sky. to measure the magnetization level, lazarian et al. suggested that the methods used in the context of the "velocity gradient technique" could also be used on the sig and spg contexts. in this work we test the two proposed methods, named "top-base" and the circular standard deviation, "s," to obtain the level of magnetization from synchrotron emission. in order to test the methods, we generate synthetic observations from magnetohydrodynamic computer simulations, with alfvénic mach numbers, ma ∈ [0.2,1.7]. using a bayesian analysis we find that the circular standard deviations for the sig and spg methods are able to recover the magnetization for cases with signal-to-noise ratio ≳5. we found that for weak faraday depolarization and different angles between the magnetic field direction and the line of sight the magnetization level can still be estimated. | synchrotron intensity and polarization gradients: tools to obtain the magnetization level in a turbulent medium |
we establish a theoretical framework for solving the equations of motion for an arbitrarily shaped, inhomogeneous dust particle in the presence of radiation pressure. the repeated scattering problem involved is solved using a state-of-the-art volume integral equation-based t-matrix method. a fortran implementation of the framework is used to solve the explicit time evolution of a homogeneous irregular sample geometry. the results are shown to be consistent with rigid body dynamics, between integrators, and comparable with predictions from an alignment efficiency potential map. also, we demonstrate the explicit effect of single-particle dynamics to observed polarization using the obtained orientational results. | dynamics of small particles in electromagnetic radiation fields: a numerical solution |
the potential energy surface is computed at the explicitly correlated coupled cluster with simple, second and perturbative triple excitation method (ccsd(t)-f12) in connection with the augmented-correlation consistent-polarized valence triple zeta (aug-cc-pvtz) gaussian basis set for the neh+ -he system. the calculations were performed by first taking into account the vibration of the molecule and then averaging the so-obtained three-dimensional potential. from this average interaction potential, cross-sections among the 11 first rotational levels of neh+ induced by collision with he are calculated for energies up to 4000 cm-1 using the quantum mechanical close coupling (cc) approach. collisional rate coefficients are obtained by thermally averaging these cross-sections at low temperature (t ≤ 300 k). the propensity rules of the rotational transitions obtained in this paper are discussed and compared with those of heh+ and arh+ in collision with electron. this work may be helpful for the eventual investigations, both theoretical and experimental, focused to detect the key cationic noble gas hydride neh+ in the interstellar and circumstellar media as well as in laboratory experiments. | collisional rates based on the first potential energy surface of the neh+ -he system |
context. many galaxies contain magnetic fields supported by galactic dynamo action. the investigation of these magnetic fields can be helpful for understanding galactic evolution; however, nothing definitive is known about magnetic fields in ring galaxies.aims: here we investigate large-scale magnetic fields in a previously unexplored context, namely ring galaxies, and concentrate our efforts on the structures that appear most promising for galactic dynamo action, i.e. outer star-forming rings in visually unbarred galaxies.methods: we use tested methods for modelling α-ω galactic dynamos, taking into account the available observational information concerning ionized interstellar matter in ring galaxies.results: our main result is that dynamo drivers in ring galaxies are strong enough to excite large-scale magnetic fields in the ring galaxies studied. the variety of dynamo driven magnetic configurations in ring galaxies obtained in our modelling is much richer than that found in classical spiral galaxies. in particular, various long-lived transients are possible. an especially interesting case is that of ngc 4513, where the ring counter-rotates with respect to the disc. strong shear in the region between the disc and the ring is associated with unusually strong dynamo drivers in such counter-rotators. the effect of the strong drivers is found to be unexpectedly moderate. with counter-rotation in the disc, a generic model shows that a steady mixed parity magnetic configuration that is unknown for classical spiral galaxies, may be excited, although we do not specifically model ngc 4513.conclusions: we deduce that ring galaxies constitute a morphological class of galaxies in which identification of large-scale magnetic fields from observations of polarized radio emission, as well as dynamo modelling, may be possible. such studies have the potential to throw additional light on the physical nature of rings, their lifetimes, and evolution. | magnetic fields in ring galaxies |
we compare two rotationally invariant decomposition techniques on linear polarization data: the spin-2 spherical harmonic decomposition in two opposite parities, the e- and b-mode, and the multiscale analysis of the gradient of linear polarization, |\nabla p|. we demonstrate that both decompositions have similar properties in the image domain and the spatial frequency domain. they can be used as complementary tools for turbulence analysis of interstellar magnetic fields in order to develop a better understanding of the origin of energy sources for the turbulence, the origin of peculiar magnetic field structures and their underlying physics. we also introduce a new quantity |∇eb| based on the e- and b-modes and we show that in the intermediate- and small-scale limit |\nabla eb| ∼eq |\nabla p|. analysis of the 2.3 ghz s-band polarization all sky survey shows many extended coherent filament-like features appearing as 'double jumps' in the |\nabla p| map that are correlated with negative and positive filaments of b-type polarization. these local asymmetries between the two polarization types, e and b, of the non-thermal galactic synchrotron emission have an influence on the e- and b-mode power spectra analyses. the wavelet-based formalism of the polarization gradient analysis allows us to locate the position of e- or b-mode features responsible for the local asymmetries between the two polarization types. in analysed subregions, the perturbations of the magnetic field are trigged by star clusters associated with h ii regions, the orion-eridanus superbubble and the north polar spur at low galactic latitude. | a new perspective on turbulent galactic magnetic fields through comparison of linear polarization decomposition techniques |
the quantum mechanical treatment of quadrupole radiation due to bremsstrahlung exact to all orders in the coulomb interaction of two nonrelativistically colliding charged spin-0 and unpolarized spin-1/2 particles is presented. we calculate the elements of the quadrupole tensor, and present analytical solutions for the double-differential photon emission cross section of identical and nonidentical particles. contact is made to born-level and quasi-classical results in the respective kinematic limits and effective energy loss rates are obtained. a generally valid formula for soft-photon emission is established and an approximate formula across the entire kinematic regime is constructed. the results apply to bremsstrahlung emission in the scattering of pairs of electrons and are hence relevant in the study of astrophysical phenomena. a definition of a gaunt factor is proposed which should facilitate broad applicability of the results. | exact theory of nonrelativistic quadrupole bremsstrahlung |
we have investigated the grain alignment and dust properties towards the direction of the cluster ngc 2345 using the multiband optical polarimetric observations. for the majority of the stars, the observed polarization is found to be due to the interstellar medium with average values of maximum polarization and wavelength corresponding to it as 1.55 per cent and 0.58 $\mu$m, respectively. this reveals a similar size distribution of dust grains to that in the general interstellar medium in the direction of ngc 2345. alteration of dust properties near the distance of 1.2 kpc towards the direction of ngc 2345 has been noticed. the dust grains located beyond this distance are found to be aligned with the galactic magnetic field, whereas a dispersion in the orientation of the dust grains lying in the foreground of this distance is found. polarizing efficiency of grains in this direction is found to be close to the average efficiency for our galaxy. the decreased grain size along with the increased polarizing efficiency towards the core region of the cluster indicates the local radiation field is higher within the cluster, which is responsible for the increased alignment efficiency of small grains. the wavelength of maximum polarization (associated with the average size of aligned grains) is also found to increase with extinction and reduces with the increase in polarizing efficiency, which can be explained by the radiative torque alignment mechanism. | polarization, polarizing efficiency, and grain alignment towards the direction of the cluster ngc 2345 |
in the laboratory, the photo-and thermochemical evolution of ices, made of simple molecules of astrophysical relevance, always leads to the formation of semi-refractory water-soluble organic residues. targeted searches for specific molecules do reveal the notable presence of two families of important molecular ''bricks of life'': amino acids, key molecules in metabolism, and sugars, including ribose, the backbone of rna molecules which support the genetic information in all living entities. most of these molecules are indeed found in primitive carbonaceous meteorites and their implication in prebiotic chemistry at the surface of the early earth must be seriously considered. these molecules are, almost all, chiral. in meteorites, some amino acids do show significant enantiomeric excesses, practically exclusively of the l-form. in our experiments, we investigate the role of circularly polarized light obtained from the desirs beamline of the synchrotron soleil, a light commonly observed in regions of star formation, in order to generate an initial symmetry breaking in chiral amino acids produced and then indeed detected in our samples. we present first a brief global description of the chemical evolution of the galaxy. then, using our laboratory simulations, we suggest the importance of cosmic ices in the build-up of complex organic matter, including enantioenrichment at the surface of telluric planets like the earth, thus establishing a link between astrochemistry and astrobiology. | interstellar ices: a possible scenario for symmetry breaking of extraterrestrial chiral organic molecules of prebiotic interest |
radiative transfer models were developed to understand the optical polarizations in edge-on galaxies, which are observed to occur even outside the geometrically thin dust disk, with a scale height of ≈0.2 kpc. in order to reproduce the vertically extended polarization structure, we find that it is essential to include a geometrically thick dust layer in the radiative transfer model, in addition to the commonly known thin dust layer. the models include polarizations due to both dust scattering and dichroic extinction, which are responsible for the observed interstellar polarization in the milky way. we also find that the polarization level is enhanced if the clumpiness of the interstellar medium, and the dichroic extinction by vertical magnetic fields in the outer regions of the dust lane are included in the radiative transfer model. the predicted degree of polarization outside the dust lane was found to be consistent with that (ranging from 1% to 4%) observed in ngc 891. | polarization as a probe of thick dust disks in edge-on galaxies: application to ngc 891 |
we present results based on the optical r-band observations of the polarization of 280 stars distributed toward the dark globule ldn 1225. parallaxes from gaia data release 2 along with the polarization data of ∼200 stars have been used to (a) constrain the distance of ldn 1225 as 830 ± 83 pc, (b) determine the contribution of interstellar polarization, and (c) characterize the dust properties and delineate the magnetic field (b-field) morphology of ldn 1225. we find that b-fields are more organized and exhibit a small dispersion of 12°. using the 12co molecular line data from the purple mountain observatory, along with the column density and dispersion in b-fields, we estimate the b-field strength to be ∼56 ± 10 μg, the ratio of magnetic to turbulent pressure to be ∼3 ± 2, and the ratio of mass to magnetic flux (in units of the critical value) to be <1. these results indicate the dominant role of b-fields in comparison to turbulence and gravity in rendering the cloud support. b-fields are aligned parallel to the low-density parts (traced by a 12co map) of the cloud; in contrast, they are neither parallel nor perpendicular to the high-density core structures (traced by 13co and c18o maps). ldn 1225 hosts two 70 μm sources, which seem to be low-mass class 0 sources. the ratio of total to selective extinction derived using optical and near-infrared photometric data is found to be anomalous (rv= 3.4), suggesting the growth of dust grains in ldn 1225. the polarization efficiency of dust grains follows a power law with an index of -0.7, implying that optical polarimetry traces b-fields in the outer parts of the cloud. | polarimetric and photometric investigation of the dark globule ldn 1225: distance, extinction law, and magnetic fields |
magnetic fields play an important role in shaping the structure and evolution of the interstellar medium (ism) of galaxies, but the details of this relationship remain unclear. with ska1, the 3d structure of galactic magnetic fields and its connection to star formation will be revealed. a highly sensitive probe of the internal structure of the magnetoionized ism is the partial depolarization of synchrotron radiation from inside the volume. different configurations of magnetic field and ionized gas within the resolution element of the telescope lead to frequency-dependent changes in the observed degree of polarization. the results of spectro-polarimetric observations are tied to physical structure in the ism through comparison with detailed modeling, supplemented with the use of new analysis techniques that are being actively developed and studied within the community such as rotation measure synthesis. the ska will enable this field to come into its own and begin the study of the detailed structure of the magnetized ism in a sample of nearby galaxies, thanks to its extraordinary wideband capabilities coupled with the combination of excellent surface brightness sensitivity and angular resolution. | magnetic field tomography in nearby galaxies with the square kilometre array |
we present the new open-source c++-based python library costuum that can be used to generate infrared absorption and emission coefficients for arbitrary mixtures of spheroidal dust grains that are (partially) aligned with a magnetic field. we outline the algorithms underlying the software, demonstrate the accuracy of our results using benchmarks from literature, and use our tool to investigate some commonly used approximative recipes. we find that the linear polarization fraction for a partially aligned dust grain mixture can be accurately represented by an appropriate linear combination of perfectly aligned grains and grains that are randomly oriented, but that the commonly used picket fence alignment breaks down for short wavelengths. we also find that for a fixed dust grain size, the absorption coefficients and linear polarization fraction for a realistic mixture of grains with various shapes cannot both be accurately represented by a single representative grain with a fixed shape, but that instead an average over an appropriate shape distribution should be used. insufficient knowledge of an appropriate shape distribution is the main obstacle in obtaining accurate optical properties. costuum is available as a standalone python library and can be used to generate optical properties to be used in radiative transfer applications. | costuum: polarized thermal dust emission by magnetically oriented spheroidal grains |
we provide a procedure for identification of dominating compressible and alfvenic mhd modes or isotropic turbulence in synchrotron emission polarization maps of galactic objects. the results for the region of north galactic pole, orion molecular cloud complex and the star-forming complex cygnus x are presented. | identification of mhd compressible modes in interstellar plasma with synchrotron emission polarization |
the discovery of interstellar communication signals is complicated by the presence of radio interference. consequently, interstellar communication signals are hypothesized to have properties that favor discovery in high levels of local planetary radio interference. a hypothesized type of interstellar signal, delta-t delta-f polarized pulse pairs, has properties that are similar to infrequent elements of random noise, while dissimilar from many types of known radio interference. discovery of delta-t delta-f polarized pulse pairs is aided by the use of interference-filtered receiver systems that are designed to indicate anomalous presence of delta-t delta-f polarized pulse pairs, when pointing a radio telescope to celestial coordinates of a hypothetical transmitter. observations reported in previous work (ref. arxiv:2105.03727) indicate an anomalous celestial pointing direction having coordinates 5.25 +- 0.15 hours right ascension and -7.6 +- 1 degrees declination. augmented interference reduction mechanisms used in the current work are described, together with reports of follow-up radio telescope beam transit measurements during 40 days. conclusions and further work are proposed. | radio interference reduction in interstellar communications: methods and observations |
pre-stellar cores within molecular clouds provide the very initial conditions in which stars are formed. fest 1-457 is a prototypical starless core and the most chemically evolved among those isolated, embedded in the most pristine part of the pipe nebula, the bowl. we use the iram 30 m telescope and the pdbi to study the chemical and physical properties of the starless core fest 1-457 (core 109) in the pipe nebula. we fit the hyperfine structure of the n2h+ (1-0) iram 30 m data. this allowed us to measure with high precision the velocity field, line widths and opacity and derive the excitation temperature and column density in the core. we used a modified bonnor-ebert sphere model adding a temperature gradient towards the center to fit the 1.2 mm continuum emission and visual extinction maps. using this model, we have estimated the abundances of the n2h+ and the rest of molecular lines detected in the 30 ghz wide line survey performed at 3 mm with iram 30 m using artist software. the core presents a rich chemistry with emission from early (c3h2, hcn, cs) and late-time molecules (e.g., n2h+), with a clear chemical spatial differentiation for nitrogen (centrally peaked), oxygen (peaking to the southwest) and sulfurated molecules (peaking to the east). for most of the molecules detected (hcn, hco+, ch3oh, cs, so, 13co and c18o), abundances are best fit with three values, presenting a clear decrease of abundance of at least one or two orders of magnitude towards the center of the core. the bonnor-ebert analysis indicates the core is gravitationally unstable and the magnetic field is not strong enough to avoid the collapse. depletion of molecules onto the dust grains occurs at the interior of the core, where dust grain growth and dust depolarization also occurs. this suggests that these properties may be related. on the other hand, some molecules exhibit asymmetries in their integrated emission maps, which appear to be correlated with a previously reported submillimetre polarization asymmetry. these asymmetries could be due to a stronger interstellar radiation field in the western side of the core. the fits files for figs. 1, 4, and 5 are only available at the cds via anonymous ftp to http://cdsarc.u-strasbg.fr (http://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?j/a+a/597/a74 | a correlation between chemistry, polarization, and dust properties in the pipe nebula starless core fest 1-457 |
multiband linear polarimetric observations of 125 stars in the region of the cluster ngc 1817 have been carried out intending to study properties of interstellar dust and grains in that direction. the polarization is found to be wavelength-dependent, being maximum in the v band with an average value of 0.95%. the foreground interstellar dust grains appear to be the main source of linear polarization of starlight toward the direction of ngc 1817. the average value of the position angle in the v band of 119°2 is found to be less than the direction of the galactic parallel in the region, indicating that the dust grains in the direction are probably not yet relaxed. spatial distribution of dust appears to be more diverse in the coronal region than the core region of the cluster. the maximum value of the degree of polarization is estimated to be 0.93% for members of the cluster using the serkowski relation. the average value of wavelength corresponding to the maximum polarization of 0.54 ± 0.02 μm indicates that the size distribution of dust grains in the line of sight is similar to that of the general interstellar medium. several variable stars in the cluster were also observed polarimetrically and pulsating variables appear to have a slightly lower value of polarization from other nonvariable member stars of the cluster. there are indications of the existence of dust layers in front of those clusters which are located close to galactic plane while for clusters located away from galactic plane no major dust layers are observed. | broadband linear polarization in the region of the open star cluster ngc 1817 |
we study the effects of ambipolar diffusion (ad) on hydromagnetic turbulence. we consider the regime of large magnetic prandtl number, relevant to the interstellar medium. in most of the cases, we use the single-fluid approximation where the drift velocity between charged and neutral particles is proportional to the lorentz force. in two cases we also compare with the corresponding two-fluid model, where ionization and recombination are included in the continuity and momentum equations for the neutral and charged species. the magnetic field properties are found to be well represented by the single-fluid approximation. we quantify the effects of ad on total and spectral kinetic and magnetic energies, the ohmic and ad dissipation rates, the statistics of the magnetic field, the current density, and the linear polarization as measured by the rotationally invariant e and b mode polarizations. we show that the kurtosis of the magnetic field decreases with increasing ad. the e mode polarization changes its skewness from positive values for small ad to negative ones for large ad. even when ad is weak, changes in ad have a marked effect on the skewness and kurtosis of e, and only a weak effect on those of b. these results open the possibility of employing e and b mode polarizations as diagnostic tools for characterizing turbulent properties of the interstellar medium. | ambipolar diffusion in large prandtl number turbulence |
the nature of dust in the diffuse interstellar medium can be best investigated by means of reddening curves where only a single interstellar cloud lies between the observer and the background source. published reddening curves often suffer from various systematic uncertainties. we merged a sample of 820 reddening curves of stars for which both fors2 polarization spectra and uves highresolution spectra are available. the resulting 111 sightlines towards ob-type stars have 175 reddening curves. for these stars, we derived their spectral-type from the uves high-resolution spectroscopy. to obtain high-quality reddening curves, we excluded stars with composite spectra in the iue/fuse data due to multiple stellar systems. likewise, we omitted stars that have uncertain spectral-type designations or stars with photometric variability. we neglected stars that show inconsistent parallaxes when comparing data releases two and three from gaia. finally, we identified stars that show differences in the space- and ground-based-derived reddening curves between 0.28 µm and the u band or in rv. in total, we find 53 stars with one or more reddening curves passing the rejection criteria. this provides the highest-quality milky way reddening curve sample available today. averaging the curves from our high-quality sample, we find rv = 3.1 ± 0.4, confirming previous estimates. a future paper in this series will use the current sample of precise reddening curves and combine them with polarization data to study the properties of dark dust. curves of fig. 9 are only available at the cds via anonymous ftp to cdsarc.cds.unistra.fr (ftp://130.79.128.5) or via https://cdsarc.cds.unistra.fr/viz-bin/cat/j/a+a/676/a132 | dark dust. iii. the high-quality single-cloud reddening curve sample: scrutinizing extinction curves in the milky way |
we present b-, v-, r-, and i-band linear polarimetric observations of 73 stars in the direction of open star cluster casado alessi 1 (hereafter alessi 1). we aim to use polarimetry as a tool to investigate the properties and distribution of dust grains toward the direction of the cluster. the polarimetric observations were carried out using the aries imaging polarimeter mounted at the 104 cm telescope of aries, nainital (india). using the gaia photometric data the age and distance of the cluster are estimated to be 0.8 ± 0.1 gyr and 673 ± 98 pc, respectively. a total of 66 stars with a 26' radius from the cluster are identified as members of the cluster using the astrometric approach. out of these 66 members, 15 stars were observed polarimetrically and found to have the same value of polarization. the majority of the stars in the region follow the general law of polarization for the interstellar medium, indicating that polarization toward the cluster alessi 1 is dominated by foreground dust grains. the average values of the maximum polarization (pmax) and the wavelength corresponding to the maximum polarization (λmax) toward the cluster are found to be 0.83 ± 0.03% and 0.59 ± 0.04 μm, respectively. also, dust grains toward the cluster appear to be aligned, possibly due to the galactic magnetic field. | optical linear polarization toward the open star cluster casado alessi 1 |
this paper presents the results of full polarization observations of the massive star-forming region w75n, conducted with ~3″ spatial resolutions at 345 ghz using the submillimeter array. the magnetic field structures in the dense cores of the region are derived using the linearly polarized continuum emission. the overall magnetic field strength and orientation are found to agree with those from the previous observations. the plane-of-sky component of the magnetic field in the region was calculated to be ~0.8 ± 0.1 mg using the angular dispersion function method. further analyses involving the polarization-intensity gradient-local gravity method and h13co+ (4-3) line data indicated that the cloud is undergoing global gravitational collapse and the magnetic field is shaped by gravity and outflows in the dense core regions. | submillimeter observations of magnetic fields in massive star-forming region w75n |
investigating interstellar (is) dust properties in external galaxies is important not only to infer the intrinsic properties of astronomical objects but also to understand the star/planet formation in the galaxies. from the non-milky way-like extinction and interstellar polarization (isp) observed in reddened type ia supernovae (sne), it has been suggested that their host galaxies contain dust grains whose properties are substantially different from the milky way (mw) dust. it is important to investigate the universality of such non-mw-like dust in the universe. here we report spectropolarimetry of two highly extinguished type ii sne (sn 2022aau and sn 2022ame). sn 2022aau shows a polarization maximum at a shorter wavelength than mw stars, which is also observed in some type ia sne. this is clear evidence for the existence of non-mw-like dust in its host galaxy (i.e., ngc 1672). this fact implies that such non-mw-like dust might be more common in some environments than expected, and thus it might affect the picture of the star/planet formation. on the other hand, sn 2022ame shows mw-like isp, implying the presence of mw-like dust in its host galaxy (i.e., ngc 1255). our findings confirm that dust properties of galaxies are diverse, either locally or globally. the present work demonstrates that further investigation of is dust properties in external galaxies using polarimetry of highly reddened sne is promising, providing a great opportunity to study the universality of such non-mw-like dust grains in the universe. | diversity of dust properties in external galaxies confirmed by polarization signals from type ii supernovae |
the large-scale magnetic fields observed in spiral disc galaxies are often thought to result from dynamo action in the disc plane. however, the increasing importance of faraday depolarization along any line of sight towards the galactic plane suggests that the strongest polarization signal may come from well above (∼0.3-1 kpc) this plane, from the vicinity of the warm interstellar medium (wim)/halo interface. we propose (see also henriksen & irwin 2016) that the observed spiral fields (polarization patterns) result from the action of vertical shear on an initially poloidal field. we show that this simple model accounts for the main observed properties of large-scale fields. we speculate as to how current models of optical spiral structure may generate the observed arm/interarm spiral polarization patterns. | the origin of the structure of large-scale magnetic fields in disc galaxies |
we present a new optical polarimetric catalog for the small magellanic cloud (smc). it contains a total of 7207 stars, located in the northeast (ne) and wing sections of the smc and part of the magellanic bridge. this new catalog is a significant improvement compared to previous polarimetric catalogs for the smc. we used it to study the sky-projected interstellar magnetic field structure of the smc. three trends were observed for the ordered magnetic field direction at position angles (pas) of (65° ± 10°), (115° ± 10°), and (150° ± 10°). our results suggest the existence of an ordered magnetic field aligned with the magellanic bridge direction and smc’s bar in the ne region, which have pas roughly at 115.°4 and 45°, respectively. however, the overall magnetic field structure is fairly complex. the trends at 115° and 150° may be correlated with the smc’s bimodal structure, observed in cepheids’ distances and hi velocities. we derived a value of {{b}sky}=(0.947+/- 0.079) μg for the ordered sky-projected magnetic field, and δ b=(1.465+/- 0.069) μg for the turbulent magnetic field. this estimate of {{b}sky} is significantly larger (by a factor of ∼10) than the line of sight field derived from faraday rotation observations, suggesting that most of the ordered field component is on the plane of the sky. a turbulent magnetic field stronger than the ordered field agrees with observed estimates for other irregular and spiral galaxies. for the smc the {{b}sky}/δ b ratio is closer to what is observed for our galaxy than other irregular dwarf galaxies. | a new optical polarization catalog for the small magellanic cloud: the magnetic field structure |
although it was found that the feh lines exist in the spectra of some stars, none of the spectral features in the interstellar medium (ism) have been assigned to this molecule. we suggest that iron atoms interact with hydrogen and produce fe-h nanoparticles which sometimes contain many h atoms. we calculate infrared spectra of hydrogenated iron nanoparticles using density functional theory methods and find broad, overlapping bands. desorption of h2 could induce spinning of these small fe-h dust grains. some of hydrogenated iron nanoparticles possess magnetic and electric moments and should interact with electromagnetic fields in the ism. fenhm nanoparticles could contribute to the polarization of the ism and the anomalous microwave emission. we discuss the conditions required to form feh and fenhm in the ism. | missing fe: hydrogenated iron nanoparticles |
we present 230 ghz continuum polarization observations with the atacama large millimeter/submillimeter array at a resolution of 0.″1 (~540 au) in the high-mass star-forming regions w51 e2 and e8. these observations resolve a network of core-connecting dust lanes, marking a departure from earlier coarser, more spherical continuum structures. at the same time, the cores do not appear to fragment further. polarized dust emission is clearly detected. the inferred magnetic field orientations are prevailingly parallel to dust lanes. this key structural feature is analyzed together with the local gravitational vector field. the direction of local gravity is found to typically align with dust lanes. with these findings, we derive a stability criterion that defines a maximum magnetic field strength that can be overcome by an observed magnetic field-gravity configuration. equivalently, this defines a minimum field strength that can stabilize dust lanes against a radial collapse. we find that the detected dust lanes in w51 e2 and e8 are stable, hence possibly making them a fundamental component in the accretion onto central sources, providing support for massive star formation models without the need of large accretion disks. when comparing to coarser resolutions, covering the scales of envelope, global, and local collapse, we find recurring similarities in the magnetic field structures and their corresponding gravitational vector fields. these self-similar structures point at a multiscale collapse-within-collapse scenario until finally the scale of core-accreting dust lanes is reached where gravity is entraining the magnetic field and aligning it with the dust lanes. | a multiscale picture of the magnetic field and gravity from a large-scale filamentary envelope to core-accreting dust lanes in the high-mass star-forming region w51 |
accurately measuring the magnetic field in the ism is essential for understanding star formation processes. we searched archival data of the submillimeter array for evidence of circular polarization in common molecular tracers, most notably co. this circular polarization possibly arises from anisotropic resonant scattering, which would imply that some background linearly polarized radiation is being converted to circular polarization. we find circular polarization in the star-forming regions ngc 7538 (in co) and orion kl (in co and sio) as well as in the carbon star irc+10216 (in cs, sis, h13cn, and co) at high enough levels to suggest that the presence of circular polarization in these spectral lines is common for such objects. this implies that measuring circular polarization is important when studying magnetic fields through the linear polarization of molecular spectral lines in the interstellar medium. we also provide a simple python wrapper for the miriad data reduction package. | non-zeeman circular polarization of molecular spectral lines in the ism |
we analyse the observational signatures of galactic magnetic fields that are self-consistently generated in magnetohydrodynamic simulations of the interstellar medium through turbulence driven by supernova (sn) explosions and differential rotation. in particular, we study the time evolution of the faraday rotation measure (rm), synchrotron radiation, and stokes parameters by characterizing the typical structures formed in the plane of observation. we do this by defining two distinct models for both thermal and cosmic ray (cr) electron distributions. our results indicate that the maps of rm have structures that are sheared and rendered anisotropically by differential rotation and that they depend on the choice of the thermal electron model as well as the sn rate. synchrotron maps are qualitatively similar to the maps of the mean magnetic field along the line of sight and structures are only marginally affected by the cr model. stokes parameters and related quantities, such as the degree of linear polarization, are highly dependent on both frequency and resolution of the observation. | observational signatures of galactic turbulent dynamos |
the intense line emission of oh masers is a perfect tracer of regions where new stars are born aswell as of evolved stars, shedding large amounts of processed matter into the interstellar medium. from ska deep surveys at 18 cm, where the maser lines from the ground-state of the oh molecule arise, we predict the discovery of more than 20000 sources of stellar and interstellar origin throughout the galaxy. the study of this maser emission has many applications, including the determination of magnetic field strengths from polarisation measurements, studies of stellar kinematics using the precisely determined radial velocities, and distance determinations from vlbi astrometry. a new opportunity to study shocked gas in different galactic environments is expected to arise with the detection of lower luminosity masers. for the first time, larger numbers of oh masers will be detected in local group galaxies. new insights are expected in structure formation in galaxies by comparing maser populations in galaxies of different metallicity, as both their properties as well as their numbers depend on it. with the full capabilities of ska, further maser transitions such as from excited oh and from methanol will be accessible, providing new tools to study the evolution of star-forming regions in particular. | oh masers in the milky way and local group galaxies in the ska era |
a high level electronic structure calculation of the born-oppenheimer (bo) potential, scalar-relativistic effect and adiabatic correction is performed for the x1σ+ ground state of astronomically important isotopologues of the arh+ cation. particular attention is paid to both short and long distances, where experimental spectroscopic data is not yet available. the bo energy is obtained over a wide range of interatomic distances (r ∈ [0.5, 20] å) in the framework of multi-reference coupled pair functional (mr-acpf) and single-reference coupled-cluster (ccsd(t)) methods. the all-electrons basis sets of the def2-nzvpp (n = 3 , 4) and nzapa-nr, aug-cc-pcvnz (n = 3 - 6) quality are used to monitor energy convergence. the bo energies are corrected for the basis set superposition error (bsse) via the counterpoise method and are reduced to the complete basis set (cbs) limit using alternative extrapolating schemes. the permanent dipole moment d(r) and static dipole polarizability α(r) are calculated using a finite-field method. the molecular structure parameters coincide with their empirical counterparts for low vibrational levels to within 0.1%. implementation of the ab initio results to circumscribe our understanding of the chemical networks in interstellar space is discussed. | an accurate ab initio electronic structure calculation for interstellar argonium |
sn 1993j is one of the best-studied type iib supernovae. spectropolarimetric data analyses were published over two decades ago at a time when the field of supernova spectropolarimetry was in its infancy. here, we present a new analysis of the spectropolarimetric data of sn 1993j and an improved estimate of its interstellar polarization (isp) as well as a critical review of isp removal techniques employed in the field. the polarization of sn 1993j is found to show significant alignment on the q - u plane, suggesting the presence of a dominant axis and therefore of continuum polarization. we also see strong line polarization features, including h β, he i λ5876, h α, he i λ6678, he i λ7065, and high velocity (hv) components of he i λ5876 and h α. sn 1993j is therefore the second example of a stripped-envelope supernova, alongside iptf13bvn, with prominent hv helium polarization features, and the first to show a likely hv h α contribution. overall, we determine that the observed features can be interpreted as the superposition of anisotropically distributed line forming regions over ellipsoidal ejecta. we cannot exclude the possibility of an off-axis energy source within the ejecta. these data demonstrate the rich structures that are inaccessible if solely considering the flux spectra but can be probed by spectropolarimetric observations. in future studies, the new isp corrected data can be used in conjunction with 3d radiative transfer models to better map the geometry of the ejecta of sn 1993j. | the shape of sn 1993j re-analysed |
how many bits of information can a single photon carry? intuition says "one", but this is incorrect. with an alphabet based on the photon's time of arrival, energy, and polarization, several bits can be encoded. in this introduction to photon information efficiency, we explain how to calculate the maximum number of bits per photon depending on the number of encoding modes, noise, and losses. | interstellar communication. v. introduction to photon information efficiency (in bits per photon) |
recent \herschel\ and \planck\ observations of submillimeter dust emission revealed the omnipresence of filamentary structures in the interstellar medium (ism). the ubiquity of filaments in quiescent clouds as well as in star-forming regions indicates that the formation of filamentary structures is a natural product of the physics at play in the magnatized turbulent cold ism. an analysis of more than 270 filaments observed with {\it herschel} in 8 regions of the gould belt, shows that interstellar filaments are characterized by a narrow distribution of central width, while they span a wide column density range. molecular line observations of a sample of these filaments show evidence of an increase in the velocity dispersion of dense filaments with column density, suggesting an evolution in mass per unit length due to accretion of surrounding material onto these star-forming filaments. the analyses of \planck\ dust polarization observations show that the mean magnetic field along the filaments is different from that of their surrounding clouds. this points to a coupling between the matter and the $\vec{b}$-field in the filament formation process. these observational results, derived from dust and gas tracers in total and polarized intensity, set strong constraints on theoretical models for filament formation and evolution. they also provide important hints on the initial conditions of the star formation process from the fragmentation of dense (supercritical) filaments. higher resolution dust polarization observations and large scale molecular line mapping are nevertheless required to investigate in more details the internal structure of interstellar filaments. | properties of interstellar filaments as derived from $herschel$, $planck$, and molecular line observations |
faraday tomography, the study of the distribution of extended polarized emission by strength of faraday rotation, is a powerful tool for studying magnetic fields in the interstellar medium of our galaxy and nearby galaxies. the strong frequency dependence of faraday rotation results in very different observational strengths and limitations for different frequency regimes. i discuss the role these effects take in faraday tomography below 1 ghz, emphasizing the 100–200 mhz band observed by the low frequency array and the murchison widefield array. with that theoretical context, i review recent faraday tomography results in this frequency regime, and discuss expectations for future observations. | the power of low frequencies: faraday tomography in the sub-ghz regime |
density functional theory calculations, using the b3lyp parameterisation, were performed to determine structures, vibrational frequencies, and binding energies for complexation of ni+ and ni2+ cations with benzene and naphthalene molecules and clusters. the calculations employed the stuttgart basis set with ecp pseudo potentials for the ni cations and basis sets of at least triple ζ plus polarisation, and diffuse quality for c and h. the effect of electron correlation on non-bonded interactions was accounted for by the grimme gd3 dispersion correction. counterpoise computations were made for bsse. comparison between experiment and theory provide fascinating new insight into the bonding for these prototypical organometallic (om) complexes. these structures have a sandwich topology, indicating major structural reorganisations occuring when benzene or naphthalene interact with ni cations. adiabatic electron affinities and ionisation potentials agree well with experiment when available. binding energies were also determined, providing insight into the stability of the complexes. the results presented here provide important information for future studies to address additional investigations of both problems of the electronic structure properties of these complexes, as well as the role of the polycyclic aromatic hydrocarbons (pahs) in the interstellar medium (ism) and soot formation in combustion. the ni+/ni2+ + aromatic organometallic bonding is of the same order of stability as an aromatic c-h bond. such bonding modifies the ir spectrum of the complexed aromatic molecules by enhancing the 3.3 μm feature and decreasing the c-h bands in the 11-12 μm range (γ c-h). organometallic complexation reactions may contribute significantly to metal depletion in the ism. | structures and binding energies for complexations of different spin states of ni+ and ni2+ to aromatic molecules |
asymptotic giant branch (agb) stars are, together with supernovae, the main contributors of stellar dust to the interstellar medium (ism). dust grains formed by agb stars are thought to be large. however, as dust nucleation and growth within their outflows are still not understood, the dust-grain size distribution (gsd) is unknown. this is an important uncertainty regarding our knowledge of the chemical and physical history of interstellar dust, as agb dust forms ${\sim} 70{{\ \rm per\ cent}}$ of the starting point of its evolution. we expand on our chemical kinetics model, which uniquely includes a comprehensive dust-gas chemistry. the gsd is now allowed to deviate from the commonly assumed canonical mathis, rumpl & nordsieck distribution. we find that the specific gsd can significantly influence the dust-gas chemistry within the outflow. our results show that the level of depletion of gas-phase species depends on the average grain surface area of the gsd. gas-phase abundance profiles and their possible depletions can be retrieved from observations of molecular emission lines when using a range of transitions. because of degeneracies within the prescription of gsd, specific parameters cannot be retrieved, only (a lower limit to) the average grain surface area. none the less, this can discriminate between dust composed of predominantly large or small grains. we show that when combined with other observables such as the spectral energy distribution and polarized light, depletion levels from molecular gas-phase abundance profiles can constrain the elusive gsd of the dust delivered to the ism by agb outflows. | chemical modelling of dust-gas chemistry within agb outflows - ii. effect of the dust-grain size distribution |
in this work we performed a spectral energy distribution (sed) analysis in the optical/infrared band of the host galaxy of a proto-brightest bluster galaxy (bcg, nvss j103023 + 052426) in a proto-cluster at z = 1.7. we found that it features a vigorous star formation rate (sfr) of ∼570 m⊙/yr and a stellar mass of m*∼3.7×1011 m⊙; the high corresponding specific sfr = 1.5±0.5gyr‑1 classifies this object as a starburst galaxy that will deplete its molecular gas reservoir in ∼3.5×108 yr. thus, this system represents a rare example of a proto-bcg caught during the short phase of its major stellar mass assembly. moreover, we investigated the nature of the host galaxy emission at 3.3 mm. we found that it originates from the cold dust in the interstellar medium, even though a minor non-thermal agn contribution cannot be completely ruled out. finally, we studied the polarized emission of the lobes at 1.4 ghz. we unveiled a patchy structure where the polarization fraction increases in the regions in which the total intensity shows a bending morphology; in addition, the magnetic field orientation follows the direction of the bendings. we interpret these features as possible indications of an interaction with the intracluster medium. this strengthens the hypothesis of positive agn feedback, as inferred in previous studies of this object on the basis of x-ray/mm/radio analysis. in this scenario, the proto-bcg heats the surrounding medium and possibly enhances the sfr in nearby galaxies. | multi-wavelength study of a proto-bcg at z = 1.7 |
we present bvri polarimetric measurements of 23 stars in the direction of the open cluster ngc 6709. fifteen stars display wavelength-dependence of polarization which is typically due to selective extinction in the interstellar medium and could be fitted to the serkwoski equation. based on these stars, we obtained an average maximum degree of polarization of 0.722 ± 0.022% and an average wavelength at a maximum polarization of 0.615 ± 0.011 μm. the observed low polarization may be due to the alignment of grains along a more longitudinally-directed magnetic field in this region which would cause the aspect ratio of the grains to be reduced. this would have the observational consequence of decreased polarization as well as shifting ${\lambda }_{\max }$ toward longer wavelengths. using the expression proposed by whittet & van breda we calculated the ratio of total-to-selective extinction as ${r}_{v}=(5.6\pm 0.3){\lambda }_{\max }=3.44\pm 0.06$ , higher than the nominal value of 3.1 for the general ism. the remaining eight sample stars display an unusual wavelength-dependence that is similar to some be stars. however, there are only two be stars currently listed in this cluster, yet, based on our data, they both present reliable fits within the serkowski equation. the overall alignment of the v-band polarization vectors implies polarization by selective extinction due to foreground dust. using the planck 353 ghz channel we find there is an agreement between polarization by emission and selective extinction. since the optical polarization traces a magnetic field, this suggests that the magnetic field in this region is inclined to the galactic plane, with an overall average galactic position angle of 104 ± 12 (st. dev.) degrees. | interstellar extinction and polarization of stars in the open cluster ngc 6709 |
models of interstellar dust alignment assume that dust grains are elongated, but none of these models explain why dust grains should be elongated. on the other hand, models of interstellar dust grain growth assume that dust grains are spherical and not elongated. we show that when dusty plasma effects and the dipole moment of water molecules are together taken into account, ice grains in interstellar space should be prolate ellipsoids and not spheres. dusty plasma analysis shows that an ice grain is charged to a negative potential that has magnitude nearly equal to that of the electron temperature. several different mechanisms causing deviation from sphericity are identified; these mechanisms involve the interaction of the dipole moment of water molecules with electric fields associated with ice grain charging. these mechanisms include the focusing of water molecule trajectories, the migration of water molecules in a quasi-liquid layer on the grain surface toward regions where the electric field is strongest, the enhancement of this migration by the bombardment of energetic protons that gain energy upon falling into the ice grain negative potential, and mutual repulsion by electric charges having the same sign. the aspect ratio is established shortly after the ice grain is formed, and then is maintained as the grain grows. | why interstellar ice dust grains should be elongated |
rotational inelastic scattering of silyl cyanide (sih3cn) molecule with helium (he) atoms is investigated. three-dimensional potential energy surface (3d-pes) for the sih3cn-he interacting system is carried out. the ab initio 3d-pes is computed using explicitly correlated coupled cluster approach with single, double, and perturbative triple excitation ccsd(t)-f12a connected to augmented-correlation consistent-polarized valence triple zeta gaussian basis set. a global minimum at (r = 6.35 bohr; θ = 90○; ϕ = 60○) with a well depth of 52.99 cm-1 is pointed out. inelastic rotational cross-sections are emphasized for the 22 first rotational levels for total energy up to 500 cm-1 via close coupling (cc) approach in the case of a-sih3cn and for the 24 first rotational levels for total energy up to 100 cm-1 via cc and from 100 to 500 cm-1 via coupled states (cs) in the case of e-sih3cn. rate coefficients are derived for temperature until 80 k for both a- and e-sih3cn-he systems. propensity rules are obtained for |δj| = 2 processes with broken parity for a-sih3cn and for |δj| = 2 processes with |δk| = 0 and unbroken parity for e-sih3cn. | inelastic scattering of interstellar silyl cyanide (sih3cn) by helium atoms : cross-sections and rate coefficients for a- and e-sih3cn |
high precision polarization measurements, such as those from the planck satellite, open new opportunities for the study of the magnetic field structure as traced by polarimetric measurements of the interstellar dust emission. the polarization parameters suffer from bias in the presence of measurement noise. it is critical to take into account all the information available in the data in order to accurately derive these parameters. in our previous work, we studied the bias on polarization fraction and angle, various estimators of these quantities, and their associated uncertainties. the goal of this paper is to characterize the bias on the polarization angle dispersion function that is used to study the spatial coherence of the polarization angle. we characterize for the first time the bias on the conventional estimator of the polarization angle dispersion function and show that it can be positive or negative depending on the true value. monte carlo simulations were performed to explore the impact of the noise properties of the polarization data, as well as the impact of the distribution of the true polarization angles on the bias. we show that in the case where the ellipticity of the noise in (q,u) varies by less than 10%, one can use simplified, diagonal approximation of the noise covariance matrix. in other cases, the shape of the noise covariance matrix should be taken into account in the estimation of the polarization angle dispersion function. we also study new estimators such as the dichotomic and the polynomial estimators. though the dichotomic estimator cannot be directly used to estimate the polarization angle dispersion function, we show that, on the one hand, it can serve as an indicator of the accuracy of the conventional estimator and, on the other hand, it can be used for deriving the polynomial estimator. we propose a method for determining the upper limit of the bias on the conventional estimator of the polarization angle dispersion function. the method is applicable to any linear polarization data set for which the noise covariance matrices are known. | polarization measurement analysis. iii. analysis of the polarization angle dispersion function with high precision polarization data |
the alignment of interstellar dust grains with magnetic fields provides a key method for measuring the strength and morphology of the fields. in turn, this provides a means to study the role of magnetic fields from diffuse gas to dense star-forming regions. the physical mechanism for aligning the grains has been a long-term subject of study and debate. the theory of radiative torques, in which an anisotropic radiation field imparts sufficient torques to align the grains while simultaneously spinning them to high rotational velocities, has passed a number of observational tests. here we use archival polarization data in dense regions of the orion molecular cloud (omc-1) at 100, 350, and 850 μm to test the prediction that the alignment efficiency is dependent upon the relative orientations of the magnetic field and radiation anisotropy. we find that the expected polarization signal, with a 180-degree period, exists at all wavelengths out to radii of 1.5 arcmin centered on the becklin-neugebauer kleinmann-low (bnkl) object in omc-1. the probabilities that these signals would occur due to random noise are low (≲1%), and are lowest toward bnkl compared to the rest of the cloud. additionally, the relative magnetic field to radiation anisotropy directions accord with theoretical predictions in that they agree to better than 15° at 100 μm and 4° at 350 μm. | a far-infrared observational test of the directional dependence in radiative grain alignment |
monitoring thermal processes occurring in molecular films on surfaces can provide insights into physical events such as morphology changes and phase transitions. here, we demonstrate that temperature-programmed contact potential difference (tp-δcpd) measurements employed by a kelvin probe under ultrahigh vacuum conditions and their temperature derivative can track films' restructure and crystallization occurring in amorphous solid water (asw) at temperatures well below the onset of film desorption. the effects of growth temperature and films' thickness on the spontaneous polarization that develops within asw films grown at 33 k-120 k on top of a ru(0001) substrate are reported. electric fields of ∼106 v/m are developed within the asw films despite low average levels of molecular dipole alignment (<0.01%) normal to the substrate plane. upon annealing, an irreversible morphology-dependent depolarization has been recorded, indicating that the asw films keep a "memory" of their thermal history. we demonstrate that tp-δcpd measurements can track the collapse of the porous structure at temperatures above the growth and the asw-ice ic and asw-ice ih transitions at 131 k and 157 k, respectively. these observations have interesting implications for physical and chemical processes that take place at the interstellar medium such as planetary formation and photon- and electron-induced synthesis of new molecules. | the role of thermal history on spontaneous polarization and phase transitions of amorphous solid water films studied by contact potential difference measurements |
polican is a near-infrared imaging linear polarimeter developed for the cananea near-infrared camera (canica) at the 2.1 m telescope of the guillermo haro astrophysical observatory (oagh) located in cananea, sonora, méxico. polican is mounted ahead of canica and consist of a rotating super-achromatic (1-2.7 μm) half-wave plate (hwp) as the modulator and a fixed wire-grid polarizer as the analyzer. canica has a 1024 × 1024 hgcdte detector with a plate scale of 0.32 arcsec/pixel and provides a field of view of 5.5 × 5.5 arcmin2. the polarimetric observations are carried out by modulating the incoming light through different steps of half-wave plate angles (0°, 22.°5, 45°, 67.°5) to establish linear stokes parameters (i, q, and u). image reduction consists of dark subtraction, polarimetric flat fielding, and sky subtraction. the astrometry and photometric calibrations are performed using the publicly available data from the two micron all sky survey. polarimetric calibration includes observations of globular clusters and polarization standards available in the literature. analysis of multiple observations of globular clusters yielded an instrumental polarization of 0.51%. uncertainties in polarization range from 0.1% to 10% from the brightest 7 mag to faintest 16 mag stars. the polarimetric accuracy achieved is better than 0.5% and the position angle errors less than 5° for stars brighter than 13 mag in h-band. polican is mainly being used to study the scattered polarization and magnetic fields in and around star-forming regions of the interstellar medium. | polican: a near-infrared imaging polarimeter at the 2.1m oagh telescope |
considering the spatially separated polarization radiation and faraday rotation regions to simulate complex interstellar media, we study synchrotron polarization gradient techniques' measurement capabilities. we explore how to trace the direction of projected magnetic field of emitting-source region at the multifrequency bands, using the gradient technique compared with the traditional polarization vector method. furthermore, we study how faraday rotation density in the foreground region, i.e. a product of electron number density and parallel component of magnetic fields along the line of sight, affects the measurement of projected magnetic field. numerical results show that synchrotron polarization gradient technique could successfully trace projected magnetic field within emitting-source region independent of radio frequency. accordingly, the gradient technique can measure the magnetic field properties for a complex astrophysical environment. | gradient measurement of synchrotron polarization diagnostic: application to spatially separated emission and faraday rotation regions |
filaments are considered to be basic structures, and molecular clouds consist of filaments. filaments are often observed as extending in the direction perpendicular to the interstellar magnetic field. the structure of filaments has been studied based on a magnetohydrostatic equilibrium model. here we simulate the expected polarization pattern for isothermal magnetohydrostatic filaments. the filament exhibits a polarization pattern in which the magnetic field is apparently perpendicular to the filament when observed from the direction perpendicular to the magnetic field. when the line of sight is parallel to the global magnetic field, the observed polarization pattern is dependent on the center-to-surface density ratio for the filament and the concentration of the gas mass toward the central magnetic flux tube. filaments with low center-to-surface density ratios have an insignificant degree of polarization when observed from the direction parallel to the global magnetic field. however, models with a large center-to-surface density ratio have polarization patterns that indicate that the filament is perpendicularly threaded by the magnetic field. when mass is heavily concentrated at the central magnetic flux tube, which can be realized by the ambipolar diffusion process, the polarization pattern is similar to that expected for a low center-to-surface density contrast. | polarization structure of filamentary clouds |
we present photometry, spectra, and spectropolarimetry of supernova (sn) 2014ab, obtained through ~200 d after peak brightness. sn 2014ab was a luminous type iin sn (mv < -19.14 mag) discovered after peak brightness near the nucleus of its host galaxy, vv 306c. pre-discovery upper limits constrain the time of explosion to within 200 d prior to discovery. while sn 2014ab declined by ~1 mag over the course of our observations, the observed spectrum remained remarkably unchanged. spectra exhibit an asymmetric emission-line profile with a consistently stronger blueshifted component, suggesting the presence of dust or a lack of symmetry between the far side and near side of the sn. the pa β emission line shows a profile very similar to that of h α, implying that this stronger blueshifted component is caused either through obscuration by large dust grains, occultation by optically thick material, or a lack of symmetry between the far side and near side of the interaction region. despite these asymmetric line profiles, our spectropolarimetric data show that sn 2014ab has little detected polarization after accounting for the interstellar polarization. we are likely seeing emission from a photosphere that has only small deviation from circular symmetry in the plane normal to our line of sight, but with either large-grain dust or significant asymmetry in the density of circumstellar material or sn ejecta along our line of sight. we suggest that sn 2014ab and sn 2010jl (as well as other sne iin) may be events with similar geometry viewed from different directions. | sn 2014ab: an aspherical type iin supernova with low polarization |
faraday rotation measures (rms) should be interpreted with caution because there could be multiple magneto-ionized medium components that contribute to the net faraday rotation along sightlines. we introduce a simple test using galactic diffuse polarized emission that evaluates whether structures evident in rm observations are associated with distant circumgalactic medium or foreground interstellar medium. we focus on the magellanic leading arm region where a clear excess of rm was previously reported. there are two gaseous objects standing out in this direction: the distant magellanic leading arm and the nearby antlia supernova remnant (snr). we recognized narrow depolarized filaments in the $2.3\, \rm ghz$ s-band polarization all sky survey image that overlaps with the reported rm excess. we suggest that there is a steep gradient in faraday rotation in a foreground screen arising from the antlia snr. the estimated strength of the line-of-sight component of the magnetic field is $b_{\parallel }\sim 5\, \rm \mu g$, assuming that the excess of rm is entirely an outcome of the magnetized supernova shell. our analysis indicates that the overlap between the rm excess and the magellanic leading arm is only a remarkable coincidence. we suggest for future rm grid studies that checking galactic diffuse polarization maps is a convenient way to identify local faraday screens. | distant probes of rotation measure structure: where is the faraday rotation towards the magellanic leading arm? |
complete studies of the radiative processes of thermal emission from the amorphous dust from microwave through far-infrared wavebands are presented by taking into account, self-consistently for the first time, the standard two-level systems (tls) model of amorphous materials. the observed spectral energy distributions (seds) for the perseus molecular cloud (mc) and w 43 from microwave through far-infrared are fitted with the seds calculated with the tls model of amorphous silicate. we have found that the model seds reproduce the observed properties of the anomalous microwave emission (ame) well. the present result suggests an alternative interpretation for the ame being carried by the resonance emission of the tls of amorphous materials without introducing new species. simultaneous fitting of the intensity and polarization seds for the perseus mc and w 43 are also performed. the amorphous model reproduces the overall observed feature of the intensity and polarization seds of the perseus mc and w 43. however, the model's predicted polarization fraction of the ame is slightly higher than the quijote upper limits in several frequency bands. a possible improvement of our model to resolve this problem is proposed. our model predicts that interstellar dust is amorphous materials with very different physical characteristics compared with terrestrial amorphous materials. | thermal emission from the amorphous dust: an alternative possibility of the origin of the anomalous microwave emission |
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