Polarimetric observations of comet Levy 1990c and of other comets: Some clues to the evolution of cometary dust

The evolution with the phase angle alpha of the polarization degree P of light scattered by comet Halley's dust is well documented. No significant discrepancy is found between Halley and Levy polarization curves near the inversion point. From all available cometary observations, we have derived polarimetric synthetic curves. Typically, a set of about 200 data points in the red wavelengths range exhibits a minimum for (alpha approximately equals 10.3 degrees, P approximately equals 1.8 percent) and an inversion point for (alpha approximately equals 22.4 degrees, P = 0 percent), with a slop of about 0.27 percent per degree. A significant spreading of some data (comets Austin 1982VI, Austin 1989c1, West 1976VI) is found at large phase angles. The analysis of our polarimetric maps of Levy reveals that the inner coma is heterogeneous. The increase of the inversion angle value with increasing distance from the photometric center is suspected to be due to the evolution with time of grains ejected from the nucleus. A fan like structure could be produced by a jet of grains freshly ejected

Scattering of light by a large, densely packed agglomerate of small silica spheres

We model the measured phase function and degree of linear polarization of a macroscopic agglomerate made of micrometer-scale silica spheres using the methodology of multiple scattering. In the laboratory work, the agglomerate is produced ballistically, characterized by scanning electron microscopy, and measured with the PROGRA(2) instrument to obtain the light scattering properties. The model phase function and degree of polarization are in satisfactory agreement with the experimental data. To our best knowledge, this is the first time the degree of linear polarization has been modeled well for a large, densely packed agglomerate composed of small particles with known sizes and shapes. The study emphasizes the relevance of the degree of linear polarization and gives insights into the effects of particle aggregation on the scattering characteristics. (C) 2020 Optical Society of AmericaPeer reviewe

Evidence of Fragmenting Dust Particles from Near-Simultaneous Optical and Near-IR Photometry and Polarimetry of Comet 73P/Schwassmann-Wachmann 3

We report imaging polarimetry of segments B and C of the Jupiter-family Comet 73P/Schwassmann-Wachmann 3 in the I and H bandpasses at solar phase angles of approximately 35 and 85deg. The level of polarization was typical for active comets, but larger than expected for a Jupiter-family comet. The polarimetric color was slightly red (dP/dL = +1.2 +/- 0.4) at a phase angle of ~ 35deg and either neutral or slightly blue at a phase angle of ~ 85deg. Observations during the closest approach from 2006 May 11-13 achieved a resolution of 35 km at the nucleus. Both segments clearly depart from a 1/rho surface brightness for the first 50 - 200 km from the nucleus. Simulations of radiation driven dust dynamics can reproduce some of the observed coma morphology, but only with a wide distribution of initial dust velocities (at least a factor of 10) for a given grain radius. Grain aggregate breakup and fragmentation are able to reproduce the observed profile perpendicular to the Sun-Comet axis, but fit the observations less well along this axis (into the tail). The required fragmentation is significant, with a reduction in the mean grain aggregate size by about a factor of 10. A combination of the two processes could possibly explain the surface brightness profile of the comet.Comment: 40 pages including 11 figure

A grid of polarization models for Rayleigh scattering planetary atmospheres

We investigate the intensity and polarization of reflected light from planetary atmospheres. We present a large grid of Monte Carlo simulations for planets with Rayleigh scattering atmospheres. We discuss the disk-integrated polarization for phase angles typical of extrasolar planet observations and for the limb polarization effect observable for solar system objects near opposition. The main parameters investigated are single scattering albedo, optical depth of the scattering layer, and albedo of an underlying Lambert surface for a homogeneous Rayleigh scattering atmosphere. We also investigate atmospheres with isotropic scattering and forward scattering aerosol particles, as well as models with two scattering layers. The model grid provides a tool for extracting quantitative results from polarimetric measurements of planetary atmospheres from solar system planets and extrasolar planets, in particular on the scattering properties and stratification of particles in the highest atmosphere layers. Spectropolarimetry of solar system planets offers complementary information to spectroscopy and polarization flux colors can be used for a first characterization of exoplanet atmospheres. From limb polarization measurements, one can set constraints on the polarization at large phase angles.Comment: 19 pages, 21 figures. Minor changes. Published in Astronomy and Astrophysic

Use of the linear absorption coefficient for absolute comparison of plasma films in the mid-IR range

In this work, we present the mid-infrared analysis of analogues of Titan's aerosols produced in a radio frequency capacitively coupled plasma (RF-CCP). The influence of the gas mixture on aerosols spectra is also studied through the analysis of the carbonaceous bands of the spectra, and its Gaussian deconvolution.Представлен анализ в среднем инфракрасном диапазоне аналогов титановых аэрозолей, которые производятся в радиочастотных источниках плазмы с емкостной связью (РЧ-ЕСС). Также изучено влияние газовой смеси на спектры аэрозолей с помощью анализа углеродсодержащих полос спектра и ее гауссовой деконволюции (обратная свертка).Представлено аналіз у середньому інфрачервоному діапазоні аналогів титанових аерозолів, які проводяться в радіочастотних джерелах плазми з ємнісним зв'язком (РЧ-ЕСС). Також вивчено вплив газової суміші на спектри аерозолів за допомогою аналізу вуглецевмісних смуг спектра і її гауссової деконволюції (зворотня згортка)

Use of the linear absorption coefficient for absolute comparison of plasma films in the mid-IR range

In this work, we present the mid-infrared analysis of analogues of Titan's aerosols produced in a radio frequency capacitively coupled plasma (RF-CCP). The influence of the gas mixture on aerosols spectra is also studied through the analysis of the carbonaceous bands of the spectra, and its Gaussian deconvolution.Представлен анализ в среднем инфракрасном диапазоне аналогов титановых аэрозолей, которые производятся в радиочастотных источниках плазмы с емкостной связью (РЧ-ЕСС). Также изучено влияние газовой смеси на спектры аэрозолей с помощью анализа углеродсодержащих полос спектра и ее гауссовой деконволюции (обратная свертка).Представлено аналіз у середньому інфрачервоному діапазоні аналогів титанових аерозолів, які проводяться в радіочастотних джерелах плазми з ємнісним зв'язком (РЧ-ЕСС). Також вивчено вплив газової суміші на спектри аерозолів за допомогою аналізу вуглецевмісних смуг спектра і її гауссової деконволюції (зворотня згортка)

The polarisation properties of the HD 181327 debris ring. Evidence for sub-micron particles from scattered light observations

This is the final version. Available on open access from EDP Sciences via the DOI in this recordContext. Polarisation is a powerful remote-sensing tool to study the nature of particles scattering the starlight. It is widely used to characterise interplanetary dust particles in the Solar System and increasingly employed to investigate extrasolar dust in debris discs’ systems. Aims. We aim to measure the scattering properties of the dust from the debris ring around HD 181327 at near-infrared wavelengths. Methods. We obtained high-contrast polarimetric images of HD 181327 in the H band with the SPHERE/IRDIS instrument on the Very Large Telescope (ESO). We complemented them with archival data from HST/NICMOS in the F 110W filter reprocessed in the context of the Archival Legacy Investigations of Circumstellar Environments (ALICE) project. We developed a combined forward-modelling framework to simultaneously retrieve the scattering phase function in polarisation and intensity. Results. We detected the debris disc around HD 181327 in polarised light and total intensity. We measured the scattering phase function and the degree of linear polarisation of the dust at 1.6 µm in the birth ring. The maximum polarisation is 23.6% ± 2.6% and occurs between a scattering angle of 70° and 82°. Conclusions. We show that compact spherical particles made of a highly refractive and relatively absorbing material in a differential power-law size distribution of exponent −3.5 can simultaneously reproduce the polarimetric and total intensity scattering properties of the dust. This type of material cannot be obtained with a mixture of silicates, amorphous carbon, water ice, and porosity, and requires a more refracting component such as iron-bearing minerals. We reveal a striking analogy between the near-infrared polarisation of comets and that of HD 181327. The methodology developed here combining VLT/SPHERE and HST/NICMOS may be applicable in the future to combine the polarimetric capabilities of SPHERE with the sensitivity of JWST