Anny-Chantal Levasseur-Regourd

Polarimetric observations of comet 67P/Churyumov-Gerasimenko during its 2008–2009 apparition

Remote observations of the light scattered by comet 67P/Churyumov-Gerasimenko dust coma are of major importance for determining the physical properties of the particles and prepare the rendezvous with the ESA/Rosetta spacecraft in 2014. Aims. Light scattering and especially linear polarization observations allow comparison between different coma regions and different comets, including comets that have been studied by space probes. Our aim is to retrieve physical properties of the dust particles and to characterize their evolution around perihelion passage. Methods. Recent imaging polarimetric observations were conducted at the Haute-Provence observatory (France) on 2009 March 17-19 at 35° phase angle and at IUCAA Girawali observatory (India) on 2008 December 25-27 at 36° phase angle and on 2009 April 30-May 1 at 29° phase angle. With the imaging technique, the intensity and linear polarization variations are studied through the various coma regions. These observations are compared to other cometary data (e.g. Jupiter family comets) and to numerical and experimental simulations. Results. The decrease in intensity as a function of the distance to nucleus in log-log scale is on average close to -1, although important variations with values down to -1.5 are noticed, in agreement with previous observations in 1982-83 and 1995-96. The intensity along the tailward direction decreases with a slope between -1.2 two months before perihelion (2009 February 28) to -1.0 two months after perihelion, and the decrease is more pronounced in the sunward direction. Before perihelion, aperture polarization values are comparable to polarization values measured on other comets at similar phase angles. The sharp decrease in intensity and the feature in the tailward direction, without any difference in polarization in the coma before perihelion, could suggest the presence of large dark particles. The post-perihelion increase in intensity and in polarization suggests that an outburst has occurred. The freshly ejected dust polarizes more the scattered light and is more sensitive to the solar radiation pressure, suggesting small micron- or submicron-sized grains. Conclusions. Polarization and intensity variations in the coma of 67P/Churyumov-Gerasimenko are reminiscent of those noticed for some comets such as comet 81P/Wild 2 and comet 9P/Tempel 1. The presence of rather large particles can thus be suggested before and just after perihelion and the ejection of post-perihelion smaller grains, eventually in fluffy aggregates. A strong seasonal effect related to the obliquity of the comet suggests that the different grains originate in different hemispheres of the nucleus.

Inferring the interplanetary dust properties - from remote observations and simulations

Since in situ studies and interplanetary dust collections only provide a spatially limited amount of information about the interplanetary dust properties, it is of major importance to complete these studies with properties inferred from remote observations of light scattered and emitted, with interpretation through simulations. Physical properties of the interplanetary dust in the near-ecliptic symmetry surface, such as the local polarization, temperature and composition, together with their heliocentric variations, may be derived from scattered and emitted light observations, giving clues to the respective contribution of the particles sources. A model of light scattering by a cloud of solid particles constituted by spheroidal grains and aggregates thereof is used to interpret the local light scattering data. Equilibrium temperature of the same particles allows us to interpret the temperature heliocentric variations. A good fit of the local polarization phase curve,

Cosmic dust optical properties: Numerical simulations and future laboratory measurements in microgravity

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Organics in cometary and interplanetary dust

, near 1.5~AU from the Sun is obtained for a mixture of silicates and more absorbing organics material (

SIMILARITIES BETWEEN IN SITU MEASUREMENTS OF LOCAL DUST LIGHT SCATTERING AND DUST FLUX IMPACT DATA WITHIN THE COMA OF 1P/HALLEY

approx

Cometary Dust Unveiled

40 % in mass) and for a realistic size distribution typical of the interplanetary dust in the 0.2 to 200 micrometre size range. The contribution of dust particles of cometary origin is at least 20% in mass. The same size distribution of particles gives a solar distance,

CONSERT constrains the internal structure of 67P at a few metres size scale

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Polarimetry of Stars and Planetary Systems: Interplanetary dust

, dependence of the temperature in

An international network of observations for the International Year of Astronomy An international network of observations for the International Year of Astronomy

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Modélisation de noyaux cométaires pour la préparation scientifique de la mission spatiale Rosetta

different than the typical black body behavior. The heliocentric dependence of