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Accueil > Séminaires > Archive des séminaires d’Utinam > Archive des séminaires d’astrophysique (jusqu’en 2011) > 2008

Polarization and Photometric Studies of comets from Mt. Abu Observatory

par Edith Burgey -

Jeudi 19 juin 2008

U.C. Joshi

Physical Research Laboratory, Ahmedabad (Inde)

Résumé :

Comets are the frozen reservoirs of ices, gases, and dust grains
that existed in the outer early solar nebula at the time of the
formation of small bodies, hence their study is very useful in our understanding of the origin of solar system. Amongst the three major constituents — ices, gases, and dust grains — the dust
grains are the most stable under solar illumination. During
perihelion passage, the dust grains are warmed to radiative
equilibrium temperatures of a few hundred degree Kelvin. At these
temperatures, the dust grains are chemically and, in the case of
the refractory silicates, morphologically stable. Thus, the dust
grains in the coma directly represent the dust grains either
entrained with the gas released directly from the nuclear surface
or entrained with the gas ejected from deeper layers of the
nucleus via jets. By determining the properties of grains in
cometary comae, we directly assess the origin of dust grains
incorporated into the small, icy bodies of the solar system.

Sun light is scattered by the cometary grains and in this process
it gets polarized. The degree of polarization mainly depends on
the size distribution of the dust particles, their chemical
composition which can be grossly represented in terms of
refractive index, wavelength of the light and the phase angle.

Recently a comet (comet 17/P Holmes) suddenly brightened from 17th
magnitude to 3rd magnitude. This phenomenon has astonished the
astronomical community. We conducted polarization and photometric
studies of the comet 17/P Holmes in visual bands. During our
observing run (November 5-7, 2007) the phase angle of the comet
was 13 deg and observations on comets at such small phase angle
are rare. Some of the results on comet 17/PHolmes and other
comets observed from Mt. Abu Observatory will be discussed.