Imaging Polarimetry of Comets 78P/Gehrels, 22P/Kopff and asteroid 5 Astraea
Résumé
The Jupiter family comets 78P/Gehrels, 22P/Kopff and the large S-type main-belt asteroid 5 Astraea were observed in different observing runs during 2009 to 2012 by Indian and French astronomers in joint campaigns. The observations were taken from the 2m IUCAA Girawali Observatory (India) and from the 0.80 m OHP telescope in France. They were mostly made in polarimetric mode with a CCD as image detector. During the observations the phase angle of 78P/Gehrels lied between 14°-15° at IGO and 28° at OHP, and that of 22P/Kopff lied between 38°-39° at IGO. Phase angle of 5 Astraea decreased from 12° to 11° at IGO. In case of comets, a broadband red filter (R, λ=630 nm, Δλ=120 nm at IGO; and λ=670 nm, Δλ=90 nm at OHP) along with a narrowband Comet-Red (CR, λ=684 nm, Δλ=9 nm at IGO) filter to avoid gaseous contaminations were used during the observations. At IGO, the asteroid 5 Astraea was observed through the following different filters: B=420 nm, ΔλB=100; V=520 nm, ΔλV=90; R=630 nm, ΔλR=120 nm; I=850 nm, ΔλI=300 nm. The linear polarization computation from the light scattered by the dust particles allowed us to suggest some physical properties for the cometary and asteroidal particles. The observed polarizations of both comets and asteroid are compared to the respective classical polarimetric phase curves at the phase angles and wavelength of the observations and that of comet 22P/Kopff to its previous observations. The polarization values of comets are also compared to those obtained previously for other Jupiter Family comets. Along with the polarization measurements of different regions of cometary coma, different enhancement techniques have been used to emphasize different dust regions in the cometary coma.