A study of comet 78P/Gehrels by imaging polarimetry

•First polarimetric observations of comet 78P/Gehrels during 2011-2012 apparitions.•Observations at 3 phase angles smaller than 30° before and after perihelion.•Slightly more negative polarization in the inner region at 15° phase angle.•Increase of activity just after perihelion observed sunward on polarization images.•Increase of activity as compared to 2004 apparition.

Comet 78P/Gehrels was observed polarimetrically from the 2 m telescope at IUCAA Girawali Observatory (IGO) in India and from the 0.8 m telescope at Haute-Provence Observatory (OHP) in France. The observations were carried out in three schedules - first in October 2011 (IGO), then in January 2012 (OHP) and lastly in February 2012 (IGO), when its phase angle was between 15.2° and 28.3°. In our knowledge, these are the first polarimetric measurements of the dust properties of Gehrels. Both broadband and narrow band cometary filters (to avoid gaseous contaminations) were used. Intensity images treated by a rotational gradient method, along with isophotes confirmed the presence of structures in the comet during the observation periods. In the polarization map, no significant difference in polarization values is noticed between the structures along the anti-solar direction and the surrounding coma, which suggests similar dust properties. After perihelion, in January, some jet activity was observed along the solar direction with higher positive polarization than the surrounding coma. The ejected particles seem to be relatively large and present with a low number density. The coma polarization measured in different apertures has been used to compare that obtained for other comets at similar phase angles. As expected, 78P/Gehrels is different from comet Hale-Bopp, but the comparison is not easy at phase angles lower than 35°.