Professor Ranjan Gupta

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Start date: 2000Subject: search.filters.subject.Polarimetric

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    Recent polarimetric observations of comet 67 P/Churyumov-Gerasimenko
    (Icy Bodies of the Solar System Proceedings IAU Symposium, 2009-11-24) Anny-Chantal Levasseur-Regourd1; Hadamcik, Edith; Sen, A.K.; et.al
    Remote observations of solar light scattered by dust in comet 67P/Churyumov- Gerasimenko coma are of major importance to assess the properties of the dust and thus to prepare the rendezvous of the Rosetta spacecraft with comet 67P/Churyumov-Gerasimenko. We present polarimetric data obtained from India in December 2008 and France in March 2009. Compared with previous observations of this comet and of other Jupiter family comets, they confirm that it is dust-poor, although it may exhibit outbursts leading to the ejection of dust particles from its subsurface, especially after its perihelion passage.
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    Polarimetric observations of comet 67P/Churyumov-Gerasimenko during its 2008–2009 apparition
    (Astronomy & Astrophysics, 2010-04-12) Hadamcik, E; Sen, A.K.; Levasseur-Regourd, A. C
    Context. 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.
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    Imaging polarimetry of some selected dark clouds?
    (Astron. Astrophys. Suppl. Ser., 2000-01-11) Sen, A.K.; Gupta, Ranjan; Ramaprakash, A.N; et al.
    A set of eight Bok Globules CB3, CB25, CB39, CB52, CB54, CB58, CB62 and CB246 were observed polarimetrically in white light, using our Imaging Polarimeter (IMPOL), from the 1.2 m IR telescope at Mount Abu, India. The observations were carried out on di erent nights during the period December 1997 and April 1998. The CCD images obtained from the instrument (IMPOL) were analyzed, to produce polarization map of the Bok Globules. The stars in the eld, which are mostly background to the cloud show typically 2% of linear polarization. Clouds which are less dynamic (having 12CO line widths V < 2:5 km s−1), in general show slightly better alignment of polarization vectors with the projected direction of galactic plane. On the other hand the more dynamic group of clouds, has the polarization vectors more scattered and poorly aligned with the projected direction of the galactic plane. However one of the clouds observed CB58 does not follow this trend very well.