IUCAA Preprints
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Item Analysis of the distribution of background star polarization in dark clouds(2005-04-01) Sen, A.K.; Mukai, T.; Gupta, Ranjan; et al.The polarization observed for stars background to dark clouds (Bok Globules) is often used as diagnostic to study the ongoing star formation processes in these clouds. Such polarization maps in the optical have been reported for eight nearby clouds CB3, CB25, CB39, CB52, CB54, CB58, CB62 and CB246 in one of our previous work (Sen et al 2000). With a view to understand the origin of this polarization, in the present work attempts are made to look for any possible relation between this observed polarization and other physical parameters in the cloud (like temperature, turbulence etc.). The observed polarization does not seem to be clearly related to the dust and gas temperatures (Td and Tg) in the cloud as expected from Davis-Greenstein grain alignment mechanism (Davis & Greenstein 1952). However, the average observed polarization (pav) appears to be related to the turbulence ∆V (measured by 12CO line width) by the mathematical relation pav = 2.95 exp(−0.24∆V ). The possible relation between the direction of polarization vector and other physical parameters are also discussed. For this analysis in addition to the data on above eight dark clouds, the data on CB4 (Kane et al. 1995) are also included for comparison.Item Photopolarimetric study of the star-forming clouds CB3,CB25,and CB39(2010-07-22) Sen, A.K.; et al.; Polcaro, V. F.; Dey, I.The small compact isolated dark clouds also known as ’Bok globules’ are believed to be ideal sites for low-mass star formation. Some of these clouds are undergoing gravitational collapse, and the ambient magnetic field plays a key role in collapse dynamics. The background star polarimetry is generally accepted as a good tool to map the magnetic field, which is responsible for the alignment of dichroic grains that produce polarization. Aims. The background star polarization when studied together with extinction is expected to help us to understand various grain properties and the role of polarimetry as a tracer of magnetic field in these star-forming clouds. With this idea, polarization and colour excess E(B − V ) values for a set of background stars have been studied together to understand various astrophysical process in some star-forming dark clouds. Methods. Optical photometric observations of the three clouds CB3, CB25, and CB39 were carried out at the 2m H.C. Telescope, India, to determine the colour excess E(B −V ) of the background stars by following a technique adopted by Barnei and Polacaro (2001). These three clouds were selected from a set of eight clouds previously observed by us in optical polarimetry ( Sen et al. 2000). Further independent spectroscopic measurements of a few selected sample stars were recently carried out during February and March 2010 from 1.52m Cassini Telescope, Loinao, Italy, to confirm the correctness of estimated E(B − V ) values obtained by this photometric technique. Results. The colour excess E(B−V ) values so obtained were compared with optical polarization values obtained for the same set of stars. It was found that the measured extinction values increase with the increase in percentage polarization for the cloud CB39 and to some extent for CB25. However, for cloud CB31 no such correlation was observed. It is normally expected that the grains causing extinction should also cause polarization of the light from background stars. Any possible deviation from this under different circumstances here has been discussed in the light of the ongoing physical processes in the star-forming clouds.