2010A&A...522A..45S

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. 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. Optical photometric observations of the three clouds CB3, CB25, and CB39 were carried out at the 2 m H.C. Telescope, India, to determine the colour excess E(B-V) of the background stars by following a technique adopted by Bernabei & Polacaro (2001A&A...371..123B). These three clouds were selected from a set of eight clouds previously observed by us in optical polarimetry (Sen et al., 2000A&AS..141..175S). 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. 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.