Proposal for UV observations of star forming clouds

A. K. Sen1,3, T. Mukai2, R. Gupta3 and Y. Okada2*
1 Department of Physics, Assam University, Silchar 788 011, India
2 Dept. of Earth and Planetary Science, Kobe University, Nada Kobe 657, Japan
3 Inter University Center for Astronomy and Astrophysics (IUCAA), Ganeshkhind,
Pune 411007, India

Abstract. The small, compact dark clouds (also known as Bok Globules), are undergoing gravitational collapse that can result in the production of low mass stars. Light from background stars is scattered in forward direction by magnetically aligned dichroic dust grains. The degree and direction of align- ment is proportional to the strength and direction of ambient magnetic field in the cloud. Background star polarimetry provides the technique to probe this field. In order to relate the physical conditions within the cloud to the back- ground star polarization and to know the dust properties, we need to determine E (B - V ) for each background star and relate it to the corresponding observed polarization (p). However, observed data, do not always show a correlation between polarization and extinction. Due to this the question arises whether the grains that produce polarization also produce observed extinction? The observation that the polarization is not related to the extinction, can be explained if polarization and extinction are caused by two different grain populations. Polarization is mainly caused by short grains, whereas the extinction is caused by larger ones. Based on these findings, justifications are made here to detect these small grains (0.0035-0.01 mum) in the star forming clouds through UV observations to be made by TAUVEX. These particles can be best detected through the UV observations, as they show far-UV excess and characteristic features of 2175 Aobump. We propose imaging of these clouds through the three bandpass filters of TAUVEX. We expect to resolve many unanswered questions associated with star forming clouds, through this set of proposed observations.

Keywords: polarization - stars: formation - SM: clouds - (ISM:) dust, extinction - ultraviolet: ISM - instrument: TAUVEX

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