Working Group Members:

• G. Maheswar, ARIES, Nainital

• Ranjan Gupta, IUCAA, Pune

• H.C. Bhatt, IIA, Bangalore

• N.K. Rao, IIA, Bangalore

• G. Pandey, IIA, Bangalore

• N. V. Sujatha, IIA, Bangalore

• J. Murthy, IIA, Bangalore

• A. Sen, Assam University, Silchar

• C. Muthumariappan, IIA, Bangalore

A number of research areas in the studies of galactic dust have been identified that will be studied with priority using the TAUVEX observations.

Dust Extinction Studies

Starlight passing through a dust cloud can be affected in two ways. The light can be totally absorbed if the dust is thick enough or it can be partially scattered by an amount that depends on the color of the light and the thickness of the dust cloud. The most extensively studied dust property may be the interstellar extinction. The amount of extinction is proportional to 1/(wavelength of the light).
The main characteristic features of the wavelength dependence of interstellar extinction "interstellar extinction curve" are:
(a) a slow and then increasingly rapid rise from the IR to the visual;
(b) an approach to leveling off in the near UV;
(c) a broad absorption feature at lambda= 2175 Å
(d) an increase to as far as has been observed in far-UV.
The extinction basically gives us information on the size distribution of the grains. The optical/UV extinction curves show considerable variations which are correlated with different regions. The ratio of total-to-selective extinction, R_v = A_v/E(B-V), is a commonly used measure of the slope of the extinction/reddening curve.

Cardelli, Clayton, and Mathis (1989) found that the extinction curves over the wavelength range of 0.125 μm - 3.5 μm can be fitted remarkably well by an analytical formula involving only R_v as the free parameter. Values of R_v as small as 2.1 (the high latitude translucent molecular cloud HD 210121; Larson, Whittet, and Hough 1996) and as large as 5.6 (the HD 36982 molecular cloud in the Orion nebula) have been observed in the Galactic regions. The Galactic mean extinction curve is characterized by R_v ~ 3.1. These different line-of-sight variations in extinction curve are more conspicuous at ultraviolet wavelengths.

The UV extinction studies with TAUVEX

The two main aims of dust extinction studies using TAUVEX are:

1. Study of variations of Rv values
   Because TAUVEX will observe stars over wide swaths of the Galaxy, we are planning study the variation of the Rv values over much of the sky with different local galactic environmental conditions.
2. Study of the 2175 Å hump
   The strongest spectroscopic extinction feature is the 2175 Å hump. Observations show that its strength and width vary with environment while its peak position is quite invariant. Through the multi-band TAUVEX observations, particularly with the use of filters optimized for extinction observations of 2175 Å feature, we will be studying line of sight variation of 2175 Å hump as a function of different environments.

Observations of star-forming clouds

The star-forming clouds are the small, compact Dark clouds (Bok Globules), 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 alignment is proportional to the strength and direction of ambient magnetic field in the cloud. Background star polarimetry provides technique to probe this field.

In order to relate the physical conditions within the cloud to the background star polarimetry and to know the dust grains properties, we need to determine E(B-V) for each background star and relate it to the corresponding observed polarization (P_v). A comparison of E(B-V) and (P_v) at different distances from the cloud centre will help to answer the following questions:

• Are the grains that produce polarization also produce observed extinction?

• If yes, we can put more constrains on grains properties and also characterize them.

The observations show that polarization is not related to the extinction which 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 upon the findings, the case is made to detect the existence of small grains (0.0035- 0.01 μm) in the star-forming clouds. These particles can be best detected through the UV observations as they show far-UV excess and characteristic features of 2175 Å bump.

We propose imaging of these clouds through the three bandpass filters of TAUVEX, namely, SF1 (160 nm), SF-2 (210 nm) and SF-3 (260 nm), along with standard stars. The program stars magnitudes are ~ 14^m. We expect to resolve many un-answered questions associated with star-forming clouds through this set of propose observations.

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