Interstellar Dust Studies
Working Group Members:
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).
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 Rv as the free parameter. Values of Rv 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 Rv ~ 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:
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 (Pv).
A comparison of E(B-V) and (Pv) at different distances from the cloud centre
will help to answer the following questions:
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 ~ 14m. We expect to resolve many un-answered questions associated with star-forming clouds through this set of propose observations.
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