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Scientific paper
Jul 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994hst..prop.5459s&link_type=abstract
HST Proposal ID #5459
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Hst Proposal Id #5459 Interstellar Medium Dust
Scientific paper
Spectropolarimetric observations in the ultraviolet give unique information relevant to the size and shape distributions, the chemical composition and alignment properties of the interstellar dust grains, and the carrier of the strong absorption feature at 2175A. This work addresses four principal observational questions: does the empirical polarization curve (Serkowski's Law) extend into the UV?; does the 2175A feature show polarization?; does polarization reflect the strong rise in extinction found in the far-UV?; are these results different in different directions, and if so how do they relate to other properties of the dust grains? Targets are selected to have a variety of ultravolet extinction and optical polarization properties. We have observed three stars in Cycle 1, and have been awarded time in Cycle 3 to observe two more, through the full UV range. The recent WUPPE results show that UV polarization can depend on direction, so we propose to increase the number of sightlines studied. With the grating G270H, it will be possible to look for polarization in the profile of the 2175A feature, and departures from the Serkowski curve should also be clearly seen. The most important result from our HST work is that none of the three stars studied shows any trace of polarization associated with the 2175A feature, with very good-quality data. This casts doubt on the excess found by WUPPE in one star only, which it is essential to reobserve with HST and is included in the present target list.
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