Spectropolarimetry of HD197770, a Search for a Graphite Signature in the Interstellar Polarization

Details Spectropolarimetry of HD197770, a Search for a Graphite Signature in the Interstellar Polarization Spectropolarimetry of HD197770, a Search for a Graphite Signature in the Interstellar Polarization

Jul 1993

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993hst..prop.4606a&link_type=abstract

HST Proposal ID #4606

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Hst Proposal Id #4606

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The wavelength dependence of the ultraviolet interstellar linear polarization for 8 stars with substantial interstellar extinction was observed by the Wisconsin Ultraviolet Photo Polarimeter Experiment (WUPPE) during the 1990 December Astro-1 mission. One star, HD197770 shows a very distinct polarization feature which coincides with the well known 2175 A extinction bump. Most models of the interstellar grains invoke graphite to explain the extinction bump. Computations by Draine (1988) based on the discrete dipole approximation show that a very good match to the bump can be produced by small (typical size of order 100A) graphite disks (axial ratios of order 1.5). Draine's model also suggests that these grains will produce a feature in the linear polarization like that observed by WUPPE. The birefringence of graphite also introduces circular polarization with a zero crossing at the feature, but its level is too small to be measured in practical exposure times. Thus we propose to utilize the HST FOS to measure linear polarization in the vicinity of 2175 A and thereby confirm the WUPPE result, further substantiate the presence or absence of a small, aligned graphite component of the interstellar medium along this line of sight and test the Draine models. THIS OBSERVATION WILL NOT BE POSSIBLE AFTER HST REFURBISHMENT

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