SPECTROPOLARIMETRY OF THE 5797-ANGSTROM DIFFUSE INTERSTELLAR BAND

Spectropolarimetry across the profiles of the diffuse interstellar bands (DIBs) is a definitive test for the presence of their carriers in the aligned grains responsible for broad-band optical interstellar extinction and polarization. Studies undertaken in the 1970s yielded negative results for the DiBs at 4430, 5780, and 6284 angstrom: these observations have often been advanced as evidence for any of the diffuse features in solid grain material. More recent work on the correlations between various DIBs and interstellar reddening has shown that the feature at 5797 angstrom is more likely to arise in interstellar grains than are those previously selected for spectropolarimetric observation. This paper reports new spectropolarimetry of lambda5797 and other DIBs in the 5600-6000 angstrom region. Results are in all cases negative: there is no detectable excess polarization in the features. We conclude that if dust grains are responsible for the 5797 angstrom, feature, they are not alignable, yet these same grains apparently give rise to a significant fraction of the observed interstellar extinction. Carbon-rich small grains appear to be the most likely solid-state carriers.