The ultraviolet extinction curve of intraclump dust in Taurus (TMC-1): Constraints on the 2175 Å bump absorber

D. C.B. Whittet, Rensselaer Polytechnic Institute
S. S. Shenoy, Rensselaer Polytechnic Institute
Geoffrey C. Clayton, Louisiana State University
Karl D. Gordon, The University of Arizona


Ultraviolet extinction curves for three early-type stars that sample dust in the Taurus dark cloud in the vicinity of TMC-1 are analyzed to examine the contribution of internal (intraclump) dust to the total line-of-sight extinction. One of our selected targets samples only dust in the "diffuse-screen" component of the cloud, where the dust grains lack ice mantles and have optical properties similar to average "diffuse- ISM" dust. Comparison with data for more heavily reddened stars that also sample dust within TMC-1 (where grains are mantled) allows us to determine the intraclump extinction. Our principal result is that the 2175 Å "extinction bump" is extremely weak or absent inside the clump. The feature present in the observed extinction curves appears to arise almost entirely in the diffuse screen. This dramatic reduction in bump amplitude in the high-density environment is consistent with the prediction of the graphite model that the feature is suppressed by hydrogenation. Amplitude reduction is accompanied by a more subtle change in the profile of the bump, specifically, an increase in width. Comparison of Taurus and other clouds strongly suggests that the increase in width results from accumulation of surface coatings on the carrier grains rather than from particle clustering.