Sachindev S. Shenoy

Interstellar Extinction and Polarization in the Taurus Dark Clouds: The Optical Properties of Dust near the Diffuse/Dense Cloud Interface

Observations of interstellar linear polarization in the spectral range 0.35-2.2??m are presented for several stars reddened by dust in the Taurus region. Combined with a previously published study by Whittet et al., these results represent the most comprehensive data set available on the spectral dependence of interstellar polarization in this nearby dark cloud (a total of 27 sight lines). Extinction data for these and other reddened stars in Taurus are assembled for the same spectral range, combining published photometry and spectral classifications with photometry from the Two Micron All Sky Survey. The polarization and extinction curves are characterized in terms of the parameters ?max (the wavelength of maximum polarization) and RV (the ratio of total to selective extinction), respectively. The data are used to investigate in detail the question of whether the optical properties of the dust change systematically as a function of environment, considering stars observed through progressively more opaque (and thus progressively denser) regions of the cloud. At low visual extinctions (0 3, real changes in grain properties occur, characterized by observed RV values in the range 3.5-4.0. A simple model for the development of RV with AV suggests that RV may approach values of 4.5 or more in the densest regions of the cloud. The transition between normal extinction and dense cloud extinction occurs at AV ~ 3.2, a value coincident with the threshold extinction above which H2O-ice is detected on grains within the cloud. Changes in RV are thus either a direct consequence of mantle growth or occur under closely similar physical conditions. Dust in Taurus appears to be in a different evolutionary state compared with other nearby dark clouds, such as ? Oph, in which coagulation is the dominant physical process.

THE ULTRAVIOLET EXTINCTION CURVE OF INTRACLUMP DUST IN TAURUS (TMC-1): CONSTRAINTS ON THE 2175 A BUMP ABSORBER

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 A˚ ‘‘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. Subject headings: dust, extinction — ISM: individual (TMC-1) — ultraviolet: ISM On-line material: color figure

A Test Case for the Organic Refractory Model of Interstellar Dust

The organic refractory model of interstellar dust postulates that the grains primarily responsible for visual extinction consist of silicate cores coated with organic refractory matter (ORM) supposed to form by photochemical processing of ice mantles. We identify the line of sight to the early-type star Elias 25, in the ρ Oph dark cloud, as a test for local production of ORM in an environment where mantled grains are exposed to a substantial ultraviolet radiation field. Infrared spectra of Elias 25 in the 2.4-4.0 μm region were used to examine the 3 μm ice profile and to search for the C–H bond absorption feature near 3.4 μm that occurs in laboratory-synthesized ORM. We find that the ice feature in Elias 25 has an unusually deep 3.1-3.7 μm wing, but no detectable 3.4 μm feature is present in the wing profile. An upper limit on the τ3.4/AV ratio is set that is a factor of 4 lower than that determined from the feature detected in the diffuse interstellar medium toward Cygnus OB2 No. 12. Our results are consistent with other evidence indicating that the carrier of the 3.4 μm feature is destroyed, rather than produced, inside dense clouds.