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

Abstract

Observations of interstellar linear polarization in the spectral range 0.35-2.2 μm 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 λ max (the wavelength of maximum polarization) and R V (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 < A V < 3), the dust has R V = 2.97 ± 0.15, implying optical properties closely similar to those of "normal" dust in the diffuse interstellar medium (ISM). However, λ max is significantly higher than the diffuse-ISM average toward some stars in this extinction range, a result which we attribute to size-dependent failure of the grain alignment mechanism. For extinctions A V > 3, real changes in grain properties occur, characterized by observed R V values in the range 3.5-4.0. A simple model for the development of R V with A V suggests that R V 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 A V ~ 3.2, a value coincident with the threshold extinction above which H 2 O-ice is detected on grains within the cloud. Changes in R V 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 ρ Oph, in which coagulation is the dominant physical process.