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 mum are presented for several stars reddened by dust in the Taurus reg ion. Combined with a previously published study by Whittet et al., these re sults represent the most comprehensive data set available on the spectral d ependence of interstellar polarization in this nearby dark cloud (a total o f 27 sight lines). Extinction data for these and other reddened stars in Ta urus are assembled for the same spectral range, combining published photome try and spectral classifications with photometry from the Two Micron All Sk y Survey. The polarization and extinction curves are characterized in terms of the parameters lambda (max) (the wavelength of maximum polarization) an d R-V (the ratio of total to selective extinction), respectively. The data are used to investigate in detail the question of whether the optical prope rties of the dust change systematically as a function of environment, consi dering stars observed through progressively more opaque (and thus progressi vely denser) regions of the cloud. At low visual extinctions the (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, lambda (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 dense st regions of the cloud. The transition between "normal" extinction and "de nse cloud" extinction occurs at A(V) similar to 3.2, a value coincident wit h the threshold extinction above which H2O-ice is detected on grains within the cloud. Changes in R-V are thus either a direct consequence of mantle g rowth or occur under closely similar physical conditions. Dust in Taurus ap pears to be in a different evolutionary state compared with other nearby da rk clouds, such as rho Oph, in which coagulation is the dominant physical p rocess.