Interstellar Polarization in the Taurus Dark Clouds: Wavelength‐dependent Position Angles and Cloud Structure near TMC‐1

We use polarimetric observations of two stars (HD29647, HD283809) in thegeneral direction of TMC-1 in the Taurus Dark Cloud to investigate grainproperties and cloud structure in this region. We show the data to beconsistent with a simple two-component model, in which general interstellarpolarization in the Taurus Cloud is produced by a widely distributed cloudcomponent with relatively uniform magnetic field orientation; the light fromstars close to TMC-1 suffers additional polarization arising in one (or more)subcloud(s) with larger average grain size and different magnetic fielddirections compared with the general trend. Towards HD29647, in particular, weshow that the unusually low degree of visual polarization relative toextinction is due to the presence of distinct cloud components in the line ofsight with markedly different magnetic field orientations. Stokes parametercalculations allow us to separate out the polarization characteristics of theindividual components. Results are fit with the Serkowski empirical formula todetermine the degree and wavelength of maximum polarization. Whereas lambda_maxvalues in the widely distributed material are similar to the average (0.55um)for the diffuse interstellar medium, the subcloud in line of sight toHD~283809, the most heavily reddened star in our study, has lambda_max approx.0.73um, indicating the presence of grains about 30% larger than this average.Our model also predicts detectable levels of circular polarization toward bothHD~29647 and HD~283809.Comment: 17 pages including 6 figures, LaTeX, to appear in the Astrophysical Journal, vol 48