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

Systematic variations with wavelength in the position angle of interstellar linear polarization of starlight may be indicative of multiple cloud structure along the line of sight. We use polarimetric observations of two stars (HD 29647 and HD 283809) in the general direction of TMC-1 in the Taurus dark cloud to investigate grain properties and cloud structure in this region. We show the data to be consistent with a simple two-component model in which general interstellar polarization in the Taurus cloud is produced by a widely distributed cloud component with relatively uniform magnetic field orientation; light from stars close to TMC-1 suffers additional polarization arising in one (or more) subcloud(s) with larger average grain size and magnetic field directions different from the general trend. Toward HD 29647 in particular, we show that the unusually low degree of visual polarization relative to extinction is due to depolarization associated with the presence of distinct cloud components in the line of sight with markedly different magnetic field orientations. Stokes parameter calculations allow us to separate the polarization characteristics of the individual components. Results are fitted with the Serkowski empirical formula to determine the degree and wavelength of maximum polarization. Whereas λmax values in the widely distributed material are similar to the average (0.55 μm) for the diffuse interstellar medium, the subcloud in the line of sight to HD 283809, the most heavily reddened star in our study, has λmax ≈ 0.73 μm, indicating the presence of grains ~30% larger than this average. Our model also predicts detectable levels of circular polarization toward both HD 29647 and HD 283809.