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, 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 eld orientation; the light from stars close to TMC{1 su ers additional polarization arising in one (or more) subcloud(s) with larger average grain size and di erent magnetic eld directions compared with the general trend. Towards 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 di erent magnetic eld orientations. Stokes parameter calculations allow us to separate out the polarization characteristics of the individual components. Results are t with the Serkowski empirical formula to determine the degree and wavelength of maximum polarization. Whereas max values in the widely distributed material are similar to the average (0.55 m) for the di use interstellar medium, the subcloud in line of sight to HD 283809, the most heavily reddened star in our study, has max 0:73 m, 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. Subject headings: ISM: dust, extinction | polarization | ISM: magnetic elds | ISM: individual (Taurus Cloud) | Stars: individual (HD 29647, HD 283809, HD 283812)