A review of interstellar polarization properties and recent measurements toward the Chamaeleon I and Taurus dark clouds

The observed linear polarization of starlight is thought to be caused by magnetically aligned, elongated dust grains existing within interstellar clouds. Typically, the magnitude of polarization as a function of wavelength may be empirically fit with great accuracy by the Serkowski law, and comparisons of the parameters of this fit to extinction data provide information about grain sizes and environments. The linear polarization properties of the Chamaeleon I and Taurus dark clouds have been studied, using data from the literature combined with some new observations. Strong trends in the amount of polarization as a function of visual extinction ( P vs A _v) in the line of sight have been observed for both clouds. Specifically, the degree of polarization per unit extinction, or polarization efficiency P/ A_v, for Taurus has been found to decrease dramatically with increasing A_v the data being well fit by a power law. For Chamaeleon, the relationship between the wavelength of maximum polarization ( λ_max and the ratio of total to selective extinction ( R_V) is significantly different from that shown previously for a variety of sources by Clayton and Mathis. It is suggested that these effects may be caused by the physical and chemical processes occurring in these clouds and may be used to further understand not only the grain alignment mechanisms but also the environment of dense interstellar clouds.