A New Mechanism for Polarizing Light from Obscured Stars

Recent spectropolarimetric observations of Herbig Ae/Be stellar systems show linear polarization variability with wavelength and epoch near their obscured Ha emission. Surprisingly, this polarization is not coincident with the Ha emission peak but is variable near the absorptive part of the line profile. With a new and novel model, we show here that this is evidence of optical pumping--anisotropy of the incident radiation that leads to a linear polarization-dependent optical depth within the intervening hydrogen wind or disk cloud. This effect can yield a larger polarization signal than scattering polarization in these systems.