Spectropolarimetry of the Hα Line in Herbig Ae/Be Stars

Abstract

Using the HiVIS spectropolarimeter built for the Haleakala 3.7 m AEOS telescope, we have obtained a large number of high-precision spectropolarimetrc observations (284) of Herbig AeBe stars collected over 53 nights totaling more than 300 hr of observing. Our sample of five HAeBe stars, AB Aurigae, MWC 480, MWC 120, MWC 158, and HD 58647, all show systematic variations in the linear polarization amplitude and direction as a function of time and wavelength near the Hα line. In all our stars, the Hα line profiles show evidence of an intervening disk or outflowing wind, evidenced by strong emission with an absorptive component. The linear polarization varies by 0.2%-1.5% with the change typically centered in the absorptive part of the line profile. These observations are inconsistent with a simple disk-scattering model or a depolarization model that produce polarization changes centered on the emissive core. We speculate that polarized absorption via optical pumping of the intervening gas may be the cause.