Spectropolarimetry of the H-alpha line in Herbig Ae/Be stars

TL;DR

This study presents high-precision spectropolarimetric observations of Herbig Ae/Be stars, revealing complex polarization variations near the H-alpha line that challenge simple scattering models and suggest optical pumping effects.

Contribution

It provides the first extensive spectropolarimetric dataset of Herbig Ae/Be stars, highlighting polarization variability and proposing a new explanation involving optical pumping.

Findings

Polarization varies by 0.2% to 1.5% near H-alpha

Variations are inconsistent with simple scattering models

Optical pumping may explain polarization features

Abstract

Using the HiVIS spectropolarimeter built for the Haleakala 3.7m 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 hours of observing. Our sample of five HAeBe stars: AB Aurigae, MWC480, MWC120, MWC158 and HD58647, all show systematic variations in the linear polarization amplitude and direction as a function of time and wavelength near the H-alpha line. In all our stars, the H-alpha 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% to 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 which produce polarization changes centered on the emmissive core. We speculate that polarized absorption via optical pumping of the intervening gas may be the cause.