Stellar spectropolarimetry with HiVIS: Herbig Ae/Be stars, circumstellar environments and optical pumping

TL;DR

This paper presents high-resolution spectropolarimetric observations of Herbig Ae/Be stars, revealing polarization effects in absorption lines that challenge traditional scattering models and support a new optical pumping explanation.

Contribution

The study introduces a new spectropolarimetric model based on optical pumping, providing a better explanation for polarization in absorption lines than traditional scattering theories.

Findings

Polarization varies from 0.1% to 2% in observed stars.

Polarization is detected mainly in absorption parts of spectral lines.

The new optical pumping model better explains observed polarization features.

Abstract

The polarization of light across spectral lines contains information about the circumstellar material on small spatial scales. Many models relate the circumstellar environment to observable polarization changes across spectral lines. However, measuring signals at the 0.1% level requires a very careful control of systematic effects. We have recently built a high-resolution spectropolarimeter for the HiVIS spectrograph on the 3.67m AEOS telescope and have obtained a large number of high precision spectropolarimetric observations of Herbig Ae/Be, Classical Be and other stars. The detected linear polarization varies from our typical detection threshold near 0.1% up to 2%. This polarization effect is generally not coincident with the H-alpha emission peak but is detected only in the absorptive part of the line profile and varies with the absorption. These detections are largely inconsistent with the traditional scattering models and inspired a new explanation of their polarization. We developed a new spectropolarimetric model and argue that polarization in absorption is evidence of optical pumping. We argue that, while scattering theory fits many Be and emission-line star observations, this new theory has much more potential to explain polarization-in-absorption as seen in Herbig Ae/Be and other stellar systems. This thesis presents a large spectropolarimetric study that combines new instrumentation, custom processing software, thorough calibrations, cross-instrument comparisons, a massive observing campaign on many targets, comparison of current theories on multiple objects and finally the creation of a new theory.