High Resolution Spectropolarimetry of the H-alpha Line: Obscured Stars and Absorptive Polarization

TL;DR

This study uses high-resolution spectropolarimetry to analyze the polarization signatures of various obscured stars, revealing absorptive effects and proposing a new optical-pumping model to understand circumstellar gas properties.

Contribution

It introduces a novel optical-pumping model to explain absorptive polarization effects observed in spectropolarimetric data of obscured stars, challenging traditional scattering models.

Findings

Absorptive polarization signatures are common in Herbig Ae/Be and evolved stars.

Traditional scattering models cannot fully explain the observed polarization effects.

A new optical-pumping model better accounts for the spectropolarimetric observations.

Abstract

The near-star environment around obscured stars is very dynamic. Many classes of stars show evidence for winds, disks, inflows and outflows with many phenomena occurring simultaneously. These processes are involved in stellar evolution, star and planet formation, and influence the formation and habitability of planets around host stars. Even for the nearest stars, this region will not be imaged even after the completion of the next generation of telescopes. Other methods for measuring the physical properties of circumstellar material must be developed. The polarization of light across spectral lines is a signature that contains information about the circumstellar material on these small spatial scales. We used the HiVIS (R=13000 to 50000) and ESPaDOnS (R=68000) spectropolarimeters to monitor several classes of stars on over a hundred nights of observing from 2004-2008. In 10/30 classical Be stars, the traditional broad depolarization morphology is reproduced, but with some additional absorptive effects in 4 of these 10 stars. In Herbig Ae/Be stars roughly 2/3 of the stars (14/20) with strong absorptive components (either central or blue-shifted) showed clear spectropolarimetric signatures typically centered on absorptive components of the spectral lines. They were typically 0.3% to 2% with some signatures being variable in time. Post-AGB and RV-Tau type evolved stars showed very strong absorptive polarimetric effects (5/6 PAGB and 4/4 RVTau) very similar to the Herbig Ae/Be stars. These observations were inconsistent with the traditional scattering models and inspired the development of a new explanation of the observed polarization. This new model, based on optical-pumping, has the potential to provide direct measurements of the circumstellar gas properties.