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

Abstract

The near-star environment around young stars is very dynamic with winds, disks, and outflows. These processes are involved in star and planet formation, and influence the formation and habitability of planets around host stars. Even for the closest young stars, this will not be imaged by the next generation of telescopes. Other proxies must be developed to probe the circumstellar environment. The polarization of light across individual spectral lines is such a proxy that contains information about the circumstellar material on small spatial scales. Many models have been created to 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 to address these issues.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 larger than most studies to date combined, comparison of current theories on multiple objects and finally the creation of a new theory.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.We have obtained a large number of high precision spectropolarimetric observations of Herbig Ae/Be, Classical Be and other emission-line stars collected on 117 nights of observations. Many stars showed systematic variations in the linear polarization amplitude and direction as a function of time and wavelength in the Halpha line. The detected linear polarization varies from our typical detection threshold near 0.1% up to 2%. Surprisingly, in several stars this polarization effect is not coincident with the Halpha 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.