Analyzing Stellar and Interstellar Contributions to Polarization: Modeling Approaches for Hot Stars

TL;DR

This paper reviews methods to distinguish stellar polarization signals from interstellar polarization in hot stars, emphasizing wavelength-dependent effects and implications for ultraviolet spectropolarimetry.

Contribution

It introduces new approaches to characterize and separate interstellar polarization from stellar signals, especially in the ultraviolet, for hot, rapidly rotating stars and binaries.

Findings

ISP declines in hot stars with near-critical rotation

Ultraviolet lines effectively trace ISP in dense winds

Temporal and chromatic effects help distinguish ISP in binaries

Abstract

Linear polarimetry of unresolved stars is a powerful method for discerning or constraining the geometry of a source and its environment, since spherical sources produce no net polarization. However, a general challenge to interpreting intrinsic stellar polarization is the contribution to the signal by interstellar polarization (ISP). Here, we review methodologies for distinguishing the stellar signal from the interstellar contribution in the context of massive stars. We first characterize ISP with distance using a recent compilation of starlight polarization catalogs. Several scenarios involving Thomson scattering, rapidly rotating stars, optically thick winds, and interacting binaries are considered specifically to contrast the wavelength-dependent effects of ISP in the ultraviolet versus optical bands. ISP is recognizable in the stellar polarization from Thomson scattering in the polarization position angle rotations. For hot stars with near-critical rotation rates, the ISP declines whereas the stellar continuum polarization sharply increases. In the case of quite dense winds, strong ultraviolet lines trace the ISP, which is not always the case in the optical. In the binary case, temporal and chromatic effects illustrate how the ISP displaces variable polarization with wavelength. This study clarifies the impacts of ISP in relation to new ultraviolet spectropolarimetry efforts such as Polstar and Pollux.