Modeling of Rayleigh Scattering and Interstellar Polarization for Evolved Late-Type Stars

TL;DR

This paper models Rayleigh scattering and interstellar polarization effects in evolved late-type stars, providing strategies to interpret polarimetric data when interstellar polarization cannot be separated, with applications to variable polarization observations.

Contribution

It introduces a hybrid polarization model combining Rayleigh and interstellar effects, enhancing interpretation of stellar polarimetric data in complex scenarios.

Findings

Polarization spectral slope can be shallower than Rayleigh prediction.

Shorter wavelengths show higher amplitude variability in stellar polarization.

Anomalous slopes occur when stellar and interstellar polarization angles differ by 90°.

Abstract

Evolved late-type stars are frequently identified as photometric and spectroscopic variables, such as Mira-type or semi-regular variable objects. These stars can also be polarimetrically variable, an indicator of non-spherical geometry for spatially unresolved sources. Departures from symmetry can arise in a number of ways, such as the presence of a binary companion (e.g., multiple illumination sources for scattered light), brightness variations in the stellar atmosphere (e.g., large convective cells), or aspherical circumstellar envelopes (e.g., disks or aspherical stellar winds). Common polarigenic opacities for cool stars include Rayleigh scattering and dust scattering. The classic wavelength dependence of lambda^-4 for Rayleigh single scattering is generally straightforward; however, that signature can be confounded by interstellar polarization (ISP). We explore strategies for interpreting polarimetric observations when the interstellar polarization (ISP) cannot be removed. We introduce a "hybrid" spectrum that includes both Rayleigh polarization for a stellar contribution and the Serkowski Law for an interstellar contribution. We find the polarization spectral slope can be more shallow than expected from Rayleigh scattering alone. For stellar variability, shorter wavelengths give higher amplitude changes when Rayleigh scattering dominates the interstellar signal. Quite anomalous slopes can occur over limited wavelength intervals if the stellar and interstellar position angles differ by 90 deg. Results of the models are discussed in the context of photopolarimetry methods, and an application is considered in terms of variable polarization from the carbon star, R Scl.