Ultraviolet Spectropolarimetry with Polstar: Clumping and Mass-loss Rate Corrections

TL;DR

This paper discusses how ultraviolet spectropolarimetry with the Polstar mission can improve understanding of stellar wind clumping and correct mass-loss rate estimates in massive stars, impacting stellar evolution models.

Contribution

It introduces the use of high-resolution UV spectropolarimetry with Polstar to study wind dynamics and quantify clumping effects on mass-loss diagnostics in massive stars.

Findings

Polstar provides high-resolution UV data for bright stars.

Time series spectroscopy reveals wind structures and clumping.

Results will improve mass-loss rate estimates by accounting for clumping.

Abstract

The most massive stars are thought to lose a significant fraction of their mass in a steady wind during the main-sequence and blue supergiant phases. This in turn sets the stage for their further evolution and eventual supernova, with consequences for ISM energization and chemical enrichment. Understanding these processes requires accurate observational constraints on the mass-loss rates of the most luminous stars, which can also be used to test theories of stellar wind generation. In the past, mass-loss rates have been characterized via collisional emission processes such as H$\alpha$ and free-free radio emission, but these so-called "density squared" diagnostics require correction in the presence of widespread clumping. Recent observational and theoretical evidence points to the likelihood of a ubiquitously high level of such clumping in hot-star winds, but quantifying its effects requires a deeper understanding of the complex dynamics of radiatively driven winds. Furthermore, large-scale structures arising from surface anisotropies and propagating throughout the wind can further complicate the picture by introducing further density enhancements, affecting mass-loss diagnostics. Time series spectroscopy of UV resonance lines with high resolution and high signal-to-noise are required to better understand this complex dynamics, and help correct "density squared" diagnostics of mass-loss rates. The proposed Polstar mission easily provides the necessary resolution at the sound-speed scale of 20 km s$^{-1}$, on three dozen bright targets with signal-to noise an order of magnitude above that of the celebrated IUE MEGA campaign, via continuous observations that track structures advecting through the wind in real time.