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

The most massive stars of populations I and II (the bridge between those being the galactic evolution that led to our solar system) are thought to lose a significant fraction of their mass in a steady wind during the main-sequence phase. 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 Hα and free-free radio emission, but these so-called “density squared” diagnostics are enhanced in the presence of local density enhancements, so 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 require a deeper understanding of the complex dynamics of radiatively driven winds, including intrinsic nonlinearities in the response to variations in the mass loading, as well as internal instabilities within the accelerating wind. Furthermore, large-scale structures arising from surface anisotropies and propagating throughout the wind can further complicate the picture by introducing further density enhancements, and by affecting key 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 would provide resolution at the thermal scale of 10 km s−1, and signal-to-noise up to an order of magnitude above that of the celebrated IUE MEGA campaign, with continuous observations that track structures advecting through the wind and correlated with rotation rates.