The Stellar Winds Atlas I: Current uncertainties in mass-loss rates

Abstract

Stellar winds are a major source of uncertainty in understanding the life and deaths of massive stars. Across studies in the field, prescriptions for stellar winds differ substantially in both their physical assumptions and implementation, making them a dominant contributor to model-to-model variation. In this work, we present a systematic analysis of the physical assumptions underlying commonly adopted wind prescriptions for optically thin and optically thick winds of hot stars, as well as the winds of cool supergiants. Our analysis reveals substantial discrepancies across all regimes: predicted mass-loss rates for optically thin winds differ by more than an order of magnitude, while rates for cool supergiants vary by several orders of magnitude, with even wider uncertainties arising in extrapolation regimes beyond the Humphreys-Davidson limit. These disparities introduce significant ambiguity into the predicted formation of Wolf-Rayet (WR) stars, a problem further compounded by the inconsistent application of transition criteria. A central issue is the "cool Wolf-Rayet problem", a temperature regime where the classical electron-scattering Eddington factor ($\Gamma_{\rm e}$) loses physical consistency. Because this factor is widely used to determine WR mass-loss rates, its failure forces models to rely on uncertain extrapolations and ad-hoc corrections. We conclude that the dominant stellar wind uncertainties arise from a mismatch between the physical assumptions in stellar wind models and the structure of the stars to which they are applied. Our framework clarifies the origins of current theoretical discrepancies and identifies the key physical bottlenecks that must be addressed to improve mass-loss modeling for massive stars.