Mass loss from inhomogeneous hot star winds II. Constraints from a combined optical/UV study

TL;DR

This study investigates how wind clumping affects mass-loss rate diagnostics in hot stars by using advanced models and radiative transfer codes, revealing that clumping significantly influences spectral line formation and mass-loss estimates.

Contribution

It introduces a comprehensive modeling approach for clumpy stellar winds, including an analytic method for line formation that accounts for optically thick clumping, improving mass-loss rate determinations.

Findings

Synthetic spectra from current wind models fail to match observed lines without increased inner wind clumping.

Mass-loss rates are approximately half of line-driven wind predictions but higher than optically thin clumping estimates.

Resonance doublets can serve as tracers for wind structure and optically thick clumping.

Abstract

Mass-loss rates currently in use for hot, massive stars have recently been seriously questioned, mainly because of the effects of wind clumping. We investigate the impact of clumping on diagnostic ultraviolet resonance and optical recombination lines. Optically thick clumps, a non-void interclump medium, and a non-monotonic velocity field are all accounted for in a single model. We used 2D and 3D stochastic and radiation-hydrodynamic (RH) wind models, constructed by assembling 1D snapshots in radially independent slices. To compute synthetic spectra, we developed and used detailed radiative transfer codes for both recombination lines (solving the "formal integral") and resonance lines (using a Monte-Carlo approach). In addition, we propose an analytic method to model these lines in clumpy winds, which does not rely on optically thin clumping. Results: Synthetic spectra calculated directly from current RH wind models of the line-driven instability are unable to in parallel reproduce strategic optical and ultraviolet lines for the Galactic O-supergiant LCep. Using our stochastic wind models, we obtain consistent fits essentially by increasing the clumping in the inner wind. A mass-loss rate is derived that is approximately two times lower than predicted by the line-driven wind theory, but much higher than the corresponding rate derived from spectra when assuming optically thin clumps. Our analytic formulation for line formation is used to demonstrate the potential impact of optically thick clumping in weak-winded stars and to confirm recent results that resonance doublets may be used as tracers of wind structure and optically thick clumping. (Abridged)