Mass loss from inhomogeneous hot star winds I. Resonance line formation in 2D models

TL;DR

This study models resonance line formation in inhomogeneous hot star winds using 2D stochastic and hydrodynamic models, revealing that realistic mass-loss rates are higher than previously inferred from simplified models, and emphasizing the importance of velocity spans and non-void inter-clump media.

Contribution

It introduces a Monte Carlo radiative transfer code for 2D wind models and demonstrates that considering velocity spans and non-void inter-clump media aligns synthetic spectra with higher, more realistic mass-loss rates.

Findings

Optically thin clumping limit applies only to very weak lines.

Intermediate strong lines are sensitive to clump velocity spans.

Realistic mass-loss rates are higher when non-void inter-clump media are considered.

Abstract

Small-scale clumping in the winds of hot, massive stars is conventionally included in spectral analyses by assuming optically thin clumps, a void inter-clump medium, and a smooth velocity field. To reconcile investigations of different diagnostics within such models, a highly clumped wind with very low mass-loss rates needs to be invoked. Particularly, unsaturated UV resonance lines seem to indicate rates an order of magnitude (or even more) lower than previously accepted values. We investigate resonance line formation in inhomogeneous hot star winds with non-monotonic velocity fields by means of 2D stochastic and pseudo-2D radiation-hydrodynamic wind models. A Monte-Carlo radiative transfer code is presented and used to produce synthetic line spectra. Results: The optically thin clumping limit is only valid for very weak lines. For intermediate strong lines, the velocity spans of the clumps are of central importance. Current hydrodynamical models predict spans that are too large to reproduce observed profiles unless a very low mass-loss rate is invoked. By simulating lower spans in 2D stochastic models, the profile strengths become drastically reduced, and are consistent with higher mass-loss rates. To simultaneously meet the constraints from strong lines, the inter-clump medium must be non-void. A first comparison to the observed PV doublet in the O6 supergiant lam Cep confirms that a stochastic 2D model reproduces observations with a mass-loss rate roughly ten times higher than that derived from the same lines but assuming optically thin clumping. Tentatively this may resolve discrepancies between theoretical predictions, evolutionary constraints, and recent derived mass-loss rates, and suggests a re-investigation of the structure predicted by current hydrodynamical models.