Mass loss from inhomogeneous hot star winds

Abstract

Context. The mass-loss rate is a key parameter of hot, massive stars. Small-scale inhomogeneities (clumping) in the winds of these stars are 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 (in particular, unsaturated UV resonance lines vs. H α /radio emission) within such models, a highly clumped wind with very low mass-loss rates needs to be invoked, where the resonance lines seem to indicate rates an order of magnitude (or even more) lower than previously accepted values. If found to be realistic, this would challenge the radiative line-driven wind theory and have dramatic consequences for the evolution of massive stars. Aims. We investigate basic properties of the formation of resonance lines in small-scale inhomogeneous hot star winds with nonmonotonic velocity fields. Methods. We study inhomogeneous wind structures by means of 2D stochastic and pseudo-2D radiation-hydrodynamic wind models, constructed by assembling 1D snapshots in radially independent slices. A Monte-Carlo radiative transfer code, which treats the resonance line formation in an axially symmetric spherical wind (without resorting to the Sobolev approximation), is presented and used to produce synthetic line spectra. Results. The optically thin clumping limit is only valid for very weak lines. The detailed density structure, the inter-clump medium, and the non-monotonic velocity field are all important for the line formation. We confirm previous findings that radiationhydrodynamic wind models reproduce observed characteristics of strong lines (e.g., the black troughs) without applying the highly supersonic "microturbulence" needed in smooth models. For intermediate strong lines, the velocity spans of the clumps are of central importance. Current radiation-hydrodynamic 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 Phosphorus V doublet in the O6 supergiant λ Cep confirms that line profiles calculated from a stochastic 2D model reproduce observations with a mass-loss rate approximately 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 clump structure predicted by current radiation-hydrodynamic models.

A quantitative study of O stars in NGC 2244 and the Monoceros OB2 association

“…Knowledge of the atmospheric structure allows the computation of the radiative forces; -recent developments in understanding the clumping properties of O stars have shown that porosity (and its effect on the velocity structure, called vorosity) could alter the shape of key diagnostic lines (e.g. Sundqvist et al 2010). Neglecting this effect could thus bias our determination.…”

“…Although theoretically expected (MacGregor et al 1979;Owocki & Rybicki 1984, 1985Runacres & Owocki 2002), clumping is currently not fully understood and characterized, although work is in progress (e.g. Puls et al 2006;Sundqvist et al 2010). This is a severe problem for stellar evolution since mass loss is a key parameter.…”

Structure in the winds of O-Type stars: observations and inferences

“…However, Sundqvist et al (2010) could not obtain satisfactory agreement between different diagnostics using the wind structure predicted by the current generation of radiation-hydrodynamics simulations of the LDI. Compared with these models, it appears that (a) structure must be initiated deeper in the wind; and (b) clumps need a greater internal velocity gradient.…”

“…This "porosity in velocity space" (dubbed "vorosity" by Owocki) is distinct from porosity in physical space, but also reduces the effective optical depth of the wind in different lines. In an impressive exploratory study, Sundqvist et al (2010) confirmed that both porosity (due to optically thick clumps) and vorosity (due to velocity gaps between clumps) play a…”

“…Thus, "black troughs" are a signature of non-monotonic velocity structure in a stellar wind, which is presumably accompanied by localized density inhomogeneities. As discussed by Puls et al (1993), a substantial amount of small-scale velocity structure is required to get extremely black profiles; see also the discussion by Sundqvist et al (2010).…”