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

J. O. Sundqvist, J. Puls+1 more|arXiv (Cornell University)|Nov 17, 2009

Astrophysics and Star Formation Studies40 references72 citations

TL;DR

This study investigates resonance line formation in 2D inhomogeneous hot star winds using stochastic and radiation-hydrodynamic models, revealing that non-monotonic velocity fields and non-void inter-clump media are essential for accurate line profiles. It finds that mass-loss rates derived from resonance lines in inhomogeneous models can be up to ten times higher than those from standard optically thin clumping assumptions, reconciling discrepancies with Hα and radio diagnostics.

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.

Motivation & Objective

• To resolve the long-standing discrepancy between mass-loss rates derived from UV resonance lines (e.g., P V) and those from Hα/radio diagnostics in hot, massive stars.
• To investigate how small-scale inhomogeneities, non-monotonic velocity fields, and non-void inter-clump media affect resonance line formation in 2D wind models.
• To test whether current radiation-hydrodynamic wind models can reproduce observed line profiles without relying on artificial 'microturbulence'.
• To assess whether the optically thin clumping approximation underestimates mass-loss rates by comparing synthetic spectra from 2D stochastic and pseudo-2D models to observations.
• To determine if the velocity span of clumps in radiation-hydrodynamic models is consistent with observed line strengths, particularly for intermediate-strength lines.

Proposed method

• Constructed 2D stochastic and pseudo-2D radiation-hydrodynamic wind models by assembling 1D snapshots in radially independent slices to simulate inhomogeneous density and velocity structures.
• Developed and applied a 3D Monte-Carlo radiative transfer code that treats resonance line formation in axially symmetric spherical winds without using the Sobolev approximation.
• Used the 'effective escape ratio' as a key parameter to quantify photon escape from resonance zones, accounting for porosity and vorosity in clump structures.
• Simulated synthetic line profiles for UV resonance lines (e.g., P V) under varying assumptions: optically thin clumps, void inter-clump medium, and non-monotonic velocity fields.
• Compared model predictions with observed P V doublet profiles in the O6 supergiant λ Cep to calibrate mass-loss rates.
• Evaluated the impact of inter-clump medium (ICM) porosity and clump velocity spans on line saturation and profile strength, especially for intermediate-strength lines.

Experimental results

Research questions

• RQ1 How do non-monotonic velocity fields and non-void inter-clump media affect the formation of resonance lines in inhomogeneous hot star winds?
• RQ2 To what extent do current radiation-hydrodynamic models reproduce observed UV resonance line profiles without requiring artificial microturbulence?
• RQ3 Why do resonance lines in standard models suggest mass-loss rates an order of magnitude lower than Hα and radio diagnostics?
• RQ4 Can 2D stochastic models with realistic velocity spans and non-void ICM reproduce observed line strengths while avoiding saturation of intermediate lines?
• RQ5 What is the true mass-loss rate implied by resonance lines when the full inhomogeneous wind structure—including clump velocity spans and ICM—is accounted for?

Key findings

• The optically thin clumping limit is invalid for strong resonance lines, as even weak lines show significant saturation due to complex density and velocity structures.
• Non-monotonic velocity fields and non-void inter-clump media are essential to reproduce observed line profiles; void ICMs lead to unphysically strong or saturated lines.
• Radiation-hydrodynamic models naturally reproduce strong line features (e.g., black troughs) without requiring supersonic microturbulence, confirming their physical realism.
• Clump velocity spans in current radiation-hydrodynamic models are too large to reproduce observed intermediate-strength line profiles unless a very low mass-loss rate is assumed.
• By reducing velocity spans in 2D stochastic models, line strengths drop significantly, allowing mass-loss rates to be up to ten times higher than those derived under the optically thin clumping assumption.
• A first comparison to the P V doublet in λ Cep shows that a 2D stochastic model with non-void ICM and realistic velocity spans reproduces observations with a mass-loss rate nearly ten times higher than standard optically thin clumping models.