Characterizing the turbulent porosity of stellar wind structure generated by the line-deshadowing instability
Abstract
We analyse recent 2D simulations of the non-linear evolution of the line-deshadowing instability (LDI) in hot-star winds, to quantify how the associated highly clumped density structure can lead to a `turbulent porosity' reduction in continuum absorption and/or scattering. The basic method is to examine the statistical variations of mass column as a function of path length, and fit these to analytic forms that lead to simple statistical scalings for the associated mean extinction. A key result is that one can characterize porosity effects on continuum transport in terms of a single `turbulent porosity length', found here to scale as H ≈ (fcl - 1)a, where fcl ≡ 〈ρ2〉/〈ρ〉2 is the clumping factor in density ρ, and a is the density autocorrelation length. For continuum absorption or scattering in an optically thick layer, we find the associated effective reduction in opacity scales as ∼ 1/√{1+τ_H}, where τH ≡ κρH is the local optical thickness of this porosity length. For these LDI simulations, the inferred porosity lengths are small, only about a couple per cent of the stellar radius, H ≈ 0.02R*. For continuum processes like bound-free absorption of X-rays that are only marginally optically thick throughout the full stellar wind, this implies τH ≪ 1, and thus that LDI-generated porosity should have little effect on X-ray transport in such winds. The formalism developed here could however be important for understanding the porous regulation of continuum-driven, super-Eddington outflows from luminous blue variables.
- Publication:
-
Monthly Notices of the Royal Astronomical Society
- Pub Date:
- March 2018
- DOI:
- 10.1093/mnras/stx3225
- arXiv:
- arXiv:1712.03457
- Bibcode:
- 2018MNRAS.475..814O
- Keywords:
-
- radiation: dynamics;
- shock waves;
- turbulence;
- stars: early-type;
- stars: winds;
- outflows;
- X-rays: stars;
- Astrophysics - Solar and Stellar Astrophysics
- E-Print:
- 9 pages, 5 figures, accepted for MNRAS