Line-driven Stellar Winds: The Dynamical Role of Diffuse Radiation Gradients and Limitations to the Sobolev Approach
Abstract
Line-driven stellar winds from hot, luminous OB stars have been most extensively modeled as smooth, steady state, supersonic outflows for which a local, Sobolev line-transfer treatment is used to compute the line-driving force. In this paper we apply a recently developed, nonlocal escape-integral source function (EISF) method for computing the line force toward time-dependent simulations. In accord with previous linear perturbation analyses, the initially most unstable flow fluctuations in numerical simulations here exhibit an outward phase propagation characterized by a positive correlation between velocity and density variations. However, such outward-mode waves quickly saturate by self-shadowing effects at a relatively low amplitude. Thus, much as in previous instability simulations, the nonlinear wind structure is still dominated by reverse shocks that arise from the somewhat slower (but less easily saturated) growth of inward-mode waves with an anticorrelated velocity-density structure. An unexpected result involves the important role that the diffuse, scattered radiation field--ignored in a Sobolev approach--plays in the dynamics around the wind sonic point. In particular, we find that the strong asymmetry in the forward and backward escape probabilities near the sonic point induces a marked depression in the scattering source function in this region. The resulting inwardly directed diffuse line force can significantly alter the mean wind conditions inferred from steady-wind models using the conventional Sobolev approach. We discuss the implications of these results and consider in particular why these effects have been overlooked in previous wind analyses.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- January 1999
- DOI:
- 10.1086/306561
- Bibcode:
- 1999ApJ...510..355O
- Keywords:
-
- STARS: CIRCUMSTELLAR MATTER;
- LINE: FORMATION;
- RADIATIVE TRANSFER;
- STARS: ATMOSPHERES;
- STARS: EARLY-TYPE;
- STARS: MASS LOSS;
- Stars: Circumstellar Matter;
- Line: Formation;
- Radiative Transfer;
- Stars: Atmospheres;
- Stars: Early-Type;
- Stars: Mass Loss