THE STEADY-STATE SOLUTIONS OF RADIATIVELY DRIVEN STELLAR WINDS FOR A NON-SOBOLEV, PURE ABSORPTION-MODEL

In their recent time-dependent simulations of an absorption-line-driven wind, Owocki, Castor, and Rybicki (OCR) found that the steady state velocity law approached by an unperturbed flow was much steeper than that obtained in previous steady flow models based either on the Sobolev approximation, or on comoving frame solutions of the line transfer. To understand the origin of this result, we have investigated the steady state solution topology for such absorption line-driven flows when one does not rely on the Sobolev approximation to compute the line force. We find that the solution topology near the sonic (critical) point is of the nodal type, with not one, but two positive slope solutions. For reasonable lower boundary conditions, the steeper of these two slopes is applicable only if one assumes (as in OCR) an artificially high ion thermal speed vth for realistic vth, as well as for the Sobolev limit vth→0 of the usual Castor, Abbott, and Klein model, the shallower slope solution applies. At finite vth, however, this shallower solution is not distinct, but consists of a family of very similar solutions converging on the sonic point, and this makes it difficult to distinguish a unique solution extending to large radii. From this we infer that a non-Sobolev, absorption line-driven flow with a realistic value of vth has no uniquely defined steady state. We discuss the implications of this result, both for interpreting results of time-dependent numerical simulations as well as for understanding the nature of the outflow in actual stellar winds. A principal conclusion is that, to the extent that a pure absorption model is applicable, radiatively driven stellar winds should be intrinsically variable.