A Simple Nozzle Analysis of Slow-Acceleration Solutions in 1-D Models of Rotating Line-Driven Stellar Winds

Stan Owocki

Bartol Research Institute, University of Delaware
Newark, DE 19716 USA

For a star rotating at more than about 75\% of the critical rate, one-dimensional (1-D) models for the equatorial regions of a line-driven stellar wind show a sudden shift to a slow-acceleration solution, implying a slower, denser equatorial outflow that might be associated with the dense disks inferred for sgB[e] stars. To clarify the nature of this solution shift, I present here a simple analysis of the 1-D flow equations based on a nozzle analogy for the terms that constrain the local mass flux. At low rotation rates the nozzle minimum (or ``throat'') occurs near the stellar surface, allowing a near-surface transition to a steeply accelerating, supercritical flow solution. But for rotations above about 75\% of the critical rate, this {\em local}, inner nozzle minimum exceeds the {\em globa}l minimum approached asymptotically at large radii, implying that near-surface supercritical solutions would now have an overloaded mass loss rate. Maintaining a monotonically positive acceleration is then only possible if the flow is kept subcritical out to large radii, where the nozzle function approaches its {\em absolute} minimum. For fixed line-driving parameters, the associated enhancements in equatorial density are typically a factor 5-30 relative to the polar (or nonrotating) wind. However, when gravity darkening and 2-D flow effects are accounted for, it still seems unlikely that rotationally modified equatorial wind outflows could account for the very large densities inferred for the disks around supergiant B[e] stars.

Reference: To appear in ``Stars with the B[e] Phenomenon'', ASP Conf. Ser. 2005, Michaela Kraus \& Anatoly S. Miroshnichenko, eds.
Status: Conference proceedings

Weblink: http://www.bartol.udel.edu/~owocki/preprints/vlieland-rotnoz.pdf


Email: owocki@bartol.udel.edu