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Title:
Two-dimensional hydrodynamical simulations of wind-compressed disks around rapidly rotating B stars
Authors:
Owocki, Stanley P.; Cranmer, Steven R.; Blondin, John M.
Affiliation:
AA(University of Delaware, Newark, DE, US), AB(University of Delaware, Newark, DE, US), AC(North Carolina State University, Raleigh, NC, US)
Publication:
Astrophysical Journal, Part 1 (ISSN 0004-637X), vol. 424, no. 2, p. 887-904 (ApJ Homepage)
Publication Date:
04/1994
Category:
Astrophysics
Origin:
STI
NASA/STI Keywords:
ACCRETION DISKS, B STARS, COMPUTERIZED SIMULATION, HYDRODYNAMICS, STELLAR MASS EJECTION, STELLAR MODELS, STELLAR ROTATION, STELLAR WINDS, TWO DIMENSIONAL MODELS, GAS PRESSURE, NUMERICAL ANALYSIS, RADIATION PRESSURE, SHOCK WAVES
DOI:
10.1086/173938
Bibliographic Code:
1994ApJ...424..887O

Abstract

We use a two-dimensional piecewise parabolic method (PPM) code to simulate numerically the hydrodynamics of a radiation-driven stellar wind from a rapidly rotating Be star. The results generally confirm predictions of the semianalytic 'wind-compressed disk' model recently proposed by Bjorkman and Cassinelli to explain the circumstellar disks inferred observationally to exist around such rapidly rotating stars. However, this numerical simulation is able to incorporate several important effects not accounted for in the simple model, including a dynamical treatment of the outward radiative driving and gas pressure, as well as a rotationally distorted, oblate stellar surface. This enables us to model quantitatively the compressed wind and shock that forms the equatorial disk. The simulation results thus do differ in several important details from the simple method, showing, for example, an inner disk inflow not possible in the heuristic approach of assuming a fixed outward velocity law. There is also no evidence for the predicted detachment of the disk that arises in the fixed outflow picture. The peak equatorward velocity in the dynamical models is furthermore about a factor of 2 smaller than the lytically predicted value of approximately 50% of the stellar equatorial rotation speed. As a result, the dynamical disks are somewhat weaker than predicted, with a wider opening angle, lower disk/pole density ratio, and smaller shock velocity jump. The principal cause of these latter differences appears to be an artificially strong equatorward drift of the subsonic outflow in the original analytic model. Much better agreement with the dynamical results can be obtained, however, from a slightly modified, analytic wind-compression model with a more detailed specification of the fixed wind outflow and a lower boundary set to the sonic radius along a rotationally oblate stellar surface. Hence, despite these detailed differences, the general predicted effect of disk formation by wind compression toward the equator is substantially confirmed.

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