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Session 59 - Pulsating/Variable Stars.
Display session, Wednesday, June 12
Hot luminous stars (O, B, W-R) are observed to have strong and variable stellar winds, and many classes of these stars are also inferred to pulsate radially or nonradially. It has been suspected for some time that these oscillations can induce periodic modulations in the surrounding stellar wind and produce observational signatures in, e.g., ultraviolet P Cygni line profiles. However, the fact that most low-order and low-degree oscillation modes are evanescent in the photosphere (i.e., damping exponentially instead of propagating sinusoidally) presents a problem to the survival of significant wave amplitude in the wind. We find, though, that the presence of an accelerating wind can provide the necessary impetus for evanescent modes to effectively ``tunnel'' their way out of the interior. First, in the subsonic, or near-static wind, the reference frame of the temporal oscillations is itself beginning to propagate, and this implies that a small degree of group velocity is imparted to the evanescent waves. Second, in the supersonic wind, the density no longer falls off exponentially, but much more slowly, so the effective scale height grows much larger. Frequencies previously evanescent here no longer ``see'' as much of an underlying density gradient, and are free to propagate nearly acoustically.
We model the propagation of oscillations into a hot-star wind via a numerical radiation-hydrodynamics code, and we find that evanescence is indeed not a hindrance to producing wind variability correlated with stellar pulsations. Preliminary models of strong (nonlinear) radial wind oscillations of the \beta Cephei variable BW Vulpeculae show good agreement between observed and modeled base ``radial velocity curves'' and wind-contaminated UV profile variability. We are currently applying this general modeling technique to other systems, especially those which rotate rapidly and exhibit nonradial oscillations (e.g., \zeta Puppis and HD 64760, extensively observed by the IUE MEGA project).
Program listing for Wednesday