Pulsation Hydrodynamics of Luminous Blue Variables and Pulsation-Driven Winds Joyce A. Guzik, Arthur N. Cox, Kate M. Despain, and Michael S. Soukup Los Alamos National Laboratory, Los Alamos, NM 87545-2345 USA Abstract. Many physical factors, including radial and nonradial pulsation, rota- tion, radiation pressure, convection, magnetic fields, or dynamical instabilities may play important roles in the hydrodynamics of Luminous Blue Variables. We review the current status of hydrodynamic modeling of LBV envelopes, and describe results of our models using the one-dimensional nonlinear hydrodynamics code of Ostlie and Cox. We find that the models pulsate in several simultaneous radial modes, driven by the helium and Fe ionization K effect. The pulsations have quasi-periods between 5 and 80 days, with radial velocity amplitudes of 50-200 km/sec, and may be identified with the LBV microvariations. In some cases, depending on luminosity- to-mass ratio and helium abundance, deep layers in the model can periodically ex- ceed the Eddington luminosity limit. The key to exceeding LE is the inclusion of the time dependence of convection: Near the regions of opacity peaks produced by Fe and helium ionization, convection is turning on and off during each pulsation cycle. If convection cannot turn on rapidly enough to transport the required luminosity through the region, the Eddington limit is exceeded. If this region of the star is sufficiently adiabatic, an "outburst" may occur. In the hydrodynamic models, an outburst is indicated by the photospheric radial velocity suddenly becoming very large, and the photospheric radius increasing monotonically over several pulsation cycles. Such pulsation-triggered outbursts may be responsible for the driving of variable, nonspherical winds. If large and infrequent enough, these outbursts may be identified with the classic LBV eruptions accompanied by episodic mass loss. 1 Mechanisms Proposed to Destabilize LBV Envelopes Many mechanisms have been proposed to destabilize the envelopes of Lumi- nous Blue Variable stars, making them susceptible to prodigious mass outflow and sporadic S Doradus-type or Eta Carinae-like outbursts. It is likely that all of these mechanisms are relevant at some stage of a massive star's evolution, or at different levels in the stellar envelope. The envelopes of LBV stars are very close to the Eddington luminosity limit, at which the outward acceleration due to radiation pressure equals the inward gravitational acceleration. As discussed by Lamers (1997) and Langer (1997), the classical Eddington luminosity L e dd = 47rcGM/K e is mod- ified for stellar envelopes by replacing the electron scattering opacity K e with the flux-mean opacity K, and by reducing the gravitational acceleration by the centripetal acceleration for rotating stars. Thus the Eddington limit can vary within a stellar envelope, and as a function of latitude, and also varies use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/S0252921100072213 Downloaded from https://www.cambridge.org/core. IP address: 3.235.21.12, on 27 May 2020 at 00:22:49, subject to the Cambridge Core terms of