Predicted Nonradial Pulsations of Luminous Blue Variables

Details Predicted Nonradial Pulsations of Luminous Blue Variables Predicted Nonradial Pulsations of Luminous Blue Variables

Dec 1993

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993aas...18310102c&link_type=abstract

American Astronomical Society, 183rd AAS Meeting, #101.02; Bulletin of the American Astronomical Society, Vol. 25, p.1441

Astronomy and Astrophysics

Astronomy

2

Scientific paper

Luminous blue variables (LBVs) are known to vary on many-decade timescales like eta Car and P Cyg, on many-year timescales as their usual observed outbursts, and with many-day period microvariations that could be stellar pulsations. These pulsations, observed only when the stars are not in outburst, may be intrinsic, just as for the less luminous beta Cephei and related variables. This investigation studies whether the new Livermore OPAL opacities can allow theoretical predictions of nonradial g-modes. A model with initial mass of 50 M_sun and current mass of 38.85 M_sun at a luminosity of 5.36x10(5) L_sun and surface effective temperature of 24,430K has unstable few day nonradial modes near g_5 with degree l=1. An approximate allowance for the effects of time-dependent convection in the driving layers between 10(-6) and 10(-5) of the mass into the model can nullify the radiation kappa effect and stabilize predicted pulsations. Thus it seems necessary to suppress convection so that the driving is not reduced to a value less than the radiation damping occurring in deeper lying layers. A likely convection suppression cause is a composition mu gradient produced by rapid helium diffusive settling in layers that originally were not convective. When the temperature gradient becomes superadiabatic in the gradually deeping layers as mass loss proceeds off the surface, a helium mu gradient can prevent convection and allow pulsations. Possibly outbursts are caused by pulsations driven so strongly that they get out of hand. They then remove any inhomogeneous composition layers. As the outburst recovers, helium settling below any new surface convection zone can reestablish the mu gradient that can again suppress convection when the deepening convection zone arrives there. Cycles of pulsations and an outburst followed by no pulsations can occur in this very thin surface layer as the star evolves to cooler F star regions with greatly deepening convection and no pulsational instabilities. The hundreds of days periods observed are probably the result of beating between our predicted adjacent few day period g-modes that could be observed individually.

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