The Theoretical Instability Strip for Luminous Blue Variables

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Session 31 -- Massive Hot Stars
Display presentation, Tuesday, 31, 1994, 9:20-6:30

[31.01] The Theoretical Instability Strip for Luminous Blue Variables

Michael S. Soukup, Arthur N. Cox, Joyce A. Guzik (Los Alamos Astrophysics), Siobahn M. Morgan (University of Northern Iowa)

Luminous blue variables (LBVs) occur in a small blue region of the H-R diagram appropriate for the most massive stars in galaxies. LBVs seem to appear only when mass loss before or during blue loops has removed the normal hydrogen-rich envelope to reveal hydrogen burning products like extra helium and nitrogen. Livermore OPAL opacities allow the observed microvariations now to be interpreted as nonradial g-mode pulsations of low angular degree. If the many unstable modes concentrate their energy into only one or a few modes, outbursts and extensive mass loss may occur as amplitudes rapidly grow. Then pulsations stop since the mass in the pulsation driving region decreases temporarily. Theoretical predictions require that convection be suppressed so that the standard kappa effect from iron line-enhanced opacities can give enough pulsation excitation to make these stars variable. A composition gradient at the surface, exposed by extensive mass loss during previous evolution, can accomplish this suppression. The blue and red edges of the pulsation instability strip could be due to the establishment of this steep enough $\mu$ gradient at the surface. However, too much mass loss, so that iron line opacity enhancement is overwhelmed by excessive helium, can prevent pulsations. In this study we find the more conventional explanation for the blue and red edges is likely. When our models are too hot and blueward of the blue edge, the driving layer at about 200,000K is too shallow to involve enough stellar material to overcome the deeper damping. When a model is too cool and redward of the red edge, driving is too deep and almost adiabatic on the pulsation time scale, giving again too little excitation. These blue and red edges depend on the stellar mass, luminosity, population type and the internal structure the evolution produces. We find that the theoretical instability strip edges between log $T_e$ of 4.4 and 4.1 are close to that observed for Population I stars. The strip is narrower for LBVs in lower metallicity galaxies.

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