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| Paper: |
Pulsations and Outbursts of Luminous Blue Variables |
| Volume: |
135, A Half Century of Stellar Pulsation Interpretations: a Tribute to Arthur N. Cox |
| Page: |
302 |
| Authors: |
Cox, A. N.; Guzik, J. A.; Soukup, M. S.; Despain, K. M. |
| Abstract: |
We have proposed an outburst mechanism for the most luminous stars in our Galaxy and other galaxies. These million solar luminosity stars, with masses (after earlier mass loss) of between 20 and maybe 70 solar masses, are pulsationally unstable for both radial and low-degree nonradial modes. Some of these modes are Rstrange,S meaning mostly that the pulsations are concentrated near the stellar surface and have very rapid growth rates in linear theory. The pulsation driving is by both the high iron line opacity (near 150,000 K) and the helium opacity (near 30,000 K) kappa effects. Periods range from 5 to 40 days. Depending on the composition, pulsations periodically produce luminosities above the Eddington limit for deep layers. The radiative luminosity creates an outward push that readily eases the very low gamma envelope to very large outburst radii. A key point is that a super-Eddington luminosity cannot be taken up by the very sluggish convection rapidly enough to prevent an outward acceleration of much of the envelope. As the helium abundance in the surface stellar material increases by ordinary wind mass loss and the LBV outbursts, the opacity in the deep pulsation driving layers decreases. This makes the current Eddington luminosity even higher so that pulsations can then no longer give radiative luminosities exceeding the limit. For the lower mass and luminosity LBVs with considerably less iron line opacity driving, the end of episodic outbursts may be caused by the loss of helium pulsation driving as the helium-depleted core is approached. Details of both linear and nonlinear hydrodynamic calculations and comparisons with observed outburst LBV stars will be presented. |
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