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Linear pulsations of strange modes in LBVs

Description: Outbursts of the luminous blue variables have been studied for a long time, but a detailed understanding of the mechanism has eluded astronomers. In the last few years it has been recognized that the dramatic increase in outburst brightness is due almost entirely to the luminosity being shifted into the visual band, rather than a true luminosity increase. Some ideas about how these very massive and very luminous stars might display their dramatic increase of visual brightness have been given by many. A sampling is given here. The strange modes we consider in this paper have been studied by the G{umlt o}ttingen group under Fricke and Glatzel. We are interested in strange modes because some are very rapidly growing when conditions are right, and amplitudes reach large radial velocity (200 km/s) and luminosity (0.1 mag). Then the radiative luminosity in deep layers can surpass the Eddington limit during each pulsation cycle, and outbursts occur.
Date: December 31, 1996
Creator: Cox, A.N.; Guzik, J.A. & Soukup, M.S.
Partner: UNT Libraries Government Documents Department

Pulsations and outbursts of luminous blue variables

Description: We propose an outburst mechanism for the most luminous stars in our 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 ``strange,`` 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 sluggish convection rapidly enough to prevent an outward acceleration of much of the envelope. As the helium abundance in the envelope stellar material increases by ordinary wind mass loss and the luminous blue variable 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 luminous blue variables there is considerably less iron line opacity driving, and pulsations are almost all caused by the helium ionization kappa effect.
Date: June 1, 1997
Creator: Cox, A.N.; Guzik, J.A.; Soukup, M.S. & Despain, K.M.
Partner: UNT Libraries Government Documents Department

A nonlinear study of luminous blue variables and possible outbursts

Description: Linear pulsation analysis of luminous blue variable models shows instability to pulsations in multiple radial and nonradial strange modes (see Glatzel, these proceedings). These modes have large linear growth rates, sometimes exceeding several hundred percent per period, which prompted us to investigate the nonlinear behavior of envelope models. While the nonradial modes are predicted in the linear analysis to have higher growth rates than the radial modes, nonlinear nonradial pulsations are beyond the capabilities of pulsation hydrodynamics codes developed to date. As for relevant radial nonlinear calculations, Stothers & Chin (1993) report briefly on nonlinear hydrodynamic calculations of one dynamically unstable massive star envelope model. Aikawa & Sreenivasan (1996) have done nonlinear oscillation modeling of strange modes in low-mass AGB stars. Kiriakidis et al. (these proceedings) present nonlinear models (not including convection) of two types of strange-mode pulsators, massive stars and Wolf-Rayet stars. They find periodic or irregular pulsations, and suggest that pulsation drives mass loss. Here we present new nonlinear hydrodynamic calculations to explore the link between strange-mode pulsations and LBV outbursts.
Date: December 31, 1996
Creator: Guzik, J.A.; Cox, A.N.; Despain, K.M. & Soukup, M.S.
Partner: UNT Libraries Government Documents Department

Deriving the structure of pre-supernovae and delta Scuti stars using nonradial oscillations

Description: This is the final report of a three-year Laboratory Directed Research and Development (LDRD) project at the Los Alamos National Laboratory (LANL). The objective is to learn more about the internal structure of two classes of variable stars, by using the observational data afforded by their pulsation properties. The authors updated the one-dimensional computer codes to calculate the evolution and pulsation frequencies of representative delta Scuti and LBV models. They compared the observed pulsation properties with model predictions in an iterative process to find a model (or models) with interior structures that matched the observational constraints for several delta Scuti stars. They carried out nonlinear hydrodynamic modeling of LBV envelopes and proposed a mechanism for their periodic outbursts. Finally, they began validation of a two-dimensional stellar evolution code that will be used to investigate the effects of rotation and hydrodynamic instabilities on the interior structure of these stars.
Date: November 1, 1998
Creator: Guzik, J.A.; Bradley, P.A.; Cox, A.N.; Swenson, F.J.; Deupree, R.G.; Soukup, M.S. et al.
Partner: UNT Libraries Government Documents Department