Pulsations and outbursts of luminous blue variables Page: 3 of 8
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Pulsations and Outbursts of Luminous Blue Variables
Arthur N. Cox, Joyce A. Guzik, Michael S. Soukup and Kate M.
Los Alamos National Laboratory, Los Alamos, NM 8754.4
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 opac-
ity (near 150,000 K) and the helium opacity (near 30,000 K) kappa ef-
fects. 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 accelera-
tion 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 n effect.
Pulsating stars have been known for over 400 years, and our understanding that
they do actually pulsate has been known now for over 80 years since the Shapley
(1914) analysis. These stars start pulsating when they evolve to conditions where
their thermodynamic properties allow small internal perturbation motions to
grow. But most of these stars evolve on to more stable conditions with hardly any
long lasting effects on their structure or evolution. The luminous blue variable
(LBV) stars, however, are so greatly affected by pulsations and consequent mass
loss that their subsequent fate is enormously changed.
This report follows earlier ones where we discuss linear theory pulsations in
LBV models (Cox et al. 1995, 1997) and a nonlinear study of LBVs and possible
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Cox, A.N.; Guzik, J.A.; Soukup, M.S. & Despain, K.M. Pulsations and outbursts of luminous blue variables, article, June 1, 1997; New Mexico. (digital.library.unt.edu/ark:/67531/metadc694877/m1/3/: accessed May 21, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.