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Session 59 - Pulsating/Variable Stars.
Display session, Wednesday, June 12
We have studied the theoretical linear nonadiabatic radial pulsation periods and amplitude growth rates of two stellar models with initial masses of 30 and 40 solar masses. The intrinsic variability and dynamical properties of massive stars are very important to the understanding of the evolutionary behavior of these stars, especially those at, or near, the Humphreys-Davidson (H-D) Line, an empirically defined boundary in the upper portion of the H- R Diagram above which no stars are observed thus far to exist. Pulsation model parameters are derived from models we evolved for each initial mass. Initial chemical compositions are Y=0.28 and Z=0.02, and mass loss (according to the de Jager - Nieuwenhuijzen parameterization) and Livermore OPAL opacities are included in the modeling. Evolution was followed to core helium exhaustion, and all models are H-R Diagram first-crossing tracks; no blue loops occurred. Convection is treated using standard mixing length theory. As expected, the models did not exhibit radial pulsations blueward of an effective temperature of 6000 K. As yellow supergiants, we found them unstable to radial pulsation in an extension of the Classical Cepheid instability strip. The initial 30 solar mass model has a blue edge at 5700 K. The fundamental mode nonadiabatic pulsation period is 161 days, with an amplitude growth rate per period of 25000 K, the period is 256 days, and the growth rate has greatly increased to 115about 25.3 solar masses, and the luminosity is about 313000 solar luminosities. The initial 40 solar mass model has a blue edge at 5900 K, a period of 195 days, and a growth rate per period of 1.74900 K, the period is 357 days, and the growth rate per period is 136units, respectively. While yellow supergiants, mass loss has not caused an enhancement of helium in the surface layers. For the initial 40 solar mass model, notable enhancement occurs when the star becomes a red supergiant, but this is not so for the lower initial mass model. For both models near their blue edges, helium ionization dominates as the pulsation driving mechanism. As the models evolve redwards, the pulsational driving contribution from the hydrogen ionization zone increases and becomes significant at about 5000 K. However, time dependent convection calculations may be necessary to model the effects of stronger convection at about this and cooler temperatures on the pulsational driving. If the very high growth rates calculated here indeed occur in real stars, it may be that the rapidly growing pulsations result in episodic ejections of mass of the tenuous outer layers of massive supergiants just below the H-D Line.
Program listing for Wednesday