Pulsation of magnetic stars

Some Ap stars with strong magnetic fields pulsate in high order p-modes; they are called roAp (rapidly oscillating Ap) stars. The p-mode frequencies are modified by the magnetic fields. Although the large frequency separations are hardly affected, small separations are modified considerably. The magnetic field also affects the latitudinal amplitude distribution on the surface. We discuss the property of axisymmetric p-mode oscillations in roAp stars.Comment: 8 pages, 7 figures, conference proceedings, IAU Symp. 301 "Precision asteroseismology" Eds. J.A., Guzik, W.J. Chaplin, G. Handler, and A. Pigulsk

Radial and nonradial oscillations of massive supergiants

Stability of radial and nonradial oscillations of massive supergiants is discussed. The kappa-mechanism and strange-mode instability exciteoscillations having various periods in wide ranges of the upper part of the HR diagram. In addition, in very luminous ($\log L/L_\odot \gtrsim 5.9$) models, monotonously unstable modes exist, which probably indicates the occurrence of optically thick winds. The instability boundary is not far from the Humphreys-Davidson limit. Furthermore, it is found that there exist low-degree($\ell = 1, 2$) oscillatory convection modes associated with the Fe-opacity peak convection zone, and they can emerge to the stellar surface so that they are very likely observable in a considerable range in the HR diagram. The convection modes have periods similar to g-modes, and their growth-times are comparable to the periods. Theoretical predictions are compared with some of the supergiant variables.Comment: 7 pages, 5 figures, IAU symposium No.272, 2010 "Active OB stars: structure, evolution, mass loss, and critical limits" Eds. C. Neiner, G. Wade, G. Maynet, & G. Pete

The evolutionary stage of an RRs star SX Phe

The evolutionary stage for a short period variable SX Phe was investigated. It was assumed that SX Phe is a mixed star with low metal abundance in which the material was mixed after the star evolved off the main sequence, and is in the second hydrogen burning stage. The validity of this hypothesis was examined by constructing two evolutionary sequences with (X,Z,M/solar mass) = (0.5,0.004,0.75) and (0.5,0.001,0.70) in the hydrogen burning phase and computed the pulsation period. Agreement between theoretical results and observational data was sufficient to conclude that the mixed model is actually adequate for SX Phe. The applicability of this model to other RRs stars is briefly discussed

Temperature and gravity of the pulsating extreme helium star LSS 3184 (BX Cir) through its pulsation cycle

We report the analysis of optical spectra of the extreme helium star LSS 3184 (BX Cir) to determine its effective temperature and gravity throughout its pulsation cycle. The spectra were also used to measure its chemical abundances. We report rest gravity, log g = 3.38 +/- 0.02, and a chemical abundance mixture consistent with those reported earlier in a study using an optical spectrum with lower spectral resolution and a lower signal to noise ratio. Our analysis decreases the upper limit for the H abundance to H < 6.0 (mass fraction < 7.1 x 10^-7). Our gravity corresponds to stellar mass M = 0.47 +/- 0.03 M_sun. We find that the effective log g varies through the pulsation cycle with an amplitude of 0.28 dex. The effective gravity is smaller than the rest gravity except when the star is very near its minimum radius. The change in effective gravity is primarily caused by acceleration of the stellar surface. Based on the optical spectra, we find the temperature varies with an amplitude of 3450 K. We find a time averaged mean temperature, 23390 +/- 90 K, consistent with that found in the earlier optical spectrum study. The mean temperature is 1750 K hotter than that found using combined ultraviolet spectra and V and R photometry and the variation amplitude is larger. This discrepancy is similar to that found for the extreme helium star V652 Her.Comment: 7 pages, 6 figures, LaTeX, to be published in A&