Variability in the extreme helium star LSS 5121

We report a photometric and spectroscopic study of the hot extreme helium star LSS 5121. We found photometric variability, but no period was evident in its periodogram. This is consistent with the previous proposal, based on spectral line variations, that LSS 5121 is a non-radial pulsator similar to other hot extreme helium stars.Comment: 5 pages, 6 figure

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

Fe-bump instability: the excitation of pulsations in subdwarf B and other low-mass stars

We consider the excitation of radial and non-radial oscillations in low-mass B stars by the iron-bump opacity mechanism. The results are significant for the interpretation of pulsations in subdwarf B stars, helium-rich subdwarfs and extreme helium stars, including the EC14026 and PG1716 variables. We demonstrate that, for radial oscillations, the driving mechanism becomes effective by increasing the contrast between the iron-bump opacity and the opacity from other sources. The location of the iron-bump instability boundary depends on the mean molecular weight in the envelope and also on the radial order of the oscillation. A bluer instability boundary is provided by increasing the iron abundance alone, explaining the observed EC14026 variables, and by higher radial order oscillations. We show that the coolest EC14026 variables may vary in the fundamental radial mode, but the hottest variables must be of higher radial order. In considering non-radial oscillations, we demonstrate that g-modes of high radial order and low spherical degree (l<4) may be excited in some blue horizontal branch stars with near-normal composition (Z=0.02). Additional iron enhancement extends the g-mode instability zone to higher effective temperatures and also creates a p-mode instability zone. With sufficient iron, the p-mode and g-mode instability zones overlap, allowing a small region where the EC14026 and PG1716-type variability can be excited simultaneously. However its location is roughly 5000 K too low compared with the observed boundary between EC14026 and PG1716 variables.Comment: MNRAS, in press, 16 pages, 13 figure

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&