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

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

Combining observational techniques to constrain convection in evolved massive star models

Recent stellar evolution computations indicate that massive stars in the range ~ 20 - 30 Msun are located in the blue supergiant (BSG) region of the Hertzsprung-Russell diagram at two different stages of their life: immediately after the main sequence (MS, group 1) and during a blueward evolution after the red supergiant phase (group 2). From the observation of the pulsationnal properties of a subgroup of variable BSGs (alpha Cyg variables), one can deduce that these stars belongs to group 2. It is however difficult to simultaneously fit the observed surface abundances and gravity for these stars, and this allows to constrain the physical processes of chemical species transport in massive stars. We will show here that the surface abundances are extremely sensitive to the physics of convection, particularly the location of the intermediate convective shell that appears at the ignition of the hydrogen shell burning after the MS. Our results show that the use of the Ledoux criterion to determine the convective regions in the stellar models leads to a better fit of the surface abundances for alpha Cyg variables than the Schwarzschild one.Comment: 5 pages, 2 figures, to appear in IAUS 307 proceeding

Time-resolved spectroscopy of the rapidly oscillating Ap star KIC 10195926

We report an analysis of high time resolution spectra of the chemically peculiar Ap star KIC 10195926 obtained with the Subaru telescope. We find that the star has low overabundances of rare earth elements compared with other rapidly oscillating Ap stars. We found only upper limits for pulsations from spectral lines of rare earth and other chemical elements. Pulsation was found only for the narrow core of the Hα line with an amplitude of 171 ± 41ms−1 and with the frequency corresponding to photometric frequency obtained from Kepler observations

Recent MOST space photometry

The Microvariability and Oscillations of STars (MOST) photometric satellite has already undertaken more than 64 primary campaigns which include some clusters and has obtained observations of >850 secondary stars of which ~180 are variable. More than half of the variables pulsate, with the majority being of B-type. Since 2006 January, MOST has operated with only a single CCD for both guiding and science. The resulting increase in read-out cadence has improved precision for the brightest stars. The 2007 light curve for Procyon confirms the lack of predicted p-modes with photometric amplitudes exceeding 8 ppm as we found in 2004 and 2005. p-modes have been detected in other solar-type stars as well as pre-main sequence objects, roAp and delta Scuti variables. g-modes have been detected in a range of slowly pulsating B stars, Be stars and beta Cephei variables. Differential rotation has been defined for several spotted solar-type stars and limits set to the albedo of certain transiting planets and the presence of other perturbing planets. The mission is expected to continue as long as the experiment operates.Comment: 9 pages, 7 figures, from HELAS-II meetin

Helium Nova on a Very Massive White Dwarf -- A Light Curve Model of V445 Puppis (2000) Revised

V445 Pup (2000) is a unique object identified as a helium nova. Color indexes during the outburst are consistent with those of free-free emission. We present a free-free emission dominated light curve model of V445 Pup on the basis of the optically thick wind theory. Our light curve fitting shows that (1) the white dwarf (WD) mass is very massive (M_WD \gtrsim 1.35 M_\sun), and (2) a half of the accreted matter remains on the WD, both of which suggest that the increasing WD mass. Therefore, V445 Pup is a strong candidate of Type Ia supernova progenitor. The estimated distance to V445 Pup is now consistent with the recent observational suggestions, 3.5 < d < 6.5 kpc. A helium star companion is consistent with the brightness of m_v=14.5 mag just before the outburst, if it is a little bit evolved hot (\log T (K) \gtrsim 4.5) star with the mass of M_He \gtrsim 0.8 M_\sun. We then emphasize importance of observations in the near future quiescent phase after the thick circumstellar dust dissipates away, especially its color and magnitude to specify the nature of the companion star. We have also calculated helium ignition masses for helium shell flashes against various helium accretion rates and discussed the recurrence period of helium novae.Comment: 8 pages including 12 figures, to appear in Ap

Gravity-mode pulsations in subdwarf B stars: a critical test of stellar opacity

The identification of non-radial g-mode oscillations as the cause of variability in cool subdwarf B stars (PG1716 variables) has been frustrated by a 5 000 K discrepancy between the observed and theoretical blue edge of the instability domain (Fontaine et al. 2003). A major component in the solution to this problem has been identified} by (a) using updated OP instead of OPAL opacities and (b) considering an enhancement of nickel, in addition to that of iron, in the driving zone. The reason for this success is that, in OP, the ``Fe-bump'' contributions from iron and nickel occur at higher temperatures than in OPAL. As well as pointing to a solution of an important problem in stellar pulsation theory, this result provides a critical test for stellar opacities and the atomic physics used to compute them.Comment: 4 figures, 2 tables, accepted for publication in MNRAS (Letters