The Beta Cephei instability domain for the new solar composition and with new OP opacities

The recent revision of the solar chemical composition (Asplund, Grevesse and Sauval 2005)is characterized by about 40 per cent decrease of C, N, O, Ne, Ar abundances and by 20 percent decrease of Fe and some other metal abundances. We tested the effect of these modifications on the instability of Beta Cephei models. For the opacities, the newest OP data from the Opacity Project (Seaton 2005) were used. We show that the Beta Cephei instability domain in the Hertzsprung-Russel diagram, when computed with new data for Z=0.012 (revised solar value), is very similar to the instability domain computed earlier using the OPAL opacities for the older solar composition with Z=0.02. Almost all observed Beta Cephei variables are located within the instability domain. Two effects are responsible for stronger instability when using the new data: (i) Metal opacity bump in the OP case is located slightly deeper in the star than that in the OPAL case, which results in more effective driving; (ii) at a fixed Z value, the new Fe-group abundances are higher than the older ones because the Z value is determined mainly by the abundances of C, N, 0, and Ne.Comment: 2 pages, 2 figures, to appear in Communications in Asteroseismology, vol.150, 2007 (Proc. Vienna Workshop on the Future of Asteroseismology, held in Vienna, Sept. 20-22, 2006, eds. G. Handler and G. Houdek

Detection of high-degree prograde sectoral mode sequences in the A-star KIC 8054146?

This paper examines the 46 frequencies found in the Delta Sct star KIC 8054146 involving a frequency spacing of exactly 2.814 c/d (32.57 microHz), which is also a dominant low-frequency peak near or equal to the rotational frequency. These 46 frequencies range up to 146 c/d. Three years of Kepler data reveal distinct sequences of these equidistantly spaced frequencies, including the basic sequence and side lobes associated with other dominant modes (i.e., small amplitude modulations). The amplitudes of the basic sequence show a high-low pattern. The basic sequence follows the equation fm = 2.8519 + m * 2.81421 c/d with m ranging from 25 to 35. The zero-point offset and the lack of low-order harmonics eliminate an interpretation in terms of a Fourier series of a non-sinusoidal light curve. The exactness of the spacing eliminates high-order asymptotic pulsation. The frequency pattern is not compatible with simple hypotheses involving single or multiple spots, even with differential rotation. The basic high-frequency sequence is interpreted in terms of prograde sectoral modes. These can be marginally unstable, while their corresponding low-degree counterparts are stable due to stronger damping. The measured projected rotation velocity (300 km/s) indicates that the star rotates with app. 70% of the Keplerian break-up velocity. This suggests a near equator-on view. We qualitatively examine the visibility of prograde sectoral high-degree g-modes in integrated photometric light in such a geometrical configuration and find that prograde sectoral modes can reproduce the frequencies and the odd-even amplitude pattern of the high-frequency sequence

Evolutionary history of four binary blue stragglers from the globular clusters \omega Cen, M55, 47 Tuc and NGC 6752

Context. Origin and evolution of blue stragglers in globular clusters is still a matter of debate. Aims. The aim of the present investigation is to reproduce the evolutionary history of four binary blue stragglers in four different clusters, for which precise values of global parameters are known. Methods. Using the model for cool close binary evolution, developed by one of us (KS), progenitors of all investigated binaries were found and their parameters evolved into the presently observed values. Results. The results show that the progenitors of the binary blue stragglers are cool close binaries with period of a few days, which transform into stragglers by rejuvenation of the initially less massive component by mass transfer from its more massive companion overflowing the inner critical Roche surface. The parameters of V209 from \omega Cen indicate that the binary is substantially enriched in helium. This is an independent and strong evidence for the existence of the helium rich subpopulation in this cluster.Comment: Accepted by Astronomy and Astrophysic