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Helium-rich subdwarfs are a rare subclass of hot subdwarf stars which constitute a small and inhomogeneous group showing varying degrees of helium enrichment. Only one star, LS IV °14 116 has been found to show multiperiodic luminosity variations. The variability of LS IV °14 116 has been explained as the consequence of nonradial g-mode oscillations, whose excitation is difficult to understand within the frame of the standard κ-mechanism driving pulsations in sdBV stars. In a recent study, we have proposed that the pulsations of LS IV °14 116 might be driven through the ϵ-mechanism acting in unstable Heburning zones in the interior of the star, that appear before the quiescent He-burning phase. One of the few accepted scenarios for the formation of He-rich subdwarfs is the merger of two He-core white dwarfs. As part of this project, we present a study of the ϵ-mechanism in post-merger remnants, and discuss the results in the light of the pulsations exhibited by LS IV °14 116. [ABSTRACT FROM AUTHOR]

We propose an scenario for the formation of DA white dwarfs with very thin He-buffers. For these stars we explore the possible ocurrence of diffusion-induced CNO-flashes in white dwarfs, during their early cooling stage. Then we calculate the consequent white dwarf cooling evolution by means of a consistent treatment of element diffusion and nuclear burning. We find that diffusion induced novas do occur in physically sounding white dwarf models. In particular, we find that these flashes do occur in some low-mass (M<=0.6 M⊙) and low metallicity (ZZAMS<=0.001) remnants about 106-107 yr after departing from the AGB. We discuss observable predictions of these models, in particular eruption lightcurves and chemical compositions. [ABSTRACT FROM AUTHOR]

We compute the emission of gravitational radiation from pulsating white dwarfs. This is done for a standard 0.6M⊙ white dwarf with a liquid CO core and a H-rich envelope, for a massive DA white dwarf with a partially crystallized core for which various l = 2 modes have been observed (BPM 37093) and for PG 1159-035 for which several quadrupole modes have been observed as well. We find that these stars do not radiate sizeable amounts of gravitational waves through their observed g-modes. We also explore the possibility of detecting gravitational waves radiated by the f-mode and the p-modes. We find that in this case the gravitational wave signal is very large. We also discuss the possible implications for the detection of gravitational waves from pulsating white dwarfs within the framework of future space-borne interferometers like LISA. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]

We present a new approach for asteroseismology of DA white dwarfs that consists in the employment of a large set of non-static, physically sound, fully evolutionary models representative of these stars. We already have applied this approach with success to pulsating PG1159 stars (GW Vir variables). Our white dwarf models, which cover a wide range of stellar masses, effective temperatures, and envelope thicknesses, are the result of fully evolutionary computations that take into account the complete history of the progenitor stars from the ZAMS. In particular, the models are characterized by self-consistent chemical structures from the centre to the surface, a crucial aspect of white dwarf asteroseismology. We apply this approach to an ensemble of 44 bright DAV (ZZ Ceti) stars. [ABSTRACT FROM AUTHOR]

NGC 6791 is an open cluster that it is so close to us that can be imaged down to very faint luminosities. The main sequence turn-off age (~8 Gyr) and the age derived from the cut-off of the white dwarf luminosity function (~6 Gyr) were found to be significantly different. Here we demonstrate that the origin of this age discrepancy lies in an incorrect evaluation of the white dwarf cooling ages, and we show that when the relevant physical separation processes are included in the calculation of white dwarf sequences both ages are coincident. [ABSTRACT FROM AUTHOR]

We present the first results of a detailed comparison between the pulsation properties of pulsating Extremely Low-Mass pre-white dwarf stars (the pre-ELMV variable stars) and δ Scuti stars. The instability domains of these very different kinds of stars nearly overlap in the log Teff vs. log g diagram, leading to a degeneracy in the classification of the stars. Our aim is to provide asteroseismic tools for their correct classification. [ABSTRACT FROM AUTHOR]