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White dwarfs are well studied objects. The relative simplicity of their physics allows to obtain very detailed models which can be ultimately compared with their observed properties. Among white dwarfs there is a specific class of stars, known as ZZ-Ceti objects, which have a hydrogen-rich envelope and show periodic variations in their light curves. G117-B15A belongs to this particular set of stars. The luminosity variations have been successfully explained as due to g-mode pulsations. G117-B15A has been recently claimed to be the most stable optical clock ever found, being the rate of change of its 215.2 s period very small: \dot{P}= (2.3 +- 1.4)x10^{-15} s s^-1, with a stability comparable to that of the most stable millisecond pulsars. The rate of change of the period is closely related to its cooling timescale, which can be accurately computed. In this paper we study the pulsational properties of G117-B15A and we use the observed rate of change of the period to impose constraints on the axion emissivity and, thus, to obtain a preliminary upper bound to the mass of the axion. This upper bound turns out to be 4cos^{2}{\beta} meV at the 95% confidence level. Although there are still several observational and theoretical uncertainties, we conclude that G117-B15A is a very promising stellar object to set up constraints on particle physics., Comment: 32 pages, 14 figures, accepted for publication in New Astronomy

The purpose of this work is to explore the evolution of helium-core white dwarf stars in a self-consistent way with the predictions of detailed non-gray model atmospheres and element diffusion. To this end, we consider helium-core white dwarf models with stellar masses of 0.406, 0.360, 0.327, 0.292, 0.242, 0.196 and 0.169 solar masses and follow their evolution from the end of mass loss episodes during their pre-white dwarf evolution down to very low surface luminosities. We find that when the effective temperature decreases below 4000K, the emergent spectrum of these stars becomes bluer within time-scales of astrophysical interest. In particular, we analyse the evolution of our models in the colour-colour and colour-magnitude diagrams and we find that helium-core white dwarfs with masses ranging from approx. 0.18 to 0.3 solar masses can reach the turn-off in their colours and become blue again within cooling times much less than 15 Gyr and then remain brighter than M_V approx. 16.5. In view of these results, many low-mass helium white dwarfs could have had time enough to evolve to the domain of collision-induced absorption from molecular hydrogen, showing blue colours., Comment: 11 pages, 9 figures. Accepted for publication in MNRAS

We investigate the effects of element diffusion on the structure and evolution of low-mass helium white dwarfs (WD). Attention is focused on the occurrence of hydrogen shell flashes induced by diffusion processes during cooling phases. Initial models from 0.406 to 0.161 solar masses are constructed by applying mass loss rates at different stages of the RGB evolution of a solar model. The multicomponent flow equations describing gravitational settling, and chemical and thermal diffusion are solved and the diffusion calculations are coupled to an evolutionary code. In addition, the same sequences are computed but neglecting diffusion. We find that element diffusion strongly affects the structure and cooling history of helium WD. In particular, diffusion induces the occurrence of hydrogen shell flashes in models with masses ranging from 0.18 to 0.41 solar masses, which is in sharp contrast from the situation when diffusion is neglected. In connection with the further evolution, these diffusion-induced flashes lead to much thinner hydrogen envelopes, preventing stable nuclear burning from being an appreciable energy source at advanced stages of evolution. This implies much shorter cooling ages than in the case when diffusion is neglected. These new WD models are discussed in light of recent observational data of some millisecond pulsar systems with WD companions. We find that age discrepancies between the predictions of standard evolutionary models and such observations appear to be the result of ignoring element diffusion in such models. Indeed, such discrepancies vanish when account is made of diffusion., Comment: 13 pages, 15 figures. Accepted for publication in MNRAS

Recent measurements made by Hipparcos (Provencal et al. 1998) present observational evidence supporting the existence of some white dwarf (WD) stars with iron - rich, core composition. In this connection, the present paper is aimed at exploring the structure and evolution of iron - core WDs by means of a detailed and updated evolutionary code. In particular, we examine the evolution of the central conditions, neutrino luminosity, surface gravity, crystallization, internal luminosity profiles and ages. We find that the evolution of iron - rich WDs is markedly different from that of their carbon - oxygen counterparts. In particular, cooling is strongly accelerated as compared with the standard case. Thus, if iron WDs were very numerous, some of them would have had time enough to evolve at lower luminosities than that corresponding to the fall - off in the observed WD luminosity function., Comment: 8 pages, 21 figures. Accepted for publication in MNRAS

The purpose of this work is to present accurate and detailed mass-radius relations for white dwarf (WD) models with helium, carbon, oxygen, silicon and iron cores, by using a fully updated stellar evolutionary code. We considered masses from 0.15 to 0.5 Mo for the case of helium core, from 0.45 to 1.2 Mo for carbon, oxygen and silicon cores and from 0.45 to 1.0 Mo for the case of an iron core. In view of recent measurements made by Hipparcos that strongly suggest the existence of WDs with an iron-dominated core, we focus our attention mainly on the finite-temperature, mass-radius relations for WD models with iron interiors. Furthermore, we explore the effects of gravitational, chemical and thermal diffusion on low-mass helium WD models with hydrogen and helium envelopes., Comment: 8 pages, 9 figures. Accepted for publication in Astronomy and Astrophysics

There are at least three ways in which a varying gravitational constant $G$ could affect the interpretation of the recent high-redhisft Type Ia supernovae results. If the local value of $G$ at the space-time location of distant supernovae is different, it would change both the thermonuclear energy release and the time scale of the supernova outburst. In both cases the effect is related to a change in the Chandrasekhar mass $M_{\rm Ch}\propto G^{-3/2}$. Moreover the integrated variation of $G$ with time would also affect cosmic evolution and therefore the luminosity distance relation. Here we investigate in a consistent way how these different effects of a varying $G$ could change the current interpretation of the Hubble diagram of Type Ia supernovae. We parametrize the variation of $G$ using scalar-tensor theories of gravity, such as the Jordan-Brans-Dicke theory or its extensions. It is remarkable that Dirac's hypothesis that $G$ should decrease with time can qualitatively explain the observed $\Delta m \simeq 0.2$ mag decrease at $z\simeq 0.5$ (with respect to a decelerating universe) and, at the same time, reduce the duration of the risetimes of distant Type Ia supernovae as recently reported., Comment: 4 pages, 2 figures, latex, revtex submitted

In this study we compute the structure and evolution of carbon-oxygen DA white dwarfs by means of a detailed and updated evolutionary code. We treat the energy transport by convection within the formalism of the full spectrum turbulence theory, as given by the Canuto, Goldman and Mazzitelli (CGM) model. We explore the effect of various hydrogen layer masses on both the surface gravity and the hydrogen burning. Convective mixing at low luminosities is also considered. One of our main interests in this work has been to study the evolution of ZZ Ceti models, with the aim of comparing the CGM and mixing length theory (MLT) predictions. In this connection, we find that the temperature profile given by the CGM model is markedly different from that of the ML1 and ML2 versions of MLT. We have also computed approximate effective temperatures for the theoretical blue edge of the DA instability strip by using thermal timescale arguments for our evolving models. In this context, we found that the CGM theory leads to blue edges that are cooler than the observed ones. However, because the determination of the atmospheric parameters of ZZ Ceti stars is dependent on the assumed convection description in model atmosphere calculation, observed blue edges computed considering the CGM theory are required in order to perform a sef consistent comparison of our results with observations. Finally, detailed pulsation calculations of ZZ Ceti models considering the CGM convection would be necessary to place the results found in this paper on a firmer basis., Comment: 11 pages, 11 figures. Uses mn.sty

We present detailed evolutionary calculations for carbon - oxygen - and helium - core white dwarf (WD) models with masses ranging from M= 0.1 to M= 1.2 solar masses and for metallicities Z= 0.001 and Z= 0. The sequences cover a wide range of hydrogen envelopes as well. We employed a detailed WD evolutionary code. In particular, the energy transport by convectcion is treated within the formalism of the full spectrum turbulence theory. The set of models presented here is very detailed and should be valuable for the interpretation of the observational data on low - mass WDs recently discovered in numerous binary configurations and also for the general problem of determining the theoretical luminosity function for WDs. In this context, we compare our cooling sequences with the observed WD luminosity function recently improved by Leggett, Ruiz and Bergeron (1998) and we obtain an age for the Galactic disc of approximately 8 Gyr. Finally, we applied the results of this paper to derive stellar masses of a sample of low - mass white dwarfs., Comment: 9 pages, 8 figures; accepted for publication in MNRAS; replaced with minor corrections to text