1,721,234 research outputs found
Calibration of white dwarf cooling sequences: Theoretical uncertainty
No full textWhite dwarf luminosities are powerful age indicators, whose calibration should be based on reliable models. We discuss the uncertainty of some chemical and physical parameters and their influence on the age estimated by means of white dwarf cooling sequences. Models at the beginning of the white dwarf sequence have been obtained on the basis of progenitor evolutionary tracks computed starting from the zero-age horizontal branch and for a typical halo chemical composition (Z = 0.0001, Y = 0.23). The uncertainties due to nuclear reaction rates, convection, mass loss, and initial chemical composition are discussed. Then, various cooling sequences for a typical white dwarf mass (M = 0.6 M-.) have been calculated under different assumptions on some input physics, namely, conductive opacity, contribution of the ion-electron interaction to the free energy, and microscopic diffusion. Finally, we present the evolution of white dwarfs having mass ranging between 0.5 and 0.9 M-.. Much effort has been spent to extend the equation of state down to the low-temperature and high-density regime. An analysis of the latest improvement in the physics of white dwarf interiors is presented. We conclude that at the faint end of the cooling sequence [log(L/L-.) similar to -5.5] the present overall uncertainty on the age is of the order of 20%, which corresponds to about 3 Gyr. We suggest that this uncertainty could be substantially reduced by improving our knowledge of the conductive opacity ( especially in the partially degenerate regime) and by xing the internal strati cation of C and O
C/O white dwarfs of very low mass: 0.33-0.5 Mo
No full textThe standard lower limit for the mass of white dwarfs (WDs) with a C/O core is roughly 0.5 Modot. In the present work we investigate the possibility to form C/O WDs with mass as low as 0.33 Modot. Both the pre-WD and the cooling evolution of such nonstandard models are described
White dwarf cooling sequences: theoretical uncertainties
No full textThe white dwarf (WD) cooling times are largely affected by the assumption on the conductive opacity. In the present work we computed various cooling sequences for a typical DA WD of mass 0.6 M_sun under different choices for the electron conductivity. We conclude that the present uncertainty on the times at the faint end of the cooling sequence (log L/Lsun ~ -5.5) is of the order of 17%, which correspond to about 2 Gyr
Very low-mass white dwarfs with a C-O core
No full textContext. The lower limit for the mass of white dwarfs (WDs) with a C-O core is commonly assumed to be roughly 0.5 M(circle dot). As a consequence, WDs of lower masses are usually identified as He-core remnants. Aims. When the initial mass of the progenitor star is between 1.8 and 3 M(circle dot), which corresponds to the so-called red giant (RGB) phase transition, the mass of the H-exhausted core at the tip of the RGB is 0.3 < M(H)/M(circle dot) < 0.5. Prompted by this well known result of stellar evolution theory, we investigate the possibility to form C-O WDs with mass M < 0.5 M(circle dot). Methods. The pre-WD evolution of stars was computed with initial mass of about 2.3 M(circle dot), undergoing anomalous mass-loss episodes during the RGB phase and leading to the formation of WDs with He-rich or CO-rich cores. The cooling sequences of the resulting WDs are also described. Results. We show that the minimum mass for a C-O WD is about 0.33 M(circle dot), so that both He and C-O core WDs can exist in the mass range 0.33-0.5 M(circle dot). The models computed for the present paper provide the theoretical tools for indentifying the observational counterpart of very low-mass remnants with a C-O core among those commonly ascribed to the He-core WD population in the progressively growing sample of observed WDs of low mass. Moreover, we show that the central He-burning phase of the stripped progeny of the 2.3 M(circle dot) star lasts longer and longer as the total mass decreases. In particular, the M = 0.33 M(circle dot) model takes about 800 Myr to exhaust its central helium, which is more than three times longer than the value for the standard 2.3 M(circle dot) star: it is, by far, the longest core-He burning lifetime. Finally, we find the occurrence of gravonuclear instabilities during the He-burning shell phase
Energy equipartition in globular clusters through the eyes of dynamical models
No full textContext. Following their birth, globular clusters (GCs) experience a very peculiar dynamical evolution. Gravitational encounters drive these systems toward energy equipartition, mass segregation, and evaporation, which alter structural, spatial, and kinematic features. Aims. We determine the dynamical state of a few GCs by means of a multi-mass King-like dynamical model. Our work focuses on the prediction of the energy equipartition degree and its relationship with model parameters. Methods. We adjusted the dynamical model parameters in order to reproduce the observed velocity dispersion - as derived from Hubble Space Telescope proper motion data - as a function of the stellar mass. By doing so, we estimated Phi 0, a measure of the gravitational potential well. We repeated the same fit by means of the Bianchini relation, a function obtained by interpolating on N-body simulation results. We studied the relationship between Phi 0 and the Bianchini equipartition mass meq and discuss the structural properties, such as concentration c, the number of core relaxation timescales Ncore, and core radius rc. To obtain an independent estimate of Phi 0, we also fitted observed surface brightness profiles using the predicted surface density and a mass-luminosity relation from isochrones. Results. The quality of the fits of the velocity dispersion-mass relationship obtained by means of our dynamical model is comparable to those obtained with the Bianchini function. Nonetheless, when the Bianchini function is used to fit the projected velocity dispersion, the resulting degree of equipartition is underestimated. On the contrary, our approach provides the equipartition degree at any radial or projected distance by means of Phi 0. As a result, a cluster in a more advanced dynamical state shows a larger Phi 0, as well as larger Ncore and c, while rc decreases. We find the estimates of Phi 0 obtained by fitting surface brightness profiles to be compatible at 2 sigma confidence level with those from internal kinematics, although further investigation of statistical and systematic errors is required. Conclusions. Our work illustrates the predicting power of dynamical models to determine the energy equipartition degree of GCs. These models are a unique tool for determining structural and kinematic properties, and can be used where observational data are poor, as is the case for the most crowded regions of a cluster, where stars are barely resolved
Theoretical uncertainties on white dwarf luminosity functions
No full textWhite dwarf stars play an important role in many fields of modern astrophysics. In the present work we discuss the limits of the available theoretical studies of cooling sequences. We analyze the variation of the age of globular clusters derived from the observed white dwarf sequence caused by different assumptions about the conductive opacity as well as that induced by changing the carbon abundance in the core. The former causes a global uncertainty of the order of 10% and the latter of about 5%. We discuss different choices of the initial-to-final mass relation, which induces an uncertainty of 8% on the globular cluster age estimate
The Initial Mass-Final Luminosity Relation of Type II Supernova Progenitors: Hints of New Physics?
No full textWe revise the theoretical initial mass-final luminosity relation for progenitors of Type IIP and IIL supernovae. The effects of the major uncertainties, such as those due to the treatment of convection, semiconvection, rotation, mass loss, nuclear reaction rates, and neutrinos production rates, are discussed in some detail. The effects of mass transfer between components of close-binary systems are also considered. By comparing the theoretical predictions to a sample of Type II supernovae for which the initial mass of the progenitors and the pre-explosive luminosity are available, we conclude that stellar rotation may explain a few progenitors that appear brighter than expected in the case of nonrotating models. In the most extreme case, SN 2012ec, an initial rotational velocity up to 300 km s-1 is required. Alternatively, these objects could be mass-losing components of close binaries. However, most of the observed progenitors appear fainter than expected. This occurrence seems to indicate that the Compton and pair neutrino energy-loss rates, as predicted by the standard electro-weak theory, are not efficient enough and that an additional negative contribution to the stellar energy balance is required. We show that axions coupled with parameters accessible to currently planned experiments, such as IAXO and, possibly, BabyIAXO and axion-like particles, may account for the missing contribution to the stellar energy loss
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