1,721,244 research outputs found
White dwarfs and fundamental physics
No full textThe evolution of white dwarfs (WDs) stars is essentially a cooling process that is easier to compute compared to other evolutionary phases. However, sizeable discrepancies are still present between the current theoretical models. In fact, the present knowledge of the physical behavior of matter under the high-density regimes typical of the WD interiors is still affected by significative uncertainties. In this contribute, I will discuss some of the main uncertainties in the input physics adopted in the computation of WD cooling evolution
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
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
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
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
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