The ages and colours of cool helium-core white dwarf stars (original) (raw)

Evolution and colors of helium-core white dwarf stars with high-metallicity progenitors

Astronomy and Astrophysics, 2009

Aims. Motivated by the recent detection of single and binary He-core white dwarfs in metal-rich clusters, we present a full set of evolutionary calculations and colors appropriate for the study of such white dwarfs. The paper is also aimed at investigating whether stable hydrogen burning may constitute a main source of energy for massive He-core white dwarfs resulting from high-metallicity progenitors.

The age and colors of massive white dwarf stars

Astronomy and Astrophysics, 2007

Aims. We present evolutionary calculations and colors for massive white dwarfs with oxygen-neon cores for masses between 1.06 and 1.28 M ⊙ . The evolutionary stages computed cover the luminosity range from log(L/L ⊙ ) ≈ 0.5 down to −5.2. Methods. Our cooling sequences are based on evolutionary calculations that take into account the chemical composition expected from massive white dwarf progenitors that burned carbon in partially degenerate conditions. The use of detailed non-gray model atmospheres provides us with accurate outer boundary conditions for our evolving models at low effective temperatures. Results. We examine the cooling age, colors and magnitudes of our sequences. We find that massive white dwarfs are characterized by very short ages to such an extent that they reach the turn-off in their colors and become blue at ages well below 10 Gyr. Extensive tabulations for massive white dwarfs, accessible from our web site, are also presented.

Evolution and colours of helium-core white dwarf stars: the case of low-metallicity progenitors

Monthly Notices of the Royal Astronomical Society, 2002

The present work is designed to explore the evolution of helium-core white dwarf (He WD) stars for the case of metallicities much lower than the solar metallicity (Z = 0.001 and 0.0002). Evolution is followed in a self-consistent way with the predictions of detailed and new nongrey model atmospheres, time-dependent element diffusion and the history of the white dwarf progenitor. Reliable initial models for low-mass He WDs are obtained by applying mass-loss rates to a 1-M stellar model in such a way that the stellar radius remains close to the Roche lobe radius. The loss of angular momentum caused by gravitational wave emission and magnetic stellar wind braking are considered. Model atmospheres, based on a detailed treatment of the microphysics entering the WD atmosphere (such as the formalism of Hummer-Mihalas to deal with non-ideal effects) and hydrogen line and pseudo-continuum opacities, enable us to provide accurate colours and magnitudes at both early and advanced evolutionary stages. We find that most of our evolutionary sequences experience several episodes of hydrogen thermonuclear flashes. In particular, the lower the metallicity, the larger the minimum stellar mass for the occurrence of flashes induced by CNO cycle reactions. The existence of a mass threshold for the occurrence of diffusion-induced CNO flashes leads to a marked dichotomy in the age of our models. Another finding of this study is that our He WD models experience unstable hydrogen burning via PP nuclear reactions at late cooling stages as a result of hydrogen chemically diffusing inwards. Such PP flashes take place in models with very low metal content. We also find that models experiencing CNO flashes exhibit a pronounced turn-off in most of their colours at M V ≈ 16. Finally, colour-magnitude diagrams for our models are presented and compared with recent observational data of He WD candidates in the globular clusters NGC 6397 and 47 Tucanae.

Improved synthetic spectra of helium-core white dwarf stars

Monthly Notices of the Royal Astronomical Society, 2002

We examine the emergent fluxes from helium-core white dwarfs following their evolution from the end of pre-white dwarf stages down to advanced cooling stages. For this purpose, we include a detailed treatment of the physics of the atmosphere, particularly an improved representation of the state of the gas by taking into account non-ideal effects according to the so-called occupation probability formalism. The present calculations also incorporate hydrogen line opacity from Lyman, Balmer and Paschen series, pseudocontinuum absorptions and new updated induced-dipole absorption from H 2-H 2 , H 2-He and H-He pairs. We find that the non-ideal effects and line absorption alter the appearance of the stellar spectrum and have a significant influence upon the photometric colours in the UBVRI-JHKL system. This occurs specially for hot models T eff 8000 due to line and pseudocontinuum opacities, and for cool models T eff 4000 where the perturbation of atoms and molecules by neighbour particles affects the chemical equilibrium of the gas. In the present study, we also include new cooling sequences for helium-core white dwarfs of very low mass (0.160 and 0.148 M) with metallicity Z = 0.02. These computations provide theoretical support to search for and identify white dwarfs of very low mass, specially useful for recent and future observational studies of globular cluster, where these objects have began to be detected.

Full evolution of low-mass white dwarfs with helium and oxygen cores

Monthly Notices of the Royal Astronomical Society, 2007

We study the full evolution of low-mass white dwarfs with helium and oxygen cores. We revisit the age dichotomy observed in many white dwarf companions to millisecond pulsar on the basis of white dwarf configurations derived from binary evolution computations. We evolve 11 dwarf sequences for helium cores with final masses of 0. 1604, 0.1869, 0.2026, 0.2495, 0.3056, 0.3333, 0.3515, 0.3844, 0.3986, 0.4160 and 0.4481 M . In addition, we compute the evolution of five sequences for oxygen cores with final masses of 0.3515, 0.3844, 0.3986, 0.4160 and 0.4481 M . A metallicity of Z = 0.02 is assumed. Gravitational settling, chemical and thermal diffusion are accounted for during the white dwarf regime. Our study reinforces the result that diffusion processes are a key ingredient in explaining the observed age and envelope dichotomy in low-mass helium-core white dwarfs, a conclusion we arrived at earlier on the basis of a simplified treatment for the binary evolution of progenitor stars. We determine the mass threshold where the age dichotomy occurs. For the oxygen white dwarf sequences, we report the occurrence of diffusion-induced, hydrogen-shell flashes, which, as in the case of their helium counterparts, strongly influence the late stages of white dwarf cooling. Finally, we present our results as a set of white dwarf mass-radius relations for helium and oxygen cores.

Formation and Evolution of a 0.242 M ⊙ Helium White Dwarf in the Presence of Element Diffusion

The Astrophysical Journal, 2001

A 0.242 M ⊙ object that finally becomes a helium white dwarf is evolved from Roche lobe detachment down to very low luminosities. In doing so, we employ our stellar code to which we have added a set of routines that compute the effects due to gravitational settling, and chemical and thermal diffusion. Initial model is constructed by abstracting mass to a 1 M ⊙ red giant branch model up to the moment at which the model begins to evolve bluewards. We find that element diffusion introduces noticeable changes in the internal structure of the star. In particular, models undergo three thermonuclear flashes instead of one flash as we found with the standard treatment. This fact has a large impact on the total mass fraction of hydrogen left in the star at entering the final cooling track. As a result, at late stages of evolution models with diffusion are characterized by a much smaller nuclear energy release, and they evolve significantly faster compared to those found with the standard treatment. We find that models in which diffusion is considered predict evolutionary ages for the white dwarf companion to the millisecond pulsar PSR B1855+09 in good agreement with the spin-down age of the pulsar.

Evolution of Helium White Dwarfs of Low and Intermediate Masses

The Astrophysical Journal, 1997

We present detailed calculations of the evolution of low-mass, helium white dwarf models with masses from M \ 0.1 to at intervals of and with a metallicity of Z \ 10~3. For this M \ 0.5 M _ 0.05 M _ purpose, we have taken fully into account Ðnite-temperature e †ects by means of a detailed and updated stellar evolutionary code, in which the convective energy transport is described according to the new model for turbulent convection developed by Canuto & Mazzitelli. Furthermore, our code considers the most recent opacity data computed by the Livermore Group (OPAL data), and also the new equation of state for helium plasmas developed by Saumon, Chabrier, & Van Horn. Neutrino emission is fully taken into account as well. For models with we started our calculations from fully convective models located at the M ¹ 0.3 M _ helium-Hayashi line for each conÐguration, far away from the white dwarf regime. By contrast, the evolutionary sequences corresponding to 0.35, 0.4, 0.45, and 0.5 were started from initial models resem-M _ bling white dwarf structures. This was necessary in order to avoid the onset of helium burning. A consequence of this constraint is the existence of a "" forbidden region ÏÏ in the HR diagram above and hotter than where helium white dwarfs can exist only for brief log (L /L _) \ [0.25 log T eff \ 4.45, intervals. All the models were evolved to log (L /L _) \ [5. The evolutionary tracks in the HR diagram have been carefully analyzed, and we found that the convective efficiency a †ects the tracks noticeably only in the high-luminosity (preÈwhite dwarf) regime. We also examined the evolution of central conditions, neutrino luminosity, radii, surface gravity, and ages. Central densities, radii, and surface gravities asymptotically approach the zero temperature Hamada-Salpeter results, as expected. Neutrino losses are important for the more massive helium white dwarf conÐgurations and should be taken into account in detailed evolutionary studies of these objects. Finally, the structure of the outer convective zone was analyzed in both the framework of the mixing length theory (for di †erent convective efficiencies) and the Canuto & Mazzitelli theory. We found that the proÐle of the outer convective zone given by the Canuto & Mazzitelli model is very di †erent from that given by any version of the mixing length theory. This behavior is critical for pulsational instability ; however, stellar parameters such as radius and surface gravity are not signiÐcantly a †ected in the white dwarf domain. These models should be especially suitable for the interpretation of the data about the recently discovered low-mass white dwarfs in systems containing another white dwarf or a millisecond pulsar.

The evolution of white dwarfs resulting from helium-enhanced, low-metallicity progenitor stars

Astronomy & Astrophysics, 2017

Context. Some globular clusters host multiple stellar populations with different chemical abundance patterns. This is particularly true for ω Centauri, which shows clear evidence of a helium-enriched subpopulation characterized by a helium abundance as high as Y = 0.4 Aims. We present a whole and consistent set of evolutionary tracks from the ZAMS to the white dwarf stage that is appropriate for the study of the formation and evolution of white dwarfs resulting from the evolution of helium-rich progenitors. Methods. We derived white dwarf sequences from progenitors with stellar mass ranging from 0.60 to 2.0 M and for an initial helium abundance of Y = 0.4. We adopted two values of metallicity: Z = 0.001 and Z = 0.0005. Results. We explored different issues of white dwarf evolution and their helium-rich progenitors. In particular, the final mass of the remnants, the role of overshooting during the thermally pulsing phase, and the cooling of the resulting white dwarfs differ markedly from the evolutionary predictions of progenitor stars with the standard initial helium abundance. Finally, the pulsational properties of the resulting white dwarfs are also explored. Conclusions. We find that, for the range of initial masses explored in this paper, the final mass of the helium-rich progenitors is markedly higher than the final mass expected from progenitors with the usual helium abundance. We also find that progenitors with initial mass lower than M 0.65 M evolve directly into helium-core white dwarfs in less than 14 Gyr, and that, for larger progenitor masses, the evolution of the resulting low-mass carbon-oxygen white dwarfs is dominated by residual nuclear burning. For heliumcore white dwarfs, we find that they evolve markedly faster than their counterparts coming from standard progenitors. Also, in contrast with what occurs for white dwarfs resulting from progenitors with the standard helium abundance, the impact of residual burning on the cooling time of white dwarfs is not affected by the occurrence of overshooting during the thermally pulsing phase of progenitor stars.

From hydrogen to helium: the spectral evolution of white dwarfs as evidence for convective mixing

Monthly Notices of the Royal Astronomical Society

We present a study of the hypothesis that white dwarfs undergo a spectral change from hydrogen- to helium-dominated atmospheres using a volume-limited photometric sample drawn from the Gaia-DR2 catalogue, the Sloan Digital Sky Survey (SDSS), and the Galaxy Evolution Explorer (GALEX). We exploit the strength of the Balmer jump in hydrogen-atmosphere DA white dwarfs to separate them from helium-dominated objects in SDSS colour space. Across the effective temperature range from 20 000 to 9000 K, we find that 22 per cent of white dwarfs will undergo a spectral change, with no spectral evolution being ruled out at 5σ. The most likely explanation is that the increase in He-rich objects is caused by the convective mixing of DA stars with thin hydrogen layers, in which helium is dredged up from deeper layers by a surface hydrogen convection zone. The rate of change in the fraction of He-rich objects as a function of temperature, coupled with a recent grid of 3D radiation-hydrodynamic simula...

The evolution of a rapidly accreting helium white dwarf to become a low‐luminosity helium star

Monthly Notices of the Royal Astronomical …, 2000

We have examined the evolution of merged low-mass double white dwarfs which become low-luminosity (or high-gravity) extreme helium stars. We have approximated the merging process by the rapid accretion of matter, consisting mostly of helium, on to a helium white dwarf. After a certain mass is accumulated, a helium shell flash occurs, the radius and luminosity increase and the star becomes a yellow giant. Mass accretion is stopped artificially when the total mass reaches a predetermined value. As the helium-burning shell moves inwards with repeating shell flashes, the effective temperature gradually increases as the star evolves towards the helium main sequence. When the mass interior to the heliumburning shell is approximately 0X25 M (Y the star enters a regime where it is pulsationally unstable. We have obtained radial pulsation periods for these models. These models have properties very similar to those of the pulsating helium star V652 Her. We have compared the rate of period change of the theoretical models with that observed in V652 Her, as well as with its position on the Hertzsprung±Russell diagram. We conclude that the merger between two helium white dwarfs can produce a star with properties remarkably similar to those observed in at least one extreme helium star, and is a viable model for their evolutionary origin. Such helium stars will evolve to become hot subdwarfs close to the helium main sequence. We also discuss the number of low-luminosity helium stars in the Galaxy expected for our evolution scenario.