Helium core white dwarf evolution -- including white dwarf companions to neutron stars (original) (raw)
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Cooling curves and initial models for low-mass white dwarfs (<0.25 Mo) with helium cores
Monthly Notices of the Royal Astronomical Society, 2000
We present a detailed calculation of the evolution of low-mass (< 0.25 M ⊙) helium white dwarfs. These white dwarfs (the optical companions to binary millisecond pulsars) are formed via long-term, low-mass binary evolution. After detachment from the Roche lobe, the hot helium cores have a rather thick hydrogen layer with mass between 0.01 to 0.06 M ⊙. Due to mixing between the core and outer envelope, the surface hydrogen content is 0.5 to 0.35, depending on the initial value of the heavy element (Z) and the initial secondary mass. We found that the majority of our computed models experience one or two hydrogen shell flashes. We found that the mass of the helium dwarf in which the hydrogen shell flash occurs depends on the chemical composition. The minimum helium white dwarf mass in which a hydrogen flash takes place is 0.213 M ⊙ (Z=0.003), 0.198 M ⊙ (Z=0.01), 0.192 M ⊙ (Z=0.02) or 0.183 M ⊙ (Z=0.03). The duration of the flashes (independent of chemical composition) is between few ×10 6 years to few ×10 7 years. In several flashes the white dwarf radius will increase so much that it forces the model to fill its Roche lobe again. Our calculations show that cooling history of the helium white dwarf depends dramatically on the thickness of the hydrogen layer. We show that the transition from a cooling white dwarf with a temporary stable hydrogen-burning shell to a cooling white dwarf in which almost all residual hydrogen is lost in a few thermal flashes (via Roche-lobe overflow) occurs between 0.183-0.213 M ⊙ (depending on the heavy element value).
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.
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.
The ages and colours of cool helium-core white dwarf stars
Monthly Notices of the Royal Astronomical Society, 2001
The purpose of this work is to explore the evolution of helium-core white dwarf stars in a selfconsistent way with the predictions of detailed non-grey 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 M (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 4000 K, the emergent spectrum of these stars becomes bluer within timescales of astrophysical interest. In particular, we analyse the evolution of our models in the colour-colour and in the colourmagnitude diagrams and find that helium-core white dwarfs with masses ranging from ,0.18 to 0.3 M (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 < 16:5. In view of these results, many low-mass helium white dwarfs could have had enough time to evolve to the domain of collision-induced absorption from molecular hydrogen, showing blue colours.
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.
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.
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.
The Ages of Very Cool Hydrogen-rich White Dwarfs
Astrophysical Journal, 2000
The evolution of white dwarfs is essentially a cooling process that depends primarily on the energy stored in their degenerate cores and on the transparency of their envelopes. In this paper we compute accurate cooling sequences for carbon-oxygen white dwarfs with hydrogen dominated atmospheres for the full range of masses of interest. For this purpose we use the most accurate available physical inputs for both the equation of state and opacities of the envelope and for the thermodynamic quantities of the degenerate core. We also investigate the role of the latent heat in the computed cooling sequences. We present separately cooling sequences in which the effects of phase separation of the carbon-oxygen binary mixture upon crystallization have been neglected, and the delay introduced in the cooling times when this mechanism is properly taken into account, in order to compare our results with other published cooling sequences which do not include a treatment of this phenomenon. We find that the cooling ages of very cool white dwarfs with pure hydrogen atmospheres have been systematically underestimated by roughly 1.5 Gyr at log(L/L ⊙ ) = −4.5 for an otherwise typical ∼ 0.6M ⊙ white dwarf, when phase separation is neglected. If phase separation of the binary mixture is included then the cooling ages are further increased by roughly 10%. Cooling tracks and cooling isochrones in several color-magnitude diagrams are presented as well.
The Astrophysical Journal, 2014
Binary millisecond pulsars (BMSPs) are thought to have evolved from lowmass X-ray binaries (LMXBs). If the mass transfer in LMXBs is driven by nuclear evolution of the donor star, the final orbital period is predicted to be well correlated with the mass of the white dwarf (WD), which is the degenerate He core of the donor. Here we show that this relation can be extended to very small WD mass (∼ 0.14 − 0.17 M ⊙) and narrow orbital period (about a few hours), mainly depending on the metallicities of the donor stars. There is also discontinuity in the relation, which is due to the temporary contraction of the donor when the H-burning shell crosses the hydrogen discontinuity. BMSPs with low-mass He WD companions in very compact binaries can be accounted for if the progenitor binary experienced very late Case A mass transfer. The WD companion of PSR J1738+0333 is likely to evolve from a Pop II star. For PSR J0348+0432, to explain its extreme compact orbit in the Roche lobe-decoupling phase, even lower metallicity (Z = 0.0001) is required.