On the angular momentum evolution of merged white dwarfs (original) (raw)
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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 Astrophysical Journal, 2013
Rotation is thought to be a major factor in the evolution of massive stars -especially at low metallicity -with consequences for their chemical yields, ionizing flux and final fate. Deriving the birth spin distribution is of high priority given its importance as a constraint on theories of massive star formation and as input for models of stellar populations in the local Universe and at high redshift. Recently, it has become clear that the majority of massive stars interact with a binary companion before they die. We investigate how this affects the distribution of rotation rates, through stellar winds, expansion, tides, mass transfer and mergers.
Rotational mixing in low-mass stars
Astronomy and Astrophysics, 2003
In this paper we study the effects of rotation in low-mass, low-metallicity RGB stars. We present the first evolutionary models taking into account self-consistently the latest prescriptions for the transport of angular momentum by meridional circulation and shear turbulence in stellar interiors as well as the associated mixing processes for chemicals computed from the ZAMS to the upper RGB. We discuss in details the uncertainties associated with the physical description of the rotational mixing and study carefully their effects on the rotation profile, diffusion coefficients, structural evolution, lifetimes and chemical signatures at the stellar surface. We focus in particular on the various assumptions concerning the rotation law in the convective envelope, the initial rotation velocity distribution, the presence of µ-gradients and the treatment of the horizontal and vertical turbulence. This exploration leads to two main conclusions : (1) After the completion of the first dredge-up, the degree of differential rotation (and hence mixing) is maximised in the case of a differentially rotating convective envelope (i.e., j CE (r) = cst), as anticipated in previous studies. (2) Even with this assumption, and contrary to some previous claims, the present treatment for the evolution of the rotation profile and associated meridional circulation and shear turbulence does not lead to enough mixing of chemicals to explain the abundance anomalies in low-metallicity field and globular cluster RGB stars observed around the bump luminosity. This study raises questions that need to be addressed in a near future. These include for example the interaction between rotation and convection and the trigger of additional hydrodynamical instabilities.
Rotational mixing in massive binaries. Detached short-period systems
Astronomy & Astrophysics, 2009
Models of rotating single stars can successfully account for a wide variety of observed stellar phenomena, such as the surface enhancements of N and He observed in massive main-sequence stars. However, recent observations have questioned the idea that rotational mixing is the main process responsible for the surface enhancements, emphasizing the need for a strong and conclusive test for rotational mixing. We investigate the consequences of rotational mixing for massive main-sequence stars in short-period binaries. In these systems the tides are thought to spin up the stars to rapid rotation, synchronous with their orbital revolution. We use a state-of-the-art stellar evolution code including the effect of rotational mixing, tides, and magnetic fields. We adopt a rotational mixing efficiency that has been calibrated against observations of rotating stars under the assumption that rotational mixing is the main process responsible for the observed surface abundances. We find that the primaries of massive close binaries (M 1 ≈ 20 M ⊙ , P orb 3 days) are expected to show significant enhancements in nitrogen (up to 0.6 dex in the Small Magellanic Cloud) for a significant fraction of their core hydrogen-burning lifetime. We propose using such systems to test the concept of rotational mixing. As these short-period binaries often show eclipses, their parameters can be determined with high accuracy. For the primary stars of more massive and very close systems (M 1 ≈ 50 M ⊙ , P orb 2 days) we find that centrally produced helium is efficiently mixed throughout the envelope. The star remains blue and compact during the main sequence evolution and stays within its Roche lobe. It is the less massive star, in which the effects of rotational mixing are less pronounced, which fills its Roche lobe first, contrary to what standard binary evolution theory predicts. The primaries will appear as "Wolf-Rayet stars in disguise": core hydrogen-burning stars with strongly enhanced He and N at the surface. We propose that this evolution path provides an alternative channel for the formation of tight Wolf-Rayet binaries with a main-sequence companion and might explain massive black hole binaries such as the intriguing system M33 X-7.
Angular Momentum Transport Within Evolved Low-Mass Stars
The Astrophysical Journal, 2014
Asteroseismology of 1.0 − 2.0M red giants by the Kepler satellite has enabled the first definitive measurements of interior rotation in both first ascent red giant branch (RGB) stars and those on the Helium burning clump. The inferred rotation rates are 10 − 30 days for the ≈ 0.2M He degenerate cores on the RGB and 30 − 100 days for the He burning core in a clump star. Using the MESA code we calculate state-of-the-art stellar evolution models of low mass rotating stars from the zero-age main sequence to the cooling white dwarf (WD) stage. We include transport of angular momentum due to rotationally induced instabilities and circulations, as well as magnetic fields in radiative zones (generated by the Tayler-Spruit dynamo). We find that all models fail to predict core rotation as slow as observed on the RGB and during core He burning, implying that an unmodeled angular momentum transport process must be operating on the early RGB of low mass stars. Later evolution of the star from the He burning clump to the cooling WD phase appears to be at nearly constant core angular momentum. We also incorporate the adiabatic pulsation code, ADIPLS, to explicitly highlight this shortfall when applied to a specific Kepler asteroseismic target, KIC8366239.
Hot Jupiters and the evolution of stellar angular momentum
Astronomy and Astrophysics, 2010
Context. Giant planets orbiting main-sequence stars closer than 0.1 AU are called hot Jupiters. They interact with their stars affecting their angular momentum. Aims. Recent observations provide evidence of excess angular momentum in stars with hot Jupiters in comparison to stars with distant and less massive planets. This has been attributed to tidal interaction, but needs to be investigated in more detail considering other possible explanations because in several cases the tidal synchronization timescales are much longer than the ages of the stars. Methods. We select stars harbouring transiting hot Jupiters to study their rotation and find that those with an effective temperature T eff > ∼ 6000 K and a rotation period P rot < ∼ 10 days are synchronized with the orbital motion of their planets or have a rotation period approximately twice that of the planetary orbital period. Stars with T eff < ∼ 6000 K or P rot > ∼ 10 days show a general trend toward synchronization with increasing effective temperature or decreasing orbital period. We propose a model for the angular momentum evolution of stars with hot Jupiters to interpret these observations. It is based on the hypothesis that a close-in giant planet affects the coronal field of its host star leading to a topology with predominantly closed field lines. An analytic linear force-free model has been adopted to compute the radial extension of the corona and its angular momentum loss rate. The corona is more tightly confined in F-type stars and in G-and K-type stars with a rotation period shorter than ∼10 days. The angular momentum loss is produced by coronal eruptions similar to solar coronal mass ejections. Results. The model predicts that F-type stars with hot Jupiters, T eff > ∼ 6000 K and an initial rotation period < ∼ 10 days suffer no or very little angular momentum loss during their main-sequence lifetime. This can explain their rotation as a remnant of their premain-sequence evolution. On the other hand, F-type stars with P rot > 10 days and G-and K-type stars experience a significant angular momentum loss during their main-sequence lifetime, but at a generally slower pace than similar stars without close-in massive planets. Considering a spread in their ages, this can explain the observed rotation period distribution of planet-harbouring stars. Conclusions. Our model can be tested observationally and has relevant consequences for the relationship between stellar rotation and close-in giant planets, as well as for the application of gyrochronology to estimate the age of planet-hosting stars.
Evolution of the First Stars: the major role of rotation for mixing and mass loss
We show that rotation plays a major role for very low metallicity stars, even if the distribution of angular velocities Ω with respect to critical values is the same as at solar Z.T h e internal gradients of Ω are much larger at lower metallicity Z, which enhance internal mixing and give rise to N-enrichments. Low Z stars easily reach break-up during MS evolution and lose mass. They also lose mass as red giants or supergiants due to their surface enrichments in heavy elements. The winds of low Z stars make peculiar contributions to the chemical yields. We suggest that the helium rich blue Main Sequence (bMS) of ω Centauri bears the signature of such enrichments by the stellar winds of rotating stars in the first stellar generations.
Monthly Notices of the Royal Astronomical Society
Orbital decay by gravitational-wave radiation will cause some close-binary white dwarfs (WDs) to merge within a Hubble time. The results from previous hydrodynamical WD-merger simulations have been used to guide calculations of the post-merger evolution of carbonoxygen + helium (CO+He) WD binaries. Our models include the formation of a hot corona in addition to a Keplerian disc. We introduce a 'destroyed-disc' model to simulate the effect of direct disc ingestion into the expanding envelope. These calculations indicate significant lifetimes in the domain of the rare R Coronae Borealis (RCB) stars, before a fast evolution through the domain of the hotter extreme helium (EHe) stars. Surface chemistries of the resulting giants are in partial agreement with the observed abundances of RCB and EHe stars. The production of 3 He, 18 O and 19 F are discussed. Evolutionary timescales combined with binary WD merger rates from binary-star population synthesis are consistent with present-day numbers of RCBs and EHes, provided that the majority come from relatively recent (<2 Gyr) star formation. However, most RCBs should be produced by CO-WD + low-mass He-WD mergers, with the He WD having a mass in the range 0.20-0.35 M. Whilst, previously, a high He-WD mass (≥0.40 M) was required to match the carbon-rich abundances of RCB stars, the 'destroyed-disc' model yields a high-carbon product with He-WD mass ≥0.30 M , in better agreement with population synthesis results.