Recent surveys have demonstrated the existence of several short-period binary systems containing two white dwarfs. Following orbital decay by gravitational-wave radiation, such binaries are expected to merge at a rate of two or three per thousand years per galaxy. The consequences of such a merger depend on the individual white dwarf masses, but are believed to include helium-rich subdwarfs, R CrB stars, extreme helium stars and also AM CVn systems and possibly Type Ia supernovae.Whilst the hydrodynamics of the merger process remains difficult to compute, it is possible to compute the evolution of a double white dwarf merger following the destruction of one component. In this paper, we describe the evolution following the merger of two helium white dwarfs. We examine three sets of assumptions concerning the distribution of debris material between a disc and a corona.Our results demonstrate that a model comprising both fast accretion to form a (hot) corona and slow accretion from a (cold) debris disc can reproduce the observed distribution of helium-rich subdwarfs in terms of their surface temperatures, gravities, nitrogen and carbon abundances.

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 carbon-oxygen + helium (CO+He) WD binaries. Our models include the formation of a hot corona in addition to a Keplerian disk. We introduce a "destroyeddisk" model to simulate the effect of direct disk 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 white-dwarf 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 (< 2Gyr) 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-disk" model yields a high-carbon product with He-WD mass 0.30 M ⊙ , in better agreement with population synthesis results.

Obtaining accurate and precise masses and ages for large numbers of giant stars is of great importance for unraveling the assemblage history of the Galaxy. In this paper, we estimate masses and ages of 6940 red giant branch (RGB) stars with asteroseismic parameters deduced from Kepler photometry and stellar atmospheric parameters derived from LAMOST spectra. The typical uncertainties of mass is a few per cent, and that of age is ∼ 20 per cent. The sample stars reveal two separate sequences in the age -[α/Fe] relation -a high-α sequence with stars older than ∼ 8 Gyr and a low-α sequence composed of stars with ages ranging from younger than 1 Gyr to older than 11 Gyr. We further investigate the feasibility of deducing ages and masses directly from LAMOST spectra with a machine learning method based on kernel based principal component analysis, taking a sub-sample of these RGB stars as a training data set. We demonstrate that ages thus derived achieve an accuracy of ∼ 24 per cent. We also explored the feasibility of estimating ages and masses based on the spectroscopically measured carbon and nitrogen abundances. The results are quite satisfactory and significantly improved compared to the previous studies.

We present a catalog of stellar age and mass estimates for a sample of 640 986 red giant branch (RGB) stars of the Galactic disk from the LAMOST Galactic Spectroscopic Survey (DR4). The RGB stars are distinguished from the red clump stars utilizing period spacing derived from the spectra with a machine learning method based on kernel principal component analysis (KPCA). Cross-validation suggests our method is capable of distinguishing RC from RGB stars with only 2 per cent contamination rate for stars with signal-to-noise ratio (SNR) higher than 50. The age and mass of these RGB stars are determined from their LAMOST spectra with KPCA method by taking the LAMOST -Kepler giant stars having asteroseismic parameters and the LAMOST-TGAS sub-giant stars based on isochrones as training sets. Examinations suggest that the age and mass estimates of our RGB sample stars with SNR > 30 have a median error of 30 per cent and 10 per cent, respectively. Stellar ages are found to exhibit positive vertical and negative radial gradients across the disk, and the age structure of the disk is strongly flared across the whole disk of 6 < R < 13 kpc. The data set demonstrates good correlations among stellar age, [Fe/H] and [α/Fe]. There are two separate sequences in the [Fe/H] -[α/Fe] plane: a high-α sequence with stars older than ∼ 8 Gyr and a low-α sequence composed of stars with ages covering the whole range of possible ages of stars. We also examine relations between age and kinematic parameters derived from the Gaia DR2 parallax and proper motions. Both the median value and dispersion of the orbital eccentricity are found to increase with age. The vertical angular momentum is found to fairly smoothly decrease with age from 2 to 12 Gyr, with a rate of about −50 kpc km s −1 Gyr −1 . A full table of the catalog is public available online.

It is predicted that orbital decay by gravitational-wave radiation and tidal interaction will cause some close-binary stars to merge within a Hubble time. The merger of a helium-core white dwarf with a main-sequence star can produce a red giant branch star that has a low-mass hydrogen envelope when helium is ignited and thus become a hot subdwarf. Because detailed calculations have not been made, we compute post-merger models with a stellar evolution code. We find the evolutionary paths available to merger remnants and find the pre-merger conditions that lead to the formation of hot subdwarfs. We find that some such mergers result in the formation of stars with intermediate helium-rich surfaces. These stars later develop helium-poor surfaces owing to diffusion. Combining our results with a model population and comparing to observed stars, we find that some observed intermediate helium-rich hot subdwarfs can be explained as the remnants of the mergers of helium-core white dwarfs with low-mass main-sequence stars.