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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 3He, 18O and 19F 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 HeWD mergers, with the He-WD having a mass in the range 0.20-0.35 Msolar. Whilst, previously, a high He-WD mass (> 0.40 Msolar) 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 Msolar, in better agreement with population synthesis results., Comment: 18 pages, 26 figures, accepted for publication in MNRAS

Early-type R stars and J stars are a special type of carbon star, having enhanced nitrogen ($\rm [N/Fe]\approx 0.5$), lithium, a low \iso{12}{C}/\iso{13}{C} ratio ($<15$) and no s-element enhancements. The merger of a helium white dwarf with a red giant is regarded to be a possible model for the origin of early-type R stars, but the details of nucleosynthesis are not clear. In this paper we investigate three possible channels for helium white-dwarf + red-giant mergers, and find that, amongst the three, only a high-mass helium white dwarf subducted into a low core-mass red giant can make an early-type R star. Nucleosynthesis of elements carbon, nitrogen, oxygen and lithium correspond well with the observations. Furthermore, we find that the J stars may represent a short and luminous stage in the evolution of an early-R star., Comment: 10 pages, 17 figures. Accepted for publication in MNRAS

Helium-rich subdwarfs are a rare subclass of hot subdwarf stars which constitute a small and inhomogeneous group showing varying degrees of helium enrichment. Only one star, LS IV $^\circ$14 116 has been found to show multiperiodic luminosity variations. The variability of LS IV $^\circ$14 116 has been explained as the consequence of nonradial g-mode oscillations, whose excitation is difficult to understand within the frame of the standard $\kappa$-mechanism driving pulsations in sdBV stars. In a recent study, we have proposed that the pulsations of LS IV $^\circ$14 116 might be driven through the $\epsilon$-mechanism acting in unstable He-burning zones in the interior of the star, that appear before the quiescent He-burning phase. One of the few accepted scenarios for the formation of He-rich subdwarfs is the merger of two He-core white dwarfs. As part of this project, we present a study of the $\epsilon$-mechanism in post-merger remnants, and discuss the results in the light of the pulsations exhibited by LS IV $^\circ$14 116., Comment: To be published in the proceedings of the 40th Li\`ege International Astrophysical Colloquium "Ageing low mass stars: from red giants to white dwarfs". 6 pages, 2 figures, 1 table

Due to orbital decay by gravitational-wave radiation, some close-binary helium white dwarfs are expected to merge within a Hubble time. The immediate merger products are believed to be helium-rich sdO stars, essentially helium main-sequence stars. We present new evolution calculations for these post-merger stars beyond the core helium-burning phase. The most massive He-sdO's develop a strong helium-burning shell and evolve to become helium-rich giants. We include nucleosynthesis calculations following the merger of $0.4 \rm M_{\odot}$ helium white-dwarf pairs with metallicities $Z = 0.0001, 0.004, 0.008$ and 0.02. The surface chemistries of the resulting giants are in partial agreement with the observed abundances of R Coronae Borealis and extreme helium stars. Such stars might represent a third, albeit rare, evolution channel for the latter, in addition to the CO+He white dwarf merger and the very-late thermal pulse channels proposed previously. We confirm a recent suggestion that lithium seen in R\,CrB stars could form naturally during the hot phase of a merger in the presence of \iso{3}{He} from the donor white dwarf., Comment: 5 pages, 3 figures. Accepted for publication in MNRAS letters

Recent theoretical and observational studies both argue that the merging of double carbon-oxygen white dwarfs (WDs) is responsible for at least some Type Ia supernovae (SNe Ia). Previous (standard) studies of the anticipated SN birthrate from this channel have assumed that the merger process is conservative and that the primary criterion for explosion is that the merged mass exceeds the Chandrasekhar mass. Han & Webbink (1999) demonstrated that mass transfer and merger in close double WDs will in many cases be non-conservative. Pakmor et al. (2011) further suggested that the merger process should be violent in order to initiate an explosion. We have therefore investigated how the SN Ia birthrate from the double-degenerate (DD) channel is affected by these constraints. Using the binary-star population-synthesis method, we have calculated the DD SN Ia birthrate under conservative and non-conservative approximations, and including lower mass and mass-ratio limits indicated by recent smoothed-particle-hydrodynamic calculations. The predicted DD SN Ia rate is significantly reduced by all of these constraints. With dynamical mass loss alone (violent merger) the birthrate is reduced to 56% of the conservative rate. Requiring that the mass ratio $q>2/3$ further reduces the birthrate to 18% that of the standard assumption. An upper limit of 0.0061 SNuM, or a Galactic rate of $4.6 \times 10^{-4}{\rm yr}^{-1}$, might be realistic., Comment: 13 pages, 2 figures; accepted by ApJ Letters

Ultraviolet radiation is a double-edged sword to life. If it is too strong, the terrestrial biological systems will be damaged. And if it is too weak, the synthesis of many biochemical compounds can not go along. We try to obtain the continuous ultraviolet habitable zones, and compare the ultraviolet habitable zones with the habitable zones of host stars. Using the boundary ultraviolet radiation of ultraviolet habitable zone, we calculate the ultraviolet habitable zones of host stars with masses from 0.08 to 4.00 \mo. For the host stars with effective temperatures lower than 4,600 K, the ultraviolet habitable zones are closer than the habitable zones. For the host stars with effective temperatures higher than 7,137 K, the ultraviolet habitable zones are farther than the habitable zones. For hot subdwarf as a host star, the distance of the ultraviolet habitable zone is about ten times more than that of the habitable zone, which is not suitable for life existence., Comment: 5 pages, 3 figures