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Binaries consisting of a hot subdwarf star and an accreting white dwarf (WD) are sources of gravitational wave radiation at low frequencies and possible progenitors of Type Ia supernovae if the WD mass is large enough. Here, we report the discovery of the third binary known of this kind: It consists of a hot subdwarf O (sdO) star and a WD with an orbital period of 3.495 h and an orbital shrinkage of 0.1 s in 6 yr. The sdO star overfills its Roche lobe and likely transfers mass to the WD via an accretion disc. From spectroscopy, we obtain an effective temperature of |$T_{\mathrm{eff}}=54\, 240\pm 1840$| K and a surface gravity of log  g  = 4.841 ± 0.108 for the sdO star. From the light curve analysis, we obtain an sdO mass of M sdO = 0.55 M⊙ and a mass ratio of q  = M WD/ M sdO = 0.738 ± 0.001. Also, we estimate that the disc has a radius of |$\sim\!0.41\ \mathrm{R}_\odot$| and a thickness of |$\sim\!0.18\ \mathrm{R}_\odot$|⁠. The origin of this binary is probably a common envelope ejection channel, where the progenitor of the sdO star is either a red giant branch star or, more likely, an early asymptotic giant branch star; the sdO star will subsequently evolve into a WD and merge with its WD companion, likely resulting in an R Coronae Borealis (R CrB) star. The outstanding feature in the spectrum of this object is strong Ca H&K lines, which are blueshifted by ∼200 km s−1 and likely originate from the recently ejected common envelope, and we estimated that the remnant common envelope (CE) material in the binary system has a density |$\sim\!6\times 10^{-10}\ {\rm g\, cm}^{-3}$|⁠. [ABSTRACT FROM AUTHOR]

Close-orbit low-mass X-ray binaries (LMXBs), radio binary millisecond pulsars (BMSPs) with extremely low-mass helium white dwarfs (ELM He WDs) and ultra-compact X-ray binaries (UCXBs) are all part of the same evolutionary sequence. It is therefore of uttermost importance to understand how these populations evolve from one specie to another. Moreover, UCXBs are important gravitational wave (GW) sources and can be detected by future space-borne GW observatories. However, the formation and evolutionary link between these three different populations of neutron star (NS) binaries are not fully understood. In particular, a peculiar fine-tuning problem has previously been demonstrated for the formation of these systems. In this investigation, we test a newly suggested magnetic braking prescription and model the formation and evolution of LMXBs. We compute a grid of binary evolution models and present the initial parameter space of the progenitor binaries which successfully evolve all the way to produce UCXBs. We find that the initial orbital period range of LMXBs, which evolve into detached NS + ELM He WD binaries and later UCXBs, becomes significantly wider compared to evolution with a standard magnetic braking prescription, and thus helps to relieve the fine-tuning problem. However, we also find that formation of wide-orbit BMSPs is prohibited for strong versions of this new magnetic braking prescription, which therefore calls for a revision of the prescription. Finally, we present examples of the properties of UCXBs as Galactic GW sources and discuss their detection by the LISA, TianQin, and Taiji observatories. [ABSTRACT FROM AUTHOR]

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 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 3He, 18O and 19F are discussed. Evolutionary time-scales 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. [ABSTRACT FROM PUBLISHER]

Blue stragglers have been found in all populations. They are important in both stellar evolution and stellar population synthesis. Much evidence shows that blue stragglers are relevant to primordial binaries. Here, we summarize the links between binary evolution and blue stragglers, describe the characteristics of blue stragglers originating from different binary evolutionary channels and show their consequences for binary population synthesis, such as for the integrated spectral-energy distribution, the colour–magnitude diagram, their specific frequency, and their influence on colours, etc. [ABSTRACT FROM PUBLISHER]

We have carried out a detailed study of the single-degenerate channel for the progenitors of type Ia supernovae (SNe Ia). In the model, a carbon-oxygen white dwarf (CO WD) accretes hydrogen-rich material from an unevolved or a slightly evolved non-degenerate companion to increase its mass to Chandrasekhar mass limit. Incorporating the prescription of Hachisu et al. (1999a) for the accretion efficiency into Eggleton's stellar evolution code and assuming that the prescription is valid for all metallicities, we performed binary stellar evolution calculations for more than 25,000 close WD binary systems with various metallicities. The initial parameter spaces for SNe Ia are presented in an orbital period-secondary mass (log Pi, M2i) plane for each Z.Adopting the results above, we studied the birth rate of SNe Ia for various Z via binary population synthesis. From the study, we see that for a high Z, SNe Ia occur systemically earlier and the peak value of the birth rate is larger if a single starburst is assumed. The Galactic birth rate from the channel is lower than (but comparable to) that inferred from observations.We also showed the distributions of the parameters of the binary systems at the moment of supernova explosion and the distributions of the properties of companions after supernova explosion. The former provides physics input to simulate the interaction between supernova ejecta and its companion, and the latter is helpful for searching the companions in supernova remnants. [ABSTRACT FROM PUBLISHER]

ABSTRACT We have completed our series of calculation of case B evolution of Population I close binaries by including a conservative case with convective overshooting and a non-conservative case without convective overshooting in this paper. To apply the results easily to binary population synthesis, we fit two formulae for the remnant mass as a function of the initial mass of the primary, the radius of the primary at the onset of Roche lobe overflow (RLOF) and the initial mass ratio, with errors less than 1.9 per cent and 3.6 per cent for the conservative and non-conservative cases, respectively. We have also made comparisons between the results in this paper and those of previous studies in this series to examine the effects of our assumptions. We find that the remnant mass depends on when RLOF begins in the Hertzsprung gap and the dependence increases with the mass of the primary. Helium–carbon–oxygen white dwarfs are formed in this series of calculation. Both the non-conservative assumption and convective overshooting make RLOF more stable: some binaries with initial mass ratios q[sub i]= 4.0 are dynamically stable during RLOF in all cases except for the conservative case without overshooting. From these models, we have discussed potential electron-degenerate oxygen–neon cores. Finally, we show the conditions under which one evolution scenario may be more likely than the others according to the results of previous authors. [ABSTRACT FROM AUTHOR]

Abstract Employing Eggleton's stellar evolution code, we carry out 150 runs of non-conservative population I binary evolution calculations with the initial primary mass between 1 and 8 M[sub ⊙], the initial mass ratio q=M[sub 1]/M[sub 2] between 1.1 and 4 and the onset of Roche lobe overflow (RLOF) at early, middle or late Hertzsprung gap. Convective overshooting is included in our models and we assume that 50 per cent of the mass lost from the primary during the RLOF is accreted on to the secondary. The other 50 per cent is lost from the system, carrying away the same specific angular momentum as the centre of mass of the primary. We find that the remnant mass depends on when the RLOF begins in the Hertzsprung gap and the dependence increases with the primary mass. The remnant mass, however, does not depend much on the initial mass ratio, as in conservative cases. For q[sub i]= 1.1 , we fit a formula for the remnant mass as a function of the initial mass M[sub 1i] of the primary and the radius of the primary at the onset of RLOF with an error of less than 2.61 per cent. We have also discussed the influences of the convective overshooting that makes the helium burning and carbon burning earlier and the final mass of the primary larger (for M[sub 1i] greater than 1.6 M[sub ⊙]). The convective overshooting has no effect for low-mass binaries, consistent with the results of previous studies. [ABSTRACT FROM AUTHOR]