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Double helium white dwarfs (He WDs) are a type of gravitational wave source and are greatly important in studies of binary interaction, particularly in common envelope (CE) ejection physics. Most double He WDs with mass ratios of q ∼ 1 are formed through a particular channel. In this channel, one He WD is initially produced from a red giant (RG) with a degenerate core via stable Roche lobe overflow, and another He WD is formed from an RG with a degenerate core via CE ejection. They may have significant implications for binary evolution processes but have not received specific studies, especially for the CE phase. This paper adopts a semianalytic method and a detailed stellar evolution simulation to model the formation of double He WDs. We find that most double He WDs show mass ratios slightly greater than 1, and their orbital period–mass ratio relations are broadly consistent with observations. There is also a relation between the mass ratios and progenitor masses of the He WDs produced via CE ejection for double He WDs with determined WD masses. Based on this relation, the mass of the He WD progenitor can be inferred from the mass ratio. Then, the CE ejection efficiency can be constrained with the orbital period. In addition, we constrain the CE ejection efficiency for two double He WDs, J1005-2249 and WD0957-666. The results show that the CE ejection efficiencies increase with the WD progenitor masses. [ABSTRACT FROM AUTHOR]

The hot subdwarf O/B stars (sdO/Bs) are known as extreme horizontal branch stars, which is of great importance in stellar evolution theory. The sdO/Bs are generally thought to have a helium-burning core and a thin hydrogen envelope (M env < 0.02 M ⊙). In the canonical binary evolution scenario, sdO/Bs are considered to be the stripped cores of red giants. However, such a scenario cannot explain the recently discovered sdO/B binary SMSS J1920, where the strong Ca H and K lines in the spectrum are found. It suggests that this binary likely originated from the recent ejection of the common envelope (CE). In this work, we propose a new formation channel of massive sdO/Bs, namely, sdO/Bs produced from a CE ejection process with an asymptotic giant branch (AGB) star (hereafter the AGB CE channel). We constructed the evolutionary model of sdO/Bs and successfully explained most of the important observed parameters of the sdO/B star in SMSS J1920, including the evolutionary age, sdO/B mass, effective temperature, surface gravity, and surface helium abundance. The minimum sdO/B mass produced from the AGB CE channel is about 0.48 M ⊙. The evolutionary tracks in the log T eff – log g plane may explain a fraction of the observational samples with high log T eff and low log g . Considering the wind mass loss of sdO/Bs, the model could produce helium-rich hot subdwarfs with log (n He / n H) ≳ − 1 . [ABSTRACT FROM AUTHOR]

The stability criteria of rapid mass transfer and common-envelope evolution are fundamental in binary star evolution. They determine the mass, mass ratio, and orbital distribution of many important systems, such as X-ray binaries, type Ia supernovae, and merging gravitational-wave sources. We use our adiabatic mass-loss model to systematically survey intermediate-mass (IM) stars' thresholds for dynamical timescale mass transfer. The impact of metallicity on the stellar responses and critical mass ratios is explored. Both tables (Z = 0.001) and fitting formulae (Z = 0.001 and Z = 0.02) of the critical mass ratios of IM stars are provided. An application of our results to intermediate-mass X-ray binaries (IMXBs) is discussed. We find that the predicted upper limit to mass ratios, as a function of orbital period, is consistent with the observed IMXBs that undergo thermal or nuclear timescale mass transfer. According to the observed peak X-ray luminosity, L X, we predict the range of L X for IMXBs as a function of the donor mass and the mass-transfer timescale. [ABSTRACT FROM AUTHOR]

The evolution and the stability of mass transfer of CO+He white dwarf (WD) binaries are not well understood. Observationally they may emerge as AM CVn binaries and are important gravitational wave (GW) emitters. In this work, we have modeled the evolution of double WD binaries with accretor masses of 0.50â€"1.30 M ⊙ and donor masses of 0.17â€"0.45 M ⊙ using the detailed stellar evolution code mesa. We find that the evolution of binaries with same donor masses but different accretor masses is very similar and binaries with same accretor masses but larger He donor masses have larger maximum mass transfer rates and smaller minimum orbital periods. We also demonstrate that the GW signal from AM CVn binaries can be detected by spaceborne GW observatories, such as LISA and TianQin. There is a linear relation between the donor mass and gravitational wave frequency during the mass transfer phase. In our calculation, all binaries can have dynamically stable mass transfer, which is very different from previous studies. The threshold donor mass of Eddington-limited mass transfer for a given accretor WD mass is lower than previous studies. Assuming that a binary may enter a common envelope if the mass transfer rate exceeds the maximum stable burning rate of He, we provide a new criterion for double WDs surviving mass transfer, which is below the threshold of the Eddington limit. Finally, we find that some systems with oxygenâ€"neon (ONe) WDs in our calculation may evolve into detached binaries consisting of neutron stars and extremely low-mass He WDs, and further ultracompact X-ray binaries. [ABSTRACT FROM AUTHOR]

Most subdwarf B (sdB) + Helium white dwarf (He WD) binaries are believed to be formed from a particular channel. In this channel, the He WDs are produced first from red giants (RGs) with degenerate cores via stable mass transfer and sdB stars are produced from RGs with degenerate cores via common-envelope (CE) ejection. They are important for the studies of CE evolution, binary evolution, and binary population synthesis. However, the relation between WD mass and orbital period of sdB + He WD binaries has not been specifically studied. In this paper, we first use a semi-analytic method to follow their formation and find a WD mass and orbital period relation. Then we use a detailed stellar evolution code to model their formation from main-sequence binaries. We find a similar relation between the WD mass and orbital period, which is in broad agreement with observations. For most sdB  + He WD systems, if the WD mass (orbital period) can be determined, the orbital period (WD mass) can be inferred with this relation and then the inclination angle can be constrained with the binary mass function. In addition, we can also use this relation to constrain the CE ejection efficiency and find that a relative large CE ejection efficiency is favoured. If both the WD and sdB star masses can be determined, the critical mass ratios of dynamically unstable mass transfer for RG binaries can also be constrained. [ABSTRACT FROM AUTHOR]

Binaries that evolved from red giant (RG) binaries through stable Roche lobe overflow follow the mass–orbital period relation if the donor's core is degenerate. This relation is important for the studies of long orbital period blue stragglers, subdwarf B stars, and millisecond pulsars. To examine the influence of mass transfer schemes on this relation, we model the evolution of RG binaries at metallicity Z = 0.02 and Z = 0.001 with two different mass transfer schemes. In one of the scheme, it is assumed that mass transfer only occurs when the donor star's radius is just larger than its Roche lobe radius ('classical' scheme). In another mass transfer scheme, the mass transfer can also occur when the donor's radius is smaller than its Roche lobe radius as proposed by Ritter (1988) and further studied by Kolb & Ritter (1990 ; 'Kolb' scheme). We find that the mass transfer scheme has a significant influence on the evolution of RG binaries. In these RG binaries initiating mass transfer on the upper part of the RG branch, the final donor masses are smaller and orbital periods are larger in the models with the 'Kolb' scheme, compared with the models with the 'classical' scheme. The difference between orbital periods (donor masses) at the end of mass transfer under two different mass transfer schemes can be up to 500 d (0.04 M⊙) for the same donor mass (orbital period). The mass–orbital period relation from these models with the 'Kolb' scheme is above the relation from models with the 'classical' scheme. Additionally, similar to previous studies, we find that the final orbital period at metallicity Z = 0.02 is larger than that at metallicity Z = 0.001 for a given donor mass. [ABSTRACT FROM AUTHOR]

Binary stars are as common as single stars. Binary stars are of immense importance to astrophysicists because that they allow us to determine the masses of the stars independent of their distances. They are the cornerstone of the understanding of stellar evolutionary theory and play an essential role in cosmic distance measurement, galactic evolution, nucleosynthesis and the formation of important objects such as cataclysmic variable stars, X-ray binaries, Type Ia supernovae, and gravitational wave-producing double compact objects. In this article, we review the significant theoretical and observational progresses in addressing binary stars in the new millennium. Increasing large survey projects have led to the discovery of enormous numbers of binary stars, which enables us to conduct statistical studies of binary populations, and therefore provide unprecedented insight into the stellar and binary evolution physics. Meanwhile, the rapid development of theoretical concepts and numerical approaches for binary evolution have made a substantial progress on the alleviation of some long-standing binary-related problems such as the stability of mass transfer and common envelope evolution. Nevertheless, it remains a challenge to have a full understanding of fundamental problems of stellar and binary astrophysics. The upcoming massive survey projects and increasingly sophisticated computational methods will lead to future progress. [ABSTRACT FROM AUTHOR]

Wide binaries with hot subdwarf-B (sdB) primaries and main sequence companions are thought to form only through stable Roche-lobe overflow (RLOF) of the sdB progenitor near the tip of the red giant branch (RGB). We present the orbital parameters of 11 new long-period composite sdB binaries based on spectroscopic observations obtained with the UVES, FEROS, and CHIRON spectrographs. Using all wide sdB binaries with known orbital parameters, 23 systems, the observed period distribution is found to match very well with theoretical predictions. A second result is the strong correlation between the orbital period (P) and the mass ratio (q) in the observed wide sdB binaries. In the P-q plane two distinct groups emerge, with the main group (18 systems) showing a strong correlation of lower mass ratios at longer orbital periods. The second group comprises systems that are thought to be formed from higher mass progenitors. Based on theoretical models, a correlation between the initial mass ratio at the start of RLOF and core mass of the sdB progenitor is found, which defines a mass-ratio range at which RLOF is stable on the RGB. [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 can significantly enhance the spectral energy towards short wavelengths, especially in ultraviolet and blue bands. Much evidence shows that blue stragglers are relevant to primordial binaries. In this paper, we systematically studied blue stragglers produced from primordial binary evolution via a binary population synthesis approach, and examined their contribution to the integrated spectral energy distributions of the host clusters. The mass transfer efficiency, β, is an important parameter for the final products (then blue stragglers) after mass transfer, and it is set to be 0.5 except for case A binary evolution. The study shows that primordial binary evolution may produce blue stragglers at any given times, and that different evolutionary channels correspond to blue stragglers in different visual magnitude regions (in V band) on the colour–magnitude diagram (CMD) of clusters. The specific frequency of blue stragglers obtained from primordial binary evolution decreases with time first, and then increases again when the age is larger than 10 Gyr, while that from angular momentum loss induced by magnetic braking in low-mass binaries increases with time and exceeds that of primordial binary evolution in a population older than 3 Gyr. Meanwhile, blue stragglers resulting from primordial binary evolution are dominant contributors to the integrated spectral energy distributions (ISEDs) in ultraviolet and blue bands in a population between 0.3 and 2.0 Gyr. The value of β significantly affects the final results, e.g. the specific frequency of blue stragglers decreasing with β, blue stragglers produced from a high value of β being more massive, then contributing more to the ISEDs of the host clusters. For old open clusters, the assumption of when the primary is in HG at the onset of mass transfer matches the observations better than that of from the locations of blue stragglers on the CMDs. Our study also shows that, for most Galactic open clusters, the specific frequency of blue stragglers obtained from our simulations is lower than that of observations, which is puzzling. Nevertheless, primordial binary evolution cannot account for all blue stragglers observed in old clusters. [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]