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Context. Supersoft X-ray sources (SSSs) have been proposed as one of the progenitors for Type Ia supernovae. However, the exact origin of the quasi-periodic variability in the optical light curve remains a mystery. Aims. In this work, our goal is to investigate the effect of the feedback of an evolved main-sequence companion star on X-ray irradiation and find whether periodic X-ray irradiation of the companion star could reproduce periodic mass transfer. Methods. Using the Modules for Experiments in Stellar Astrophysics (MESA) code, we modeled the evolutionary track of the companion star under the influence of supersoft X-ray irradiation, and we calculated the resulting mass transfer rate. Results. We find that the supersoft X-ray heating of the companion star can result in the expansion of the companion, causing it to greatly overflow its Roche lobe and thereby increasing the mass transfer rate. The periodic X-ray irradiation on the companion stars leads to periodic changes in the mass transfer rate. For a given companion star, higher irradiation efficiencies result in a higher mass transfer rate. Additionally, the mass transfer rate increases as the mass of the companion star decreases for a given irradiation efficiency. Conclusions. The companion star undergoing thermal timescale mass transfer is periodically irradiated by the X-rays from the WD, which can lead to periodic enhancement of the mass transfer rate. The mechanism could be the origin of the quasi-periodic optical light curve in supersoft X-ray sources. [ABSTRACT FROM AUTHOR]

Context. The vast majority of close binaries containing a compact object, including the progenitors of supernovae Ia and at least a substantial fraction of all accreting black holes in the Galaxy, form through common-envelope (CE) evolution. Despite this importance, we struggle to even understand the energy budget of CE evolution. For decades, observed long-period post-CE binaries have been interpreted as evidence of additional energies contributing during CE evolution. We have recently shown that this argument is based on simplified assumptions for all long-period post-CE binaries containing massive white dwarfs (WDs). The only remaining post-CE binary star that has been claimed to require contributions from additional energy sources to understand its formation is KOI 3278. Aims. Here, we address in detail the potential evolutionary history of KOI 3278. In particular, we investigate whether extra energy sources, such as recombination energy, are indeed required to explain its existence. Methods. We used the 1D stellar evolution code MESA to carry out binary evolution simulations and searched for potential formation pathways for KOI 3278 that are able to explain its observed properties. Results. We find that KOI 3278 can be explained if the WD progenitor filled its Roche lobe during a helium shell flash. In this case, the orbital period of KOI 3278 can be reproduced if the CE binding energy is calculated taking into account gravitational energy and thermodynamic internal energy. While the CE evolution that led to the formation of KOI 3278 must have been efficient – that is, most of the available orbital energy must have been used to unbind the CE – recombination energy is not required. Conclusions. We conclude that currently not a single observed post-CE binary requires one to assume that energy sources other than gravitational and thermodynamic energy are contributing to CE evolution. KOI 3278, however, remains an intriguing post-CE binary as, unlike its siblings, understanding its existence requires highly efficient CE ejection. [ABSTRACT FROM AUTHOR]

Post-common envelope binaries (PCEBs) containing a white dwarf (WD) and a main-sequence (MS) star can constrain the physics of common envelope evolution and calibrate binary evolution models. Most PCEBs studied to date have short orbital periods (P orb ≲ 1 d), implying relatively inefficient harnessing of binaries' orbital energy for envelope expulsion. Here, we present follow-up observations of five binaries from 3rd data release of Gaia mission containing solar-type MS stars and probable ultramassive WDs (⁠|$M\gtrsim 1.2\ {\rm M}_{\odot}$|⁠) with significantly wider orbits than previously known PCEBs, P orb = 18–49 d. The WD masses are much higher than expected for systems formed via stable mass transfer at these periods, and their near-circular orbits suggest partial tidal circularization when the WD progenitors were giants. These properties strongly suggest that the binaries are PCEBs. Forming PCEBs at such wide separations requires highly efficient envelope ejection, and we find that the observed periods can only be explained if a significant fraction of the energy released when the envelope recombines goes into ejecting it. Our one-dimensional stellar models including recombination energy confirm prior predictions that a wide range of PCEB orbital periods, extending up to months or years, can potentially result from Roche lobe overflow of a luminous asymptotic giant branch (AGB) star. This evolutionary scenario may also explain the formation of several wide WD + MS binaries discovered via self-lensing, as well as a significant fraction of post-AGB binaries and barium stars. [ABSTRACT FROM AUTHOR]

Black widows and redbacks are binary millisecond pulsars with close low-mass companions that are irradiated and gradually ablated by the pulsar's high-energy luminosity L irr. These binaries evolve primarily through magnetic braking, which extracts orbital angular momentum and pushes the companion to overflow its Roche lobe. Here, we use the stellar evolution code mesa to examine how the irradiation modifies the companion's structure. Strong L irr inhibits convection to the extent that otherwise fully convective stars become almost fully radiative. By computing the convective velocities and assuming a dynamo mechanism, we find that the thin convective envelopes of such strongly irradiated companions (⁠|$L_{\rm irr}\gtrsim 3\, {\rm L}_\odot$|⁠) generate much weaker magnetic fields than previously thought – halting binary evolution. With our improved magnetic braking model, we explain most observed black widow and redback companions as remnants of main-sequence stars. We also apply our model (with L irr) to evolved companions that overflow their Roche lobe close to the end of their main-sequence phase. The evolutionary tracks of such companions bifurcate, explaining the shortest period systems (which are potential gravitational wave sources) as well as the longest period ones (which are the progenitors of common pulsar–white dwarf binaries). The variety of black widow structures and evolutionary trajectories may be utilized to calibrate the dependence of magnetic braking on the size of the convective layer and on the existence of a radiative–convective boundary, with implications for single stars as well as other binaries, such as cataclysmic variables and AM Canum Venaticorum stars. [ABSTRACT FROM AUTHOR]

Helium white dwarfs (HeWDs) are thought to form from low-mass red giant stars experiencing binary interaction. Because the helium core mass of a red giant star is closely related to the stellar radius, there exists well-known relation between the orbital period (P orb) and the mass (M WD) of the HeWDs, which is almost independent of the type of the companion star. Traditional derivation of the M WD– P orb relation generally neglected the effect of wind mass-loss from the red giants, while observations show that wind mass-loss from red giants in binary systems is systematically higher than that from isolated stars. In this work, we calculate binary evolution with tidally enhanced stellar wind (TEW) and find that it causes significantly scatter of the traditional M WD– P orb relation. The TEW can prevent the red giants from overflowing their Roche lobes and slow down the growth of the helium core, leaving a lower mass HeWD for given orbital period. This scenario may account for some of the HeWD binaries that deviate from the traditional M WD– P orb relation. However, we point out that observations of more HeWD binaries in wide orbits are needed to test the TEW model and to constrain the enhanced wind factor. [ABSTRACT FROM AUTHOR]

The origin of current angular momentum (AM) of the black hole (BH) in X-ray binary (XRB) is still unclear, which is related with the birth and/or the growth of the BH. Here, we collect the spin parameters a * measured in BH XRBs and find an apparent bimodal distribution centered at ∼ 0.17 and 0.83. We find a positive relation between the spin parameter and the orbital period/orbital separation through combining distinct XRB categories, including neutron star (NS) low-mass X-ray binaries (LMXBs), Roche lobe overflow (RLOF) BH XRBs, and wind-fed BH XRBs. It seems that the AM of the compact star and the binary orbit correlates by combining the different XRB systems. These positive relations imply that accretion process is a common mechanism for spinning up the compact star in these diverse XRB systems. We infer that the low and high spin BH XRBs may experience different evolution and accretion history, which corresponds to the bimodal distribution of the BH spin parameters. The low spin BHs (a * < 0.3) are similar to the NS LMXBs, the compact star of which is spun-up by the low-level accretion, and the high spin BHs (a * > 0.5) had experienced a short hypercritical accretion (⁠|$\gg \dot{M}_\mathrm{Edd}$|⁠) period, during which, the BH spin dramatically increased. [ABSTRACT FROM AUTHOR]

It has been proposed that accretion-induced collapse (AIC) of massive white-dwarfs (WDs) is an indispensable path for the formation of neutron star (NS) binaries. Although there are still no direct evidence for the existence of AIC events, several kinds of NS systems are suggested to originate from the AIC processes. One of the representative evidence is the detection of the strong magnetic field and slow spin NSs with ultra-light companions (⁠|${\le}0.1\, \rm M_{\odot }$|⁠) in close orbits. However, previous studies cannot explain the formation of AIC events with such low-mass companions. In the present work, we evolve a series of ONe WD+He WD systems to the formation of AIC events (named as the He WD donor channel), and the NS binaries behave as ultra-compact X-ray binaries when the He WDs refill their Roche lobes. We found that the ONe WD+He WD systems a possible channel for the formation of the newly formed NS+ultra-light companion systems just after the AIC event. Although there are some other inconsistent properties, the detected companion mass and orbital period of 4U 1626–67 (one of the newly formed NS binaries with ultra-light companions) can be reproduced by the He WD donor channel. In addition, combined with previous asteroseismology results, we speculate that an UCXB source (XTE J1751−305) may originate from the He WD donor channel. [ABSTRACT FROM AUTHOR]

Many stars live in hierarchical triple systems, but the physics of such systems are still poorly understood. One understudied physical aspect of these systems is tertiary tides, wherein the tidal deformation of a tertiary in a hierarchical triple drains energy from the inner binary, causing the inner binary's orbital separation to decrease. This tidal process is difficult to observe directly, since such an observation requires a very compact hierarchical triple, the tertiary of which must be almost large enough to fill its Roche lobe at the epoch of observation. Concurrently, the recently discovered stellar system TIC242132789 is the fourth most compact observed hierarchical triple, and the most compact in which the tertiary is a giant. In this paper, we demonstrate that TIC242132789 provides a rare opportunity to place constraints on the model parameters for tertiary tides, and can even be a rare opportunity to directly observe tertiary-tides-induced orbital shrinkage of the inner binary. We calculate our expectations of how fast the inner orbit will shrink, and demonstrate that our estimates of this rate of shrinkage should be observable using current techniques. We conclude with a call for relevant observations of this system to commence. [ABSTRACT FROM AUTHOR]

Various interactions affect the population of close-in planets. Among them, the tidal and magnetic interactions drive orbital decay and star-planet angular momentum exchange, leading to stellar spin-up. As a result of the above processes, a planet may initiate the mass transfer to the host star once it encounters the Roche limit. Another mechanism providing substantial mass-loss is associated with the atmospheric escape caused by photoevaporation followed by orbital expansion, which is thought to be important for hot Neptunes and super-Earths. Thus, the fraction of the initial number of hot Jupiters may transform into lower-mass planets through the Roche lobe overflow (RLO) phase and continue secular evolution under the effect of photoevaporation. In this paper, we compile the latest prescriptions for tidal and magnetic migration and mass-loss rates to explore the dynamics of hot Jupiter systems. We study how the implemented interactions shape the orbital architecture of Jovian planets and whether their impact is enough to reproduce the observational sample. Our models suggest that the tidal interaction is able to generate the upper boundary of the hot Jupiter population in the mass–separation diagram. To recreate the sub-Jovian desert, we need to make additional assumptions regarding the RLO phase or the influence of the protoplanetary disc's inner edge on the initial planetary location. According to our estimates, 12–15 per cent of hot Jupiters around solar-mass stars have been engulfed or become lower-mass planets. 0.20–0.25 per cent of the present-day giant planet population undergoes decay intense enough to be detected with modern facilities. [ABSTRACT FROM AUTHOR]

Barium stars have been studied extensively over the past few decades, yet our current understanding of how these intriguing objects formed leaves much to be desired. Many trends observed in systems containing barium stars cannot be satisfactorily explained by classical binary evolution models, naturally raising the question of whether triples and other higher order multiples can give rise to such exotic objects. In this paper, we study the possibility that a Roche Lobe overflow from a tertiary in a hierarchical triple system can potentially lead to surface barium enrichment within the inner binary, while at the same time causing the inner binary to merge, thereby producing a barium star. This possibility has the potential to form a large proportion of Barium stars, as Roche Lobe overflow from a tertiary is typically much more stable for close orbits than that from a binary companion. Various formation channels and mechanisms by which this can be achieved are considered, and constraints on relative formation rates are placed on each scenario. Three recently discovered triple systems containing Ba stars further support our proposed formation mechanism. We conclude that a significant portion of barium stars may be formed from hierarchical triple systems, and that further studies are required in this area before a complete understanding of Barium star populations can be achieved. [ABSTRACT FROM AUTHOR]

Black widows (BWs) are a type of eclipsing millisecond pulsars (MSPs) with companion masses |$M_2\lesssim 0.05\, \rm M_\odot$|⁠ , which can be used to study the accretion history and the radiation of pulsars, as well as the origin of isolated MSPs. Recent observations indicate that there are two subtypes of BWs. One is the BWs with |$M_2 \sim 0.01\!-\!0.05\, \rm M_\odot$|⁠ , whereas another with |$M_2 \lesssim 0.01\, \rm M_\odot$|⁠. However, the origin of the latter is still highly uncertain. In this paper, we investigated the formation of BWs with |$M_2 \lesssim 0.01\, \rm M_\odot$| through ultracompact X-ray binaries (UCXBs) with He star companions, in which a neutron star (NS) accretes material from a He star through Roche lobe overflow. By considering different He star masses and evaporation efficiencies with the stellar evolution code Modules for Experiments in Stellar Astrophysics (mesa), we evolved a series of NS+He star systems that can undergo UCXB stage. This channel can explain the formation of the BWs with |$M_2 \lesssim 0.01\, \rm M_\odot$| within the Hubble time, especially three widely studied BWs, i.e. PSRs J1719−1438, J2322−2650, and J1311−3430. We found that X-ray irradiation feedback does not affect the evolutionary tracks of evaporation process. The simulations indicate that the UCXBs with He star companions are the potential progenitors of isolated MSPs, and that the origin of BWs with |$M_2 \lesssim 0.01\, \rm M_\odot$| is different with another subtype of BWs. In addition, this work suggests that the BWs with |$M_2 \lesssim 0.01\, \rm M_\odot$| may not be produced by redback systems. [ABSTRACT FROM AUTHOR]

V375 Cassiopeia (V375 Cas) is an early B-type close binary with an orbital period of 1.4734273 d. Based on high-quality and continuous light curves from the Transiting Exoplanet Survey Satellite (TESS) and new low-resolution spectra, we have determined photometric solutions of the eclipsing binary using the Wilson–Devinney method. We have found that V375 Cas is a semidetached binary where the secondary component fills the critical Roche lobe. Meanwhile, its O − C diagram was constructed with the data spanning 122 yr. An upward parabolic variation is discovered to be superimposed on a cyclic modulation with a semi-amplitude of 0.0938 (±0.0143) d and a period of 118.74 (±12.87) yr. The parabolic change reveals a period increase at a rate of |${\rm d}p/{\rm d}t = +5.42 \times {10^{-7}}\, {\rm d} \,\mathrm{yr}^{-1}$|⁠. Both the semidetached configuration and the period increase suggest that V375 Cas is undergoing a late case A mass transfer from the less-massive component to the more-massive component. The cyclic change is attributed to be the effect of the light travel time via the presence of a third body. The minimum mass of the tertiary companion is estimated as M 3 = 4.24 (±0.87) M⊙, which orbits around the central eclipsing binary with a nearly circular orbit (e  = 0.184). It is detected that the light contribution of the third body is about 8.0 per cent, indicating that it is a massive main-sequence star. All the results reveal that V375 Cas is a hierarchical triple system where a massive main-sequence star accompanies a massive semidetached mass-transfer binary at an orbital separation of 60.3 au. [ABSTRACT FROM AUTHOR]

We analyse two binary systems containing giant stars, V723 Mon ('the Unicorn') and 2M04123153+6738486 ('the Giraffe'). Both giants orbit more massive but less luminous companions, previously proposed to be mass-gap black holes. Spectral disentangling reveals luminous companions with star-like spectra in both systems. Joint modelling of the spectra, light curves, and spectral energy distributions robustly constrains the masses, temperatures, and radii of both components: the primaries are luminous, cool giants (⁠|$T_{\rm eff,\, giant} = 3800$| and |$4000\, \rm K$|⁠ , |$R_{\rm giant}= 22.5$| and |$25\, {\rm R}_{\odot }$|⁠) with exceptionally low masses (⁠|$M_{\rm giant} \approx 0.4\, {\rm M}_{\odot }$|⁠) that likely fill their Roche lobes. The secondaries are only slightly warmer subgiants (⁠|$T_{\rm eff,\, 2} = 5800$| and |$5150\, \rm K$|⁠ , |$R_2= 8.3$| and |$9\, {\rm R}_{\odot }$|⁠) and thus are consistent with observed UV limits that would rule out main-sequence stars with similar masses (⁠|$M_2 \approx 2.8$| and |${\approx}1.8\, {\rm M}_{\odot }$|⁠). In the Unicorn, rapid rotation blurs the spectral lines of the subgiant, making it challenging to detect even at wavelengths where it dominates the total light. Both giants have surface abundances indicative of CNO processing and subsequent envelope stripping. The properties of both systems can be reproduced by binary evolution models in which a |$1{-}2\, {\rm M}_{\odot }$| primary is stripped by a companion as it ascends the giant branch. The fact that the companions are also evolved implies either that the initial mass ratio was very near unity, or that the companions are temporarily inflated due to rapid accretion. The Unicorn and Giraffe offer a window into into a rarely observed phase of binary evolution preceding the formation of wide-orbit helium white dwarfs, and eventually, compact binaries containing two helium white dwarfs. [ABSTRACT FROM AUTHOR]

We measure rotational broadening in spectra taken by the Apache Point Observatory Galactic Evolution Experiment (APOGEE) survey to characterize the relationship between stellar multiplicity and rotation. We create a sample of 2786 giants and 24 496 dwarfs with stellar parameters and multiple radial velocities from the APOGEE pipeline, projected rotation speeds v sin  i determined from our own pipeline, and distances, masses, and ages measured by Sanders & Das. We use the statistical distribution of the maximum shift in the radial velocities, ΔRVmax, as a proxy for the close binary fraction to explore the interplay between stellar evolution, rotation, and multiplicity. Assuming that the minimum orbital period allowed is the critical period for Roche Lobe overflow and rotational synchronization, we calculate theoretical upper limits on expected v sin  i and ΔRVmax values. These expectations agree with the positive correlation between the maximum ΔRVmax and v sin  i values observed in our sample as a function of log(g). We find that the fast rotators in our sample have a high occurrence of short-period [log(P /d) ≲ 4] companions. We also find that old, rapidly rotating main-sequence stars have larger completeness-corrected close binary fractions than their younger peers. Furthermore, rapidly rotating stars with large ΔRVmax consistently show differences of 1–10 Gyr between the predicted gyrochronological and measured isochronal ages. These results point towards a link between rapid rotation and close binarity through tidal interactions. We conclude that stellar rotation is strongly correlated with stellar multiplicity in the field, and caution should be taken in the application of gyrochronology relations to cool stars. [ABSTRACT FROM AUTHOR]

We analyse properties of the mass outflow from the Roche lobe filling component of a semidetached binary system. We follow the approaches published by Paczyński & Sienkiewicz and by Lubow & Shu, which we compare with other simplified approaches. We find that the density of the flow at L 1 is orders of magnitude lower than the density on the same equipotential but away from L 1. Furthermore, the effective cross-section of the flow, after averaging over its profile of the momentum density, is much lower than some published estimates done without accounting for the averaging. Thus, the use of some simplified formulae for the density and the flow cross-section can lead to overestimates of the accretion rate and of the mass contained in the L 1 regions by very large factors unless they are supported by simultaneous integrations of the equations of stellar structure for the outer layers of the donor. [ABSTRACT FROM AUTHOR]

The majority of close massive binary stars with initial periods of a few days experience a contact phase, in which both stars overflow their Roche lobes simultaneously. We perform the first dedicated study of the evolution of massive contact binaries and provide a comprehensive prediction of their observed properties. We compute 2790 detailed binary models for the Large and Small Magellanic Clouds each, assuming mass transfer to be conservative. The initial parameter space for both grids span total masses from 20 to 80 |$\, \mathrm{M}_\odot$|  , orbital periods of 0.6–2 d and mass ratios of 0.6–1.0. We find that models that remain in contact over nuclear time-scales evolve towards equal masses, echoing the mass ratios of their observed counterparts. Ultimately, the fate of our nuclear-time-scale models is to merge on the main sequence. Our predicted period–mass ratio distributions of O-type contact binaries are similar for both galaxies, and we expect 10 such systems together in both Magellanic Clouds. While we can largely reproduce the observed distribution, we overestimate the population of equal-mass contact binaries. This situation is somewhat remedied if we also account for binaries that are nearly in contact. Our theoretical distributions work particularly well for contact binaries with periods <2 d and total masses |$\lessapprox 45\, \mathrm{M}_\odot \,$|⁠. We expect stellar winds, non-conservative mass transfer, and envelope inflation to have played a role in the formation of the more massive and longer-period contact binaries. [ABSTRACT FROM AUTHOR]

We present ULTRACAM multiband optical photometry of two transitional millisecond pulsars (tMSPs), PSR J1023+0038 and PSR J1227−4853, taken while both were in their radio pulsar states. The light curves show significant asymmetry about the flux maxima in all observed bands, suggesting an asymmetric source of heating in the system. We model the light curves using the icarus binary code, using models with an additional 'hotspot' heating contribution and an anisotropic heat redistribution model to treat the asymmetry. Our modelling reveals companion stars with underfilled Roche lobes in both PSRs J1023+0038 and J1227−4853, with Roche lobe filling factors in the range f ∼ 0.82−0.92. While the volume-averaged filling factors are closer to unity, significant underfilling is unexpected from tMSPs as they must rapidly overfill their Roche lobes to start transferring mass, which occurs on time-scale of weeks or months. We discuss the motivation and validity of our extensions to the models and the implications of the underfilled Roche lobe, and suggest future work to further investigate the role of the filling factor in the tMSP cycle. [ABSTRACT FROM AUTHOR]

Blue large-amplitude pulsators (BLAPs) are a recently discovered class of pulsating star, believed to be proto-white dwarfs, produced by mass stripping of a red giant when it has a small helium core. An outstanding question is why the stars in this class of pulsator seem to form two distinct groups by surface gravity, despite predictions that stars in the gap between them should also pulsate. We use a binary population synthesis model to identify potential evolutionary pathways that a star can take to become a BLAP. We find that BLAPs can be produced either through common envelope evolution or through Roche lobe overflow, with a main-sequence star or an evolved compact object being responsible for the envelope stripping. The mass distribution of the inferred population indicates that fewer stars would be expected in the range of masses intermediate to the two known groups of pulsators, suggesting that the lack of observational discoveries in this region may be a result of the underlying population of pre-white dwarf stars. We also consider metallicity variation and find evidence that BLAPs at Z = 0.010 (half-solar) would be pulsationally unstable and may also be more common. Based on this analysis, we expect the Milky Way to host around 12 000 BLAPs and we predict the number density of sources expected in future observations such as the Legacy Survey of Space and Time at the Vera Rubin Observatory. [ABSTRACT FROM AUTHOR]

Ultracompact X-ray binaries (UCXBs) are low-mass X-ray binaries with hydrogen-deficient mass donors and ultrashort orbital periods. They have been suggested to be the potential Laser Interferometer Space Antenna (LISA) sources in the low-frequency region. Several channels for the formation of UCXBs have been proposed so far. In this paper, we carried out a systematic study on the He star donor channel, in which a neutron star (NS) accretes matter from a He main-sequence (MS) star through Roche lobe overflow, where the mass transfer is driven by the gravitational wave radiation. First, we followed the long-term evolution of the NS+He MS binaries by employing the stellar evolution code Modules for Experiments in Stellar Astrophysics (mesa), and thereby obtained the initial parameter spaces for the production of UCXBs. We then used these results to perform a detailed binary population synthesis approach to obtain the Galactic rates of UCXBs through this channel. We estimate the Galactic rates of UCXBs appearing as LISA sources to be |${\sim} 3.1\!-\!11.9\, \rm Myr^{-1}$| through this channel, and the number of such UCXB- LISA sources in the Galaxy can reach about 1–26 calibrated by observations. This work indicates that the He star donor channel may contribute significantly to the Galactic UCXB formation rate. We found that the evolutionary tracks of UCXBs through this channel can account for the location of the five transient sources with relatively long orbital periods quite well. We also found that such UCXBs can be identified by their locations in the mass-transfer rate versus the orbital period diagram. [ABSTRACT FROM AUTHOR]

GK Cep is an eclipsing binary that contains two A-type components with an orbital period of 0.936 d. The high mass ratio and the contact or near-contact configuration make it a key target to understand binary evolution. However, the physical properties and evolutionary state are still unknown. Here we present the UV light curve obtained by using the lunar-based ultraviolet telescope (LUT) and several eclipse times obtained with the Sino-Thai 70-cm telescope and the 1.0-m telescope at Yunnan Observatories. Analysis of the LUT light curve suggests that GK Cep is a near-contact semidetached system where the slightly less massive component is the hotter one that is filling the critical Roche lobe. Meanwhile, a hotspot is found on the neck of the more massive but cooler component. By using all available times for the light minimum, it is detected that the O–C curve shows a cyclic oscillation while it undergoes an upward parabolic change. The increase of the orbital period, the high mass ratio, the semidetached configuration with a lobe-filling less massive component, and the presence of a hotspot on the more massive component all suggest that GK Cep is in a key evolutionary state just after the shortest-period evolutionary stage during the rapid mass transfer. The cyclic change in the O–C curve reveals that GK Cep contains a solar-type stellar companion in a triply hierarchical system that is in agreement with the spectroscopic result. The third body contributes about 2.6 per cent to the total light of the triple system. [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]

Black widows are millisecond pulsars with low-mass companions, a few per cent the mass of the sun, on orbits of several hours. These companions are presumably the remnants of main-sequence stars that lost their mass through a combination of Roche lobe overflow and ablation by the host pulsar's high-energy radiation. While ablation itself is too weak to significantly reduce the mass of the companion star, the ablated wind couples to its magnetic field, removes orbital angular momentum, and thus maintains stable Roche lobe overflow. We use the mesa stellar evolution code, complemented by analytical estimates, to track initially main-sequence companions as they are reduced to a fraction of their original mass by this ablation-driven magnetic braking. We argue that magnetic braking remains effective even for low-mass companions. A key ingredient of our model is that the irradiating luminosity of the pulsar L irr deposits energy in the companion's atmosphere and thereby slows down its Kelvin–Helmholtz cooling. We find that the high-energy luminosities measured by Fermi   |$L_{\rm irr}=0.1\rm {-}3$| L⊙ can explain the span of black widow orbital periods. The same L irr range reproduces the companions' night-side temperatures, which cluster around 3000 K, as inferred from optical light curves. [ABSTRACT FROM AUTHOR]

We model the Galactic population of detached binaries that harbour black holes with 0.5–1.7 M⊙ companions – remnants of case B mass exchange that rapidly cross Hertzsprung gap after the termination of the Roche lobe overflow or as He-shell burning stars. Several such binaries can be currently present in the Galaxy. The range of M BH in them is about 4–10 M⊙, and the orbital periods are tens to hundreds of days. The unique black hole binary LB-1 fits well into this extremely rare class of double stars. [ABSTRACT FROM AUTHOR]

We present a comparative study between the results of most hydrodynamic simulations of the common envelope binary interaction to date and observations of post-common envelope binaries. The goal is to evaluate whether this data set indicates the existence of a formula that may predict final separations of post-common envelope systems as a function of pre-common envelope parameters. Some of our conclusions are not surprising while others are more subtle. We find that: (i) Values of the final orbital separation derived from common envelope simulations must at this time be considered upper limits. Simulations that include recombination energy do not seem to have systematically different final separations; these and other simulations imply αCE < 0.6–1.0. At least one simulation, applicable to double-degenerate systems, implies αCE < 0.2. (ii) Despite large reconstruction errors, the post-RGB observations reconstructed parameters are in agreement with some of the simulations. The post-AGB observations behave instead as if they had a systematically lower value of αCE. The lack of common envelope simulations with low-mass AGB stars leaves us with no insight as to why this is the case. (iii) The smallest mass companion that survives the common envelope with intermediate mass giants is 0.05–0.1 M⊙. (iv) Observations of binaries with separations larger than ∼10 R⊙, tend to have high M 2/ M 1 mass ratios and may go through a relatively long phase of unstable Roche lobe mass transfer followed by a weakened common envelope (or with no common envelope at all). (v) The effect of the spatial resolution and of the softening length on simulation results remains poorly quantified. [ABSTRACT FROM AUTHOR]

Detached eclipsing binaries are remarkable systems to provide accurate fundamental stellar parameters. Fundamental stellar parameters and the metallicity values of stellar systems are needed to deeply understand stellar evolution and formation. In this study, we focus on the detailed spectroscopic and photometric studies of three detached eclipsing binary systems, V372 And, V2080 Cyg, and CF Lyn, to obtain their accurate stellar and atmospheric parameters, and chemical compositions. An analysis of light and radial velocity curves was carried out to derive the orbital and stellar parameters. The disentangled spectra of component stars were obtained for the spectroscopic analysis. Final T eff, log  g , ξ, v sin  i  parameters and the element abundances of component stars were derived by using the spectrum synthesis method. The fundamental stellar parameters were determined with a high certainty for V372 And, V2080 Cyg (∼1–2 per cent) and with accuracy for CF Lyn (∼2–6 per cent). The evolutionary status of the systems was examined and their ages were obtained. It was found that the component stars of V2080 Cyg have similar iron abundances, slightly lower than the solar iron abundance. Additionally, we show that the primary component of CF Lyn exhibits a non-spherical shape with an 80 per cent Roche lobe filling factor. It could be estimated that CF Lyn will start its first Roche overflow in the next 0.02 Gyr. [ABSTRACT FROM AUTHOR]

We find that applying a theoretical wind mass-loss rate from Monte Carlo radiative transfer models for hydrogen-deficient stars results in significantly more leftover hydrogen following stable mass transfer through Roche lobe overflow than when we use an extrapolation of an empirical fit for Galactic Wolf-Rayet stars, for which a negligible amount of hydrogen remains in a large set of binary stellar evolution computations. These findings have implications for modelling progenitors of Types Ib and IIb supernovae. Most importantly, our study stresses the sensitivity of the stellar evolution models to the assumed mass-loss rates and the need to develop a better theoretical understanding of stellar winds. [ABSTRACT FROM AUTHOR]

Symbiotic stars are interacting binaries with one of the longest known orbital periods. Because they can contain a massive white dwarf with a high accretion rate they are considered possible Type Ia supernova (SN Ia) progenitors. Among symbiotic binaries there are systems containing a Mira donor, which can have orbital periods of a few tens of years or more. This subclass of symbiotic stars has not usually been considered to provide promising SN Ia progenitors owing to their very large separation. We analyse the evolution of a well-studied symbiotic star with a Mira donor, V407 Cyg, and show that the standard evolution model predicts that the system will not become a SN Ia. However, with the addition of wind Roche lobe overflow as one of the mass-transfer modes, we predict that the white dwarf in V407 Cyg will reach the Chandrasekhar limit in 40–200 Myr. [ABSTRACT FROM AUTHOR]

We have simulated a common envelope interaction between a 0.88 M⊙, 90 R⊙, red giant branch star, and a 0.6 M⊙, compact companion with the smoothed particle hydrodynamics code, phantom, from the beginning of the Roche lobe overflow phase to the beginning of the self-regulated inspiral, at three different resolutions. The duration of the Roche lobe overflow phase is resolution dependent and would lengthen with increased resolution beyond the ∼20 yr observed, while the inspiral phase and the post-common envelope separation are largely independent of (average) resolution. Mass transfer rates through the Lagrangian points drive the orbital evolution during the Roche lobe overflow phase, as predicted analytically. The absolute mass transfer rate is resolution dependent, but always within a factor of two of the analytical value. Similarly, the gravitational drag in the simulations is close to the analytical approximation. This verifies simulations and shows that these analytical approximations are reasonable. The L 2 and L 3 outflows observed during Roche lobe overflow remain bound, forming a circumbinary disc that is largely disrupted by the common envelope ejection. However, a longer phase of Roche lobe overflow and weaker common envelope ejection typical of a more stable Roche lobe phase may result in a surviving circumbinary disc. Finally, we examine the density distribution resulting from the interaction for simulations that include or omit the phase of Roche lobe overflow. We conclude that the degree of stability of the Roche lobe phase modulates the shape of the subsequent planetary nebula, explaining the wide range of post-common envelope planetary nebula shapes observed. [ABSTRACT FROM AUTHOR]

We present a population study of low-mass X-ray binaries (LMXBs) and intermediate-mass X-ray binaries with neutron star accretors, performed using the detailed one-dimensional stellar evolution code MESA. We identify all plausible Roche lobe overflowing binaries at the start of mass transfer, and compare our theoretical mass-transfer tracks to the population of well-studied Milky Way LMXBs. The mass-transferring evolution depends on the accepted magnetic braking (MB) law for angular momentum loss. The most common MB prescription ('Skumanich MB') originated from observations of the time-dependence of rotational braking of Sun-type stars, where the angular momentum loss rate depends on the donor mass M d, donor radius R d, and rotation rate Ω, |$\dot{J} \propto M_{\rm d} R_{\rm d}^{\gamma } \Omega ^3$|⁠. The functional form of the Skumanich MB can also be obtained theoretically assuming a radial magnetic field, isotropic isothermal winds, and boosting of the magnetic field by rotation. Here, we show that this simple form of the Skumanich MB law gives mass transfer rates an order of magnitude too weak to explain most observed persistent LMXBs. This failure suggests that the standard Skumanich MB law should not be employed to interpret Galactic, or extragalactic, LMXB populations, with either detailed stellar codes or rapid binary population synthesis codes. We investigate modifications for the MB law, and find that including a scaling of the magnetic field strength with the convective turnover time, and a scaling of MB with the wind mass-loss rate, can reproduce persistent LMXBs, and does a better job at reproducing transient LMXBs. [ABSTRACT FROM AUTHOR]

We consider the evolution of stellar hierarchical quadruple systems in the 2+2 (two binaries orbiting each other's barycentre) and 3+1 (triple orbited by a fourth star) configurations. In our simulations, we take into account the effects of secular dynamical evolution, stellar evolution, tidal evolution, and encounters with passing stars. We focus on Type Ia supernovae (SNe Ia) driven by collisions of carbon–oxygen (CO) white dwarfs (WDs). Such collisions can arise from several channels: (1) collisions due to extremely high eccentricities induced by secular evolution, (2) collisions following a dynamical instability of the system, and (3) collisions driven by semisecular evolution. The systems considered here have initially wide inner orbits, with initial semilatus recti larger than $12\,\textrm {au}$ , implying no interaction if the orbits were isolated. However, taking into account dynamical evolution, we find that ≈0.4 (≈0.6) of 2+2 (3+1) systems interact. In particular, Roche lobe overflow can be triggered possibly in highly eccentric orbits, dynamical instability can ensue due to mass-loss-driven orbital expansion or secular evolution, or a semisecular regime can be entered. We compute the delay-time distributions (DTDs) of collision-induced SNe Ia, and find that they are flatter compared to the observed DTD. Moreover, our combined SNe Ia rates are $(3.7\pm 0.7) \times 10^{-6} \, \mathrm{M}_{\odot }^{-1}$ and $(1.3\pm 0.2) \times 10^{-6} \, \mathrm{M}_{\odot }^{-1}$ for 2+2 and 3+1 systems, respectively, three orders of magnitude lower compared to the observed rate, of the order of $10^{-3} \, \mathrm{M}_{\odot }^{-1}$. The low rates can be ascribed to interactions before the stars evolve to CO WDs. However, our results are lower limits given that we considered a subset of quadruple systems. [ABSTRACT FROM AUTHOR]

W UMa-type contact binaries belong to close binary systems whose components exactly overflow their Roche lobes and share a common convective envelope (CCE). In the last twenty years, the long-term variations of their orbital periods have been thought to depend on several mechanisms. Now, we suggest a new mechanism: CCE-dominated mechanism. The CCE-dominated mechanism is found based on our numerical result, especially at high mass ratios, that the orbital periods (P) of contact binaries change very much with their fill-out factors (f). Because f is taken as a measurement of the thickness of CCE, the physical cause for the variation of P is a mass transfer between CCE and components. Further, an f-dominated simplification model for this mechanism is introduced. According to it, P may change in a long-term oscillation way with a similar time scale of the thermal modulation, meanwhile q is decreasing slowly till the two components merge. It could be also applied to explain the presence of extremely short period, high mass ratio and deep contact binaries. Moreover, the CCE-dominated mechanism should always work due to mass transfer and mass loss both occurring via CCE. Therefor, the effect of CCE on the variations of orbital periods may have been underestimated before. [ABSTRACT FROM AUTHOR]

We have revisited the sdB+K-star long-period binary PG1018-047 based on 20 new highresolution Very Large Telescope/Ultraviolet and Visual Echelle Spectrograph spectra that provided regular coverage over a period of more than 26 m. We refine the period and establish that the orbit is significantly eccentric (P = 751.6 ± 1.9 d and e=0.049 ± 0.008). A simultaneous fit derived from the narrow metal lines visible in the spectrum of the sdB star and the metal lines in the red part of the spectrum that originate from the companion provides the mass ratio, MMS/MsdB = 1.52 ± 0.04, for the system. From an NLTE model atmosphere analysis of the combined spectra, we find Teff =29900 ± 330 K, log g= 5.65 ± 0.06 dex and log(nHe/nH)=-3.98 ± 0.16 dex for the primary, consistent with a B-type hot subdwarf star. The spectral contribution of the companion is consistent with a K5V-type star. With the companion having a mass of only ~0.7M⊙, this system lies close to the boundary below which stable Roche lobe overflow (RLOF) cannot be supported. To model the evolution of such a system, we have extended earlier MESA models towards lower companion masses. We find that both phase-dependent mass loss during RLOF, when 30 to 40 per cent of the available mass is lost through the outer Lagrange point and phase-dependent mass loss during RLOF in combination with a circumbinary disc of maximum MCB = 0.001M⊙ could have formed the PG 1018-047 binary system. [ABSTRACT FROM AUTHOR]

We present hydrodynamic simulations of the common envelope binary interaction between a giant star and a compact companion carried out with the adaptive mesh refinement code ENZO and the smooth particle hydrodynamics code PHANTOM. These simulations mimic the parameters of one of the simulations by Passy et al. but assess the impact of a larger, more realistic initial orbital separation on the simulation outcome. We conclude that for both codes the post-common envelope separation is somewhat larger and the amount of unbound mass slightly greater when the initial separation is wide enough that the giant does not yet overflow or just overflows its Roche lobe. PHANTOM has been adapted to the common envelope problem here for the first time and a full comparison with ENZO is presented, including an investigation of convergence as well as energy and angular momentum conservation. We also set our simulations in the context of past simulations. This comparison reveals that it is the expansion of the giant before rapid in-spiral and not spinning up of the star that causes a larger final separation. We also suggest that the large range in unbound mass for different simulations is difficult to explain and may have something to do with simulations that are not fully converged. [ABSTRACT FROM AUTHOR]

The black holes (BH) in merging BH-BH binaries are likely progeny of binary O stars. Their properties, including their spins, will be strongly influenced by the evolution of their progenitor Ostars. The remarkable observation that many singleOstars spin very rapidly can be explained if they accreted angular momentum from a mass-transferring, O-type or Wolf-Rayet (WR) companion before that star blew up as a supernova. To test this prediction, we have measured the spin rates of eight O stars in WR + O binaries, increasing the total sample size of such O stars' measured spins from 2 to 10. Polarimetric and other determinations of these systems' sin i allow us to determine an average equatorial rotation velocity from He I (He II) lines of ve = 348 (173) km s-1 for these O stars, with individual star's ve from He I (He II) lines ranging from 482 (237) to 290 (91) km s-1. We argue that the ~100 per cent difference between He I and He II speeds is due to gravity darkening. Supersynchronous spins, now observed in all 10 O stars in WR + O binaries where it has been measured, are strong observational evidence that Roche lobe overflow mass transfer from a WR progenitor companion has played a critical role in the evolution of WR + OB binaries. While theory predicts that this mass transfer rapidly spins up the O-type mass gainer to a nearly breakup rotational velocity of ve ~ 530 km s-1, the observed average ve of the O-type stars in our sample is 65 per cent of that speed. This demonstrates that, even over the relatively short WR-phase time-scale, tidal and/or other effects causing rotational spin-down must be efficient. A challenge to tidal synchronization theory is that the two longest period binaries in our sample (with periods of 29.7 and 78.5 d) unexpectedly display supersynchronous rotation. [ABSTRACT FROM AUTHOR]

The most frequently seen binary companions to millisecond pulsars (MSPs) are helium white dwarfs. The standard rejuvenation mechanism, in which a low- to intermediate-mass companion to a neutron star fills its Roche lobe between central hydrogen exhaustion and core helium ignition, is the most plausible formation mechanism. We have investigated whether the observed population can realistically be formed via this mechanism. We used the Cambridge STARS code to make models of Case B Roche-lobe overflow with Reimers' mass-loss from the donor. We find that the range of initial orbital periods required to produce the currently observed range of orbital periods of MSPs is extremely narrow. To reduce this fine tuning, we introduce a companion enhanced wind (CEW) that strips the donor of its envelope more quickly so that systems can detach at shorter periods. Our models indicate that the fine tuning can be significantly reduced if a CEW is active. Because significant mass is lost owing to a CEW, we expect some binary pulsars to accrete less than the 0.1Mʘ needed to spin them up to millisecond periods. This can account for mildly recycled pulsars present along the entire Mc-Porb relation. Systems with Pspin > 30 ms are consistent with this, but too few of these mildly recycled pulsars have yet been observed to make a significant comparison. [ABSTRACT FROM AUTHOR]

We have used the tidal equations of Zahn to determine the maximum orbital distance at which companions are brought into Roche lobe contact with their giant primary, when the primary expands during the giant phases. This is a key step when determining the rates of interaction between giants and their companions. Our stellar structure calculations are presented as maximum radii reached during the red and asymptotic giant branch (RGB and AGB, respectively) stages of evolution for masses between 0.8 and 4.0 M⊙ (Z=0.001–0.04) and compared with other models to gauge the uncertainty on radii deriving from details of these calculations. We find overall tidal capture distances that are typically 1–4 times the maximum radial extent of the giant star, where companions are in the mass range from 1 MJ to a mass slightly smaller than the mass of the primary. We find that only companions at initial orbital separations between ∼320 and ∼630 R⊙ will be typically captured into a Roche lobe-filling interaction or a common envelope on the AGB. Comparing these limits with the period distribution for binaries that will make planetary nebula (PN), we deduce that in the standard scenario where all ∼1–8 M⊙ stars make a PN, at most 2.5 per cent of all PN should have a post-common envelope central star binary, at odds with the observational lower limit of 15–20 per cent. The observed overabundance of post-interaction central stars of PN cannot be easily explained considering the uncertainties. We examine a range of explanations for this discrepancy. [ABSTRACT FROM AUTHOR]

HD 170582 is an interacting binary of the double periodic variable (DPV) type, showing ellipsoidal variability with a period of 16.87 d along with a long photometric cycle of 587 d. It was recently studied by Mennickent et al., who found a slightly evolved B-type star surrounded by a luminous accretion disc fed by a Roche lobe overflowing A-type giant. Here we extend their analysis presenting new spectroscopic data and studying the Balmer emission lines. We find orbitally modulated double-peak Ha and Hβ emissions whose strength also vary in the long term. In addition, Doppler maps of the emission lines reveal sites of enhanced line emission in the first and fourth velocity quadrants, the first one consistent with the position of one of the bright zones detected by the light-curve analysis. We find a difference between Doppler maps at high and low stage of the long cycle; evidence that the emission is optically thicker at high state in the stream-disc impact region, possibly reflecting a larger mass transfer rate. We compare the system parameters with a grid of synthetic binary evolutionary tracks and find the best-fitting model. The system is found to be semi-detached, in a conservative Case-B mass transfer stage, with age 7.68 × 107 yr and mass transfer rate 1.6 × 10-6 M⊙ yr-1. For five well-studied DPVs, the disc luminosity scales with the primary mass and is much larger than the theoretical accretion luminosity. [ABSTRACT FROM AUTHOR]

The Rotten Egg Nebula has at its core a binary composed of a Mira star and an A-type companion at a separation >10 au. It has been hypothesized to have formed by strong binary interactions between the Mira and a companion in an eccentric orbit during periastron passage ~800 yr ago. We have performed hydrodynamic simulations of an asymptotic giant branch (AGB) star interacting with companions with a range of masses in orbits with a range of initial eccentricities and periastron separations. For reasonable values of the eccentricity, we find that Roche lobe overflow can take place only if the periods are ⪡100 yr. Moreover, mass transfer causes the system to enter a common envelope phase within several orbits. Since the central star of the Rotten Egg nebula is an AGB star, we conclude that such a common envelope phase must have lead to a merger, so the observed companion must have been a tertiary companion of a binary that merged at the time of nebula ejection. Based on the mass and time-scale of the simulated disc formed around the companion before the common envelope phase, we analytically estimate the properties of jets that could be launched. Allowing for super-Eddington accretion rates, we find that jets similar to those observed are plausible, provided that the putative lost companion was relatively massive. [ABSTRACT FROM AUTHOR]

Many classes of objects and events are thought to form in binary star systems after a phase in which a core and companion spiral to smaller separation inside a common envelope (CE). Such a phase can end with the merging of the two stars or with the ejection of the envelope to leave a surviving binary system. The outcome is usually predicted by calculating the separation to which the stars must spiral to eject the envelope, assuming that the ratio of the core–envelope binding energy to the change in orbital energy is equal to a constant efficiency factor α. If either object would overfill its Roche lobe at this end-of-CE separation, then the stars are assumed to merge. It is unclear what critical radius should be compared to the end-of-CE Roche lobe for stars which have developed cores before the start of a CE phase. After improving the core radius formulae in the widely used bse rapid evolution code, we compare the properties of populations in which the critical radius is chosen to be the pre-CE core radius or the post-CE stripped remnant radius. Our improvements to the core radius formulae and the uncertainty in the critical radius significantly affect the rates of merging in CE phases of most types. We find the types of systems for which these changes are most important. [ABSTRACT FROM AUTHOR]

We examine the growing data set of binary millisecond pulsars that are thought to have a helium white dwarf companion. These systems are believed to form when a low- to intermediate-mass companion to a neutron star fills its Roche lobe between central hydrogen exhaustion and core helium ignition. We confirm that our own stellar models reproduce a well-defined period–companion mass relation irrespective of the details of the mass transfer process. With magnetic braking, this relation extends to periods of less than 1 d for a 1 M⊙ giant donor. With this and the measured binary mass functions, we calculate the orbital inclination of each system for a given pulsar mass. We expect these inclinations to be randomly oriented in space. If the masses of the pulsars were typically 1.35 M⊙, then there would appear to be a distinct dearth of high-inclination systems. However, if the pulsar masses are more typically from 1.55 to 1.65 M⊙, then the distribution of inclinations is indeed indistinguishable from random. If it were as much as 1.75 M⊙, then there would appear to be an excess of high-inclination systems. Thus, with the available data, we can argue that the neutron star masses in binary millisecond pulsars recycled by mass transfer from a red giant typically lie around 1.6 M⊙ and that there is no preferred inclination at which these systems are observed. Hence, there is reason to believe that pulsar beams are either sufficiently broad or show no preferred direction relative to the pulsar's spin axis which is aligned with the binary orbit. This is contrary to some previous claims, based on a subset of the data available today, that there might be a tendency for the pulsar beams to be perpendicular to their spin. [ABSTRACT FROM AUTHOR]

Context. An important ingredient in binary evolution is the common-envelope (CE) phase. Although this phase is believed to be responsible for the formation of many close binaries, the process is not well understood. Aims. We investigate the characteristics of the population of post-common-envelope binaries (PCEB). As the evolution of these binaries and their stellar components are relatively simple, this population can be directly used to constraint CE evolution. Methods. We use the binary population synthesis code SeBa to simulate the current-day population of PCEBs in the Galaxy. We incorporate the selection effects in our model that are inherent to the general PCEB population and that are specific to the SDSS survey, which enables a direct comparison for the first time between the synthetic and observed population of visible PCEBs. Results. We find that selection effects do not play a significant role on the period distribution of visible PCEBs. To explain the observed dearth of long-period systems, the ɑ-CE efficiency of the main evolutionary channel must be low. In the main channel, the CE is initiated by a red giant as it fills its Roche lobe in a dynamically unstable way. Other evolutionary paths cannot be constrained more. Additionally our model reproduces well the observed space density, the fraction of visible PCEBs amongst white dwarf (WD)-main sequence (MS) binaries, and the WD mass versus MS mass distribution, but overestimates the fraction of PCEBs with helium WD companions. [ABSTRACT FROM AUTHOR]

Context. Studies of interacting binary systems typically assume that tidal forces have circularized the orbit by the time Roche lobe overflow (RLOF) commences. However, recent observations of ellipsoidal variables have challenged this assumption. Aims. We present the first calculations of mass transfer via RLOF for a binary system with a significant eccentricity using our new binary stellar evolution code. The study focuses on a 1.50+1.40 M⊚ main sequence binary with an eccentricity of 0.25, and an orbital period of Porb ≈ 0.7 d. The reaction of the stellar components due to mass transfer is analysed, and the evolution of mass transfer during the periastron passage is compared to recent smooth particle hydrodynamics (SPH) simulations. The impact of asynchronism and non-zero eccentricity on the Roche lobe radius, and the effects of tidal and rotational deformation on the stars' structures, are also investigated. Methods. Calculations were performed using the state-of-the-art binary evolution code BINSTAR, which calculates simultaneously the structure of the two stars and the evolution of the orbital parameters. Results. The evolution of the mass transfer rate during an orbit has a Gaussian-like shape, with a maximum at periastron, in qualitative agreement with SPH simulations. The Roche lobe radius is modified by the donor star's spin and the orbital eccentricity. This has a significant impact on both the duration and the rate of mass transfer. We find that below some critical rotation rate, mass transfer never occurs, while above some threshold, mass is transferred over the entire orbit. Tidal and rotational deformation of the donor star causes it to become over-sized, enhancing the mass transfer rate further by up to about a factor of ten, leading to non-conservative mass transfer. The modulation of the mass transfer rate with orbital phase produces short-term variability in the surface luminosity and radius of each star. The longer-term behaviour shows, in accordance with studies of circular systems with radiative stars, that the donor becomes ever small and under-luminous, while the converse is the case for the accretor. [ABSTRACT FROM AUTHOR]

The common-envelope (CE) phase is an important stage in the evolution of binary stellar populations. The most common way to compute the change in orbital period during a CE is to relate the binding energy of the envelope of the Roche-lobe filling giant to the change in orbital energy. Especially in population-synthesis codes, where the evolution of millions of stars must be computed and detailed evolutionary models are too expensive computationally, simple approximations are made for the envelope binding energy. In this study, we present accurate analytic prescriptions based on detailed stellar-evolution models that provide the envelope binding energy for giants with metallicities between Z = 10-4 and Z = 0.03 and masses between 0.8 M⊙ and 100 M⊙, as a function of the metallicity, mass, radius, and evolutionary phase of the star. Our results are also presented in the form of electronic data tables and Fortran routines that use them. We find that the accuracy of our fits is better than 15% for 90% of our model data points in all cases, and better than 10% for 90% of our data points in all cases except the asymptotic giant branches for three of the six metallicities we consider. For very massive stars (M ≳ 50 M⊙), when stars lose more than ~20% of their initial mass due to stellar winds, our fits do not describe the models as accurately. Our results are more widely applicable--covering wider ranges of metallicity and mass--and are of higher accuracy than those of previous studies. [ABSTRACT FROM AUTHOR]

A photoelectric light curve of BL And is presented along with the first CCD light curve of GW Tau. Both objects are short-period eclipsing binaries and were observed in 2003 or 2004. Photometric elements were computed using the latest version of the Wilson–Van Hamme code. The results reveal that BL And is a semidetached system with the primary component filling its Roche lobe and the secondary one almost filling but still detached, while GW Tau is a marginal-contact binary system with a small degree of contact ( f= 10.9 per cent) and a large temperature difference of about 3100 K. All available eclipse times, including new ones, were analysed for each system. It was found that the orbital period of BL And is decreasing at the rate of while that of GW Tau may be decreasing or oscillating. We think period decrease is more probable. The derived configuration and secular period decrease for BL And combined with the asymmetry of the light curve indicate that this system may evolve from the present semidetached phase into a contact stage, with mass transfer from the primary component to the secondary one through the L1 point, or that it might just undergo the broken stage predicted by the theory of thermal relaxation oscillations. In contrast, GW Tau is a marginal-contact binary in poor thermal contact and may be at the beginning of the contact phase. [ABSTRACT FROM AUTHOR]

The concept of Roche lobe overflow is fundamental to the theory of interacting binaries. Based on potential theory, it is dependent on all the relevant material corotating in a single frame of reference. Therefore if the mass losing star is asynchronous with the orbital motion or the orbit is eccentric, the simple theory no longer applies and no exact analytical treatment has been found. We use an analytic approximation whose predictions are largely justified by smoothed particle hydrodynamic simulations (SPH). We present SPH simulations of binary systems with the same semi-major axis, massesand radiusfor the primary star but with different eccentricitiesand 0.7. In each case the secondary star is treated as a point mass. Whenno mass is lost from the primary while atcatastrophic mass transfer, partly through the L2 point, takes place near periastron. This would probably lead to common-envelope evolution if star 1 were a giant or to coalescence for a main-sequence star. In between, at, some mass is lost through the L1 point from the primary close to periastron. However, rather than being all accreted by the secondary, some of the stream appears to leave the system. Our results indicate that the radius of the Roche lobe is similar to circular binaries when calculated for the separation and angular velocity at periastron. Part of the mass loss occurs through the L2 point. [ABSTRACT FROM AUTHOR]

The structure and evolution of low-mass W UMa-type contact binaries are discussed by employing Eggleton's stellar evolution code. Assuming that these systems completely satisfy Roche geometry for contact binaries with every kind of mass ratio (0.02–1.0) we calculate the relative radii ( , where are the radii of both stars and A is the orbital separation) of both components of contact binaries with different contact depths between inner and outer Roche lobes. We obtain a radius grid of contact binaries and can ensure the surfaces of two components lying on an equipotential surface by interpolation using this radius grid when we follow the evolution of the contact binaries. There are serious uncertainties concerning mainly the transfer of energy in these systems, i.e. it is unclear how and where the energy is transferred. We assume that the energy transfer takes place in different regions of the common envelope to investigate the effects of the region of energy transfer on the structure and evolution of contact binaries. We find that the region of energy transfer has a significant influence on the structure and evolution of contact binaries, and conclude that the energy transfer may occur in the outermost layers of the common convective envelope for W-type systems, and that this transfer takes place in the deeper layers of the common envelope for A-type systems. Meanwhile, if we assume that the energy transfer takes place in the outermost layers for our model with low total mass, and find that our model steadily evolves towards a system with a smaller mass ratio and a deeper envelope, suggesting that some A-type W UMa systems with low total mass could be considered as the later evolutionary stages of W-subtype systems, and that the surface temperature of the secondary exceeds that of the primary during the time when the primary expands rapidly, or the secondary contracts rapidly, suggesting that W-subtype systems may be caused by expansion of the primary, or by the contraction of the secondary. [ABSTRACT FROM AUTHOR]

We have carried out a detailed study of one of the most favoured evolutionary channels for the production of Type Ia supernova (SN Ia) progenitors, the single-degenerate channel (CO + MS), where a carbon/oxygen (CO) white dwarf (WD) accretes matter from an unevolved or slightly evolved non-degenerate star until it reaches the Chandrasekhar mass limit. Employing Eggleton's stellar evolution code and adopting the prescription of Hachisu et al. for the accretion efficiency, we performed binary stellar evolution calculations for about 2300 close WD binary systems and mapped out the initial parameters in the orbital period–secondary mass plane (for a range of WD masses) which lead to a successful Type Ia supernova. We obtained accurate, analytical fitting formulae to describe this parameter range which can be used for binary population synthesis (BPS) studies. The contours in the plane differ from those obtained by Hachisu et al. for low-mass CO WDs, which are more common than massive CO WDs. We confirm that WDs with a mass as low as 0.67 M⊙ can accrete efficiently and reach the Chandrasekhar limit. We have implemented these results in a BPS study to obtain the birth rates for SNe Ia and the evolution of birth rates with time of SNe Ia for both a constant star formation rate and a single starburst. The birth rates are lower than (but comparable to) those inferred observationally. [ABSTRACT FROM AUTHOR]