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We address the challenge of running thermally pulsing-(super)asymptotic giant branch [TP-(S)AGB] models, with a 1D hydrostatic stellar evolution code, without suffering instabilities that terminate the evolution. We investigate two instabilities that usually occur during the luminosity peak following a thermal pulse: the hydrogen recombination instability and the Fe-peak instability. Both instabilities occur when the stellar mass is significantly reduced (⁠|$M \lesssim M_\mathrm{i}/2$|⁠) at the end of the TP-(S)AGB in our models with initial mass |$M_\mathrm{i}\gtrsim 2~\mathrm{M}_\odot$|⁠. The hydrogen recombination instability occurs due to the difficulty of modelling a thermally and dynamically unstable envelope in a 1D hydrostatic code, and is prevented by damping the energy released by hydrogen recombination in the outer envelope. The Fe-peak instability occurs when the radiation pressure drops at the base of the convective envelope and is prevented by boosting the convective energy transport in this region. We provide custom routines to prevent these instabilities in the stellar evolution code mesa. The impact of these routines on the stellar structure is minimized so as to not affect the efficiency of third dredge-up, hot-bottom burning, or the wind mass-loss rate. We find only a modest reduction in third dredge-up efficiency at small envelope masses (⁠|$M_\mathrm{env}\lesssim 1.0~\mathrm{M}_\odot$|⁠). Consequently, our |$M_\mathrm{i}=5~\mathrm{M}_\odot$| star, with hot-bottom burning, becomes a carbon star for the last |$\sim 10~{{\ \rm per\ cent}}$| of its thermally pulsing lifetime. The largest stellar radii are reached during the final thermal pulses, which may have important consequences for binary–star interactions. [ABSTRACT FROM AUTHOR]

The Thermally Pulsing- (Super) Asymptotic Giant Branch is a late stage in the evolution of low- and intermediate-mass stars. These stars undergo strong wind mass-loss and diverse nucleosynthesis. Third dredge-up events, that occur following thermal pulses, are responsible for enriching the surfaces of Asymptotic Giant Branch stars, hence an understanding of this process is crucial for constraining galactic chemical evolution. Using a custom numerical scheme, we investigate the temporal and spatial resolution required to resolve the third dredge-up in the 1D stellar evolution code mesa. With mesa 's default controls, the third dredge-up efficiency is underestimated by as much as |$\approx 76~{{\ \rm per\ cent}}$|⁠. In stars that undergo hot third dredge-up (M ≳ 6 M⊙), the third dredge-up efficiency is overestimated by |$\approx 55~{{\ \rm per\ cent}}$|⁠. The Thermally Pulsing- (Super) Asymptotic Giant Branch (TP-(S)AGB) evolution is computed for models with initial masses 1 ≤ M i/M⊙ ≤ 8 at Solar metallicity (Z = 0.014). The minimum initial mass for carbon stars falls in the range 1.5–1.75 M⊙, compatible with observations. The use of mesa for TP-(S)AGB evolution is validated by comparison to the widely used monash models which show good agreement in the maximum third dredge-up efficiency at initial masses M i > 2 M⊙. We also compare the third dredge-up efficiency in models produced using two independent stellar evolution codes, fruity and aton , which were computed with various differences in input physics including mass-loss, and which exhibit weaker third dredge-up episodes. [ABSTRACT FROM AUTHOR]

X-ray binaries (XRBs) are thought to regulate cosmic thermal and ionization histories during the Epoch of Reionization and Cosmic Dawn (z ∼ 5–30). Theoretical predictions of the X-ray emission from XRBs are important for modelling such early cosmic evolution. Nevertheless, the contribution from Be-XRBs, powered by accretion of compact objects from decretion discs around rapidly rotating O/B stars, has not been investigated systematically. Be-XRBs are the largest class of high-mass XRBs (HMXBs) identified in local observations and are expected to play even more important roles in metal-poor environments at high redshifts. In light of this, we build a physically motivated model for Be-XRBs based on recent hydrodynamic simulations and observations of decretion discs. Our model is able to reproduce the observed population of Be-XRBs in the Small Magellanic Cloud with appropriate initial conditions and binary stellar evolution parameters. We derive the X-ray output from Be-XRBs as a function of metallicity in the (absolute) metallicity range Z ∈ [10−4, 0.03] with a large suite of binary population synthesis (BPS) simulations. The simulated Be-XRBs can explain a non-negligible fraction (⁠|$\gtrsim 30{{\ \rm per\ cent}}$|⁠) of the total X-ray output from HMXBs observed in nearby galaxies for Z ∼ 0.0003–0.02. The X-ray luminosity per unit star formation rate from Be-XRBs in our fiducial model increases by a factor of ∼8 from Z  = 0.02 to Z  = 0.0003, which is similar to the trend seen in observations of all types of HMXBs. We conclude that Be-XRBs are potentially important X-ray sources that deserve greater attention in BPS of XRBs. [ABSTRACT FROM AUTHOR]

We present detailed implementations of (i) binary stellar evolution (BSE; using binary_c) and (ii) dust production and destruction into the cosmological semi-analytic galaxy evolution simulation, l-galaxies. This new version of l-galaxies is compared to a version assuming only single stars and to global and spatially-resolved observational data across a range of redshifts (z). We find that binaries have a negligible impact on the stellar masses, gas masses, and star formation rates of galaxies if the total mass ejected by massive stars is unchanged. This is because massive stars determine the strength of supernova (SN) feedback, which in turn regulates galaxy growth. Binary effects, such as common envelope ejection and novae, affect carbon and nitrogen enrichment in galaxies; however, heavier alpha elements are more affected by the choice of SN and wind yields. Unlike many other simulations, the new l-galaxies reproduces observed dust-to-metal (DTM) and dust-to-gas (DTG) ratios at z ∼ 0–4. This is mainly due to shorter dust accretion time-scales in dust-rich environments. However, dust masses are under-predicted at z ≳ 4, highlighting the need for enhanced dust production at early times in simulations, possibly accompanied by increased star formation. On sub-galactic scales, there is very good agreement between l-galaxies and observed dust and metal radial profiles at z  = 0. A drop in DTM ratio is also found in diffuse low-metallicity regions, contradicting the assumption of a universal value. We hope that this work serves as a useful template for BSE implementations in other cosmological simulations in future. [ABSTRACT FROM AUTHOR]

Binary stars evolve into chemically peculiar objects and are a major driver of the galactic enrichment of heavy elements. During their evolution they undergo interactions, including tides, that circularize orbits and synchronize stellar spins, impacting both individual systems and stellar populations. Using Zahn's tidal theory and mesa main-sequence model grids, we derive the governing parameters λ lm and E 2, and implement them in the new mint library of the stellar population code binary_c. Our mint equilibrium tides are two to five times more efficient than the ubiquitous bse prescriptions, while the radiative-tide efficiency drops sharply with increasing age. We also implement precise initial distributions based on bias-corrected observations. We assess the impact of tides and initial orbital-parameter distributions on circularization and synchronization in eight open clusters, comparing synthetic populations and observations through a bootstrapping method. We find that changing the tidal prescription yields no statistically significant improvement as both calculations typically lie within 0.5σ. The initial distribution, especially the primordial concentration of systems at log10(P /d) ≈ 0.8, e ≈ 0.05 dominates the statistics even when artificially increasing tidal strength. This confirms the inefficiency of tides on the main sequence and shows that constraining tidal-efficiency parameters using the e − log10(P /d) distribution alone is difficult or impossible. Orbital synchronization carries a more striking age-dependent signature of tidal interactions. In M35 we find twice as many synchronized rotators in our mint calculation as with bse. This measure of tidal efficiency is verifiable with combined measurements of orbital parameters and stellar spins. [ABSTRACT FROM AUTHOR]

We use the rapid binary stellar evolution code binary_c to estimate the rate of merging neutron stars with numerous combinations of envelope ejection efficiency and natal kick dispersion. We find a peak in the local rate of merging neutron stars around α ≈ 0.3–0.4, depending on the metallicity, where α is the efficiency of utilizing orbital energy to unbind the envelope. The peak height decreases with increasing electron-capture supernova kick dispersion σECSN. We explain the peak as a competition between the total number of systems that survive the common-envelope phase increasing with α and their separation, which increases with α as well. Increasing α reduces the fraction of systems that merge within a time shorter than the age of the Universe and results in different mass distributions for merging and non-merging double neutron stars. This offers a possible explanation for the discrepancy between the Galactic double neutron star mass distribution and the observed massive merging neutron star event GW190425. Within the α–σECSN parameter space that we investigate, the rate of merging neutron stars spans several orders of magnitude up to more than |$1\times 10^{3} \, \mathrm{Gpc}^{-3}\, \mathrm{yr}^{-1}$| and can be higher than the observed upper limit or lower than the observed lower limit inferred thus far from merging neutron stars detected by gravitational waves. Our results stress the importance of common-envelope physics for the quantitative prediction and interpretation of merging binary neutron star events in this new age of gravitational wave astronomy. [ABSTRACT FROM AUTHOR]

We develop a rapid algorithm for the evolution of stable, circular, circumbinary discs suitable for parameter estimation and population synthesis modelling. Our model includes disc mass and angular momentum changes, accretion on to the binary stars, and binary orbital eccentricity pumping. We fit our model to the post-asymptotic giant branch (post-AGB) circumbinary disc around IRAS 08544−4431, finding reasonable agreement despite the simplicity of our model. Our best-fitting disc has a mass of about |$0.01\, \mathrm{M}_{\odot }$| and angular momentum |$2.7\times 10^{52}\, \mathrm{g}\, \mathrm{cm}^{2}\, \mathrm{s}^{-1}\simeq 9 \,\mathrm{M}_{\odot }\, \mathrm{km}\, \mathrm{s}^{-1}\, \mathrm{au}$|⁠ , corresponding to 0.0079 and 0.16 of the common-envelope mass and angular momentum, respectively. The best-fitting disc viscosity is αdisc = 5 × 10−3 and our tidal torque algorithm can be constrained such that the inner edge of the disc R in ∼ 2 a. The inner binary eccentricity reaches about 0.13 in our best-fitting model of IRAS 08544−4431, short of the observed 0.22. The circumbinary disc evaporates quickly when the post-AGB star reaches a temperature of |$\sim \! 6\times 10^4\, \mathrm{K}$|⁠ , suggesting that planetismals must form in the disc in about |$10^{4}\, \mathrm{yr}$| if secondary planet formation is to occur, while accretion from the disc on to the stars at ∼10 times the inner-edge viscous rate can double the disc lifetime. [ABSTRACT FROM AUTHOR]

The metallicity of a star affects its evolution in a variety of ways, changing stellar radii, luminosities, lifetimes, and remnant properties. In this work, we use the population synthesis code binary_c to study how metallicity affects novae in the context of binary stellar evolution. We compute a 16-point grid of metallicities ranging from Z  = 10−4 to 0.03, presenting distributions of nova white dwarf masses, accretion rates, delay-times, and initial system properties at the two extremes of our 16-point metallicity grid. We find a clear anticorrelation between metallicity and the number of novae produced, with the number of novae at Z  = 0.03 roughly half that at Z  = 10−4. The white dwarf mass distribution has a strong systematic variation with metallicity, while the shape of the accretion rate distribution is relatively insensitive. We compute a current nova rate of approximately 33 novae per year for the Milky Way, a result consistent with observational estimates relying on extra-Galactic novae but an under-prediction relative to observational estimates relying on Galactic novae. However, the shape of our predicted Galactic white dwarf mass distribution differs significantly to existing observationally derived distributions, likely due to our underlying physical assumptions. In M31, we compute a current nova rate of approximately 36 novae per year, under-predicting the most recent observational estimate of |$65^{+15}_{-16}$|⁠. Finally, we conclude that when making predictions about currently observable nova rates in spiral galaxies, or stellar environments where star formation has ceased in the distant past, metallicity can likely be considered of secondary importance compared to uncertainties in binary stellar evolution. [ABSTRACT FROM AUTHOR]

Novae are some of the most commonly detected optical transients and have the potential to provide valuable information about binary evolution. Binary population synthesis codes have emerged as the most effective tool for modelling populations of binary systems, but such codes have traditionally employed greatly simplified nova physics, precluding detailed study. In this work, we implement a model treating H and He novae as individual events into the binary population synthesis code binary_c. This treatment of novae represents a significant improvement on the 'averaging' treatment currently employed in modern population synthesis codes. We discuss the evolutionary pathways leading to these phenomena and present nova event rates and distributions of several important physical parameters. Most novae are produced on massive white dwarfs, with approximately 70 and 55 per cent of nova events occurring on O/Ne white dwarfs for H and He novae, respectively. Only 15 per cent of H-nova systems undergo a common-envelope phase, but these systems are responsible for the majority of H nova events. All He-accreting He-nova systems are considered post-common-envelope systems, and almost all will merge with their donor star in a gravitational-wave-driven inspiral. We estimate the current annual rate of novae in M31 (Andromeda) to be approximately 41 ± 4 for H novae, underpredicting the current observational estimate of |$65^{+15}_{-16}$|⁠ , and 0.14 ± 0.015 for He novae. When varying common-envelope parameters, the H nova rate varies between 20 and 80 events per year. [ABSTRACT FROM AUTHOR]

The first gravitational wave detections of mergers between black holes and neutron stars represent a remarkable new regime of high-energy transient astrophysics. The signals observed with LIGO-Virgo detectors come from mergers of extreme physical objects which are the end products of stellar evolution in close binary systems. To better understand their origin and merger rates, we have performed binary population syntheses at different metallicities using the new grid-based binary population synthesis code ComBinE. Starting from newborn pairs of stars, we follow their evolution including mass-loss, mass transfer and accretion, common envelopes, and supernova explosions. We apply the binding energies of common envelopes based on dense grids of detailed stellar structure models, make use of improved investigations of the subsequent Case BB Roche lobe overflow and scale supernova kicks according to the stripping of the exploding stars. We demonstrate that all the double black hole mergers, GW150914, LVT151012, GW151226, GW170104, GW170608, and GW170814, as well as the double neutron star merger GW170817, are accounted for in our models in the appropriate metallicity regime. Our binary interaction parameters are calibrated to match the accurately determined properties of Galactic double neutron star systems, and we discuss their masses and types of supernova origin. Using our default values for the input physics parameters, we find a double neutron star merger rate of |${3.0}\, \mathrm{Myr}^{-1}$| for Milky-Way equivalent galaxies. Our upper limit to the merger-rate density of double neutron stars is |$R\simeq {400} \, \mathrm{yr}^{-1}\, {\rm Gpc}^{-3}$| in the local Universe (⁠|$z$| = 0). [ABSTRACT FROM AUTHOR]

We evolve stellar models to study the common envelope (CE) interaction of an early asymptotic giant branch star of initial mass |$5\, \rm M_{\odot }$| with a companion star of mass ranging from 0.1 to |$2\, \rm M_{\odot }$|⁠. We model the CE as a fast stripping phase in which the primary experiences rapid mass loss and loses about 80 per cent of its mass. The post-CE remnant is then allowed to thermally readjust during a Roche lobe overflow (RLOF) phase and the final binary system and its orbital period are investigated. We find that the post-CE RLOF phase is long enough to allow nuclear burning to proceed in the helium shell. By the end of this phase, the donor is stripped of both its hydrogen and helium and ends up as carbon–oxygen white dwarf of mass about |$0.8\, \rm M_{\odot }$|⁠. We study the sensitivity of our results to initial conditions of different companion masses and orbital separations at which the stripping phase begins. We find that the companion mass affects the final binary separation and that helium-shell burning causes the star to refill its Roche lobe leading to post-CE RLOF. Our results show that double mass transfer in such a binary interaction is able to strip the helium and hydrogen layers from the donor star without the need for any special conditions or fine tuning of the binary parameters. [ABSTRACT FROM AUTHOR]

The combination of asteroseismologically measured masses with abundances from detailed analyses of stellar atmospheres challenges our fundamental knowledge of stars and our ability to model them. Ancient red-giant stars in the Galactic thick disc are proving to be most troublesome in this regard. They are older than 5Gyr, a lifetime corresponding to an initial stellar mass of about 1.2M⊙. So why do the masses of a sizeable fraction of thick-disc stars exceed 1.3M⊙, with some as massive as 2.3M⊙? We answer this question by considering duplicity in the thick-disc stellar population using a binary population-nucleosynthesis model. We examine how mass transfer and merging affect the stellar mass distribution and surface abundances of carbon and nitrogen. We show that a few per cent of thick-disc stars can interact in binary star systems and become more massive than 1.3M⊙. Of these stars, most are single because they are merged binaries. Some stars more massive than 1.3M⊙ form in binaries by wind mass transfer. We compare our results to a sample of the APOKASC data set and find reasonable agreement except in the number of these thick-disc stars more massive than 1.3M⊙. This problem is resolved by the use of a logarithmically flat orbital-period distribution and a large binary fraction. [ABSTRACT FROM AUTHOR]

Classical Cepheids are key probes of both stellar astrophysics and cosmology as standard candles and pulsating variable stars. It is important to understand Cepheids in unprecedented detail in preparation for upcoming Gaia, James Webb Space Telescope (JWST) and extremely-large telescope observations. Cepheid eclipsing binary stars are ideal tools for achieving this goal, however there are currently only three known systems. One of those systems, OGLE-LMC-CEP1812, raises new questions about the evolution of classical Cepheids because of an apparent age discrepancy between the Cepheid and its red giant companion. We show that the Cepheid component is actually the product of a stellar merger of two main sequence stars that has since evolved across the Hertzsprung gap of the HR diagram. This post-merger product appears younger than the companion, hence the apparent age discrepancy is resolved. We discuss this idea and consequences for understanding Cepheid evolution. [ABSTRACT FROM AUTHOR]

Many core-collapse supernova progenitors are presumed to be in binary systems. If a star explodes in a binary system, the early supernova light curve can be brightened by the collision of the supernova ejecta with the companion star. The early brightening can be observed when the observer is in the direction of the hole created by the collision. Based on a population synthesis model, we estimate the fractions of core-collapse supernovae in which the light-curve brightening by the collision can be observed. We find that 0.19 per cent of core-collapse supernova light curves can be observed with the collisional brightening. Type Ibc supernova light curves are more likely to be brightened by the collision (0.53 per cent) because of the high fraction of the progenitors being in binary systems and their proximity to the companion stars. Type II and IIb supernova light curves are less affected (~10-3 and ~10-2percent, respectively). Although the early, slow light-curve declines of some Type IIb and Ibc supernovae are argued to be caused by the collision with the companion star (e.g. SN 2008D), the small expected fraction, as well as the unrealistically small separation required, disfavour the argument. The future transient survey by the Large Synoptic Survey Telescope is expected to detect ~10 Type Ibc supernovae with the early collisional brightening per year, and they will be able to provide information on supernova progenitors in binary systems. [ABSTRACT FROM AUTHOR]

Classical Cepheids, like binary stars, are laboratories for stellar evolution and Cepheids in binary systems are especially powerful ones. About one-third of Galactic Cepheids are known to have companions and Cepheids in eclipsing binary systems have recently been discovered in the Large Magellanic Cloud (LMC). However, there are no known Galactic binary Cepheids with orbital periods less than one year. We compute population synthesis models of binary Cepheids to compare to the observed period and eccentricity distributions of Galactic Cepheids as well as to the number of observed eclipsing binary Cepheids in the LMC. We find that our population synthesis models are consistent with observed binary properties of Cepheids. Furthermore, we show that binary interaction on the red giant branch prevents some red giant stars from becoming classical Cepheids. Such interactions suggest that the binary fraction of Cepheids should be significantly less than that of their main-sequence progenitors, and that almost all binary Cepheids have orbital periods longer than one year. If the Galactic Cepheid spectroscopic binary fraction is about 35%, then the spectroscopic binary fraction of their intermediate mass main sequence progenitors is about 40-45%. [ABSTRACT FROM AUTHOR]

Many binary star systems are not wide enough to contain the progenitor stars from which they were made. One explanation for this is that when one star becomes a red giant a common envelope forms around both stars in the binary system. The core of the giant and its companion star continue to orbit one another inside the envelope. Frictional energy deposited into the common envelope may lead to its ejection and, if so, a close binary system is formed from the core of the former giant star and its relatively untouched companion. When the primary is an asymptotic giant branch star the core becomes a hot carbon-oxygen white dwarf which may ionise the ejected envelope and illuminate a planetary nebula. Many other types of binary systems form through common envelope evolution such as low-mass X-ray binaries and cataclysmic variables. In the case of a failed envelope ejection when the cores merge, rapidly-rotating solitary giants similar to FK Comae stars form. In this short review we focus on attempts to constrain parameters of common envelope evolution models and also describe the latest efforts to model this elusive phase of binary stellar evolution. [ABSTRACT FROM PUBLISHER]

We compare our latest single and binary stellar model results from the Cambridgestars code to several sets of observations. We examine four stellar population ratios: the number of blue to red supergiants, the number of Wolf–Rayet stars to O supergiants, the number of red supergiants to Wolf–Rayet stars and the relative number of Wolf–Rayet subtypes, WC to WN stars. These four ratios provide a quantitative measure of nuclear burning lifetimes and the importance of mass loss during various stages of the stars' lifetimes. In addition, we compare our models to the relative rate of Type Ib/c to Type II supernovae to measure the amount of mass lost over the entire lives of all stars. We find reasonable agreement between the observationally inferred values and our predicted values by mixing single and binary star populations. However, there is evidence that extra mass loss is required to improve the agreement further, to reduce the number of red supergiants and increase the number of Wolf–Rayet stars. [ABSTRACT FROM AUTHOR]

We have calculated a large set of detailed binary models and used them to test the observed stellar population ratios that compare the relative populations of blue supergiants, red supergiants and Wolf-Rayet stars at different metallicities. We have also used our models to estimate the relative rate of type Ib/c to type II supernovae. We find, with an interacting binary fraction of about two thirds, that we obtain better agreement between our models and observations than with single stars. We discuss the use of models in determining the nature of supernova progenitors and show the surprising result that many type Ib/c supernova progenitors are less luminous and less massive in our models than the observed population of Wolf-Rayet stars. [ABSTRACT FROM PUBLISHER]

A long standing problem with asymptotic giant branch (AGB) star models has been their inability to produce the low-luminosity carbon stars in the Large and Small Magellanic Clouds (LMC and SMC, respectively). Dredge-up must begin earlier and extend deeper. We find this for the first time in our models of LMC metallicity. Such features are not found in our models of SMC metallicity. The fully implicit and simultaneous stellar evolution codestars has been used to calculate the evolution of AGB stars with metallicities of and , corresponding to the observed metallicities of the LMC and SMC, respectively. Third dredge-up occurs in stars of and above and carbon stars were found for models between and . We use the detailed models as input physics for a population synthesis code and generate carbon star luminosity functions. We now find that we are able to reproduce the carbon star luminosity function of the LMC without any manipulation of our models. The SMC carbon star luminosity function still cannot be produced from our detailed models unless the minimum core mass for third dredge-up is reduced by . [ABSTRACT FROM AUTHOR]

We present a synthetic model for thermally pulsing asymptotic giant branch (TPAGB) evolution constructed by fitting expressions to full evolutionary models in the metallicity range 0.0001 ≤ Z ≤ 0.02. Our model includes parametrizations of third dredge-up and hot-bottom burning with mass and metallicity. The Large Magellanic Cloud and Small Magellanic Cloud carbon star luminosity functions are used to calibrate third dredge-up. We calculate yields appropriate for galactic chemical evolution models for 1H, 4He, 12C, 13C, 14N, 15N, 16O and 17O. The initial—final mass relation is examined for our stars and found to fit to within 0.1 M[This symbol cannot be presented in ASCII format] of the observations. We also reproduce well the white dwarf mass function for masses above about 0.58 M[This symbol cannot be presented in ASCII format]. The new model is to be implemented in a rapid binary star evolution code. [ABSTRACT FROM AUTHOR]

Core collapse of massive stars with a relativistic jet expulsion along the rotation axis is a widely discussed scenario for gamma-ray burst (GRB) production. However the nature of the stellar progenitor remains unclear. We study the evolution of stars that may be the progenitors of long-soft GRBs – rotating naked helium stars presumed to have lost their envelopes to winds or companions. Our aim is to investigate the formation and development of single and binary systems and from this population evaluate the rates of interesting individual species. Using a rapid binary-evolution algorithm that enables us to model the most complex binary systems and to explore the effect of metallicity on GRB production, we draw the following conclusions. First, we find that, if we include an approximate treatment of angular momentum transport by mass loss, the resulting spin rates for single stars become too low to form a centrifugally supported disc that can drive a GRB engine, although they do have sufficiently massive cores to form black holes. Secondly, massive stars in binaries result in enough angular momentum – due to spin–orbit tidal interactions – to form a centrifugally supported disc and are thus capable of supplying a sufficient number of progenitors. This holds true even if only a small fraction of bursts are visible to a given observer and the GRB rate is several hundred times larger than the observed rate. Thirdly, low-metallicity stars aid the formation of rapidly rotating, massive helium cores at collapse and so their evolution is likely to be affected by the local properties of the interstellar medium (ISM). This effect could increase the GRB formation rate by a factor of 5–7 at . Finally we quantify the effects of mass loss, common-envelope evolution and black-hole formation and show that more stringent constraints to many of these evolution parameters are needed in order to draw quantitative conclusions from population synthesis work. [ABSTRACT FROM AUTHOR]

Post-asymptotic giant branch (post-AGB) stars with discs are all binaries. Many of these binaries have orbital periods between 100 and 1000 days so cannot have avoided mass transfer between the AGB star and its companion, likely through a common-envelope type interaction. We report on preliminary results of our project to model circumbinary discs around post-AGB stars using our binary population synthesis code binary_c. We combine a simple analytic thin-disc model with binary stellar evolution to estimate the impact of the disc on the binary, and vice versa, fast enough that we can model stellar population and hence explore the rather uncertain parameter space involved with disc formation. We find that, provided the discs form with sufficient mass and angular momentum, and have an inner edge that is relatively close to the binary, they can both prolong the life of their parent post-AGB star and pump the eccentricity of orbits of their inner binaries. [ABSTRACT FROM AUTHOR]

The very bright red star HV2112 in the Small Magellanic Cloud could be a massive Thorne–Żytkow object (TŻO), a supergiant-like star with a degenerate neutron core. With a luminosity of over 105 L⊙, it could also be a super asymptotic giant branch (SAGB) star, a star with an oxygen/neon core supported by electron degeneracy and undergoing thermal pulses with third dredge up. Both TŻOs and SAGB stars are expected to be rare. Abundances of heavy elements in HV2112's atmosphere, as observed to date, do not allow us to distinguish between the two possibilities based on the latest models. Molybdenum and rubidium can be enhanced by both the irp-process in a TŻO or by the s-process in SAGB stars. Lithium can be generated by hot bottom burning at the base of the convective envelope in either. HV2112's enhanced calcium could thus be the key determinant. Neither SAGB stars nor TŻOs are known to be able to synthesize their own calcium but it may be possible to produce it in the final stages of the process that forms a TŻO, when the degenerate electron core of a giant star is tidally disrupted by a neutron star. Hence, it is more likely, on a fine balance, that HV2112 is indeed a genuine TŻO. [ABSTRACT FROM PUBLISHER]

We show that mass transfer from thermally pulsing asymptotic giant branch stars in binary systems naturally leads to a population of dim, extrinsic carbon stars in the Magellanic Clouds. This explains the existence of the dim tail in the carbon star luminosity functions of the clouds. We also conclude that the majority of these extrinsic stars are first giant-branch stars rather than early asymptotic giant branch stars. [ABSTRACT FROM AUTHOR]