Subdwarf B (sdB) stars (and related sdO/sdOB stars) are believed to be helium‐core‐burning objects with very thin hydrogen‐rich envelopes. In recent years it has become increasingly clear from observational surveys that a large fraction of these objects are members of binary systems. To understand their formation better, we present the results of a detailed investigation of the three main binary evolution channels that can lead to the formation of sdB stars: the common‐envelope (CE) ejection channel, the stable Roche lobe overflow (RLOF) channel, and the double helium white dwarfs (WDs) merger channel. The CE ejection channel leads to the formation of sdB stars in short‐period binaries with typical orbital periods between 0.1 and 10 d, very thin hydrogen‐rich envelopes and a mass distribution sharply peaked around ∼0.46 M⊙. On the other hand, under the assumption that all mass transferred is soon lost, the stable RLOF channel produces sdB stars with similar masses but long orbital periods (400–1500 d) and with rather thick hydrogen‐rich envelopes. The merger channel gives rise to single sdB stars whose hydrogen‐rich envelopes are extremely thin but which have a fairly wide distribution of masses (0.4−0.65 M⊙). We obtained the conditions for the formation of sdB stars from each of these channels using detailed stellar and binary evolution calculations where we modelled the detailed evolution of sdB stars and carried out simplified binary population synthesis simulations. The observed period distribution of sdB stars in compact binaries strongly constrains the CE ejection parameters. The best fits to the observations are obtained for very efficient CE ejection where the envelope ionization energy is included, consistent with previous results. We also present the distribution of sdB stars in the Teff−log g diagram, the Hertzsprung–Russell diagram and the distribution of mass functions.

With recent advances in gravitational-wave astronomy, the direct detection of gravitational waves from the merger of two stellarmass compact objects has become a realistic prospect. Evolutionary scenarios towards mergers of various double compact objects generally invoke so-called common-envelope evolution, which is poorly understood and leads to large uncertainties in the predicted merger rates. Here we explore, as an alternative, the scenario of massive overcontact binary (MOB) evolution, which involves two very massive stars in a very tight binary that remain fully mixed as a result of their tidally induced high spin. While many of these systems merge early on, we find many MOBs that swap mass several times, but survive as a close binary until the stars collapse. The simplicity of the MOB scenario allows us to use the efficient public stellar-evolution code MESA to explore it systematically by means of detailed numerical calculations. We find that, at low metallicity, MOBs produce double-black-hole (BH+BH) systems that will merge within a Hubble time with mass-ratios close to one, in two mass ranges, about 25 . . . 60 M and > ∼ 130 M , with pairinstability supernovae (PISNe) being produced at intermediate masses. Our models are also able to reproduce counterparts of various stages in the MOB scenario in the local Universe, providing direct support for the scenario. We map the initial binary parameter space that produces BH+BH mergers, determine the expected chirp mass distribution, merger times, and expected Kerr parameters, and predict event rates. We find typically one BH+BH merger event for ∼1000 core-collapse supernovae for Z < ∼ Z /10 . The advanced LIGO (aLIGO) detection rate is more uncertain and depends on the cosmic metallicity evolution. From deriving upper and lower limits from a local and a global approximation for the metallicity distribution of massive stars, we estimate aLIGO detection rates (at the aLIGO design limit) of ∼19−550 yr −1 for BH-BH mergers below the PISN gap and of ∼2.1−370 yr −1 above the PISN gap. Even with conservative assumptions, we find that aLIGO will probably soon detect BH+BH mergers from the MOB scenario. These could be the dominant source for aLIGO detections.

We present the results of a systematic study of the evolution of low-and intermediate-mass X-ray binaries (LMXBs and IMXBs). Using a standard Henyey-type stellar evolution code and a standard model for binary interactions, we have calculated 100 binary evolution sequences containing a neutron star and a normal-type companion star, where the initial mass of the secondary ranges from 0.6 to 7 M _ and the initial orbital period from D4 hr to D100 days. This range samples the entire range of parameters one is likely to encounter for LMXBs and IMXBs. The sequences show an enormous variety of evolutionary histories and outcomes, where di †erent mass transfer mechanisms dominate in di †erent phases. Very few sequences resemble the classical evolution of cataclysmic variables, where the evolution is driven by magnetic braking and gravitational radiation alone. Many systems experience a phase of mass transfer on a thermal timescale and may brieÑy become detached immediately after this phase (for the more massive secondaries). In agreement with previous results (Tauris & Savonije 1999), we Ðnd that all sequences with (sub)giant donors up to D2 are stable against dynamical mass transfer. Sequences M _ where the secondary has a radiative envelope are stable against dynamical mass transfer for initial masses up to D4 For higher initial masses, they experience a delayed dynamical instability after a M _ . stable phase of mass transfer lasting up to D106 yr. Systems where the initial orbital period is just below the bifurcation period of D18 hr evolve toward extremely short orbital periods (as short as D10 minutes). For a 1 secondary, the initial period range that leads to the formation of ultracompact M _ systems (with minimum periods less than D40 minutes) is 13È18 hr. Since systems that start mass transfer in this period range are naturally produced as a result of tidal capture, this may explain the large fraction of ultracompact LMXBs observed in globular clusters. The implications of this study for our understanding of the population of X-ray binaries and the formation of millisecond pulsars are also discussed.

The explosion of ultra-stripped stars in close binaries can lead to ejecta masses < 0.1 M ⊙ and may explain some of the recent discoveries of weak and fast optical transients. In Tau ris et al. (2013), it was demonstrated that helium star companions to neutron stars (NSs) may experience mass transfer and evolve into naked ∼ 1.5 M ⊙ metal cores, barely above the Chandrasekhar mass limit. Here we elaborate on this work and present a systematic investigation of the progenitor evolution leading to ultra-stripped supernovae (SNe). In particular, we examine the binary parameter space leading to electron-capture (EC SNe) and iron core-collapse SNe (Fe CCSNe), respectively, and determine the amount of helium ejected with applications to their observational classification as Type Ib or Type Ic. We mainly evolve systems where the SN progenitors are helium star donors of initial mass M He = 2.5 − 3.5 M ⊙ in tight binaries with orbital periods of P orb = 0.06 − 2.0 days, and hosting an accreting NS, but we also discuss the evolution of wider systems and of both more massive and lighter -as well as single -helium stars. In some cases we are able to follow the evolution until the onset of silicon burning, just a few days prior to the SN explosion. We find that ultra-stripped SNe are possible for both EC SNe and Fe CCSNe. EC SNe only occur for M He = 2.60 − 2.95 M ⊙ depending on P orb . The general outcome, however, is an Fe CCSN above this mass interval and an ONeMg or CO white dwarf for smaller masses. For the exploding stars, the amount of helium ejected is correlated with P orb -the tightest systems even having donors being stripped down to envelopes of less than 0.01 M ⊙ . We estimate the rise time of ultra-stripped SNe to be in the range 12 hr − 8 days, and light curve decay times between 1 and 50 days. A number of fitting formulae for our models are provided with applications to population synthesis. Ultrastripped SNe may produce NSs in the mass range 1.10 − 1.80 M ⊙ and are highly relevant for LIGO/VIRGO since most (possibly all) merging double NS systems have evolved through this phase. Finally, we discuss the low-velocity kicks which might be imparted on these resulting NSs at birth.

Type Ia supernovae have been used empirically as 'standard candles' to demonstrate the acceleration of the expansion of the Universe even though fundamental details, such as the nature of their progenitor systems and how the stars explode, remain a mystery. There is consensus that a white dwarf star explodes after accreting matter in a binary system, but the secondary body could be anything from a main-sequence star to a red giant, or even another white dwarf. This uncertainty stems from the fact that no recent type Ia supernova has been discovered close enough to Earth to detect the stars before explosion. Here we report early observations of supernova SN 2011fe in the galaxy M101 at a distance from Earth of 6.4 megaparsecs. We find that the exploding star was probably a carbon-oxygen white dwarf, and from the lack of an early shock we conclude that the companion was probably a main-sequence star. Early spectroscopy shows high-velocity oxygen that slows rapidly, on a timescale of hours, and extensive mixing of newly synthesized intermediate-mass elements in the outermost layers of the supernova. A companion paper uses pre-explosion images to rule out luminous red giants and most helium stars as companions to the progenitor.

Double neutron stars (DNSs) have been observed as Galactic radio pulsars, and the recent discovery of gravitational waves from the DNS merger GW170817 adds to the known DNS population. We perform rapid population synthesis of massive binary stars and discuss model predictions, including DNS formation rates, mass distributions, and delay time distributions. We vary assumptions and parameters of physical processes such as mass transfer stability criteria, supernova natal kick distributions, remnant mass prescriptions and common-envelope energetics. We compute the likelihood of observing the orbital period-eccentricity distribution of the Galactic DNS population under each of our population synthesis models, allowing us to quantitatively compare the models. We find that mass transfer from a stripped post-helium-burning secondary (case BB) onto a neutron star is most likely dynamically stable. We also find that a natal kick distribution composed of both low (Maxwellian σ = 30 km s −1 ) and high (σ = 265 km s −1 ) components is preferred over a single high-kick component. We conclude that the observed DNS mass distribution can place strong constraints on model assumptions.

The massive star which underwent core-collapse to produce SN1993J was identified as a non-variable red supergiant star in images of the galaxy M81 taken before explosion 1,2 . However the stellar source showed an excess in UV and Bband colours that suggested it had either a hot, massive companion star or was embedded in an unresolved young stellar association 1 . The spectra of SN1993J underwent a remarkable transformation between a hydrogen-rich Type II supernova and a helium-rich (hydrogen-deficient) Type Ib 3,4 . The spectral and photometric peculiarities were explained by models in which the 13-20 solar mass supergiant had lost almost its entire hydrogen envelope to a close binary companion 5-7 . The binary scenario is currently the best fitting model for the production of such "type IIb" supernovae, however the hypothetical massive companion stars have so far eluded discovery. Here we report the results of new photometric and spectroscopic observations of SN1993J, 10 years after explosion.

We present the results of a systematic study of the formation and evolution of binaries containing black holes and normal-star companions with a wide range of masses. We first reexamine the standard formation scenario for close black hole binaries, where the progenitor system, a binary with at least one massive component, experienced a common-envelope phase and where the spiral-in of the companion in the envelope of the massive star caused the ejection of the envelope. We estimate the formation rates for different companion masses and different assumptions about the common-envelope structure and other model parameters. We find that black hole binaries with intermediate-and high-mass secondaries can form for a wide range of assumptions, while black hole binaries with low-mass secondaries can only form with apparently unrealistic assumptions (in agreement with previous studies).We then present detailed binary evolution sequences for black hole binaries with secondaries of 2 to 17 M and demonstrate that in these systems the black hole can accrete appreciably even if accretion is Eddington-limited (up to 7 M for an initial black hole mass of 10 M ) and that the black holes can be spun up significantly in the process. We discuss the implications of these calculations for well-studied black hole binaries (in particular GRS 1915+105) and ultraluminous X-ray sources of which GRS 1915+105 appears to represent a typical Galactic counterpart. We also present a detailed evolutionary model for Cygnus X-1, a massive black hole binary, which suggests that at present the system is most likely in a wind mass-transfer phase following an earlier Roche-lobe overflow phase. Finally, we discuss how some of the assumptions in the standard model could be relaxed to allow the formation of low-mass, short-period black hole binaries, which appear to be very abundant in nature.