We systematically examine how the presence in a binary affects the final core structure of a massive star and its consequences for the subsequent supernova explosion. Interactions with a companion star may change the final rate of rotation, the size of the helium core, the strength of carbon burning and the final iron core mass. Stars with initial masses larger than ∼ 11 M ⊙ that experience core collapse will generally have smaller iron cores at the point of explosion if they lost their envelopes due to a binary interaction during or soon after core hydrogen burning. Stars below ∼ 11M ⊙ , on the other hand, can end up with larger helium and metal cores if they have a close companion, since the second dredge-up phase which reduces the helium core mass dramatically in single stars does not occur once the hydrogen envelope is lost. We find that the initially more massive stars in binary systems with masses in the range 8 − 11M ⊙ are likely to undergo an electron-capture supernova, while single stars in the same mass range would end as ONeMg white dwarfs. We suggest that the core collapse in an electron-capture supernova (and possibly in the case of relatively small iron cores) leads to a prompt or fast explosion rather than a very slow, delayed neutrino-driven explosion and that this naturally produces neutron stars with low-velocity kicks. This leads to a dichotomous distribution of neutron star kicks, as inferred previously, where neutron stars in relatively close binaries attain low kick velocities. We illustrate the consequences of such a dichotomous kick scenario using binary population synthesis simulations and discuss its implications. This scenario has also important consequences for the minimum initial mass of a massive star that becomes a neutron star. For single stars the critical mass may be as high as 10 -12 M ⊙ , while for close binaries, it may be as low as 6 -8 M ⊙ . These critical masses depend on the treatment of convection, the amount of convective overshooting and the metallicity of the star and will generally be lower for larger amounts of convective overshooting and lower metallicity.

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.

We investigate an interesting new class of high-mass X-ray binaries (HMXBs) with long orbital periods (P orb > 30 d) and low eccentricities (e 0.2). The orbital parameters suggest that the neutron stars in these systems did not receive a large impulse, or "kick," at the time of formation. After considering the statistical significance of these new binaries, we develop a self-consistent phenomenological picture wherein the neutron stars born in the observed wide HMXBs receive only a small kick ( 50 km s −1 ), while neutron stars born in isolation, in the majority of low-mass X-ray binaries, or in many of the wellknown HMXBs with P orb 30 d receive the conventional large kicks, with a mean speed of ∼ 300 km s −1 . Assuming that this basic scenario is correct, we discuss a physical process that lends support to our hypothesis, whereby the magnitude of the natal kick to a neutron star born in a binary system depends on the rotation rate of its immediate progenitor following mass transfer -the core of the initially more massive star in the binary. Specifically, the model predicts that rapidly rotating pre-collapse cores produce NSs with relatively small kicks, and vice versa for slowly rotating cores. If the envelope of the NS progenitor is removed before it has become deeply convective, then the exposed core is likely to be a rapid rotator. However, if the progenitor becomes highly evolved prior to mass transfer, then a strong magnetic torque, generated by differential rotation between the core and the convective envelope, may cause the core to spin down to the very slow rotation rate of the envelope. Our model, if basically correct, has important implications for the dynamics of stellar core collapse, the retention of neutron stars in globular clusters, and the formation of double neutron star systems in the Galaxy.

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We use a collection of 14 well-measured neutron star masses to strengthen the case that a substantial fraction of these neutron stars was formed via electron-capture supernovae (SNe) as opposed to Fe-core collapse SNe. The e-capture SNe are characterized by lower resultant gravitational masses and smaller natal kicks, leading to lower orbital eccentricities when the e-capture SN has led to the formation of the second neutron star in a binary system. Based on the measured masses and eccentricities, we identify four neutron stars, which have a mean post-collapse gravitational mass of ∼1.25 M , as the product of e-capture SNe. We associate the remaining ten neutron stars, which have a mean mass of ∼1.35 M , with Fe-core collapse SNe. If the ecapture supernova occurs during the formation of the first neutron star, then this should substantially increase the formation probability for double neutron stars, given that more systems will remain bound with the smaller kicks. However, this does not appear to be the case for any of the observed systems, and we discuss possible reasons for this. tron star masses into the two principal evolutionary scenarios: the "standard" channel and the double-core channel. We summarize our results and draw some general conclusions in §7.In particular, we find that (i) a substantial fraction of neutron stars are formed in e-capture SNe, and (ii) there is evidence from our work that the double-core formation scenario is less unlikely than previously thought by most workers in the field. EVOLUTIONARY HISTORY