Philipp Podsiadlowski

Abstract:

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 \sim 11\Ms that experiece core collapse will generally have smaller iron cores at the time of the explosion if they lost their envelopes due to a previous binary interaction. Stars below \sim 11\Ms, 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 - 11\Ms 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 explosion rather than a 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. (Abbreviated.)

Abstract:

One promising method to test progenitor models for Type Ia supernovae is to identify surviving companion stars in historical supernova remnants. A surviving companion will have been strongly affected by its interaction with the supernova ejecta. Here we systematically explore the evolution and appearance of a typical companion star that has been stripped and heated by the supernova interaction during the post-impact re-equilibration phase. We show that, depending on the amount of heating and the amount of mass stripped by the impact or the previous binary mass transfer, such a star may be significantly overluminous or underluminous 10^3 - 10^4 yr after the supernova relative to its pre-supernova luminosity (by up to two orders of magnitude) and discuss the implications of these results for the strategies to be employed in searches for such companions.

Abstract:

(abridged) We present the first study that combines binary population synthesis in the Galactic disk and detailed evolutionary calculations of low- and intermediate-mass X-ray binaries (L/IMXBs). We show that the formation probability of IMXBs with initial donor masses of 1.5--4 Msun is typically >~5 times higher than that of standard LMXBs, and suggest that the majority of the observed systems may have descended from IMXBs. Distributions at the current epoch of the orbital periods, donor masses, and mass accretion rates have been computed, as have orbital-period distributions of BMPs. Several significant discrepancies between the theoretical and observed distributions are discussed. The orbital-period distribution of observed BMPs strongly favors cases where the envelope of the neutron-star progenitor is more easily ejected during the common-envelope phase. However, this leads to a >~100-fold overproduction of the theoretical number of luminous X-ray sources relative to the total observed number of LMXBs. X-ray irradiation of the donor star may result in a dramatic reduction in the X-ray active lifetime of L/IMXBs, thus possibly resolving the overproduction problem, as well as the long-standing BMP/LMXB birthrate problem.

Abstract:

We have carried out a detailed binary populations synthesis (BPS) study of the formation of subdwarf B (sdB) stars and related objects (sdO, sdOB stars) using the latest version of the BPS code developed by Han et al.(1994, 1995a, 1995b, 1998, 2001). We systematically investigate the importance of the five main evolutionary channels in which the sdB stars form after one or two common-envelope (CE) phases, one or two phases of stable Roche-lobe overflow (RLOF) or as the result of the merger of two helium white dwarfs (WD) (see Han et al. 2002, Paper I). Our best BPS model can satisfactorily explain the main observational characteristics of sdB stars, in particular their distributions in the orbital period - minimum companion mass diagram and in the effective temperature - surface gravity diagram, their distributions of orbital period, log (g theta^4), and mass function, their binary fraction and the fraction of sdB binaries with WD companions, their birthrates and their space density. We obtain a Galactic formation rate, a total number in the Galaxy, the intrinsic binary fraction for sdB stars. We also predict a distribution of masses for sdB stars that is wider than is commonly assumed and that some sdB stars have companions of spectral type as early as B. The percentage of A type stars with sdB companions can in principle be used to constrain some of the important parameters in the binary evolution model. We conclude that (a) the first RLOF phase needs to be more stable than is commonly assumed; (b) mass transfer in the first stable RLOF phase is non-conservative, and the mass lost from the system takes away a specific angular momentum similar to that of the system; (c) common-envelope ejection is very efficient.

Abstract:

We present the results of a systematic study of how the CNO abundances in the envelopes of massive supergiants change as a result of the complete merger of two massive stars inside a common envelope, a relatively common event in the lives of massive stars. Three qualitatively different types of mergers have been investigated in detail. The evolution of the surface composition has been followed from the merger to the immediate presupernova stage. We apply these models in particular to the progenitor of SN 1987A and the anomalous carbon star V Hydrae, both of which are likely products of mergers.