Philipp Podsiadlowski

Abstract:

We study the complete merger of two massive stars inside a common envelope and the subsequent evolution of the merger products a rapidly rotating massive supergiant. Three qualitatively different types of mergers have been identified and investigated in detail, and the post-merger evolution has been followed to the immediate presupernova stage. The "quiet merger" case does not lead to significant changes in composition, and the star remains a red supergiant. In the case of a "moderate merger", the star May become a blue supergiant and end its evolution as a blue supergiant, depending on the core to total mass ratio (as may be appropriate for the progenitor of SN 1987A). In the case of the most effective "explosive merger", the merger product stays a red giant. In the last two cases, the He abundance in the envelope is increased drastically, but significant s-processing. is mainly expected in the "explosive merger" case.

Abstract:

Millisecond pulsars are widely believed to be the descendants of low-mass X-ray binaries (LMXBs). We review the main problems with this standard paradigm and how this is affected by the recently recognized importance of intermediate-mass X-ray binaries (IMXBs). Combining realistic binary-evolution calculations with a binary population synthesis approach, we show that the predicted properties of the population of L/IMXBs are inconsistent with the observed ones. We then demonstrate how irradiation-driven mass-transfer cycles can provide a simultaneous solution to the main problems with the standard model, in particular the luminosity distribution of L/IMXBs and the ins pulsar birthrate problem.

Abstract:

We study the complete merger of two massive stars inside a common envelope and the subsequent evolution of the merger product, a rapidly rotating massive supergiant. Three qualitatively different types of mergers have been identified and investigated in detail, and the post-merger evolution has been followed to the immediate presupernova stage. The ``quiet merger'' case does not lead to significant changes in composition, and the star remains a red supergiant. In the case of a ``moderate merger'', the star may become a blue supergiant and end its evolution as a blue supergiant, depending on the core to total mass ratio (as may be appropriate for the progenitor of SN 1987A). In the case of the most effective ``explosive merger'', the merger product stays a red giant. In last two cases, the He abundance in the envelope is increased drastically, but significant s-processing is mainly expected in the ``explosive merger'' case.

Abstract:

We present unique radio observations of SS433, using MERLIN, the VLBA, and the VLA, which allow us to, for the first time, properly image and derive a meaningful spectral index for the `ruff' of equatorial emission which surrounds SS433's jet. We interpret this smooth ruff as a wind-like outflow from the binary.

Abstract:

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 spiral-in of the companion in the envelope of a massive star causes 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 Msun and demonstrate that in these systems the black hole can accrete appreciably even if accretion is Eddington limited (up to 7 Msun for an initial black-hole mass of 10 Msun) 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), ultra-luminous X-ray sources and Cygnus X-1. 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. (Abstract abridged)