Philipp Podsiadlowski

Abstract:

We present results of a multi-wavelength program to study the faint discrete X-ray source population discovered by Chandra in the Galactic Centre (GC). From IR imaging obtained with the VLT we identify candidate K-band counterparts to 75% of the X-ray sources in our sample. By combining follow-up VLT K-band spectroscopy of a subset of these candidate counterparts with the magnitude limits of our photometric survey, we suggest that only a small percentage of the sources are HMXBs, while the majority are likely to be canonical LMXBs and CVs at the distance of the GC. In addition, we present our discovery of highly structured small-scale (5-15 '') extinction towards the Galactic Centre. This is the finest-scale extinction study of the Galactic Centre to date.

Abstract:

Some 10% of all massive stars are predicted to merge completely with a companion star. Here we review the different types of mergers and how they may produce unusual objects such as the anomalous progenitor of SN 1987A, gamma-ray burst sources and Thorne-Zytkow objects.

Abstract:

Symbiotic binaries form a diverse class of interacting binaries that can be used to constrain binary evolution theory. In this contribution we will review some of the major unsolved issues concerning symbiotic binaries, in particular the origin of the orbital-period distribution of S-type symbiotic binaries, their possible relation to a sub-class of Type la supernovae and the stability of mass transfer. We will then present some recent observations of Mira suggesting that even very wide symbiotic experience a new form of Roche-lobe overflow. "wind Roche-lobe overflow". We will present preliminary hydrodynamical simulations of this type of mass transfer and discuss its implications for the shaping of (proto-)planetary nebulae, the origin of barium stars and potentially Type la supernova progenitors.

Abstract:

The nature of the progenitor has been one of the major mysteries of SN 1987A. It has long been suspected that the various anomalies of the supernova, the blue color of the progenitor, the chemical anomalies in the envelope and the triple-ring nebula surrounding it, could be the direct consequence of the merger of two massive stars some 20,000yr before the explosion. Here we present the results of detailed recent numerical simulations of such mergers and show that these can indeed explain all of the main anomalies. In particular, we present detailed 3-dimensional hydrodynamical simulations that show how the triple-ring nebula is likely to have formed, providing the best evidence for a binary merger to date. We also discuss how a merger may naturally explain other structures in the nebula, further strengthening the model and providing future observational tests.

Abstract:

Most symbiotic binaries consist of an evolved giant star and a hot, accreting companion, typically a white dwarf. In wide symbiotic binaries, the companion may accrete part of the wind emitted by the giant by Bondi-Hoyle accretion. Here we present preliminary simulations of a new mass-transfer mode - wind Roche-lobe overflow (RLOF) - that can occur when the wind acceleration region of the evolved star occurs at several stellar radii (as, e.g., in Mira variables). If this region is relatively close to the Roche-lobe, wind material fills the giant's Roche-lobe and is transferred to the hot component through the inner Lagrangian point (similar to standard RLOF). We show that the accretion rate can be 100 times larger than the rate expected from standard Bondi-Hoyle accretion. Furthermore, mass loss from the system tends to be strongly concentrated towards the binary plane, producing an equatorial outflow. We discuss the implications of this study for the shaping of planetary nebulae, for symbiotic binaries as potential type la supernova progenitors and for the origin of barium stars.