Philipp Podsiadlowski

Abstract:

In this talk, we present the general principles of binary evolution and give two examples. The first example is the formation of subdwarf B stars (sdBs) and their application to the long-standing problem of ultraviolet excess (also known as UV-upturn) in elliptical galaxies. The second is for the progenitors of type Ia supernovae (SNe Ia). We discuss the main binary interactions, i.e., stable Roche lobe overflow (RLOF) and common envelope (CE) evolution, and show evolutionary channels leading to the formation of various binary-related objects. In the first example, we show that the binary model of sdB stars of Han et al. (2002, 2003) can reproduce field sdB stars and their counterparts, extreme horizontal branch (EHB) stars, in globular clusters. By applying the binary model to the study of evolutionary population synthesis, we have obtained an ``a priori'' model for the UV-upturn of elliptical galaxies and showed that the UV-upturn is most likely resulted from binary interactions. This has major implications for understanding the evolution of the UV excess and elliptical galaxies in general. In the second example, we introduce the single degenerate channel and the double degenerate channel for the progenitors of SNe Ia. We give the birth rates and delay time distributions for each channel and the distributions of companion stars at the moment of SN explosion for the single degenerate channel, which would help to search for the remnant companion stars observationally.

Abstract:

The collapsar model requires rapidly rotating Wolf-Rayet stars as progenitors of long gamma-ray bursts. However, Galactic Wolf-Rayet stars rapidly lose angular momentum due to their intense stellar winds. We investigate whether the tidal interaction of a Wolf-Rayet star with a compact object in a binary system can spin up the Wolf-Rayet star enough to produce a collapsar. We compute the evolution of close Wolf-Rayet binaries, including tidal angular momentum exchange, differential rotation of the Wolf-Rayet star, internal magnetic fields, stellar wind mass loss, and mass transfer. The Wolf-Rayet companion is approximated as a point mass. We then employ a population synthesis code to infer the occurrence rates of the various relevant binary evolution channels. We find that the simple scenario -- i.e., the Wolf-Rayet star being tidally spun up and producing a collapsar -- does not occur at solar metallicity and may only occur with low probability at low metallicity. It is limited by the widening of the binary orbit induced by the strong Wolf-Rayet wind or by the radius evolution of the Wolf-Rayet star that most often leads to a binary merger. The tidal effects enhance the merger rate of Wolf-Rayet stars with black holes such that it becomes comparable to the occurrence rate of long gamma-ray bursts.

Abstract:

The excess of far-ultraviolet (far-UV) radiation in elliptical galaxies has remained one of their most enduring mysteries. On the other hand, the origin of old blue stars in the Milky Way, hot subdwarfs, is now reasonably well understood: they are hot stars that have lost their hydrogen envelopes by various binary interactions. Here, we review the main evolutionary channels that produce hot subdwarfs in the Galaxy and present binary population synthesis simulations that reproduce the main properties of the Galactic hot-subdwarf population. Applying the same model to elliptical galaxies, we show how this model can explain the major observational properties of the far-UV excess, including the far-UV spectrum, without the need for ad hoc physical assumptions. The model implies that the UV excess is not a sign of age, as has been postulated previously, and predicts that it should not be strongly dependent on the metallicity of the population.

Abstract:

We investigate the evolution of the properties of model populations of ultraluminous X-ray sources (ULXs) consisting of a black-hole accretor in a binary with a donor star. We have computed models corresponding to two different. populations of black-hole binaries: one invokes stellar-mass (similar to 10 M-circle dot) black hole accretors(1) (LMBHs), and the second utilizes intermediate-mass (similar to 1000M(circle dot)) black holes (IMBHs). For each of the two populations, we computed 30,000 binary evolution sequences using a full Henyey stellar evolution code. The optical flux from the model ULXs includes contributions from the donor star and the accretion disk (the latter being due to X-ray irradiation as well as intrinsic viscous heating). We present. "probability images" for the ULX systems in planes of color-magnitude, orbital period vs. X-ray luminosity, and luminosity vs. evolution time. Estimates of the numbers of ULXs in a typical galaxy as a function of X-ray luminosity are also presented. Our model CMDs are compared with six ULX counterparts that have been discussed in the literature. Overall, the observed systems seem more closely related to model systems with very high-mass donors (greater than or similar to 25 M-circle dot) in binaries with IMBH accretors. However, significant difficulties remain with both the IMBH and stellar-mass black hole models.