Philipp Podsiadlowski

Abstract:

BeppoSAX has detected a population of faint transient Xray sources in the Galactic Centre. I show that a simple irradiated-disc picture gives a consistent fit to the properties of this population, and that it probably consists of low-mass X-ray binaries which have evolved beyond their minimum orbital periods similar to 80 min. Since all post-minimum systems are transient, and neutron-star LMXBs are more common than black-hole LMXBs in the Galaxy, the majority of these systems should contain neutron stars, as observed. This picture predicts that the Galactic Centre transients should have orbital periods in the range similar to 80-120 minutes, and that most of them should repeat in the next few years. In this case, the total number of post-minimum transients in the Galaxy would be considerably smaller than the usual estimates of its total LMXB population. I discuss possible reasons for this.

Abstract:

Type Ia supernovae (SNe Ia) are being used as standard candles to measure the distant reaches of the Universe. Although accretion on to a white dwarf, that explodes in a thermonuclear runaway, is almost certainly the direct precursor of the supernova itself, there remains considerable doubt as to the identity of the progenitor systems. We consider exploding Chandrasekhar-mass CO white dwarfs and sub-Chandrasekhar edge lit detonations to be the most viable candidates. Based on detailed binary star population synthesis calculations we propose that many SNe la could be such ELDs of CO white dwarfs that have accreted a critical mass of helium. Both the actual mass of the ELD and its composition, a function of its progenitor's mass, can affect its peak luminosity and light curve shape. In general these two masses are correlated and an empirical relation between peak luminosity and light curve shape can be expected. However when we perform the population synthesis for progenitors of different metallicities we find a systematic shift in this relation that would systematically affect the use of SNe la as cosmic distance indicators.

Abstract:

We present ellipsoidal lightcurve fits to the quiescent BVRI lightcurves of GRO J1655-40 (Nova Scorpii 1994). The fits are based on a simple model consisting of a Roche-lobe filling secondary and an accretion disk around the black-hole primary. Unlike previous studies, no assumptions are made about the interstellar extinction or the distance to the source; instead these are determined self-consistently from the observed lightcurves. We obtain a lower value for the extinction than was assumed previously, which leads to lower masses for both the black hole and the secondary star of 5.4 +/- 0.3 Mcircle dot and 1.45 +/- 0.3 Mcircle dot, respectively. These results are, however, quite sensitive to the assumed disk structure. A lower mass of the secondary naturally explains the transient nature of the system if it is either in a late case A or early case B mass-transfer phase.

Abstract:

Cygnus X-2 appears to be the descendant of an intermediate-mass X-ray binary (IMXB). We compute detailed evolutionary sequences for the system and find that its prehistory is sensitive to stellar input parameters, in particular the amount of core overshooting during the main-sequence phase. With standard assumptions a case B mass transfer starting with a 3.5 Mcircle dot donor star is the most likely evolutionary solution for Cygnus X-2. This makes the currently observed state rather short-lived, of order 3 Myr, and requires a formation rate 10(-7)-10(-6) yr(-1) of such systems in the Galaxy. Our calculations show that neutron star IMXBs with initially more massive donors (greater than or similar to 4 Mcircle dot) encounter a delayed dynamical instability; they are unlikely to survive this rapid mass transfer phase. We determine limits for the age and initial parameters of Cygnus X-2 and calculate possible dynamical orbits of the system in a realistic Galactic potential, given its observed radial velocity. We find trajectories which are consistent with a progenitor binary on a circular orbit in the Galactic plane inside the solar circle that received a kick velocity less than or equal to 200 km s(-1) at the birth of the neutron star. The simulations suggests that about 7% of IMXBs receiving an arbitrary kick velocity from a standard kick velocity spectrum would end up in an orbit similar to Cygnus X-2, while about 10% of them reach yet larger Galactocentric distances.

Abstract:

We review our current understanding of thermally pulsing AGB evolution, with emphasis on quantitative uncertainties in the third dredge-up process. We describe results obtained with an implicit and simultaneous method of solving for stellar structure, convective mixing and nuclear burning, using Eggleton's stellar evolution code, and compare these with other recent results. We also briefly discuss the importance of binary stars in the context of AGB evolution and nucleosynthesis.