Philipp Podsiadlowski

Abstract:

A detailed study has been started to investigate the role of thermal-timescale mass transfer in compact binaries. The properties of systems with initially thermally unstable mass ratios are discussed both analytically and using full computations of binary evolution, in which the mass transfer rate is allowed to adjust itself accordingly. It is shown that in such a model supersoft X-ray sources in the period range similar to6 - 20 hr appear as natural progenitors of CVs bearing the signature of unusual chemical composition. As a particular well constrained example, the case of AE Aqr is discussed.

Abstract:

Contrary to what is generally assumed, resonant tidal interaction with gravity modes seems a common phenomenon during the evolution of massive eccentric binary systems. This is because non-resonant low order retrograde orbital harmonics drive weak high order harmonics up into resonance until the resonant interaction counterbalances the driving and the prograde harmonic gets locked. This frequently occurring near-resonant tidal interaction gives rise to strongly enhanced tidal evolution whereby large orbital eccentricities decay to small values in only a few million years in systems with orbital periods less than about 10 days.

Abstract:

The existence of pulsars with spin period below one millisecond is expected, though they have not been detected up to now. Their formation depends on the quantity of matter accreted from the companion which, in turn, is limited by the mechanism of mass ejection from the binary. Mass ejection must be efficient, at least in some cases, in order to produce the observed population of moderately fast spinning millisecond pulsars. First we demonstrate, in the framework of the widely accepted recycling scenario, using a population synthesis approach, that a significant number of pulsars with spin period below one millisecond is expected. Then we propose that significant variations in the mass-transfer rate may cause, in systems with orbital periods greater than or similar to 1 hr, the switch-on of a radio pulsar whose radiation pressure is capable of ejecting out of the system most of the matter transferred by the companion and prevent any further accretion. We show how this mechanism could dramatically alter the binary evolution since the mechanism that drives mass overflow from the inner Lagrangian point is still active while the accretion is inhibited. Moreover we demonstrate that the persistence of this "radio ejection" phase depends on the binary orbital period, demonstrating that close systems (orbital periods P-orb less than or similar to 1 hr) are the only possible hosts for ultra fast spinning neutron stars. This could explain why submillisecond pulsars have not been detected so far, as current radio surveys are hampered by computational limitations with respect to the detection of very short spin period pulsars in short orbital period binaries.

Abstract:

The X-ray binary Cygnus X-2 (Cyg X-2) has recently been shown to contain a secondary that is much more luminous and hotter than is appropriate for a low-mass subgiant. We present detailed binary-evolution calculations which demonstrate that the present evolutionary state of Cyg X-2 can be understood if the secondary had an initial mass of around 3.5 M_sun and started to transfer mass near the end of its main-sequence phase (or, somewhat less likely, just after leaving the main sequence). Most of the mass of the secondary must have been ejected from the system during an earlier rapid mass-transfer phase. In the present phase, the secondary has a mass of around 0.5 M_sun with a non-degenerate helium core. It is burning hydrogen in a shell, and mass transfer is driven by the advancement of the burning shell. Cyg X-2 therefore is related to a previously little studied class of intermediate-mass X-ray binaries (IMXBs). We suggest that perhaps a significant fraction of X-ray binaries presently classified as low-mass X-ray binaries may be descendants of IMXBs and discuss some of the implications.