Philipp Podsiadlowski

Abstract:

In this contribution I review the theory of resonance and stability. I then discuss how one can predict, from first principles, the dynamical stability of arbitrary three-body configurations. Tidal stability of binaries and other tidally interacting systems can be predicted using an identical formulation which uses the chaos theory concept of resonance overlap.

Abstract:

An algorithm for objective Least Squares solution of star cluster HR diagrams (HRDs) is outlined, stimulated by the limited usefulness of 1-dimensional isochrones. The underlying model includes distributions of multiple stars and of quantities such as metallicity and interstellar extinction that vary from star to star. The synthesized quantity compared with observations is Areal density (A) of HRD points. Computational speed and precision are indispensible for overall success and are gained by use of a stellar evolution program based on approximation functions. Differentiability of A is greatly enhanced by two sub-algorithms, designated as pixel sharing and re-scaling. Distributions are handled by a new statistical method called Functional Statistics that was developed for HRDs but applies as well to many other problems traditionally treated by Monte Carlo methods. Applications of this method to synthetic and real clusters were mainly successful. Schemes to promote flexibility and several presently deferred problems are discussed.

Abstract:

I review the problem of lithium depletion in pre-Main Sequence (pre-MS) stars, and the curious points of view which accompanied the changes in the results due to the changes in the stellar physics inputs in the last 35 years. Today, the problem of explaining the present solar lithium abundance is reversed with respect to the problem of some years ago: new standard stellar models deplete too much lithium during the pre-MS evolution. We have recently attempted to model the solar pre-MS by including the thermal effect of the magnetic field which is due to the rotationally induced dynamo in these stars. I show that this new physical input can not only help to correctly describe the pre-MS lithium depletion, but it can also provide the correct HR diagram location for the first eclipsing pre-MS binary having a component of mass close to solar, RXJ 0529.4+0041. We show that the same model can be applied to the fast rotating secondaries of cataclysmic binaries and help to solve the problem of matching the observed minimum orbital period with the theoretical period.

Abstract:

Cataclysmic binaries first reaching mass transfer with marginally stable mass ratios can achieve mass transfer rates high enough to sustain stable hydrogen burning and become supersoft X-ray sources. Such systems are concentrated in orbital period just above the period gap, and can, in the most favorable circumstances, achieve X-ray lifetimes in excess of 3 x 10(5) yr. They have white dwarfs of moderate mass (0.7 - 0.9 M.), and may therefore tend to lower X-ray temperatures and luminosities than the more populous, longer-period supersoft sources with donor masses exceeding similar to 1.5 M.. Their properties offer a direct probe of orbital angular momentum loss rates.

Abstract:

We carried out medium resolution spectroscopy of close, detached white dwarf - red dwarf binaries without planetary nebulae and cataclysmic variables in the infra-red K-band (2.3 - 2.4 mum). We have detected the (CO)-C-12 bands at 2.3535 mum and 2.3829 mum and, for the first time, the (CO)-C-13 bands at 2.3448 mum and 2.3739 mum from the secondary stars. Comparison with synthetic spectral profiles of these lines indicate that the C-12/C-13 isotopic ratio in these binaries are different from solar. Our preliminary estimates of the isotopic ratios agree well with the theoretical predictions from the common-envelope binary evolution model by Marks et al. (1997). Thus we may have the first direct proof that these objects have indeed undergone common-envelope evolution.