Philipp Podsiadlowski

Abstract:

We use the hot spot eclipse times of the newly discovered deeply-eclipsing dwarf nova IY UMa to trace out the shape of its disc during the late superhump era. We find an eccentric disc. The brightness of the stream-disc impact region varies as expected with \Delta(V) over right arrow \(2), where Delta(V) over right arrow is the differential velocity of the stream with respect to the velocity of the disc at the impact point. We conclude that the hot spot is the source of late superhump light.

Abstract:

Low-mass red giants may be viewed as stars - or even binary stars - with two distinct but very important centres: (i) a geometrical centre, and (ii) a separate nuclear centre, residing in a shell outside a zero-luminosity dense core. This two-centre or two-star perspective leads to an explicit, analytic, asymptotic theory of low-mass red-giant structure. In this theory, both rho(sh)/rhoc and rhosh . rhoc prove to be important quantities. I present new theorems and results involving these quantities, and then sketch a viewpoint which (i) links the structural and evolutionary behaviour of stars from the main-sequence through horizontal branch phases of evolution, and (ii) also has implications for post-main-sequence developments in more massive stars.

Abstract:

Hydromagnetic dynamos in close binaries with outer convective envelopes produce large scale and intense magnetic fields which affect all atmospheric levels, from the photosphere to the chromosphere and corona. In the photosphere, magnetic fields give rise to huge cool structures, similar to sunspots, which are called starspots. Their surface distribution and area changes can be used to study stellar rotation and activity cycles. The information on cycle lengths and differential rotation can be complemented with observations of the orbital period modulation in eclipsing systems. We discuss the recently proposed connection between the activity cycles and the modulation of the orbital period based on the variation of the gravitational quadrupole moment of the active component as a consequence of the redistribution of the kinetic and magnetic energies in its interior along the activity cycle. The variation of the gravitational potential felt by the companion can explain the observed variations of the orbital period of the order of DeltaP/P similar to 10(-6) - 10(-5). We present some recent results obtained by the study of a sample of late-type binaries and discuss their implication for the energy balance of hydromagnetic dynamos.

Abstract:

We determine the projected rotational velocities, V-rot sin i of the secondary stars in the PCEBs EG UMa, PG1026+002, REJ2013+400 and GD123. We find that the results are inconsistent with the values expected from other information we have about the binaries, particularly when V-rot sin i is measured from the NaI doublet.

Abstract:

Optically thick winds are used in recent binary evolution models for Type la supernova as well as in reproducing light curves of recurrent novae that are strong candidates of SN la progenitor. Here, I present the metallicity dependence of optically thick winds for a wide range of parameters of WD mass and chemical composition, X and Z. In the case of low metallicity, the wind is very weak, the theoretical light curve develops slowly, and the expansion velocity of the wind is small.