Evolutionary history of four binary blue stragglers from the globular clusters \omega Cen, M55, 47 Tuc and NGC 6752

Context. Origin and evolution of blue stragglers in globular clusters is still a matter of debate. Aims. The aim of the present investigation is to reproduce the evolutionary history of four binary blue stragglers in four different clusters, for which precise values of global parameters are known. Methods. Using the model for cool close binary evolution, developed by one of us (KS), progenitors of all investigated binaries were found and their parameters evolved into the presently observed values. Results. The results show that the progenitors of the binary blue stragglers are cool close binaries with period of a few days, which transform into stragglers by rejuvenation of the initially less massive component by mass transfer from its more massive companion overflowing the inner critical Roche surface. The parameters of V209 from \omega Cen indicate that the binary is substantially enriched in helium. This is an independent and strong evidence for the existence of the helium rich subpopulation in this cluster.Comment: Accepted by Astronomy and Astrophysic

HII regions in symbiotic binaries and their radio emission

The slow and dense wind from a symbiotic red giant can be significantly deflected toward the orbital plane by the gravitational pull of the companion star. In such an environment, the ionizing radiation from the companion creates a highly asymmetric HII region. We present three-dimensional models of HII regions in symbiotic S-type systems, for which we calculate radio maps and radio spectra. We show that the standard assumption of spherically symmetric RG wind results in wrong shapes, sizes and spectra of ionized regions, which in turn affects the observational estimates of orbital separation and mass loss rate. A sample of radio maps and radio spectra of our models is presented and the results are discussed in relation to observational data.Comment: Accepted for publication in A&

Two-dimensional models of layered protoplanetary discs - II. The effect of a residual viscosity in the dead zone

We study axisymmetric models of layered protoplanetary discs taking radiative transfer effects into account, and allowing for a residual viscosity in the dead zone. We also explore the effect of different viscosity prescriptions. In addition to the ring instability reported in the first paper of the series we find an oscillatory instability of the dead zone, accompanied by variations of the accretion rate onto the central star. We provide a simplified analytical description explaining the mechanism of the oscillations. Finally, we find that the residual viscosity enables stationary accretion in large regions of layered discs. Based on results obtained with the help of a simple 1-D hydrocode we identify these regions, and discuss conditions in which layered discs can give rise to FU~Orionis phenomena.Comment: 9 pages, 5 figures, accepted for publication in MNRA