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Abstract:

Most symbiotic binaries consist of an evolved giant star and a hot, accreting companion, typically a white dwarf. In wide symbiotic binaries, the companion may accrete part of the wind emitted by the giant by Bondi-Hoyle accretion. Here we present preliminary simulations of a new mass-transfer mode - wind Roche-lobe overflow (RLOF) - that can occur when the wind acceleration region of the evolved star occurs at several stellar radii (as, e.g., in Mira variables). If this region is relatively close to the Roche-lobe, wind material fills the giant's Roche-lobe and is transferred to the hot component through the inner Lagrangian point (similar to standard RLOF). We show that the accretion rate can be 100 times larger than the rate expected from standard Bondi-Hoyle accretion. Furthermore, mass loss from the system tends to be strongly concentrated towards the binary plane, producing an equatorial outflow. We discuss the implications of this study for the shaping of planetary nebulae, for symbiotic binaries as potential type la supernova progenitors and for the origin of barium stars.

Abstract:

The discovery of an excess of light in the far-ultraviolet (UV) in 1969 in elliptical galaxies was a major surprise. While it is now clear that this UV excess (UV-upturn) is probably caused by an old population of helium-burning stars. Han et al. (2002, 2003) proposed a binary model for the formation of hot subdwarfs (helium burning stars) and the model can reproduce the observations in our Galaxy. By applying the binary model to the study of evolutionary population synthesis, we have obtained an a priori model for the UV-upturn of elliptical galaxies. The model shows that the UV-upturn is most likely resulted from binary interactions and it is universal (not very much metallicity- dependant) in ellipticals. This has major implications for understanding the evolution of the UV-upturn and elliptical galaxies in general; contrary to previous postulates, it implies that the UV-upturn is not a sign of age, but could be a potentially powerful indicator for a recent minor burst of star-forming activity. © 2007 International Astronomical Union.

Abstract:

We present results of a multi-wavelength program to study the faint discrete X-ray source population discovered by Chandra in the Galactic Centre (GC). From IR imaging obtained with the VLT we identify candidate K-band counterparts to 75% of the X-ray sources in our sample. By combining follow-up VLT K-band spectroscopy of a subset of these candidate counterparts with the magnitude limits of our photometric survey, we suggest that only a small percentage of the sources are HMXBs, while the majority are likely to be canonical LMXBs and CVs at the distance of the GC. In addition, we present our discovery of highly structured small-scale (5-15′′) extinction towards the Galactic Centre. This is the finest-scale extinction study of the Galactic Centre to date. Finally, from these VLT observationswe are able to place constraints on the stellar counterpart to the "bursting pulsar" GRO J1744-28.

Abstract:

Theoretical models predict that the initial metallicity of the progenitor of a Type Ia supernova (SN Ia) affects the peak of the supernova light curve. This can cause a deviation from the standard light curve calibration employed when using SNe Ia as standardizable distance candles and, if there is a systematic evolution of the metallicity of SN Ia progenitors, could affect the determination of cosmological parameters. Here we show that this metallicity effect can be substantially larger than has been estimated previously, when the neutronisation in the immediate pre-explosion phase in the CO white dwarf is taken into account, and quantitatively assess the importance of metallicity evolution for determining cosmological parameters. We show that, in principle, a moderate and plausible amount of metallicity evolution could mimic a lambda-dominated, flat Universe in an open, lambda-free Universe. However, the effect of metallicity evolution appears not large enough to explain the high-z SN Ia data in a flat Universe, for which there is strong independent evidence, without a cosmological constant. We also estimate the systematic uncertainties introduced by metallicity evolution in a lambda-dominated, flat Universe. We find that metallicity evolution may limit the precision with which Omega_m and w can be measured and that it will be difficult to distinguish evolution of the equation of state of dark energy from metallicity evolution, at least from SN Ia data alone.