The chemical composition of donors in AM CVn stars and ultra-compact X-ray binaries: observational tests of their formation

We study the formation of ultra-compact binaries (AM CVn stars and ultra-compact X-ray binaries) with emphasis on the surface chemical abundances of the donors in these systems. Hydrogen is not convincingly detected in the spectra of these systems. Three different proposed formation scenarios involve different donor stars, white dwarfs, helium stars or evolved main-sequence stars. Using detailed evolutionary calculations we show that the abundances of helium WD donors and evolved main-sequence stars are close to equilibrium CNO-processed material, and the detailed abundances correlate with the core temperature and thus mass of the MS progenitors. Evolved MS donors typically have traces of H left. For hybrid or CO white dwarf donors, the carbon and oxygen abundances depend on the temperature of the helium burning and thus on the helium core mass of the progenitors. For helium star donors in addition to their mass, the abundances depend strongly on the amount of helium burnt before mass transfer starts and can range from unprocessed and thus almost equal to CNO-processed matter, to strongly processed and thus C/O rich and N-deficient. We briefly discuss the relative frequency of these cases for helium star donors, based on population synthesis results. Finally we give diagnostics for applying our results to observed systems and find that the most important test is the N/C ratio, which can indicate the formation scenario as well as, in some cases, the mass of the progenitor of the donor. In addition, if observed, the N/O, O/He and O/C ratios can distinguish between helium star and WD donors. Applied to the known systems we find evidence for WD donors in the AM CVn systems GP Com, CE 315 and SDSS J0804+16 and evidence for hybrid WD or very evolved helium star donors in the UCXBs 4U 1626-67 and 4U 0614+09. [Abridged]Comment: Accepted for publication in MNRA

A survey for post-common-envelope binary stars using GALEX and SDSS photometry star

We report the first results of our programme to obtain multi-epoch radial velocity measurements of stars with a strong far-ultraviolet excess to identify post-common-envelope binaries (PCEBs). The targets have been identified using optical photometry from Sloan Digital Sky Survey (SDSS) DR4, ultraviolet photometry from Galaxy Evolution Explorer (GALEX) GR2 and proper motion information from SDSS DR5. We have obtained spectra at two or more epochs for 36 targets. Three of our targets show large radial velocity shifts (> 50 km s-1) on a time-scale of hours or days and are almost certainly PCEBs. For one of these targets (SDSS J104234.77+644205.4) we have obtained further spectroscopy to confirm that this is a PCEB with an orbital period of 4.74 h and semi-amplitude K = 165 km s-1. Two targets are rapidly rotating K-dwarfs which appear to show small radial velocity shifts and have strong Ca ii H&K emission lines. These may be wind-induced rapidly rotating (WIRRing) stars. These results show that we can use GALEX and SDSS photometry to identify PCEBs that cannot be identified using SDSS photometry alone, and to identify new WIRRing stars. A more comprehensive survey of stars identified using the methods developed in this paper will lead to a much improved understanding of common envelope evolution