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

Abstract

We study the formation of ultracompact binaries (AM CVn stars and ultracompact 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 any 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 white dwarf 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 main‐sequence progenitors. Evolved main‐sequence donors typically have traces of H left. For hybrid or carbon/oxygen 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 white dwarf donors. Applied to the known systems, we find evidence for white dwarf donors in the AM CVn systems GP Com, CE 315 and SDSS J0804+16 and evidence for hybrid white dwarf or very evolved helium star donors in the ultracompact X‐ray binaries 4U 1626−67 and 4U 0614+09.

What ignites on the neutron star of 4U 0614+091?

“…First, the amount of He required to achieve the required ignition conditions without stably burning away is significantly larger than the < ∼ 10% limit from the optical spectra. Recently, however, formation studies using evolutionary calculations indicate that the donor in 4U 0614+091 may be a hybrid white dwarf or very evolved helium star (Nelemans et al 2010). This suggests the donor still to be a possible supplier of a significant amount of He to the accretion disk, onto the neutron star.…”

“…Since it is thought to be an UCXB (in 't Zand et al 2007;Shahbaz et al 2008, and references therein), it is supposed to be so compact that the donor star can only be a non-degenerate H-deficient star or a white dwarf. Evolutionary models suggest that the accreted material could be rich in C/O or He (Nelemans et al 2010). However, no evidence of He (or H) has been found so far in optical spectra, with rather stringent upper limits: He and/or H are at most present at the 10% level (Werner et al 2006).…”

1000 cataclysmic variables from the Catalina Real-time Transient Survey

“…Podsiadlowski, Han & Rappaport (2003) proposed this mechanism as a possible formation channel for AM CVn binaries, but because of the lack of observed progenitor binaries, as well as long evolutionary time-scales required, this formation channel is considered less likely than alternatives (e.g. Nelemans et al 2010). Population synthesis models suggest that less than two per cent of compact binaries could form this way (Nelemans, Yungelson & Portegies Zwart 2004), favouring instead models which involve two phases of common envelope evolution to reduce the binary to its compact size.…”

Massive double white dwarfs and the AM CVn birthrate

“…However, it is difficult to distinguish between the different formation channels based on the observed AM CVn population, since all three channels essentially lead to very low-mass degenerate He-rich donors. Nelemans et al (2010) demonstrate that N/C abundance ratios significantly differ between the He-white dwarf and He star donors, and use this to identify He white dwarf donors in three AM CVn systems. Breedt et al (2012) present the first compelling evidence for an AM CVn progenitor with an evolved main-sequence donor.…”