The White Dwarf Binary Pathways Survey −III. Contamination from hierarchical triples containing a white dwarf

Abstract

The White Dwarf Binary Pathways Survey aims at increasing the number of known detached A, F, G and K main sequence stars in close orbits with white dwarf companions (WD+AFGK binaries) to refine our understanding about compact binary evolution and the nature of Supernova Ia progenitors. These close WD+AFGK binary stars are expected to form through common envelope evolution, in which tidal forces tend to circularize the orbit. However, some of the identified WD+AFGK binary candidates show eccentric orbits, indicating that these systems are either formed through a different mechanism or perhaps they are not close WD+AFGK binaries. We observed one of these eccentric WD+AFGK binaries with SPHERE and find that the system TYC 7218-934-1 is in fact a triple system where the WD is a distant companion. The inner binary likely consists of the G-type star plus an unseen low mass companion in an eccentric orbit. Based on this finding, we estimate the fraction of triple systems that could contaminate the WD+AFGK sample. We find that less than 15 per cent of our targets with orbital periods shorter than 100 days might be hierarchical triples.

The White Dwarf Binary Pathways Survey VII: Evidence for a bi-modal distribution of post mass transfer systems?

“…Following this strategy, we have so far identified the first presupersoft X-ray binary system (Parsons et al 2015), confirmed that our target selection is reliable and in virtually all cases the UV excess is indeed indicative of a white dwarf (Parsons et al 2016), published the first results of our radial velocity campaign (Rebassa-Mansergas et al 2017), estimated the limited contamination from triple systems motivated by the discovery of the first hierarchical triple system in the survey (Lagos et al 2020), and identified and characterised 26 new close WD+AFGK systems (Ren et al 2020) through radial velocity variations. Most importantly for this work, we measured the periods of six systems and found all of them to be PCEBs with periods below 2.5 days and circular orbits.…”

“…These systems can easily be interpreted as PCEBs. At somewhat longer orbital periods (weeks), we have so far not yet identified a single PCEB but instead four systems, three in this work and one in Lagos et al (2020), with eccentric orbits, which are most likely main sequence binaries or the inner binaries of hierarchical triple systems, either with no white dwarf at all or with the white dwarf being the distant tertiary companion candidate. In what follows we discuss the implications of this finding for our survey as well as for our understanding of white dwarf binary star formation.…”

“…The presence of triple systems with white dwarfs in the survey has been previously discussed in Lagos et al (2020), who found that the white dwarf detected in TYC 7218 is in fact a tertiary companion to a main sequence binary star. They derived an upper limit of 15 per cent for contaminants (i.e.…”

“…The three targets we present in this paper turned out to have eccentric orbits, which together with TYC 7218 (Lagos et al 2020) brings the number of measured eccentric orbits with periods of the order of weeks to four. Given their eccentric nature, these systems are not probably formed though CE evolution (e.g.…”

“…The fraction of AFGK/M+WD triples in the survey was derived in Lagos et al (2020), ranging from 1 up to 15 per cent of the sample of PCEB candidates. To roughly assess the fraction of type-(b) contaminants, we use an approach similar to that in section 5 of Lagos et al (2020).…”