A Volume-limited Sample of Cataclysmic Variables from Gaia DR2: Space Density and Population Properties

Abstract

We present the first volume-limited sample of cataclysmic variables (CVs), selected using the accurate parallaxes provided by the second data release (DR2) of the European Space Agency Gaia space mission. The sample is composed of 42 CVs within 150 pc, including two new systems discovered using the Gaia data, and is $(77 \pm 10)$ per cent complete. We use this sample to study the intrinsic properties of the Galactic CV population. In particular, the CV space density we derive, $\rho =(4.8^{+0.6}_{-0.8}) \times 10^{-6}\, \mbox{$\mathrm{pc}^{-3}$}$, is lower than that predicted by most binary population synthesis studies. We also find a low fraction of period bounce CVs, seven per cent, and an average white dwarf mass of $\langle M_\mathrm{WD} \rangle = (0.83 \pm 0.17)\, \mathrm{M}_\odot$. Both findings confirm previous results, ruling out the presence of observational biases affecting these measurements, as has been suggested in the past. The observed fraction of period bounce CVs falls well below theoretical predictions, by at least a factor of five, and remains one of the open problems in the current understanding of CV evolution. Conversely, the average white dwarf mass supports the presence of additional mechanisms of angular momentum loss that have been accounted for in the latest evolutionary models. The fraction of magnetic CVs in the 150 pc sample is remarkably high at 36 per cent. This is in striking contrast with the absence of magnetic white dwarfs in the detached population of CV progenitors, and underlines that the evolution of magnetic systems has to be included in the next generation of population models.

Spectroscopic and photometric periods of six ultracompact accreting binaries

“…For M 1 , we assumed 0.8±0.1M ⊙ . This value is typical of CV accretors (Pala et al 2020) and consistent with the two AM CVn accretors which have measured masses (Copperwheat et al 2011;Green et al 2018a).…”

“…The formation path of a He CV is as follows. In a volume-limited sample, 5±3 per cent of CVs appear to have evolved donors (Pala et al 2020). The majority of these donor stars still have hydrogen-dominated atmospheres and orbital periods longer than the canonical period minimum.…”

X-rays observations of a super-Chandrasekhar object reveal an ONe and a CO white dwarf merger product embedded in a putative SN Iax remnant

“…It should be noted that while mergers may produce magnetic WDs, there is no strong evidence that they always do so. Conversely, there are strongly magnetic WDs in binaries (Pala et al 2020). Whether IRAS 00500+6713 is indeed magnetic is not known.…”

Spectroscopy of the helium-rich binary ES Ceti reveals accretion via a disc and evidence of eclipses

“…There are also differences between the two types; obviously their abundances and orbital periods are very different, but two other distinctions can be drawn. As yet there are no known examples of magnetic white dwarfs amongst almost 60 known AM CVn stars whereas around one third of CVs are magnetic (Pala et al 2020); there is no obvious explanation for this difference. The other difference is that AM CVn stars can reach such short periods that a novel form of accretion becomes possible in which no accretion disc forms but matter directly impacts the accreting white dwarf instead (Marsh & Steeghs 2002;Marsh, Nelemans & Steeghs 2004).…”