SDSS unveils a population of intrinsically faint cataclysmic variables at the minimum orbital period

Abstract

We discuss the properties of 137 cataclysmic variables (CVs) which are included in the Sloan Digital Sky Survey (SDSS) spectroscopic data base, and for which accurate orbital periods have been measured. 92 of these systems are new discoveries from SDSS and were followed‐up in more detail over the past few years. 45 systems were previously identified as CVs because of the detection of optical outbursts and/or X‐ray emission, and subsequently re‐identified from the SDSS spectroscopy. The period distribution of the SDSS CVs differs dramatically from that of all the previously known CVs, in particular it contains a significant accumulation of systems in the orbital period range 80–86 min. We identify this feature as the elusive ‘period minimum spike’ predicted by CV population models, which resolves a long‐standing discrepancy between compact binary evolution theory and observations. We show that this spike is almost entirely due to the large number of CVs with very low accretion activity identified by SDSS. The optical spectra of these systems are dominated by emission from the white dwarf photosphere, and display little or no spectroscopic signature from the donor stars, suggesting very low mass companion stars. We determine the average absolute magnitude of these low‐luminosity CVs at the period minimum to be 〈Mg〉= 11.6 ± 0.7. Comparison of the SDSS CV sample to the CVs found in the Hamburg Quasar Survey and the Palomar Green Survey suggests that the depth of SDSS is the key ingredient resulting in the discovery of a large number of intrinsically faint short‐period systems.

1000 cataclysmic variables from the Catalina Real-time Transient Survey

“…In CRTS J075414.5+313216, CRTS J130030.3+115101, CRTS J132103.2+015329, CRTS J140454.0−102702 and CRTS J172515.5+073249, the flux is strongly dominated by the white dwarf. Such a strong signature of the white dwarf in the optical spectrum is often seen in old, short-period CVs with low accretion rates (Gänsicke et al 2009). Photometric observations are available for four of these five CVs, confirming that they have short orbital periods.…”

“…As a system evolves towards the period minimum, the accretion rate drops and the evolution slows down, so a key prediction of these evolutionary models is an accumulation of CVs at the minimum period. Despite it being a long-standing prediction, the first observational confirmation of such a spike in the orbital period histogram was only found recently, in the Sloan Digital Sky Survey (SDSS) sample of CVs (Gänsicke et al 2009). There are two main reasons for the success of the SDSS-based work in uncovering the previously 'missing' CVs.…”

“…The superhump period is closely related to the mass ratio of the two component stars and is a good proxy for the orbital period (e.g. Patterson et al 2005;Gänsicke et al 2009;Kato et al 2009). …”

New cataclysmic variables and other exotic binaries in the globular cluster 47 Tucanae*

“…However the folded light curve using the longer period is asymmetric, which could be due to ellipsoidal variations. If indeed the period is 52 min, W21UV should have a fully or partially degenerate secondary (the minimum period for CVs with a non degenerate donor is ∼ 75-80 mins; Gänsicke et al 2009). H-rich CVs with periods below 75 mins have been detected, examples are 485 Cen (59 min period, Augusteijn et al 1996) and 1RXS J232953.9+062814 (64 min period, Thorstensen et al 2002).…”

Dynamical masses of a nova-like variable on the edge of the period gap

“…The orbital period distribution of CVs shows a significant paucity of systems in the ∼ 2-3 hour range, the so-called period gap (see e.g. Gänsicke et al 2009, and references therein). CVs with orbital periods above the period gap are expected to have M-dwarf secondary stars up to a period of about ∼ 5.8 h, while secondary stars of earlier type are to be found for longer orbital periods (Knigge 2006).…”