CSS100603:112253−111037: a helium-rich dwarf nova with a 65 min orbital period

Abstract

We present time‐resolved optical spectroscopy of the dwarf nova CSS100603:112253−111037. Its optical spectrum is rich in helium, with broad, double‐peaked emission lines produced in an accretion disc. We measure a line flux ratio He i λ5876/Hα = 1.49 ± 0.04, a much higher ratio than is typically observed in dwarf novae. The orbital period, as derived from the radial velocity of the line wings, is 65.233 ± 0.015 min. In combination with the previously measured superhump period, this implies an extreme mass ratio of M2/M1 = 0.017 ± 0.004. The Hα and He i λ 6678 emission lines additionally have a narrow central spike, as is often seen in the spectra of AM CVn‐type stars. Comparing their properties with cataclysmic variables (CVs), AM CVn systems and hydrogen binaries below the CV period minimum, we argue that CSS100603:112253−111037 is the first compelling example of an AM CVn system forming via the evolved CV channel. With the addition of this system, evolved CVs now account for 7 per cent of all known semidetached white dwarf binaries with Porb < 76 min. Two recently discovered binaries may further increase this figure. Although the selection bias of this sample is not yet well defined, these systems support the evolved CV model as a possible formation channel for ultracompact accreting binaries. The orbital periods of the three ultracompact hydrogen accreting binaries overlap with those of the long‐period AM CVn stars, but there are currently no known systems in the period range of 67–76 min.

1000 cataclysmic variables from the Catalina Real-time Transient Survey

“…Its orbital period is still unknown, but given the similarity of its spectrum to CRTS J112253.3−111037 and CRTS J111126.9+571239, we expect it to be well below 80 min as well. With the addition of the recently discovered BOKS 45906, a 56 min CV in the Kepler field (Ramsay et al 2014), evolved CVs now account for 10 per cent of all semidetached white dwarf binaries with orbital periods below the CV period minimum (Breedt et al 2012). This fraction increases to 12 per cent if CRTS J080853.7+355053 is included as well.…”

“…Two similar systems have recently been discovered, CRTS J112253.3−111037 (Breedt et al 2012) and CRTS J111126.9+ 571239 (Carter et al 2013b;Littlefield et al 2013). Both these binaries have orbital periods well below the orbital period minimum of CVs and enhanced helium in their optical spectra.…”

UVES and X-Shooter spectroscopy of the emission line AM CVn systems GP Com and V396 Hya

“…The fact that these 'central-spikes' contribute to the flare spectrum and follow a low-amplitude radial velocity curve as a function of the orbital phase suggests an origin on, or near, the white dwarf accretor (Marsh 1999;Morales-Rueda et al 2003). Such so-called 'central-spikes' have so far only been observed in AM CVn systems and He-rich dwarf novae but never in hydrogen dominated cataclysmic variables (Breedt et al 2012). If the accretor origin of the central-spikes can be confirmed, the central-spikes would be a powerful tool to trace the motion of the accreting white dwarf.…”

Massive double white dwarfs and the AM CVn birthrate

“…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. They constrain the orbital period of CSS1122-1110, a cataclysmic variable with unusually strong He lines, to 65.2 min and use the superhump period excess to infer a mass ratio of 0.017.…”