CSS100603:112253-111037: A helium-rich dwarf nova with a 65 minute orbital period

TL;DR

This paper reports the discovery of a helium-rich dwarf nova with an extremely short 65-minute orbital period, providing evidence for its classification as an AM CVn system formed via the evolved CV channel, and discusses its implications for ultracompact binary formation.

Contribution

It presents the first compelling example of an AM CVn system formed through the evolved CV channel, expanding understanding of ultracompact binary formation pathways.

Findings

Orbital period of 65.233 minutes derived from spectroscopy.

High helium-to-hydrogen emission line ratio indicating helium-rich nature.

Evolved CVs now constitute 7% of known ultracompact white dwarf binaries.

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 HeI5876/H_alpha = 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 minutes. In combination with the previously measured superhump period, this implies an extreme mass ratio of M_2/M_1 = 0.017 +/- 0.004. The H_alpha and HeI6678 emission lines additionally have a narrow central spike, as is often seen in the spectra of AM CVn type stars. Comparing its properties with 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 cataclysmic variables (CVs) now account for seven per cent of all known semi-detached 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 67 - 76 minutes.