Abstract
We present results from a spectroscopic survey designed to uncover AM Canum Venaticorum (AM CVn) binaries hidden in the photometric database of the Sloan Digital Sky Survey (SDSS). The discovery of only 7 new AM CVns in the observed part of our sample suggests a lower space density than previously predicted. Based on the complete g≤19 sample, we calculate an observed space density for AM CVns of (5 ± 3) × 10<sup>−7</sup> pc<sup>−3</sup>. We also compare the cataclysmic variables (CVs) discovered via this survey to those found in the SDSS spectroscopy, and we discuss SBSS 1108+574, an unusually helium-rich CV that has a spectroscopically confirmed orbital period of 55 minutes, well below the CV period minimum (~80 min). SBSS 1108+574 may represent an AM CVn forming via the ‘evolved CV’ formation channel.
Highlights
The AM Canum Venaticorum (AM CVn) binaries are a rare group of hydrogen-deficient, ultra-compact, masstransferring white dwarf binaries
Three formation channels have been proposed for the AM CVn binaries, each characterised by the donor
As AM CVns should spend only a few percent of their lifetime as mass-transferring systems at orbital periods below 30 minutes, our selection should include the vast majority of AM CVns in the Sloan Digital Sky Survey (SDSS) footprint [6]
Summary
The AM Canum Venaticorum (AM CVn) binaries are a rare group of hydrogen-deficient, ultra-compact, masstransferring white dwarf binaries They have orbital periods in the range 5 – 65 minutes, well below the observed period minimum of hydrogen accreting cataclysmic variables (∼80 min, Gansicke et al 10). The third channel is known as the ‘evolved CV’ channel, and has generally been considered to be unimportant in comparison to the double white dwarf and helium star channels The evolution of these systems is thought to be governed by gravitational wave radiation [e.g. 22], causing the mass accretion rate to be a steeply decreasing function of orbital separation. AM CVns with periods greater than ∼40 min, are thought to be in a stable low-state, the spectra of these systems are characterised by emission lines from the accretion disc [e.g. 30]. We discuss estimates of the AM CVn space density, and results from our spectroscopic survey designed to uncover new AM CVn binaries amongst colour-selected objects from the SDSS photometric database [6, 25, 29]
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