Abstract
The AM Canum Venaticorum (AM CVn) binaries are a rare group of hydrogen-deficient, ultrashort period, mass-transferring white dwarf binaries and are possible progenitors of Type Ia supernovae. We present time-resolved spectroscopy of the recently discovered AM CVn binary SDSS J173047.59+554518.5. The average spectrum shows strong double-peaked helium emission lines, as well as a variety of metal lines, including neon; this is the second detection of neon in an AM CVn binary, after the much brighter system GP Com. We detect no calcium in the accretion disc, a puzzling feature that has been noted in many of the longer period AM CVn binaries. We measure an orbital period, from the radial velocities of the emission lines, of 35.2 ± 0.2 min, confirming the ultracompact binary nature of the system. The emission lines seen in SDSS J1730 are very narrow, although double-peaked, implying a low-inclination, face-on accretion disc; using the measured velocities of the line peaks, we estimate i ≤ 11°. This low inclination makes SDSS J1730 an excellent system for the identification of emission lines.
Highlights
The AM Canum Venaticorum (AM CVn) binaries are a class of ultracompact systems, characterised by their short orbital periods, which range from 5 to 65 minutes, and their hydrogen-deficient spectra
Metal lines are often seen in quiescent AM CVn spectra (e.g. Roelofs et al 2006a, 2009; Levitan et al 2013; Kupfer et al 2013), neon has only previously been identified in GP Com (Marsh et al 1991; Nelemans et al 2010)
We measure an orbital period Porb = 35.2 ± 0.2 minutes, confirming Sloan Digital Sky Survey (SDSS) J1730 as an ultracompact binary. This puts SDSS J1730 in the longer period region of the outbursting AM CVn binary period distribution, and we would expect that the system would spend the majority of its time in a low state, with infrequent, longer-duration outbursts, similar to those observed in V406 Hya (Porb = 33.8 min; Roelofs et al 2006a) and SDSS J0129 (Porb = 37.6 min; Kupfer et al 2013) (Ramsay et al 2012)
Summary
The AM Canum Venaticorum (AM CVn) binaries are a class of ultracompact systems, characterised by their short orbital periods, which range from 5 to 65 minutes, and their hydrogen-deficient spectra. They consist of a white dwarf accreting helium-rich material from a (semi-)degenerate companion. It is the degenerate nature of the mass donor that allows their periods to lie well below the believed minimum period (∼80 min) of hydrogen-rich cataclysmic variables (CVs; Rappaport et al 1982; Gansicke et al 2009).
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