Phase-Resolved Spectroscopy of Gaia14aae: Line Emission From Near the White Dwarf Surface

Abstract

AM CVn binaries are a class of ultracompact, hydrogen-deficient binaries, each consisting of a white dwarf accreting helium-dominated material from a degenerate or semi-degenerate donor star. Of the 56 known systems, only Gaia14aae undergoes complete eclipses of its central white dwarf, allowing the parameters of its stellar components to be tightly constrained. Here, we present phase-resolved optical spectroscopy of Gaia14aae. We use the spectra to test the assumption that the narrow emission feature known as the `central spike' traces the motion of the central white dwarf. We measure a central spike velocity amplitude of $13.8\pm 3.2$ km/s, which agrees at the 1 $\sigma$ level with the predicted value of $17.6\pm 1.0$ km/s based on eclipse-derived system parameters. The orbital phase offset of the central spike from its expected position is $4\pm 15$ ${}^{\circ }$, consistent with 0 ${}^{\circ }$. Doppler maps of the He I lines in Gaia14aae show two accretion disc bright spots, as seen in many AM CVn systems. The formation mechanism for the second spot remains unclear. We detect no hydrogen in the system, but we estimate a 3 $\sigma$ limit on H$\alpha$ emission with an equivalent width of -1.14 Å. Our detection of nitrogen and oxygen with no corresponding detection of carbon, in conjunction with evidence from recent studies, mildly favours a formation channel in which Gaia14aae is descended from a cataclysmic variable with a significantly evolved donor.

16 pages, accepted by MNRAS

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Cited by in corpus (7)

• Discovery and characterization of five new eclipsing AM CVn systems
• Spectroscopic and Photometric Periods of Six Ultracompact Accreting Binaries
• Reversing the verdict: Cataclysmic variables could be the dominant progenitors of AM CVn binaries after all
• Evolved cataclysmic variables as progenitors of AM CVn stars
• A Catalogue of Ultracompact Mass-Transferring White Dwarf Binaries
• New insights into the helium star formation channel of AM CVn systems with explanations of Gaia14aae and ZTFJ1637+49
• Spectroscopy of the helium-rich binary ES Ceti reveals accretion via a disc and evidence for eclipse