Phase-resolved spectroscopy of Gaia14aae: line emission from near the white dwarf surface

AM CVn binaries are a class of ultracompact, hydrogen-deficient binaries, each consisting of a white dwarfaccreting 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 phaseoffset 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 discbright 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 widthof -1.14 \AA. 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 cataclysmicvariable with a significantly evolved donor.