High-speed photometry of Gaia14aae: an eclipsing AM CVn that challenges formation models

Abstract

AM CVn-type systems are ultra-compact, hydrogen-deficient accreting binaries with degenerate or semi-degenerate donors. The evolutionary history of these systems can be explored by constraining the properties of their donor stars. We present high-speed photometry of Gaia14aae, an AM CVn with a binary period of 49.7 minutes and the first AM CVn in which the central white dwarf is fully eclipsed by the donor star. Modelling of the lightcurves of this system allows for the most precise measurement to date of the donor mass of an AM CVn, and relies only on geometric and well-tested physical assumptions. We find a mass ratio q = M 2 /M 1 = 0.0287 ± 0.0020 and masses M 1 = 0.87 ± 0.02M and M 2 = 0.0250 ± 0.0013M . We compare these properties to the three proposed channels for AM CVn formation. Our measured donor mass and radius do not fit with the contraction that is predicted for AM CVn donors descended from white dwarfs or helium stars at long orbital periods. The donor properties we measure fall in a region of parameter space in which systems evolved from hydrogen-dominated cataclysmic variables are expected, but such systems should show spectroscopic hydrogen, which is not seen in Gaia14aae. The evolutionary history of this system is therefore not clear. We consider a helium-burning star or an evolved cataclysmic variable to be the most likely progenitors, but both models require additional processes and/or finetuning to fit the data. Additionally, we calculate an updated ephemeris which corrects for an anomalous time measurement in the previously published ephemeris.

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

“…This confirms the widely used assumption that the spike originates on or near the surface of the white dwarf, and traces its motion (Marsh 1999;Kupfer et al 2016). This agreement can also be taken as an independent test of the stellar properties measured by Green et al (2018a), producing a reasonably secure confirmation.…”

“…x + K 2 y = 13.8 ± 3.2 km/s. This can be compared to our prediction of K WD = 17.6 ± 1.0 km/s based on the photometry-derived masses and inclination of Green et al (2018a). The two values agree at the level of 1.1 σ .…”

“…The upper limit on Hα EW measured in Section 3.1.4 is less constraining than was assumed by Green et al (2018a). Allowing the accreted material to contain a higher fraction of hydrogen makes the evolved cataclysmic variable channel more favourable for Gaia14aae.…”

“…This gravitational redshift can be used to further constrain the origin site of the central spike. Given the mass and radius of the central white dwarf measured photometrically (Green et al 2018a, Table 1), a redshift of 60 ± 2 km/s would be expected at the white dwarf surface. The redshift we measure for the central spike disagrees with this.…”

Detection of a 23.6 min periodic modulation in the optical counterpart of 3XMMJ051034.6–670353

“…We find only evidence of a narrow feature at a wavelength consistent with Hα in absorption (see Figure 4) and mea- sure an equivalent width of 14 ± 2Å of that line. However, we note that a Heii line at 6560Å has been seen in some AM CVn systems (see for instance Green et al (2018)). A higher signalto-noise spectrum is needed to search for the presence of Hβ and Hγ which would be expected if the line near 6563Å is indeed hydrogen.…”