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

Creators: Green, M. J.; Marsh, T. R.; Steeghs, D. T. H.; Kupfer, T.; Ashley, R. P.; Bloemen, S.; Breedt, E.; Campbell, H. C.; Chakpor, A.; Copperwheat, C. M.; Dhillon, V. S.; Hallinan, G.; Hardy, L. K.; Hermes, J. J.; Kerry, P.; Littlefair, S. P.; Milburn, J.; Parsons, S. G.; Prasert, N.; van Roestel, J.; Sahman, D. I.; Singh, N.

Abstract

AM CVn-type systems are ultracompact, hydrogen-deficient accreting binaries with degenerate or semidegenerate 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 min and the first AM CVn in which the central white dwarf is fully eclipsed by the donor star. Modelling of the light curves 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 ± 0.02 M⊙ and M_2 = 0.0250 ± 0.0013 M⊙. 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 fine-tuning to fit the data. Additionally, we calculate an updated ephemeris which corrects for an anomalous time measurement in the previously published ephemeris.

Additional Information

© 2018 The Author(s). Published by Oxford University Press on behalf of the Royal Astronomical Society. Accepted 2018 January 30. Received 2018 January 12; in original form 2017 December 7. Published: 05 February 2018. The authors would like to thank Lorne Nelson and Lev Yungelson for exceedingly helpful discussions and insight, and the anonymous referee for their constructive feedback. MJG acknowledges funding from an the Science and Technology Facilities Council (STFC) studentship via grant ST/N504506/1. TRM, DTHS, and EB acknowledge STFC via grants ST/L000733/1 and ST/P000495/1. SB acknowledges funding from NWO VIDI grant 639.042.218, financed by the Netherlands Organisation for Scientific Research (NWO). VSD, SPL, ULTRACAM, and ULTRASPEC are funded by STFC via consolidated grant ST/J001589. Support for this work was provided by National Aeronautics and Space Administration (NASA) through Hubble Fellowship grant no. HST-HF2-51357.001-A, awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Incorporated, under NASA contract NAS5-26555. Part of the research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration. The research leading to these results has received funding from the European Research Council under the European Union's Seventh Framework Programme (FP/2007-2013)/ERC Grant Agreement n. 320964 (WDTracer). This publication made use of the packages LCURVE (Copperwheat et al. 2010), NUMPY, MATPLOTLIB, ASTROPY, SCIPY, EMCEE (Foreman-Mackey et al. 2013), and CORNER (Foreman-Mackey et al. 2016). DA and DB model white dwarf atmospheres for Fig. 10 were taken from http://www.astro.umontreal.ca/~bergeron/CoolingModels (last accessed 2016 June 21). The data presented in this work were obtained using the WHT operated by the Isaac Newton Group at the Roque de los Muchachos Observatory on La Palma, the 2.4 m TNT operated by the National Astronomy Research Institute of Thailand at the Thai National Observatory on Doi Inthanon, and the 200-inch Hale Telescope at Palomar Observatory operated by the California Institute of Technology.

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