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Using the Hubble Space Telescope, we detail the first abundance analysisenabled by far-ultraviolet spectroscopy of a low-mass (~0.19 Msun) white dwarf(WD), GALEX J1717+6757, which is in a 5.9-hr binary with a fainter,more-massive companion. We see absorption from nine metals, including roughlysolar abundances of Ca, Fe, Ti, and P. We detect a significantly sub-solarabundance of C, and put upper limits on N and O that are also markedlysub-solar. Updated diffusion calculations indicate that all metals shouldsettle out of the atmosphere of this 14,900 K, log(g) = 5.67 WD in the absenceof radiative forces in less than 20 yr, orders of magnitude faster than thecooling age of hundreds of Myr. We demonstrate that ongoing accretion of rockymaterial that is often the cause of atmospheric metals in isolated, moremassive WDs is unlikely to explain the observed abundances in GALEX J1717+6757.Using new radiative levitation calculations, we determine that radiative forcescan counteract diffusion and support many but not all of the elements presentin the atmosphere of this WD; radiative levitation cannot, on its own, explainall of the observed abundance patterns, and additional mechanisms such asrotational mixing may be required. Finally, we detect both primary andsecondary eclipses using ULTRACAM high-speed photometry, which we use toconstrain the low-mass WD radius and rotation rate as well as update theephemeris from the discovery observations of this WD+WD binary.

Heavy metals in a light white dwarf: Abundances of the metal-rich, extremely low-mass GALEX J1717+6757