August 11-15, 2014


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

JJ Hermes (University of Warwick)

Boris Gaensicke (U. of Warwick), Detlev Koester (U. Kiel), M. C. P. Bours (U. of Warwick), D. M. Townsley (U. of Alabama), J. Farihi (University College London), Stuart Littlefair (U. of Sheffield), T. R. Marsh (U. of Warwick), V. S. Dhillon (U. of Sheffield), A. Gianninas (U. of Oklahoma), E. Breedt (U. of Warwick), and R. Raddi (U. of Warwick)

White dwarfs are dense stars with strong surface gravities, which cause metals to rapidly diffuse out of their photospheres. However, a significant number of extremely low-mass (<0.3 Msun) white dwarfs show some form of atmospheric metals, despite relatively rapid settling times. Using the Hubble Space Telescope, we detail the first abundance analysis enabled by far-ultraviolet spectroscopy of a low-mass (0.19 Msun) white dwarf, GALEX J1717+6757, which is in a 5.9-hr binary with a cooler, more-massive companion. We see absorption from at least nine metals, including roughly solar abundances of Ca, Fe, Ti, and P. We detect a significantly sub-solar abundance of C, and put upper limits on N and O that are also markedly sub-solar. Updated diffusion calculations indicate that all metals should settle out of the atmosphere of this 14 900 K, log g = 5.67 white dwarf in the absence of radiative levitation in less than 20 yr, orders of magnitude faster than the cooling age of hundreds of Myr. We demonstrate that the observed abundances in GALEX J1717+6757 are incompatible with accretion from rocky material, which is often the cause of atmospheric metals in canonical-mass white dwarfs. Using new radiative levitation calculations, we determine that radiative forces can counteract diffusion and support many but not all of the elements present in the atmosphere of this white dwarf; radiative levitation cannot, on its own, explain all of the observed abundances. We detect both primary and secondary eclipses using ULTRACAM high-speed photometry, and update the ephemeris from the discovery observations of Vennes et al. (2011).

Mode of presentation: poster