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

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Hermes, J. J., Gänsicke, B. T. (Boris T.), Koester, Detlev, Bours, Madelon C. P., Townsley, D. M., Farihi, J., Marsh, T. R., Littlefair, S., Dhillon, V. S., Gianninas, A., Breedt, E. and Raddi, R. (2014) Heavy metals in a light white dwarf : abundances of the metal-rich, extremely low-mass GALEX J1717+6757. Monthly Notices of the Royal Astronomical Society, Volume 444 (Number 2). pp. 1674-1682. doi:10.1093/mnras/stu1518 ISSN 0035-8711.

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Official URL: http://dx.doi.org/10.1093/mnras/stu1518

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Abstract

Using the Hubble Space Telescope, we detail the first abundance analysis enabled by far-ultraviolet spectroscopy of a low-mass (≃0.19 M⊙) white dwarf (WD), GALEX J1717+6757, which is in a 5.9-h binary with a fainter, more-massive companion. We see absorption from 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 WD in the absence of radiative forces in less than 20 yr, orders of magnitude faster than the cooling age of hundreds of Myr. We demonstrate that ongoing accretion of rocky material that is often the cause of atmospheric metals in isolated, more massive WDs is unlikely to explain the observed abundances in GALEX J1717+6757. 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 WD; radiative levitation cannot, on its own, explain all of the observed abundance patterns, and additional mechanisms such as rotational mixing may be required. Finally, we detect both primary and secondary eclipses using ULTRACAM high-speed photometry, which we use to constrain the low-mass WD radius and rotation rate as well as update the ephemeris from the discovery observations of this WD+WD binary.

Item Type: Journal Article
Divisions: Faculty of Science, Engineering and Medicine > Science > Physics
Journal or Publication Title: Monthly Notices of the Royal Astronomical Society
Publisher: Oxford University Press
ISSN: 0035-8711
Official Date: 30 August 2014
Dates:
DateEvent
30 August 2014Published
28 July 2014Accepted
28 July 2014Submitted
Volume: Volume 444
Number: Number 2
Page Range: pp. 1674-1682
DOI: 10.1093/mnras/stu1518
Status: Peer Reviewed
Publication Status: Published
Access rights to Published version: Restricted or Subscription Access
Adapted As: .

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