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The chemical diversity of exo-terrestrial planetary debris around white dwarfs

Gänsicke, BT; Koester, D; Farihi, J; Girven, J; Parsons, SG; Breedt, E; (2012) The chemical diversity of exo-terrestrial planetary debris around white dwarfs. Monthly Notices of the Royal Astronomical Society , 424 (1) pp. 333-347. 10.1111/j.1365-2966.2012.21201.x. Green open access

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Abstract

We present Hubble Space Telescope (HST) ultraviolet spectroscopy of the white dwarfs PG0843+516, PG1015+161, SDSS1228+1040, and GALEX1931+0117, which accrete circumstellar planetary debris formed from the destruction of asteroids. Combined with optical data, a minimum of five and a maximum of 11 different metals are detected in their photospheres. With metal sinking time-scales of only a few days, these stars are in accretion/diffusion equilibrium, and the photospheric abundances closely reflect those of the circumstellar material. We find C/Si ratios that are consistent with that of the bulk Earth, corroborating the rocky nature of the debris. Their C/O values are also very similar to those of bulk Earth, implying that the planetary debris is dominated by Mg and Fe silicates. The abundances found for the debris at the four white dwarfs show substantial diversity, comparable at least to that seen across different meteorite classes in the Solar system. PG0843+516 exhibits significant overabundances of Fe and Ni, as well as of S and Cr, which suggests the accretion of material that has undergone melting, and possibly differentiation. PG1015+161 stands out by having the lowest Si abundance relative to all other detected elements. The Al/Ca ratio determined for the planetary debris around different white dwarfs is remarkably similar. This is analogous to the nearly constant abundance ratio of these two refractory lithophile elements found among most bodies in the Solar system. Based on the detection of all major elements of the circumstellar debris, we calculate accretion rates of ≃1.7 × 10 8 to ≃1.5 × 10 9gs -1. Finally, we detect additional circumstellar absorption in the Siiv1394, 1403Å doublet in PG0843+516 and SDSS1228+1040, reminiscent to similar high-ionization lines seen in the HST spectra of white dwarfs in cataclysmic variables. We suspect that these lines originate in hot gas close to the white dwarf, well within the sublimation radius.

Type: Article
Title: The chemical diversity of exo-terrestrial planetary debris around white dwarfs
Open access status: An open access version is available from UCL Discovery
DOI: 10.1111/j.1365-2966.2012.21201.x
Publisher version: http://dx.doi.org/10.1111/j.1365-2966.2012.21201.x
Language: English
Additional information: This article has been accepted for publication in the Monthly Notices of the Royal Astronomical Society ©: 2012 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.
Keywords: stars: individual: PG 0843+516; stars: individual: PG 1015+161; stars: individual: SDSS J122859.93+104032.9; stars: individual: GALEX J193156.8+011745; planetary systems; white dwarfs
UCL classification: UCL
UCL > Provost and Vice Provost Offices > UCL BEAMS
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences > Dept of Physics and Astronomy
URI: https://discovery.ucl.ac.uk/id/eprint/1477436
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