Surface abundance and the hunt for stratification in chemically peculiar hot subdwarfs: PG 0909+276 and UVO 0512−08

Abstract Edelmann identified two chemically peculiar hot subdwarfs,  PG 0909+276 and  UVO 0512−08, as having very high overabundances of iron-group elements. We obtained high-resolution ultraviolet spectroscopy in order to measure abundances of species not observable in the optical, and to seek evid...

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Bibliographic Details
Published in:Monthly notices of the Royal Astronomical Society 2018-01, Vol.473 (3), p.4021-4032
Main Authors: Wild, J. F., Jeffery, C. S.
Format: Article
Language:English
Subjects:
Abundance
Chemical stratification
Cobalt
Gallium
Germanium
Helium
Hubble Space Telescope
Iron
Local thermodynamic equilibrium
Organic chemistry
Photosphere
Space telescopes
Spectrum analysis
Stars
Subdwarf stars
Ultraviolet spectra
Ultraviolet spectroscopy
Zinc
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Summary:Abstract Edelmann identified two chemically peculiar hot subdwarfs,  PG 0909+276 and  UVO 0512−08, as having very high overabundances of iron-group elements. We obtained high-resolution ultraviolet spectroscopy in order to measure abundances of species not observable in the optical, and to seek evidence for chemical stratification in the photosphere. Abundances were measured in three wavelength regions; the optical 3900–6900 Å range was re-analysed to confirm consistency with that analysed by Edelmann. Ultraviolet spectra were obtained with the Space Telescope Imaging Spectrograph on the Hubble Space Telescope, covering the far-UV (1140–1740 Å) and the near-UV (1740–2500 Å). We computed a grid of theoretical local thermodynamic equilibrium spectra to find basic parameters (effective temperatures, surface gravity, surface hydrogen and helium fractions). We measured abundances using a spectral-synthesis approach in each wavelength range. We confirm that several iron-group metals are highly enriched, including cobalt, copper and zinc, relative to typical sdB stars. We detect gallium, germanium, tin and lead, similar to analysis of ultraviolet spectra of some other sdB stars. Our results confirm that  PG 0909+276 and  UVO 0512−08 exhibit peculiarities which make them distinct from both the normal H-rich sdB and intermediate He-rich sdB stars. The process which leads to this particular composition has still to be identified.
ISSN:0035-8711
1365-2966
DOI:10.1093/mnras/stx2593