Volume 546, October 2012
|Number of page(s)||12|
|Published online||08 October 2012|
The nature of the high Galactic latitude O-star HD 93521: new results from X-ray and optical spectroscopy⋆,⋆⋆
Groupe d’Astrophysique des Hautes Énergies, Institut d’Astrophysique et de
Géophysique, Université de Liège,
Allée du 6 Août, Bât. B5c,
Received: 22 June 2012
Accepted: 4 September 2012
Context. Owing to its unusual location and its isolation, the nature of the high Galactic latitude O9.5 Vp object HD 93521 is still uncertain.
Aims. We have collected X-ray and optical observations to characterize the star and its surroundings.
Methods. X-ray images and spectra are analysed to search for traces of a recent star formation event around HD 93521 and to search for the signature of a possible compact companion. Optical echelle spectra are analysed with plane-parallel model atmosphere codes, assuming either a spherical star or a gravity darkened rotationally flattened star, to infer the effective temperature and surface gravity, and to derive the He, C, N and O abundances of HD 93521.
Results. The X-ray images reveal no traces of a population of young low-mass stars coeval with HD 93521. The X-ray spectrum of HD 93521 is consistent with a normal late O-type star although with subsolar metallicity. No trace of a compact companion is found in the X-ray data. In the optical spectrum, He and N are found to be overabundant, in line with the effect of rotational mixing in this very fast rotator, whilst C and O are subsolar. A critical comparison with the properties of subdwarf OB stars, indicates that, despite some apparent similarities, HD 93521 does not belong to this category.
Conclusions. Despite some ambiguities on the runaway status of the star, the most likely explanation is that HD 93521 is a Population I massive O-type star that was ejected from the Galactic plane either through dynamical interactions or a result of a supernova event in a binary system.
Key words: stars: early-type / stars: fundamental parameters / stars: abundances / stars: massive / stars: individual: HD 93521 / X-rays: stars
Based on observations collected at the Observatoire de Haute Provence (France) and with XMM-Newton, an ESA science mission with instruments and contributions directly funded by ESA member states and the USA (NASA).
Hα images are only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (126.96.36.199) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/546/A77
© ESO, 2012
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