EDP Sciences
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Volume 505, Number 1, October I 2009
Page(s) 281 - 286
Section Stellar structure and evolution
DOI http://dx.doi.org/10.1051/0004-6361/200912480
Published online 16 July 2009
A&A 505, 281-286 (2009)
DOI: 10.1051/0004-6361/200912480

Spectral classification of the mass donors in the high-mass X-ray binaries EXO 1722-363 and OAO 1657-415

A. B. Mason1, J. S. Clark1, A. J. Norton1, I. Negueruela2, and P. Roche1, 3, 4

1  Department of Physics & Astronomy, The Open University, Milton Keynes MK7 6AA, UK
    e-mail: a.mason@open.ac.uk
2  Departamento de Física, Ingeniería de Sistemas y Teoría de la Señal, Universidad de Alicante, Apdo. 99, 03080 Alicante, Spain
3  School of Physics & Astronomy, Cardiff University, The Parade, Cardiff, CF24 3AA, UK
4  Division of Earth, Space & Environment, University of Glamorgan, Pontypridd, CF37 1DL, UK

Received 13 May 2009 / Accepted 9 July 2009

Aims. We report near-infrared (NIR) observations of the mass donors of the eclipsing high-mass X-ray binary (HMXB) systems EXO 1722-363 and OAO 1657-415 in order to derive their accurate spectral classifications.
Methods. ESO/VLT observations of the targets with the NIR spectrometer ISAAC were compared with several published NIR spectral atlases of O and B supergiants, an identification of each object's spectral characteristics was made, enabling the refinement of spectral classification of the mass donors.
Results. We determined that EXO 1722-363 was of spectral type B0–B1Ia, positioned at a distance $8.0_{\rm -2.0}^{+2.5}$ kpc with a progenitor mass in the range 30–40 $M_{\odot}$. Luminosity calculations imply that LX ~ 1035–1037 erg s-1 for this distance range. We conclude that EXO 1722-363 shares many of the properties associated with other X-ray binary B-type supergiant donors.
We found that OAO 1657-415 correlates closely with the spectra of a class of transitional objects, the Ofpe/WNL stars, an intermediate evolutionary stage between massive O type stars leaving the main sequence and evolving into Wolf-Rayets. Due to the wide range in luminosity displayed by Ofpe/WNL stars, (log $(L/L_{\odot})$ ~ 5.3–6.2) distance determinations are problematic. For OAO 1657-415 we report a distance of 4.4 $\le$ d $\le$ 12 kpc, implying an X-ray luminosity of 1.5 $\times$ 1036 $\le$ LX $\le$ 1037 erg s-1. We have used our new classification of OAO 1657-415 to explain the physical processes responsible for its unique position within the Corbet diagram. Ofpe/WNL stars demonstrate a high rate of mass-loss through a dense stellar wind combined with a low terminal velocity. This combination of wind properties leads to a high accretion rate and transfer of angular momentum to the neutron star in this system. We believe this in turn leads to a smaller instantaneous equilibrium spin period with respect to normal OB supergiants.

Key words: binaries: eclipsing -- binaries: general -- X-rays: binaries -- stars: winds, outflows -- stars: individual: OAO 1657-415 -- stars: individual: EXO 1722-363

© ESO 2009

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