Astronomy and Astrophysics, volume 505, 281-286 (2009/10-1)
Spectral classification of the mass donors in the high-mass X-ray binaries EXO 1722-363 and OAO 1657-415.
MASON A.B., CLARK J.S., NORTON A.J., NEGUERUELA I. and ROCHE P.
Abstract (from CDS):
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.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.We determined that EXO 1722-363
was of spectral type B0-B1Ia, positioned at a distance 8.0–2.0+2.5
kpc with a progenitor mass in the range 30-40M☉
. Luminosity calculations imply that LX
erg/s 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☉
)∼5.3-6.2) distance determinations are problematic. For OAO 1657-415
we report a distance of 4.4≤d≤12kpc, implying an X-ray luminosity of 1.5x1036
erg/s. 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.
binaries: eclipsing - binaries: general - X-rays: binaries - stars: winds, outflows - stars: individual: OAO1657-415 - stars: individual: EXO1722-363
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