P44	February 2000

ASCA spectroscopy of the hard X-ray emission from the colliding wind interaction in ² Velorum

¹ Institut d'Astrophysique et de Géophysique - Université de Liège, Avenue de Cointe 5, 4000 Liège, Belgium
² School of Physics & Astronomy, University of Birmingham, Edgbaston Birmingham B15 2TT, United Kingdom
³ Dept. of Physics & Astronomy, University of Leeds, Woodhouse Lane Leeds LS2 9JT, United Kingdom
⁴ USRA/LHEA, Goddard Space Flight Center, Greenbelt, MD 20771, USA

^* Chargé de Recherches FNRS, Belgium

Abstract

     We discuss an ASCA observation of the eccentric WC8 + O7.5III binary ² Velorum near apastron. The X-ray spectrum is compared to two previous observations obtained when the system was near periastron. All three spectra display a hard emission component that undergoes strong variability over the orbital cycle. The properties of the hard X-ray emission of ² Vel are constrained by taking into account the contribution from contaminating soft X-ray sources in the vicinity of ² Vel. We find that the observed variations are in qualitative agreement with the predictions of colliding wind models. We investigate for the first time the effect of uncertainties in the chemical composition of the X-ray emitting plasma on our understanding of the high energy properties of the wind interaction region. Our results indicate that these uncertainties significantly affect the derived shock temperature and absorption column, but play a smaller role in determining the intrinsic X-ray luminosity of the colliding wind zone. We further find that the intrinsic luminosity from the hard X-ray component in ² Vel does not follow the 1/D distance relation expected from simple models of adiabatic shocks.

Key Words

binaries: general - stars: Wolf-Rayet - stars: individual (² Velorum) - stars: mass-loss - X-rays: stars