X-RAY EMISSION FROM THE DOUBLE-BINARY OB-STAR SYSTEM QZ CAR (HD 93206)
E.R. Parkin, P.S. Broos, L.K. Townsley, J.M. Pittard, A.F.J. Moffat, Y. Naze, G. Rauw, L.M. Oskinova, W.L. Waldron
X-ray observations of the double-binary OB-star system QZ Car (HD 93206)
obtained with the Chandra X-ray Observatory over a period of roughly 2
years are presented. The orbit of systems A (O9.7I+B2V, P_A = 21 d) and
B (O8III + O9V, P_B = 6 d) are reasonably well sampled by the
observations, allowing the origin of the X-ray emission to be examined
in detail. The X-ray spectra can be well fitted by an attenuated three
temperature thermal plasma model, characterised by cool, moderate, and
hot plasma components at kT ~ 0.2, 0.7 and 2 keV, respectively, and a
circumstellar absorption of ~ 0.2e22 cm-2. Although the hot
plasma component could be indicating the presence of wind-wind collision
shocks in the system, the model fluxes calculated from spectral fits,
with an average value of ~ 7e-13 erg s-1 cm-2, do
not show a clear correlation with the orbits of the two constituent
binaries. A semi-analytical model of QZ Car reveals that a stable
momentum balance may not be established in either system A or B. Yet,
despite this, system B is expected to produce an observed X-ray flux
well in excess of the observations. If one considers the wind of the
O8III star to be disrupted by mass transfer the model and observations
are in far better agreement, which lends support to the previous
suggestion of mass-transfer in the O8III + O9V binary. We conclude that
the X-ray emission from QZ Car can be reasonably well accounted for by a
combination of contributions mainly from the single stars and the mutual
wind-wind collision between systems A and B.
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