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

Universite de Liege et al.

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 (O9.7~I+b2~v,
P_A=21 d) and B (O8~III+o9~v, 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 O8~III 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 O8~III + o9~v 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.

Reference: Accepted for the ApJS Special Issue on the Chandra Carina Complex Project (CCCP), scheduled for publication in May 2011. All 16 CCCP Special Issue papers are available at the listed weblink.
Status: Manuscript has been accepted

Weblink: http://cochise.astro.psu.edu/Carina_public/special_issue.html

Comments: 11 pages, 7 figures.

Email: parkin@mso.anu.edu.au