Chandra detection of a close X-ray companion and rich emission-line spectrum in the Wolf-Rayet binary γ velorum

We present first results of a high-resolution X-ray observation of the nearby Wolf-Rayet binary system gamma (2) Velorum (WC8 + O7.5) using the Chandra High-Energy Transmission Grating (HETG). Emission lines from Mg, Si, S, Ne, and Fe dominate the spectrum and imply a range of plasma temperatures from similar to4 MK up to at least similar to 25 MK. The strongest lines are broadened, but no Doppler shifts are detected. He-like triplets show strong forbidden lines with no significant weakening from collisional effects or photoexcitation, contrasting sharply with the diluted forbidden lines of single O-type supergiants such as zeta Puppis. These results imply that some lines such as the Ne ix triplet are formed in cooler plasma at tens of stellar radii or more from the O star, well outside of the central wind interaction region located near the O star surface. Lastly, we report the discovery of a new X-ray source lying only 4."8 north of gamma (2) Vel that is very likely a low-mass pre-main-sequence star.