Understanding the Unusual X-Ray Emission Properties of the Massive, Close Binary WR 20a: A High Energy Window into the Stellar Wind Initiation Region

摘要

The problem of explaining the X-ray emission properties of the massive, closebinary WR 20a is discussed. Located near the cluster core of Westerlund 2, WR20a is composed of two nearly identical Wolf- Rayet stars of 82 and 83 solarmasses orbiting with a period of only 3.7 days. Although Chandra observationswere taken during the secondary optical eclipse, the X-ray light curve shows nosigns of a flux decrement. In fact, WR 20a appears slightly more X-ray luminousand softer during the optical eclipse, opposite to what has been observed inother binary systems. To aid in our interpretation of the data, we compare withthe results of hydrodynamical simulations using the adaptive mesh refinementcode Mezcal that includes radiative cooling and a radiative acceleration forceterm. It is shown that the X-ray emission can be successfully explained inmodels where the wind-wind collision interface in this system occurs while theoutflowing material is still being accelerated. Consequently, WR 20a serves asa critical test-case for how radiatively-driven stellar winds initiate andinteract. Our models not only procure a robust description of current Chandradata, which cover the orbital phases between 0.3 to 0.6, but provide detailedpredictions over the entire orbit.