High‐Resolution Radio Observations of the Colliding‐Wind Binary WR 140

Abstract

Milliarcsecond resolution Very Long Baseline Array (VLBA) observationsof the archetype W-R+O star colliding-wind binary (CWB) system WR140 are presented for 23 epochs between orbital phases 0.74 and 0.97.At 8.4 GHz, the emission in the wind-collision region (WCR) is clearlyresolved as a bow-shaped arc that rotates as the orbit progresses.We interpret this rotation as due to the O star moving from southeastto approximately east of the W-R star, which leads to solutions forthe orbital inclination of 122deg+/-5deg, longitude of the ascendingnode of 353deg+/-3deg, and an orbit semimajor axis of 9.0+/-0.5 mas.The distance to WR 140 is determined to be 1.85+/-0.16 kpc, whichrequires the O star to be a supergiant. The inclination implies thatthe mass of the W-R and O star is 20+/-4 and 54+/-10 Msolar, respectively.We determine a wind momentum ratio of 0.22, with an expected half-openingangle for the WCR of 63°, consistent with 65deg+/-10deg derivedfrom the VLBA observations. Total flux measurements from Very LargeArray (VLA) observations show that the radio emission from WR 140is very closely the same from one orbit to the next, pointing stronglytoward emission, absorption, and cooling mechanism(s) that are controlledlargely by the orbital motion. The synchrotron spectra evolve dramaticallythrough the orbital phases observed, exhibiting both optically thinand optically thick emission. We discuss a number of absorption andcooling mechanisms that may determine the evolution of the synchrotronspectrum with orbital phase.