THE BECKLIN-NEUGEBAUER OBJECT AS A RUNAWAY B STAR, EJECTED 4000 YEARS AGO FROM THE θ~1 ORIONIS C SYSTEM

摘要

We attempt to explain the properties of the Becklin-Neugebauer (BN) object as a runaway B star, as originally proposed by Plambeck et al. This is one of the best-studied bright infrared sources, located in the Orion Nebula Cluster—an important testing ground for massive star formation theories. From radio observations of the BN object's proper motion, we trace its trajectory back to Trapezium star θ~1 Ori C, the most massive (45 solar mass) in the cluster and a relatively tight (17 AU) visual binary with a B star secondary. This origin would be the most recent known runaway B star ejection event, occurring only ～4000 years ago and providing a unique test of ejection models. Although highly obscured, we can constrain the BN object's mass (approx=7 solar mass) from both its bolometric luminosity and the recoil of θ~1 Ori C. Interaction of a runaway B star with dense ambient gas should produce a compact wind bow shock. We suggest that X-ray emission from this shocked gas may have been seen by Chandra: the offset from the radio position is approx=300 AU in the direction of the BN object's motion. Given this model, we constrain the ambient density, wind mass-loss rate, and wind velocity. The BN object made its closest approach to the massive protostar, source "I," 500 years ago. This may have triggered enhanced accretion and thus outflow, consistent with previous interpretations of the outflow being a recent (～10~3 yr) "explosive" event.