The Becklin-Neugebauer Object as a Runaway B Star, Ejected 4000 years ago from the theta^1C system
Abstract
We attempt to explain the properties of the Becklin-Neugebauer (BN) object as a runaway B star, as originally proposed by Plambeck et al. (1995). This is one of the best-studied bright infrared sources, located in the Orion Nebula Cluster (ONC) - an important testing ground for massive star formation theories. From radio observations of BN's proper motion, we trace its trajectory back to the Trapezium star theta^1C, the most massive (45Msun) in the cluster and a relatively tight (17AU) visual binary with a B star secondary. This origin would be the most recent known runaway B star ejection event, occurring only ~4000yr ago and providing a unique test of models of ejection from multiple systems of massive stars. Although BN is highly obscured, we can constrain its mass (~7Msun) both from its bolometric luminosity and from the recoil of theta^1C. The interaction of a runaway B star with dense ambient gas should produce a compact wind bow shock. We suggest that emission from this shocked gas may have been seen in X-rays by Chandra. The offset from the radio position is about 300AU in a direction consistent with BN's motion. Given this interpretation, we constrain the ambient gas density, the wind mass-loss rate and the wind velocity. We describe the necessary conditions to achieve a self-consistent model. BN made a close passage to the massive protostar, source "I", about 500 years ago. This may have triggered enhanced accretion and thus outflow, consistent with previous interpretations of the outflow as an "explosive" event occurring in the last ~1000 years.Keywords
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