A MULTI-EPOCH STUDY OF THE RADIO CONTINUUM EMISSION OF ORION SOURCE. I. CONSTRAINTS ON THE DISK EVOLUTION OF A MASSIVE YSO AND THE DYNAMICAL HISTORY OF ORION BN/KL

We present new lambda 7 mm continuum observations of Orion BN/KL with the Very Large Array. We resolve the emission from the young stellar objects radio Source I and BN at several epochs. Radio Source I is highly elongated northwest-southeast, and remarkably stable in flux density, position angle, and overall morphology over nearly a decade. This favors the extended emission component arising from an ionized edge-on disk rather than an outwardly propagating jet. We have measured the proper motions of Source I and BN for the first time at 43 GHz. We confirm that both sources are moving at high speed (12 and 26 km s(-1), respectively) approximately in opposite directions, as previously inferred from measurements at lower frequencies. We discuss dynamical scenarios that can explain the large motions of both BN and Source I and the presence of disks around both. Our new measurements support the hypothesis that a close (similar to 50 AU) dynamical interaction occurred around 500 years ago between Source I and BN as proposed by Gomez et al. From the dynamics of encounter, we argue that Source I today is likely to be a binary with a total mass on the order of 20M(circle dot) and that it probably existed as a softer binary before the close encounter. This enables preservation of the original accretion disk, though truncated to its present radius of similar to 50 AU. N-body numerical simulations show that the dynamical interaction between a binary of 20M(circle dot) total mass (Source I) and a single star of 10M(circle dot) mass (BN) may lead to the ejection of both and binary hardening. The gravitational energy released in the process would be large enough to power the wide-angle, high-velocity flow traced by H-2 and CO emission in the BN/KL nebula. Assuming that the proposed dynamical history is correct, the smaller mass for Source I recently estimated from SiO maser dynamics (greater than or similar to 7M(circle dot)) by Matthews et al., suggests that non-gravitational forces (e.g., magnetic) must play an important role in the circumstellar gas dynamics.