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

TL;DR

This study uses multi-epoch 7mm VLA observations to analyze the stability, proper motions, and dynamical history of Orion Source I and BN, revealing a past close encounter and its implications for disk evolution and star ejection.

Contribution

It provides new proper motion measurements at 43 GHz and supports a dynamical interaction scenario involving a binary Source I and BN, explaining their high velocities and disk properties.

Findings

Source I is stable and elongated, indicating an ionized disk rather than a jet.

Both Source I and BN are moving rapidly in opposite directions, consistent with a past close encounter.

Dynamical simulations suggest a binary-single star interaction leading to ejections and energy release.

Abstract

We present new 7mm continuum observations of Orion BN/KL with the VLA. We resolve the emission from the protostar radio Source I and BN at several epochs. Source I is highly elongated NW-SE, 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 disk rather than a 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, 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 (~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 20 Msun, 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 ~50 AU. N-body numerical simulations show that the dynamical interaction between a binary of 20 Msun total mass (I) and a single star of 10 Msun 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 flow traced by H2 and CO emission in the BN/KL nebula. Assuming the proposed dynamical history is correct, the smaller mass for Source I recently estimated from SiO maser dynamics (>7 Msun) by Matthews et al., suggests that non-gravitational forces (e.g. magnetic) must play an important role in the circumstellar gas dynamics.