Kinematics of the Orion Nebula Cluster: Velocity Substructure and Spectroscopic Binaries

TL;DR

This study provides a detailed kinematic analysis of the Orion Nebula Cluster, revealing its non-relaxed state, velocity substructure, and the presence of spectroscopic binaries, supporting a cold collapse formation scenario.

Contribution

It offers new radial velocity measurements for over 1600 stars, identifies 89 spectroscopic binaries, and compares stellar and gas kinematics to test cluster formation models.

Findings

ONC is not in dynamical equilibrium.

Stars follow the gas velocity structure.

Data supports cold collapse formation model.

Abstract

We present a kinematic study of the Orion Nebula Cluster based upon radial velocities measured by multi-fiber echelle spectroscopy at the 6.5 meter MMT and Magellan telescopes. Velocities are reported for 1613 stars, with multi-epoch data for 727 objects as part of our continuing effort to detect and analyze spectroscopic binaries. We confirm and extend the results of Furesz et al. showing that the ONC is not relaxed, consistent with its youth, and that the stars generally follow the position-velocity structure of the moderate density gas in the region, traced by $^{13}$CO. The additional radial velocities we have measured enable us to probe some discrepancies between stellar and gaseous structure which can be attributed to binary motion and the inclusion of non-members in our kinematic sample. Our multi-epoch data allow us to identify 89 spectroscopic binaries; more will be found as we continue monitoring. Our results reinforce the idea that the ONC is a cluster in formation, and thus provides a valuable testing ground for theory. In particular, our observations are not consistent with the quasi-equilibrium or slow contraction models of cluster formation, but are consistent with cold collapse models.