Kinematic and Spatial Substructure in NGC 2264

TL;DR

This study provides an expanded kinematic analysis of NGC 2264, revealing complex stellar and gas motions, including blueshifted and redshifted populations, which shed light on the cluster's formation and dynamics.

Contribution

The paper significantly increases the number of stars with measured radial velocities in NGC 2264, improving understanding of its kinematic relationship with molecular gas.

Findings

Identification of a blueshifted stellar population relative to molecular gas

Detection of Lithium absorption and infrared excesses in blue-shifted stars

Clarification of the redshifted stellar population influenced by stellar wind bubbles

Abstract

We present an expanded kinematic study of the young cluster NGC 2264 based upon optical radial velocities measured using multi-fiber echelle spectroscopy at the 6.5 meter MMT and Magellan telescopes. We report radial velocities for 695 stars, of which approximately 407 stars are confirmed or very likely members. Our results more than double the number of members with radial velocities from F{\H u}r{\'e}sz et al., resulting in a much better defined kinematic relationship between the stellar population and the associated molecular gas. In particular, we find that there is a significant subset of stars that are systematically blueshifted with respect to the molecular ($^{13}$CO) gas. The detection of Lithium absorption and/or infrared excesses in this blue-shifted population suggests that at least some of these stars are cluster members; we suggest some speculative scenarios to explain their kinematics. Our results also more clearly define the redshifted population of stars in the northern end of the cluster; we suggest that the stellar and gas kinematics of this region are the result of a bubble driven by the wind from O7 star S Mon. Our results emphasize the complexity of the spatial and kinematic structure of NGC 2264, important for eventually building up a comprehensive picture of cluster formation.