Kinematics of NGC 2264: signs of cluster formation

TL;DR

This study analyzes the kinematics of NGC 2264 using high-resolution spectra, revealing substructure and velocity gradients that suggest the cluster's formation involved gravitational amplification of initial inhomogeneities.

Contribution

It provides detailed radial velocity measurements and identifies subclusters in NGC 2264, linking stellar kinematics with molecular gas structure to understand cluster formation.

Findings

Non-Gaussian velocity distribution with ~3.5 km/s dispersion

Detection of a north-south velocity gradient

Identification of at least three subclusters correlated with molecular gas structures

Abstract

We present results from 1078 high resolution spectra of 990 stars in the young open cluster NGC 2264, obtained with the Hectochelle multiobject echelle spectrograph on the 6.5m MMT. We confirm 471 stars as members, based on their radial velocity and/or H-alpha emission. The radial velocity distribution of cluster members is non-Gaussian with a dispersion of approx 3.5 km/s. We find a substantial north-south velocity gradient and spatially coherent structure inthe radial velocity distribution, similar to that seen in the molecular gas in the region. Our results suggest that there are at least three distinguishable subclusters in NGC 2264, correlated with similar structure seen in 13CO emission, which is likely to be a remnant of initial structure in this very young cluster. We propose that this substructure is the result of gravitational amplification of initial inhomogeneities during overall collapse to a filamentary distribution of gas and stars, as found in simulations by Burkert & Hartman (2004).