Kinematics of NGC 2264: Signs of cluster formation

We present results from 1078 high-resolution spectra of 990 stars in the young open cluster NGC2264, obtained with the Hectochelle multiobject echelle spectrograph on the 6.5 m MMT. We confirm 471 stars as members on the basis of their radial velocity and/or H alpha emission. The radial velocity distribution of cluster members is non-Gaussian, with a dispersion of sigma approximate to 3.5 kms(-1). We find a substantial north-south velocity gradient and spatially coherent structure in the 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 (CO)-C-13 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 & Hartmann (2004).