CO in the Draco Nebula: The Atomic-Molecular Transition

Abstract

This paper presents maps of the J=2-1 transition of CO toward the Draco Nebula Intermediate Velocity Cloud (IVC). The maps cover 8500 square arcmin with a velocity resolution of 0.33 km~s$^{-1}$ and angular resolution of 38", or 0.11 pc at the cloud distance of 600 pc. The mapped area includes all the emission detected by the {\it Herschel} satellite with 250 $\mu$m intensity >5 MJy/sr. Previously published observations of the far-IR emission and the 21 cm line of HI are used to derive the column density distribution of H$_2$ and the abundance ratio CO/H$_2$, as well as the distribution of the molecular fraction of hydrogen, which approaches 90\% over much of the brighter parts of the nebula. The CO emission is highly clumpy and closely resembles the structures seen in far-IR images. The kinematics of the CO show supersonic motions between clumps but near-thermal to trans-sonic motions within clumps, consistent with model predictions that the scale length for dissipation of supersonic turbulence should be $\sim0.1$ pc, mediated by kinematic viscosity and/or ambipolar diffusion. Different parts of the nebula show evidence for a spread of molecular formation timescales of a few 10$^5$ years, comparable to the dynamical timescale of the infalling gas. The IVC will likely merge with the Galactic interstellar medium in $\sim 10^7$ years, and the densest clumps may form an unbound cluster of low-mass stars.