CO in the Draco Nebula: The Atomic-Molecular Transition

TL;DR

This study maps CO emission in the Draco Nebula IVC, revealing detailed structure, kinematics, and molecular formation processes, and discusses the nebula's evolution and potential star formation.

Contribution

First detailed CO J=2-1 maps of the Draco Nebula IVC, linking molecular structure and kinematics with IR and HI data to understand turbulence and formation timescales.

Findings

CO emission is highly clumpy and resembles far-IR structures.

Supersonic motions between clumps; near-thermal within clumps.

Molecular formation timescales are a few 10^5 years.

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.