Rotational excitation of CO in a cool, mixed atomic and molecular hydrogen gas

National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903-2475, USA e-mail: hliszt@nrao.edu

Received: 11 June 2006
Accepted: 4 August 2006

Abstract

Context.Rate constants for rotational excitation of CO by atomic hydrogen have recently been recalculated and found to be much larger than was previously believed, larger even than for excitation by H₂.

Aims.To clarify the effect upon CO rotational excitation of an admixture of atomic hydrogen in diffuse molecule-bearing gas.

Methods.The equations of statistical equilibrium in the CO rotation ladder are solved under a variety of conditions typical of CO-bearing gas in the diffuse ISM.

Results. Atomic hydrogen, if present, substantiantially increases the brightness and excitation temperatures of the CO rotational transitions, and increases the /, /, etc. line brightness ratios. A modest admixture of atomic hydrogen mimics a pure H₂-gas of substantially higher ambient thermal pressure.

Conclusions.Rotational excitation of CO by atomic hydrogen is important if even a few percent of the hydrogen is in atomic form.