Back to the Abstract

	1 Introduction
	2 Observational material
	3 Variation of molecular abundance with H2 and the source function for carbon monoxide
	4 Mechanisms of CO formation and fractionation
	5 Observed N(12CO) and N(13CO)
	6 J = 1-0 rotational excitation and line brightness
	7 Loose ends and unanswered questions
	References

List of figures