THE PARA-H2 - ORTHO-H2 RATIO AND THE DEUTERATION OF INTERSTELLAR-MOLECULES

We show that there is a close correlation between the degree of deuteration of interstellar molecules and the para-H2:ortho-H2 ratio in dense clouds. This correlation stems from the reactions HD(H+, D+)H2 and H2(H2D+, H3+)HD, whose endothermicities differ according as H2(j = 0) or H2(j = 1) is involved. Both the level of deuteration and the ratio H2(j = 0):H2(j = 1) depend on the chemical and thermal evolution and hence on the age of the cloud. Illustrative results are given for an isobaric model.