  Interstellar dark clouds are the sites of star formation. Their main
component, dihydrogen, exists under two states, ortho and para. H2 is supposed
to form in the ortho:para ratio (OPR) of 3:1 and to subsequently decay to
almost pure para-H2 (OPR <= 0.001). Only if the H2 OPR is low enough, will
deuteration enrichment, as observed in the cores of these clouds, be efficient.
The second condition for strong deuteration enrichment is the local
disappearance of CO, which freezes out onto grains in the core formation
process. We show that this latter condition does not apply to DCO+, which,
therefore, should be present all over the cloud. We find that an OPR >= 0.1 is
necessary to prevent DCO+ large-scale apparition. We conclude that the
inevitable decay of ortho-H2 sets an upper limit of ~6 million years to the age
of starless molecular clouds under usual conditions.
