Ortho-H2 and the Age of Interstellar Dark Clouds

TL;DR

This paper investigates how the ortho-to-para ratio of dihydrogen influences star formation in dark clouds, concluding that ortho-H2 decay limits cloud ages to about 6 million years.

Contribution

It demonstrates that the decay of ortho-H2 sets an upper age limit for starless clouds and clarifies the conditions for deuteration enrichment.

Findings

Ortho-H2 decay constrains cloud age to ~6 million years.

DCO+ presence indicates OPR >= 0.1.

CO freeze-out is not necessary for DCO+ formation.

Abstract

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.