Ortho-H2 and the Age of Interstellar Dark Clouds

Details Ortho-H2 and the Age of Interstellar Dark Clouds Ortho-H2 and the Age of Interstellar Dark Clouds

2011-09-29

arxiv.org/abs/1109.6495v1

The Astrophysical Journal Letters 739 (2011) L35

Astronomy and Astrophysics

Astrophysics

Galaxy Astrophysics

Scientific paper

10.1088/2041-8205/739/2/L35

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.

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