2009A&A...506.1243T

The ortho-to-para ratio (OPR) of molecular hydrogen is a fundamental parameter in understanding the physics and chemistry of molecular clouds. In dark and cold regions, however, H₂ is not directly observable and the OPR of H₂ in these sources has so far remained elusive. We show that the 6cm absorption line of ortho-formaldehyde (H₂CO) can be employed to constrain both the density and the OPR of H₂ in dark clouds.Green Bank Telescope (GBT) observations of ortho-H₂CO toward the molecular cloud Barnard 68 (B68) are reported. Non-LTE radiative transfer calculations combined with the well-constrained structure of B68 are then employed to derive the physical conditions in the absorption region.We provide the first firm confirmation of the Townes & Cheung mechanism: propensity rules for the collisions of H₂CO with H₂ molecules are responsible for the sub-2.7K cooling of the 6cm doublet. Non-LTE calculations show that in the absorption region of B68, the kinetic temperature is ∼10K, the ortho-H₂CO column density amounts to ∼2.2x10¹³cm^–2, the H₂ density is in the range 1.4-2.4x10⁴cm^–3, and the OPR of H₂ is close to zero. Our observations thus provide fresh evidence that H₂ is mostly in its para form in the cold gas, as expected from theoretical considerations. Our results also suggest that formaldehyde absorption originates in the edge of B68, at visual extinctions A_V≲0.5mag.