Constraining the ortho-to-para ratio of H₂ with anomalous H₂CO absorption^*

Laboratoire d'Astrophysique de Grenoble, Université Joseph Fourier, UMR 5571-CNRS, Grenoble, France e-mail: Nicolas.troscompt@obs.ujf-grenoble.fr

Received: 26 June 2009
Accepted: 5 August 2009

Abstract

Context. 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.

Aims. We show that the 6 cm absorption line of ortho-formaldehyde (H₂CO) can be employed to constrain both the density and the OPR of H₂ in dark clouds.

Methods. 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.

Results. 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-2.7 K cooling of the 6 cm doublet. Non-LTE calculations show that in the absorption region of B68, the kinetic temperature is ∼10 K, the ortho-H₂CO column density amounts to ∼ cm^-2, the H₂ density is in the range  cm^-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  mag.