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

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, H 2 is not directly observable and the OPR of H 2 in these sources has so far remained elusive. Aims. We show that the 6 cm absorption line of ortho-formaldehyde (H 2 CO) can be employed to constrain both the density and the OPR of H 2 in dark clouds. Methods. Green Bank Telescope (GBT) observations of ortho-H 2 CO 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 H 2 CO with H 2 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-H 2 CO column density amounts to ∼2.2 × 10 13 cm −2 , the H 2 density is in the range 1.4−2.4 × 10 4 cm −3 , and the OPR of H 2 is close to zero. Our observations thus provide fresh evidence that H 2 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 < ∼ 0.5 mag.

The relation of H2CO,12CO, and13CO in molecular clouds

“…The optical depths of 13 CO and the column densities of both 13 CO and H 2 were estimated using Sato (1994) calculations, on the assumption that the cloud is in LTE. (Young et al 2004;Troscompt et al 2009). This is less than our velocity resolution.…”

Formaldehyde Silhouettes Against the Cosmic Microwave Background: A Mass-Limited, Distance-Independent, Extinction-Free Tracer of Star Formation Across the Epoch of Galaxy Evolution

“…We also assume an H 2 CO/H 2 abundance of 10 −9 , which is reasonable in molecular gas-rich galaxies, even at high redshift (Zeiger & Darling 2010 obtain an abundance of 0.1-2.5 × 10 −9 in the gravitational lens galaxy toward B0218+357 at z = 0.68), and an H 2 CO OPR of 3. Troscompt et al (2009aTroscompt et al ( , 2009b show that the anti-inversion of ortho-H 2 CO is principally driven by collisions with para-H 2 . Since the H 2 CO line excitation temperatures critically determine the detectability and interpretation of lines, the effect of the unknown H 2 OPR is a concern.…”

“…When not varying, we assume n(H 2 ) = 10 4 cm −3 , T kin = 40 K, and H 2 OPR 10 −2 (e.g., Maret & Bergin 2007, Pagani et al 2009, and Troscompt et al 2009a infer ratios in the range 0.01-0.1 in cold Galactic pre-stellar molecular gas). We focus on the first five K-doublet lines because these produce the strongest signals (millimeter lines thermalize in typical conditions), and the Troscompt et al (2009b) calculations do not include higher states.…”

H2 Ortho-to-para Conversion on Grains: A Route to Fast Deuterium Fractionation in Dense Cloud Cores?

“…While CO depletion could be estimated from observations, the H 2 OPR cannot be inferred (Pagani et al 2009(Pagani et al , 2013. H 2 does not emit cooling radiation in cold regions and cannot be detected unless probed indirectly through other tracers such as H 2 CO (Troscompt et al 2009). This means that the knowledge of the H 2 OPR merely comes from theoretical studies.…”