Constraining the ortho-to-para ratio of H2 with anomalous H2COabsorption

摘要

Context. The ortho-to-para ratio (OPR) of molecular hydrogen is a fundamental parameter in understanding the physics and chemistryof molecular clouds. In dark and cold regions, however, H_2is not directly observable and the OPR ofH_2in these sources has so farremained elusive.Aims. We show that the 6 cm absorption line of ortho-formaldehyde (H_2CO) can be employed to constrain both the density and theOPR of H_2in dark clouds.Methods. Green Bank Telescope (GBT) observations of ortho-H_2CO 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 physicalconditions in the absorption region.Results. We provide the first firm confirmation of the Townes & Cheung mechanism: propensity rules for the collisions of H_2CO withH_2molecules are responsible for the sub-2.7 K cooling of 6 cm doublet. Non-LTE calculations show that in the absorption regionof B68, the kinetic temperature is ~10 K, the ortho-H_2CO column density amounts to ~2.2x 10~(13)cm~(-2), theH_2density is in the range1.4-2.4 x 10~4cm~(-3), and the OPR ofH_2is close to zero. Our observations thus provide fresh evidence thatH_mostly in its paraform in the cold gas, as expected from theoretical considerations. Our results also suggest that formaldehyde absorption originates inthe edge of B68, at visual extinctions Av ≤ 0.5 mag.