Hunting for the elusive methylene radical

Vol. 647
6. Interstellar and circumstellar matter

Hunting for the elusive methylene radical

by A. M. Jacob, K. M. Menten, Y. Gong, et al. 2021, A&A, 647, A42

The methylene radical, CH2, is of considerable astrophysical interest because it is both produced and destroyed at an early stage in the sequence of ion-molecule reactions that govern interstellar chemistry. It has only been detected in emission toward the high-mass star-forming regions Orion KL and W51, and the characteristics of the gas responsible for this emission are still poorly known. To investigate the nature of the CH2-emitting gas, Jacob and collaborators have carried out observations of this radical toward multiple positions in Orion and other high-mass star-forming regions using the Kit Peak, Onsala, and APEX radio telescopes. The observations indicate that the CH2 emission closely follows the emission from the [CII] 158 μm line but is undetected toward the hot core. This suggests that the CH2 emission arises from hot but dilute layers of photodissociation regions (PDRs) but not from the denser parts, such as the Orion Bar. Further non-local thermodynamic equilibrium radiative transfer analysis of the emission from W3 IRS5 confirms the above interpretation and suggests that the observed lines are masering with weak level inversion. The resulting amplification of the lines' spontaneous emission greatly aides their detection.