Hunting for the elusive methylene radical

TL;DR

This study detects and analyzes CH2 emission in various star-forming regions, revealing its origin in warm, dilute PDR layers rather than dense, hot cores, and suggests maser amplification of these lines.

Contribution

It provides the first extensive observational evidence that CH2 emission arises from PDR layers and not hot dense regions, with detailed physical condition constraints.

Findings

CH2 emission closely follows [CII] 158 um emission.

CH2 is undetected in hot, dense cores like Orion Hot Core.

CH2 lines are likely masering with weak level inversion.

Abstract

CH2 transitions between 68 and 71 GHz were first detected toward the Orion-KL and W51 Main SFRs. Given their upper level energies of 225 K, they were thought to arise in dense, hot molecular cores near newly formed stars. However, this has not been confirmed by further observations of these lines and their origin has remained unclear. Generally, there is a scarcity of observational data for CH2 and, while it is an important compound in the astrochemical context, its actual occurrence in astronomical sources is poorly constrained. The present study, along with other recent observations of the Orion region we report, rule out the possibility of an association with gas that is both hot and dense. We find that the distribution of the CH2 emission closely follows that of the [CII] 158 um emission, while CH2 is undetected toward the hot core itself. The observations suggest, rather, that its extended emission arises from hot but dilute layers of PDRs and not from the denser parts of such regions as in the case of the Orion Bar. This hypothesis was corroborated by comparisons of the observed CH2 line profiles with those of CRRLs, well-known PDR tracers. In addition, we report the detection of the 70 GHz fine- and hfs lines of o-CH2 toward the W51E, W51M, W51N, W49N, W43, W75N, DR21, and S140 SFRs, and three of the fine- and hfs lines between 68-71 GHz toward W3 IRS5. Furthermore, using a non-LTE radiative transfer analysis, we can constrain the gas temperatures and H2 density to 163 K and 3.4e3 cm^-3, respectively. This analysis confirms our hypothesis that CH2 originates in warm and dilute PDR layers. Our analysis suggests that for the excitation conditions under the physical conditions that prevail in such regions, these lines are masering, with weak level inversion. The resulting amplification of the lines' spontaneous emission greatly aids in their detection.