Hunting for the elusive methylene radical

Journal article, 2021

Context.
The NKaKc = 404-313 transitions of ortho-CH2 between 68 and 71 GHz were first detected toward the Orion-KL and W51 Main star-forming regions. Given their high upper level energies (225 K) above the ground state, they were naturally 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.

Aims.
In this work, we aim to investigate the nature of the elusive CH2 emission, address its association with hot cores, and examine alternative possibilities for its origin. Owing to its importance in carbon chemistry, we also extend the search for CH2 lines by observing an assortment of regions, guided by the hypothesis that the observed CH2 emission is likely to arise from the hot gas environment of photodissociation regions (PDRs).

Methods.
We carried out our observations first using the Kitt Peak 12 m telescope to verify the original detection of CH2 toward different positions in the central region of the Orion Molecular Cloud 1. These were followed-up by deep integrations using the higher angular resolution of the Onsala 20 m telescope. We also searched for the NKaKc = 212-303 transitions of para-CH2 between 440-445 GHz toward the Orion giant molecular cloud complex using the APEX 12 m telescope. We also obtained auxiliary data for carbon recombination lines with the Effelsberg 100 m telescope and employing archival far infrared data.

Results.
The present study, along with other recent observations of the Orion region reported here, 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 μm 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 carbon radio recombination lines (CRRLs), which are well-known PDR tracers. In addition, we report the detection of the 70 GHz fine- and hyperfine structure components of ortho-CH2 toward the W51 E, W51 M, W51 N, W49 N, W43, W75 N, DR21, and S140 star-forming regions, and three of the NKaKc = 404-313 fine- and hyperfine structure transitions between 68-71 GHz toward W3 IRS5. While we have no information on the spatial distribution of CH2 in these regions, aside from that in W51, we again see a correspondence between the profiles of CH2 lines and those of CRRLs. We see a stronger CH2 emission toward the extended HII region W51 M rather than toward the much more massive and denser W51 E and N regions, which strongly supports the origin of CH2 in extended dilute gas. We also report the non-detection of the 212-303 transitions of para-CH2 toward Orion. Furthermore, using a non-LTE radiative transfer analysis, we can constrain the gas temperatures and H2 density to (163 ± 26) K and (3.4 ± 0.3) × 103 cm-3, respectively, for the 68-71 GHz ortho-CH2 transitions toward W3 IRS5, for which we have a data set of the highest quality. This analysis confirms our hypothesis that CH2 originates inwarm and dilute PDR layers. Our analysis suggests that for the excitation conditions under the physical conditions that prevail in such an environment, these lines are masering, with weak level inversion. The resulting amplification of the lines' spontaneousemission greatly aids in their detection.