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Context. The N_KaKc=4₀₄-3₁₃ transitions of ortho-CH₂ between 68 and 71GHz 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 N_KaKc=2₁₂-3₀₃ transitions of para-CH₂ between 440-445GHz toward the Orion giant molecular cloud complex using the APEX 12m telescope. We also obtained auxiliary data for carbon recombination lines with the Effelsberg 100m 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 70GHz 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 N_KaKc=4₀₄-3₁₃ fine- and hyperfine structure transitions between 68-71GHz 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)x10³cm^–3, respectively, for the 68-71GHz 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.