Hunting for the elusive methylene radical
  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.
