|
Description
The N_KaKc_=4_04_-3_13_ transitions of ortho-CH_2_ between 68 and 71GHz were first detected toward the Orion-KL and W51 Main star-forming regions. Given their high upper level energies (225K) 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. Quite in general, only a paucity of observational data exist for CH_2_ and, while astrochemically an important compound, its actual occurrence in astronomical sources is poorly constrained. In this work, we aim to investigate the nature of the elusive CH_2_ emission and 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 CH_2_ lines by observing an assortment of regions guided by the hypothesis that the observed CH_2_ emission likely arises from the hot gas environment of photodissociation regions (PDRs). We carried out observations using, first, the Kitt Peak 12m telescope to verify the original detection of CH_2_ toward different positions in the central region of Orion Molecular Cloud 1. These were followed-up by deep integrations using the higher angular resolution of the Onsala 20 m telescope. We have also searched for the N_KaKc_=2_12_-3_03_ transitions of para-CH_2_ 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 employed archival far infrared data. Our and other recent observations of the Orion region reported here, rule out an association with hot and dense gas. We find that the distribution of the CH_2_ emission follows closely that of the [CII] 158 um emission while CH_2_ is undetected toward the hot core itself. The observations suggest that its extended emission rather arises from hot but dilute layers of PDRs, but not from the denser parts of such regions, in particular the Orion Bar. This hypothesis was corroborated by comparisons of the observed CH_2_ line profiles with those of carbon radio recombination lines (CRRLs), well known PDR tracers. In addition, we report the detection of the 70GHz fine-, and hyperfine structure components of ortho-CH_2_ 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_=4_04_-3_13_ fine- and hyperfine structure transitions between 68-71GHz toward W3 IRS5. While we have no information on CH_2_'s spatial distribution in these regions, except for W51, we again see a correspondence of the profiles of CH_2_ lines with those of CRRLs. We see stronger CH_2_ emission toward the extended HII region W51 M than toward the much more massive and denser W51 E and N regions, strongly supporting an origin of CH_2_ in extended dilute gas. We also report the non-detection of the 2_12_-3_03_ transitions of para-CH_2_ toward Orion. Furthermore, using a non-LTE radiative transfer analysis, we can constrain the gas temperatures and H_2_ density to (163+/-26)K and (3.4+/-0.3)x10^3^cm^-3^, respectively, for the 68-71GHz ortho-CH_2_ transitions toward W3 IRS5, for which we have the highest quality data set. This analysis confirms our hypothesis that CH_2_ originates in warm 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' spontaneous emission greatly aides their detection.
|
