Hunting for the elusive methylene radical
arXiv:2101.02654 · doi:10.1051/0004-6361/202039906
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.
Accepted in A&A, 25 pages, 18 figures, 5 tables
References in corpus (24)
- Array Programming with NumPy
- The distance to the Orion Nebula
- Early Science with SOFIA, the Stratospheric Observatory for Infrared Astronomy
- Ultraviolet Survey of CO and H_2 in Diffuse Molecular Clouds: The Reflection of Two Photochemistry Regimes in Abundance Relationships
- Herschel Survey of Galactic OH+, H2O+, and H3O+: Probing the Molecular Hydrogen Fraction and Cosmic-Ray Ionization Rate
- The Gould's Belt Distances Survey (GOBELINS) II. Distances and structure towards the Orion Molecular Clouds
- The chemistry and spatial distribution of small hydrocarbons in UV-irradiated molecular clouds: the Orion Bar PDR
- [CII] absorption and emission in the diffuse interstellar medium across the Galactic Plane
- Chemical probes of turbulence in the diffuse medium: the TDR model
- Discovery of Interstellar CF+
- Complex organic molecules in strongly UV-irradiated gas
- Trans-cis molecular photoswitching in interstellar Space
- The chemistry of interstellar argonium and other probes of the molecular fraction in diffuse clouds
- CH observations toward the Orion Bar
- The ISO LWS high resolution spectral survey towards Sagittarius B2
- The cooling of atomic and molecular gas in DR21
- Gaia-DR2 distance to the W3 Complex in the Perseus Arm
- Carbon radio recombination lines from gigahertz to megahertz frequencies towards Orion A
- Small-scale physical and chemical structure of diffuse and translucent molecular clouds along the line of sight to Sgr B2
- HD 62542: Probing the Bare, Dense Core of a Translucent Interstellar Cloud
- Low Frequency Carbon Radio Recombination Lines II: The Diffuse Interstellar Medium
- Hot Ammonia around Young O-type Stars. II. JVLA imaging of highly-excited metastable ammonia masers in W51-North
- Origin of hydrogen fluoride emission in the Orion Bar: An excellent tracer for CO-dark H2 gas clouds
- The GBT 67 -- 93.6 GHz Spectral Line Survey of Orion-KL