The CH radical at radio wavelengths: Revisiting emission in the 3.3 GHz ground-state lines

Abstract

Context. The intensities of the three widely observed radio-wavelength hyperfine structure (HFS) lines between the Λ-doublet components of the rotational ground state of CH are inconsistent with local thermodynamic equilibrium (LTE) and indicate ubiquitous population inversion. While this can be qualitatively understood assuming a pumping cycle that involves collisional excitation processes, the relative intensities of the lines and in particular the dominance of the lowest frequency satellite line has not been well understood. This has limited the use of CH radio emission as a tracer of the molecular interstellar medium. Aims. We aim to investigate the nature of the (generally) weak CH ground state masers by employing synergies between the ground state HFS transitions themselves and with the far-infrared lines, near 149 μm (2 THz), that connect these levels to an also HFS split rotationally excited level. Methods. We present the first interferometric observations, with the Karl G. Jansky Very Large Array, of the CH 9 cm ground state HFS transitions at 3.264 GHz, 3.335 GHz, and 3.349 GHz toward the four high mass star-forming regions (SFRs) Sgr B2 (M), G34.26+0.15, W49 (N), and W51. We combine this data set with our high spectral resolution observations of the N, J =2, 3/2→1, 1/2 transitions of CH near 149 μm observed toward the same sources made with the upGREAT receiver on SOFIA, which share a common lower energy levels with the HFS transitions within the rotational ground state. Results. Toward all four sources, we observe the 3.264 GHz lower satellite line in enhanced emission with its relative intensity higher than its expected value at LTE by a factor between 4 and 20. Employing recently calculated collisional rate coefficients, we perform statistical equilibrium calculations with the non-LTE radiative transfer code MOLPOP-CEP in order to model the excitation conditions traced by the ground state HFS lines of CH and to infer the physical conditions in the emitting regions. The models account for effects of far-infrared line overlap with additional constraints provided by reliable column densities of CH estimated from the 149 μm lines. Conclusions. The derived gas densities indicate that the CH radio emission lines (and the far-infrared absorption) arise from the diffuse and translucent outer regions of the SFRs’ envelopes as well as in such clouds located along the lines of sight. We infer temperatures ranging from 50 to 125 K. These elevated temperatures, together with astrochemical considerations, may indicate that CH is formed in material heated by the dissipation of interstellar turbulence, which has been invoked for other molecules. The excitation conditions we derive reproduce the observed level inversion in all three of the ground state HFS lines of CH over a wide range of gas densities with an excitation temperature of ∼−0.3 K, consistent with previous theoretical predictions.