Revisiting rotationally excited CH at radio wavelengths: A case study towards W51
Authors: Jacob Arshia M. , Nandakumar Meera , Roy Nirupam , Menten Karl M. , Neufeld David A. , Faure Alexandre , Tiwari Maitraiyee , Pillai Thushara G. S. , Robishaw Timothy , Durán Carlos A. 
Published: November 2024

Context. Ever since they were first detected in the interstellar medium, the radio wavelength (3.3 GHz) hyperfine-structure splitting transitions in the rotational ground state of CH were observed to show anomalous excitation. Astonishingly, this behaviour was uniformly observed towards a variety of different sources probing a wide range of physical conditions. While the observed level inversion could be explained globally by a pumping scheme involving collisions, a description of the extent of ‘over-excitation’ observed in individual sources required the inclusion of radiative processes, involving transitions at higher rotational levels. Therefore, a complete description of the excitation mechanism in the CH ground state, observed towards individual sources entails observational constraints from the rotationally excited levels of CH and in particular that of its first rotationally excited state (2Π3/2, N = 1, J = 3/2).
Aims. Given the limited detections of these lines, the objective of this work is to characterise the physical and excitation properties of the rotationally excited lines of CH between the Λ-doublet levels of its 2Π3/2, N = 1, J = 3/2 state near 700 MHz, and investigate their influence on the pumping mechanisms of the ground-state lines of CH.
Methods. This work presents the first interferometric search for the rotationally excited lines of CH between the Λ-doublet levels of its 2Π3/2, N = 1, J = 3/2 state near 700 MHz carried out using the upgraded Giant Metrewave Radio Telescope (uGMRT) array towards six star-forming regions, W51 E, Sgr B2 (M), M8, M17, W43, and DR21 Main.
Results. We detected the two main hyperfine structure lines within the first rotationally excited state of CH, in absorption towards W51 E. To jointly model the physical and excitation conditions traced by lines from both the ground and first rotationally excited states of CH, we performed non-local thermodynamic equilibrium (LTE) radiative transfer calculations using the code MOLPOP-CEP. These models account for the effects of line overlap and are aided by column density constraints from the far-infrared (FIR) wavelength rotational transitions of CH that connect to the ground state and use collisional rate coefficients for collisions of CH with H, H2 and electrons (the latter was computed in this work using cross-sections estimated within the Born approximation).
Conclusions. The non-LTE analysis revealed that physical properties typical of diffuse and translucent clouds best reproduced the higher rates of level inversion seen in the ground-state lines at 3.3 GHz, observed at velocities near 66 km s−1 along the sightline towards W51 E. In contrast, the excited lines near 700 MHz were only excited in much denser environments with nH ~ 105 cm−3 towards which the anomalous excitation in two of the three ground state lines is quenched, but not in the 3.264 GHz line. This is in alignment with our observations and suggests that while FIR pumping and line overlap effects are essential for exciting and producing line inversion in the ground state, excitation to the first rotational level is dominated by collisional excitation from the ground state. For the rotationally excited state of CH, the models indicated low excitation temperatures and column densities of 2 × 1014 cm−2. Furthermore, modelling these lines helps us understand the complexities of the spectral features observed in the 532/536 GHz rotational transitions of CH. These transitions, connecting sub-levels of the first rotationally excited state to the ground state, play a crucial role in trapping FIR radiation and enhancing the degree of inversion seen in the ground state lines. Based on the physical conditions constrained, we predict the potential of detecting hyperfine-splitting transitions arising from higher rotationally excited transitions of CH in the context of their current non-detections.