Revisiting rotationally excited CH at radio wavelengths: A case study towards W51

TL;DR

This study investigates the excitation mechanisms of CH molecules in space by analyzing the first rotationally excited lines near 700 MHz using the uGMRT, providing new insights into molecular excitation processes in interstellar environments.

Contribution

It presents the first interferometric observations of CH's excited lines near 700 MHz and models their impact on understanding CH excitation in interstellar medium.

Findings

Detection of rotationally excited CH lines near 700 MHz

Insights into the excitation and pumping mechanisms of CH

Constraints on physical conditions in W51 region

Abstract

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 have been observed to show anomalous excitation. Astonishingly, this behaviour has been uniformly observed towards a variety of different sources probing a wide range of physical conditions. While the observed level inversion can be explained globally by a pumping scheme involving collisions, a description of the extent of 'over-excitation' observed in individual sources requires 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. 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 near 700 MHz, and investigate their influence on the pumping mechanisms of the ground-state lines of CH. This work presents the first interferometric search for the rotationally excited lines of CH near 700 MHz carried out using the uGMRT array and jointly models the physical and excitation conditions traced by lines from both the ground and first rotationally excited states of CH.