Arshia Maria Jacob
A renaissance of CH observations: synergies between the FIR and radio wavelengths
Whilst the 9 cm ground state radio lines of CH are widely observed, their excitation has been poorly understood as their intensities have long been found to be inconsistent with the assumptions of local thermodynamic equilibrium (LTE). We present the first interferometric observations of these CH lines using the Karl G. Jansky Very Large Array (VLA) and provide constraints on the physical and excitation conditions traced by these lines, through non-LTE radiative transfer models which invoke the effects of far-infrared (FIR) line overlap. Furthermore, by taking advantage of the synergies between the FIR and radio transitions of CH we constrain our models using reliable column densities determined from the FIR lines of CH at 149 μm observed using the high spectral resolution upGREAT receiver on board the Stratospheric Observatory for Infrared Astronomy (SOFIA). Our results reveal that the physical conditions traced by CH are consistent with those found within the warm layers of photodissociation regions (PDRs), which along with astrochemical considerations suggest that CH may be formed via the dissipation of turbulence, possible even in dark clouds. This analysis establishes the use of the widely observed ground state transitions of CH as a powerful tool to probe the diffuse and translucent ISM at radio wavelengths. By further exploiting the unique capabilities provided by SOFIA, we discuss our search for, and successful detection, of 13 CH in the ISM for the first time. Combined with observations of its main isotopologue we highlight its use as a potentially unbiased tracer of the 12C/ 13C isotopic ratio, an important diagnostic tool for probing the nucleosynthesis history of the Galaxy.