2020A&A...640A.125J

In recent years, a plethora of observations with high spectral resolution of sub-millimetre and far-infrared transitions of methylidene (CH), conducted with Herschel and SOFIA, have demonstrated this radical to be a valuable proxy for molecular hydrogen that can be used for characterising molecular gas within the interstellar medium on a Galactic scale, including the CO-dark component. We report the discovery of the ¹³CH isotopologue in the interstellar medium using the upGREAT receiver on board SOFIA. We have detected the three hyperfine structure components of the ≃2THz frequency transition from its X²Π_1/2 ground-state towards the high-mass star-forming regions Sgr B2(M), G34.26+0.15, W49(N), and W51E and determined ¹³CH column densities. The ubiquity of molecules containing carbon in the interstellar medium has turned the determination of the ratio between the abundances of the two stable isotopes of carbon, ¹²C/¹³C, into a cornerstone for Galactic chemical evolution studies. Whilst displaying a rising gradient with galactocentric distance, this ratio, when measured using observations of different molecules (CO, H₂CO, and others), shows systematic variations depending on the tracer used. These observed inconsistencies may arise from optical depth effects, chemical fractionation, or isotope-selective photo-dissociation. Formed from C⁺ either through UV-driven or turbulence-driven chemistry, CH reflects the fractionation of C⁺, and does not show any significant fractionation effects, unlike other molecules that were previously used to determine the ¹²C/¹³C isotopic ratio. This makes it an ideal tracer for the ¹²C/¹³C ratio throughout the Galaxy. By comparing the derived column densities of ¹³CH with previously obtained SOFIA data of the corresponding transitions of the main isotopologue ¹²CH, we therefore derive ¹²C/¹³C isotopic ratios toward Sgr B2(M), G34.26+0.15, W49(N) and W51E. Adding our values derived from ^12/13CH to previous calculations of the Galactic isotopic gradient, we derive a revised value of ¹²C/¹³C=5.87(0.45)R_GC+13.25(2.94).