First detection of 13CH in the interstellar medium
  In recent years, a plethora of high spectral resolution observations of
sub-mm and FIR transitions of methylidene (CH), have demonstrated this radical
to be a valuable proxy for H2, that can be used for characterising molecular
gas within the interstellar medium (ISM) on a Galactic scale, including the
CO-dark component. Here we report the discovery of the 13CH isotopologue in the
ISM using the upGREAT receiver on board SOFIA. We have detected the three
hyperfine structure components of the 2THz frequency transition from its
ground-state toward four high-mass star-forming regions and determine 13CH
column densities. The ubiquity of molecules containing carbon in the ISM has
turned the determination of the ratio between the abundances of carbon's two
stable isotopes, 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
via UV-driven or turbulence-driven chemistry, CH reflects the fractionation of
C+, and does not show any significant fractionation effects unlike other
molecules previously used to determine the 12C/13C isotopic ratio which make it
an ideal tracer for the 12C/13C ratio throughout the Galaxy. Therefore, by
comparing the derived column densities of 13CH with previously obtained SOFIA
data of the corresponding transitions of the main isotopologue 12CH, we 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 = 5.85(0.50)R_GC +
15.03(3.40).
