First detection of 13CH in the interstellar medium

Arshia M Jacob,Friedrich Wyrowski,Rolf Güsten,Karl M Menten,Bernd Klein,Helmut Wiesemeyer

doi:10.1051/0004-6361/201937385

Abstract

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 ≈2 THz frequency transition from its X2Π1∕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)RGC + 13.25(2.94).

Highlights

The methylidene radical CH has received widespread attention as a general probe of diffuse and translucent interstellar clouds and in particular as a surrogate for the H2 column density determinations in such environments (e.g. Federman 1982; Sheffer et al 2008, and references therein)
We report the discovery of the 1139C9H7 GHz 0.02 (13CH) isotopologue in the interstellar medium using the upGREAT receiver on board SOFIA
We mention that because first, the expected 12CH/ 13CH ratios, which range from ≈20 in the Galactic centre (GC) to ≈90 at the solar circle and to even higher values beyond (Wilson & Rood 1994, and references therein), and second, because even 12CH lines are predominantly optically thin, we expect a detection of 13CH absorption only towards the local standard of rest (LSR) velocities of the star-forming regions (SFRs) studied because these sources have far greater column densities than the intervening diffuse clouds along their lines of sight

Summary

Introduction

The methylidene radical CH has received widespread attention as a general probe of diffuse and translucent interstellar clouds and in particular as a surrogate for the H2 column density determinations in such environments (e.g. Federman 1982; Sheffer et al 2008, and references therein). We mention that because first, the expected 12CH/ 13CH ratios, which range from ≈20 in the GC to ≈90 at the solar circle and to even higher values beyond (Wilson & Rood 1994, and references therein), and second, because even 12CH lines are predominantly optically thin, we expect a detection of 13CH absorption only towards the LSR velocities of the SFRs studied because these sources have far greater column densities than the intervening diffuse clouds along their lines of sight. HIFI/Herschel data of the N, J = 1, 3/2 → 1, 1/2 transitions of CH and 13CH near 532 GHz. While the 532 GHz CH line displays a deep absorption feature at the systemic velocity of the Sgr B2(M) molecular cloud with a peak temperature of 1.64 K (Qin et al 2010), the corresponding 13CH transition is not detected above a 3σ noise level of 0.33 K (at a spectral resolution of 0.5 km s−1) and yields a lower limit of 5 on the 12C/13C isotopic ratio. The formal lower limits for the 12C/13C ratio derived from archival HIFI/Herschel data quoted above should be regarded with some caution

Analysis and discussion

Conclusions