Unveiling the chemistry of interstellar CH

Abstract

Context. The methylidyne radical CH is commonly used as a proxy for molecular hydrogen in the cold, neutral phase of the interstellar medium. The optical spectroscopy of CH is limited by interstellar extinction, whereas far-infrared observations provide an integral view through the Galaxy. While the HF ground state absorption, another H2 proxy in diffuse gas, frequently suffers from saturation, CH remains transparent both in spiral-arm crossings and high-mass star forming regions, turning this light hydride into a universal surrogate for H2. However, in slow shocks and in regions dissipating turbulence its abundance is expected to be enhanced by an endothermic production path, and the idea of a “canonical” CH abundance needs to be addressed. Aim. The N = 2 ← 1 ground state transition of CH at λ149 μm has become accessible to high-resolution spectroscopy thanks to the German Receiver for Astronomy at Terahertz Frequencies (GREAT) aboard the Stratospheric Observatory for Infrared Astronomy (SOFIA). Its unsaturated absorption and the absence of emission from the star forming regions makes it an ideal candidate for the determination of column densities with a minimum of assumptions. Here we present an analysis of four sightlines towards distant Galactic star forming regions, whose hot cores emit a strong far-infrared dust continuum serving as background signal. Moreover, if combined with the sub-millimeter line of CH at λ560 μm , environments forming massive stars can be analyzed. For this we present a case study on the “proto-Trapezium” cluster W3 IRS5. Methods. While we confirm the global correlation between the column densities of HF and those of CH, both in arm and interarm regions, clear signposts of an over-abundance of CH are observed towards lower densities. However, a significant correlation between the column densities of CH and HF remains. A characterization of the hot cores in the W3 IRS5 proto-cluster and its envelope demonstrates that the sub-millimeter/far-infrared lines of CH reliably trace not only diffuse but also dense, molecular gas. Results. In diffuse gas, at lower densities a quiescent ion-neutral chemistry alone cannot account for the observed abundance of CH. Unlike the production of HF, for CH+ and CH, vortices forming in turbulent, diffuse gas may be the setting for an enhanced production path. However, CH remains a valuable tracer for molecular gas in environments reaching from diffuse clouds to sites of high-mass star formation.

First detection of 13CH in the interstellar medium

“…The DSB-continuum level was then determined by accounting for contributions from the signal and image bands, which was added back to the spectra to obtain the correct line-to-continuum ratio. This calibration technique was applied not only to the 13 CH data presented in this work, but also to the previously published 12 CH data (Wiesemeyer et al 2018;Jacob et al 2019), which thus were re-calibrated for use in our analysis to avoid any inconsistencies. For the 12 CH spectra, we note that the continuum levels derived using the closure products are compatible with those derived using the standard calibration methods, except for W49(N).…”

“…Federman 1982;Sheffer et al 2008, and references therein). CH has been observed in widely different wavelength regimes, from the radio at 9 cm (Rydbeck et al 1973) to the sub-millimetre (sub-mm) and farinfrared (FIR) ranges, at 560 µm (Gerin et al 2010) and 150 µm (Stacey et al 1987;Wiesemeyer et al 2018) and the optical (4300.3 Å. e.g. Danks et al 1984;Sheffer et al 2008) into the far-ultraviolet (FUV) regimes (1369.13 Å, Watson 2001).…”

“…2. In particular, the hyperfine structure (hfs) components of the N, J = 1, 1/2 → 2, 3/2 transitions of CH near 2 THz have recently been observed in generally unsaturated absorption, which is ideal for column density determinations (Wiesemeyer et al 2018;Jacob et al 2019). Models and simulations by for photo-dissociation regions (PDRs) show that the fractionation present in CH is dominated by the fractionation of its parental species.…”

Molecular ion abundances in the diffuse ISM: CF+, HCO+, HOC+, and C3H+

“…The ground state line of hydrides observed with the Herschel satellite provided a wealth of new informations on the properties of the diffuse interstellar gas , including sensitive tracers of molecular gas (HF, CH, and H 2 O), of the cosmic ray ionization rate (OH + , H 2 O + , H 3 O + ), and strong supporting evidence for the role of turbulent dissipation in triggering the formation of the first molecule building blocks like CH + and SH + (Godard et al 2012(Godard et al , 2014. The GREAT receiver on SOFIA has enabled further studies of CH, OH, and SH absorption (Wiesemeyer et al 2016(Wiesemeyer et al , 2018Neufeld et al 2015).…”

Fingerprinting the effects of hyperfine structure on CH and OH far infrared spectra using Wiener filter deconvolution

“…CH has been widely studied in various spectral regimes to infer the thermal, chemical and dynamical evolution of diffuse and translucent cloud regions (Rydbeck et al 1973;Turner & Zuckerman 1974;Stacey et al 1987). Moreover, the advent of space-and air-borne telescopes like Herschel and SOFIA have established the use of CH alongside other interstellar hydrides as tracers for H 2 in the diffuse regions of the ISM (Gerin et al 2010;Wiesemeyer et al 2018). Figure 1 shows the low-lying energy level structure of the ground electronic state of the CH molecule, abiding by Hund's case b.…”