arXiv subjects

Arshia M. Jacob

Publications and source records attributed to Arshia M. Jacob.

16 recordsLinked to original sources

arXiv2026

First detection of C2H+ in the interstellar medium

Despite the detection of nearly 350 molecules in the interstellar medium, almost half of which are carbon chains, the pathways that build molecular complexity remain poorly understood. Observed abundances of carbon-chain and aromatic species are difficult to reconcile with existing top-down or bottom-up formation scenarios, due in part to limited observational constraints and incomplete theoretical understanding. In particular, small intermediary ions, key drivers of ion-molecule reactions capable of seeding larger hydrocarbons and aromatic rings, could provide critical support for the bottom-up formation scenario. Constraining the abundance and chemistry of these ions is therefore essential to test whether bottom-up growth can operate efficiently under interstellar conditions. Here, we report the first detection of the small hydrocarbon cation ethynylium, C2H+, toward the Orion Bar, based on observations with the APEX 12m sub-mm telescope of its lowest-lying J=3-2 rotational transition near 211GHz, which exhibits a unique spectroscopic fingerprint through resolved Lambda-doubling and hyperfine splitting components, as recently measured in the laboratory. Meudon PDR models successfully reproduce these values, placing C2H+ formation at the outer edges of PDR fronts. Our results link C2H+ production to CH+ and CH3+ within a network of ion-molecule reactions driven by vibrationally excited H2, a scenario now further supported by recent detections of these species in PDRs like the Orion Bar with JWST observations. The importance of C2H+ lies in its role as a key intermediate: it produces C2H2+ and subsequently C2H3+, effectively channelling small C2 building blocks toward larger hydrocarbons and facilitating bottom-up growth at the PDR surface. Targeted searches for C2H+ in other regions promise to provide a potentially decisive probe of ion-driven bottom-up chemistry in the ISM.

astro-ph.GA↗

arXiv2026

Hyperfine-Resolved Rovibrational and Rotational Spectroscopy of OH$^+$ ($X ^3Σ^-$)

The OH$^+$ ($X ^3Σ^-$) radical cation has been investigated by combining a 4 K 22-pole ion trap apparatus with high-resolution IR and THz radiation sources. Applying different types of action spectroscopic methods, the fundamental vibrational band in the 3 $μ$m range and the spin manifold of the $N=1 \leftarrow 0$ rotational transition around 1 THz have been extended and refined. Additionally, the spin manifold of the $N=2 \leftarrow 1$ rotational transition, scattered around 2 THz, has been measured for the first time with microwave accuracy. Although all hyperfine components of the pure rotational transitions are affected by considerable Zeeman splittings, a simulation of their contours allowed us to extract the field-free center frequencies with high accuracy. A global fit combining rovibrational and pure rotational transitions from the literature with those newly obtained in this work was performed, leading to improvements in the spectroscopic constants of OH$^+$, particularly those in the ground vibrational state.

physics.atm-clus↗

arXiv2025

FIRESTORM I: Stellar Feedback and Gas Kinematics in the Evolved W40 Hub-Filament System

The FIRESTORM project--Feedback-Induced Regions and Emission from Star-forming Tracers of ObseRvable Molecular Gas--has targeted four star-forming regions to quantify the impact of stellar feedback on star formation. In this paper, we present multiwavelength results for one of the targets, the nearby high-mass star-forming region W40. Using dense-gas tracers C$^{18}$O(1--0) and H$^{13}$CO$^+$(1--0), we identified six velocity-coherent filaments: five at \vlsr $\sim$\,7.5\kms\! and one at \vlsr $\sim$\,5\kms. Four of these converge towards an infrared-bright cluster hosting the most massive star of the region (IRS 1A South, O9.5V), forming a hub-filament system (HFS). Key physical parameters, including filament lengths, widths, masses, velocity dispersions, and line masses, are derived. Five dense clumps traced by N$_2$H$^+$(1--0) exhibit subsonic to transonic turbulence, contrasting with the supersonic motions of their parental filaments, indicating turbulence dissipation. A deficit of emission at \vlsr $\sim$\,7\kms\! in several molecular lines, along with a blueshifted absorption dip in the HCN(1--0) profile, suggests that emission from OB-heated gas is being absorbed by a cold foreground cloud. A bridge-like feature in position-velocity space connects the \vlsr $\sim$\,5 and $\sim$\,7.5\kms\! filaments, and spatially coinciding with dense condensations and radio continuum peaks. These findings suggest that a past interaction--likely a cloud-cloud collision--triggered the formation of HFS and ultimately the central massive cluster.

astro-ph.GA↗

arXiv2025

APEX survey of interstellar HCl: $^{35}$Cl/$^{37}$Cl isotopic ratios in dense cores and outflows

Despite being only the 19th most abundant element in the interstellar medium, chlorine's reactivity and volatility give rise to a unique interstellar chemistry, favouring the formation of several chlorine-bearing hydrides. Further, the $^{35}\text{Cl}/ ^{37}$Cl ratio probes nucleosynthesis across the Galaxy. Yet, studies of Cl-bearing molecules have remained limited to a few sightlines due to observational challenges. We systematically investigated the Galactic distribution of HCl and the [H$^{35}$Cl]/[H$^{37}$Cl] ratio in high-mass star-forming regions. As a probe of a region's nucleosynthesis history, this ratio may constrain predictions of Galactic chemical evolution models. We observed the ground-state $J=1$$-$0 lines of H$^{35}$Cl and H$^{37}$Cl toward 28 high-mass star-forming regions with SEPIA660 on APEX, more than doubling the number of known HCl detections and revealing with XCLASS models emission from both cores and outflows. H$^{35}$Cl was detected in all sources, H$^{37}$Cl in all but two, with spectral line profiles ranging from those with only emission to complex emission-absorption mixtures. We find column densities of the order of $10^{13}\,\mathrm{cm}^{-2}$ for H$^{35}$Cl and isotopic ratios between $1.6$ and $3.5$ in emission-only sources. The derived [H$^{35}$Cl]/[H$^{37}$Cl] aligns with Galactic chemical evolution models and shows no trend with Galactocentric radius. However, local variations may reflect recent nucleosynthesis. Overall, the results suggest that most Galactic chlorine was synthesized during epochs of lower average metallicity in the Galaxy. Notably, we detect H$^{35}$Cl emission arising from outflows - particularly explosive ones - hinting at its presence in a broader range of environments. The present single-dish observations cannot reveal the origin of HCl in outflows; necessitating interferometric follow-up observations.

astro-ph.GA↗

arXiv2025

The evolution of CH in Planck Galactic Cold Clumps

Methylidyne (CH) has long been considered a reliable tracer of molecular gas in the low-to-intermediate extinction range. Although extended CH 3.3 GHz emission is commonly observed in diffuse and translucent clouds, observations in cold, dense clumps are rare. In this work, we conducted high-sensitivity CH observations toward 27 PGCCs with the Arecibo 305m telescope. Toward each source, the CH data were analyzed in conjunction with $^{13}$CO (1--0), HINSA, and H$_2$ column densities. Our results revealed ubiquitous subsonic velocity dispersions of CH, in contrast to $^{13}$CO, which is predominantly supersonic. The findings suggest that subsonic CH emissions may trace dense, low-turbulent gas structures in PGCCs. To investigate environmental effects, particularly the cosmic-ray ionization rate (CRIR), we estimated CRIR upper limits from HINSA, yielding values from $(8.1\pm4.7)\times10^{-18}$ to $(2.0\pm0.8)\times10^{-16}$ s$^{-1}$ ($N_{H_2}$ from $(1.7\pm0.2)\times10^{21}$ to $(3.6\pm0.4)\times10^{22}$~cm$^{-2}$). This result favors theoretical predictions of a cosmic-ray attenuation model, in which the interstellar spectra of low-energy CR protons and electrons match {\it Voyager} measurements, although alternative models cannot yet be ruled out. The abundance of CH decreases with increasing column density, while showing a positive dependence on the CRIR, which requires atomic oxygen not heavily depleted to dominate CH destruction in PGCCs. By fitting the abundance of CH with an analytic formula, we place constraints on atomic O abundance ($2.4\pm0.4\times10^{-4}$ with respect to total H) and C$^+$ abundance ($7.4\pm0.7\times10^{13}ζ_2/n_{\rm H_2}$). These findings indicate that CH formation is closely linked to the C$^+$ abundance, regulated by cosmic-ray ionization, while other processes, such as turbulent diffusive transport, might also contribute a non-negligible effect.

astro-ph.GA↗

arXiv2025

The first detection of cosmic-ray excited H$_2$ in interstellar space

Stars and planets form within cold, dark molecular clouds. In these dense regions, where starlight cannot penetrate, cosmic rays (CRs) are the dominant source of ionization -- driving interstellar chemistry(Dalgarno (2006, PNAS, 103, 12269)), setting the gas temperature(Goldsmith et al. (1969, ApJ, 158, 173)), and enabling coupling to magnetic fields(McKee & Ostriker (2007, ARA&A, 45, 565; arXiv:0707.3514)). Together, these effects regulate the collapse of clouds and the onset of star formation. Despite this importance, the cosmic-ray ionization rate, $ζ$, has never been measured directly. Instead, this fundamental parameter has been loosely inferred from indirect chemical tracers and uncertain assumptions, leading to published values that span nearly two orders of magnitude and limiting our understanding of star formation physics. Here, we report the first direct detection of CR-excited vibrational H$_2$ emission, using \textit{James Webb Space Telescope} (JWST) observations of the starless core Barnard 68 (B68). The observed emission pattern matches theoretical predictions for CR excitation precisely, confirming a decades-old theoretical proposal long considered observationally inaccessible. This result enables direct measurement of $ζ$, effectively turning molecular clouds into natural, light-year-sized, cosmic-ray detectors. It opens a transformative observational window into the origin, propagation, and role of cosmic rays in star formation and galaxy evolution.

astro-ph.GA↗

arXiv2024

Revisiting rotationally excited CH at radio wavelengths: A case study towards W51

Ever since they were first detected in the interstellar medium, the radio wavelength (3.3 GHz) hyperfine-structure splitting transitions in the rotational ground state of CH have been observed to show anomalous excitation. Astonishingly, this behaviour has been uniformly observed towards a variety of different sources probing a wide range of physical conditions. While the observed level inversion can be explained globally by a pumping scheme involving collisions, a description of the extent of 'over-excitation' observed in individual sources requires the inclusion of radiative processes, involving transitions at higher rotational levels. Therefore, a complete description of the excitation mechanism in the CH ground state, observed towards individual sources entails observational constraints from the rotationally excited levels of CH and in particular that of its first rotationally excited state. Given the limited detections of these lines, the objective of this work is to characterise the physical and excitation properties of the rotationally excited lines of CH near 700 MHz, and investigate their influence on the pumping mechanisms of the ground-state lines of CH. This work presents the first interferometric search for the rotationally excited lines of CH near 700 MHz carried out using the uGMRT array and jointly models the physical and excitation conditions traced by lines from both the ground and first rotationally excited states of CH.

astro-ph.GA↗

arXiv2024

First detection of CF$^{+}$ in the Large Magellanic Cloud

CF$^{+}$ has been established as a valuable diagnostic tool for investigating photo-dissociation regions (PDRs) and fluorine abundances in the Milky Way. However, its role in extragalactic environments remains largely uncharted. Our objective is to explore the significance of CF$^{+}$ in the Large Magellanic Cloud (LMC) and assess its utility as a valuable probe for examining C$^{+}$ and fluorine abundances in external galaxies. We performed pointed CF$^{+}$ observations toward an active star-forming region, N113 in the LMC, using the Atacama Pathfinder EXperiment 12~m sub-millimeter telescope. We report the first discovery of CF$^{+}$ in the LMC through the successful detection of the CF$^{+}$ (2$\to$1) and (3$\to$2) lines. The excitation models indicate that CF$^{+}$ emission originates from dense PDRs characterized by an H$_{2}$ number density of $(0.5-7.9)\times 10^{4}$~cm$^{-3}$ in N113. Our observations provide the first constraint on the fluorine abundance in molecular clouds in the LMC, disclosing a value of $\lesssim 1.7\times 10^{-9}$. This value is about an order of magnitude lower than those previously measured toward red giants in the LMC, indicative of fluorine deficiency in the molecular gas. The estimated column density ratio between C$^{+}$ and CF$^{+}$ appears to be lower than the anticipated equilibrium ratio derived from the fluorine abundance in red giants. Both phenomena can be explained by the deficiency of CF$^{+}$ caused by the freeze-out of its primary chemical precursor, HF, onto dust grains. The deficiency of CF$^{+}$ within molecular clouds suggests that the measurements presented in this work serve exclusively as conservative estimates, establishing lower bounds for both the fluorine abundance and C$^{+}$ column densities in external galaxies.

astro-ph.GA↗

arXiv2023

First detection of deuterated methylidyne (CD) in the interstellar medium

While the abundance of elemental deuterium is relatively low (D/H ~ a few 1E-5), orders of magnitude higher D/H abundance ratios have been found for many interstellar molecules, enhanced by deuterium fractionation. In cold molecular clouds (T < 20K) deuterium fractionation is driven by the H2D+ ion, whereas at higher temperatures (T > 20-30K) gas-phase deuteration is controlled by reactions with CH2D+ and C2HD+. While the role of H2D+ in driving cold interstellar deuterium chemistry is well understood, thanks to observational constraints from direct measurements of H2D+, deuteration stemming from CH2D+ is far less understood, caused by the absence of direct observational constraints of its key ions. Therefore, making use of chemical surrogates is imperative for exploring deuterium chemistry at intermediate temperatures. Formed at an early stage of ion-molecule chemistry, directly from the dissociative recombination of CH3+ (CH2D+), CH (CD) is an ideal tracer for investigating deuterium substitution initiated by reactions with CH2D+. This paper reports the first detection of CD in the interstellar medium, carried out using the APEX 12m telescope toward the widely studied low-mass protostellar system IRAS 16293-2422. Gas-phase chemical models reproducing the observed CD/CH abundance ratio of 0.016 suggests that it reflects `warm deuterium chemistry' (which ensues in moderately warm conditions of the interstellar medium) and illustrates the potential use of the CD/CH ratio in constraining the gas temperatures of the envelope gas clouds it probes.

astro-ph.GA↗

arXiv2021

ArH+ and H2O+ absorption towards luminous galaxies

Along several sight lines within the Milky Way ArH+ has been ubiquitously detected with only one detection in extragalactic environments, namely along two sight lines in the red shift z=0.89 absorber towards the lensed blazar PKS 1830-211. Being formed in predominantly atomic gas by reactions between Ar+, which were initially ionised by cosmic rays and molecular hydrogen, ArH+ has been shown to be an excellent tracer of atomic gas as well as the impinging cosmic-ray ionisation rates. In this work, we attempt to extend the observations of ArH+in extragalactic sources to examine its use as a tracer of the atomic interstellar medium (ISM) in these galaxies. We report the detection of ArH+ towards two luminous nearby galaxies, NGC 253 and NGC 4945, and the non-detection towards Arp 220 observed using the SEPIA660 receiver on the APEX 12 m telescope. In addition, the two sidebands of this receiver allowed us to observe the NKaKc=1_1,0-1_0,1 transitions of another atomic gas tracer p-H2O+ at 607.227 GHz with the ArH+ line, simultaneously. We modelled the optically thin spectra of both species and compared their observed line profiles with that of other well-known atomic gas tracers such as OH+ and o-H2O+ and diffuse and dense molecular gas tracers HF and CO, respectively. By further assuming that the observed absorption from the ArH+, OH+, and H2O+ molecules are affected by the same flux of cosmic rays, we investigate the properties of the different cloud layers based on a steady-state analysis of the chemistry of these three species.

astro-ph.GA↗

arXiv2021

The CH radical at radio wavelengths: Revisiting emission in the 3.3GHz ground state lines

The intensities of the three widely observed radio-wavelength hyperfine structure (HFS) lines between the Λ-doublet components of the rotational ground state of CH are inconsistent with LTE and indicate ubiquitous population inversion. While this can be qualitatively understood assuming a pumping cycle that involves collisional excitation processes, the relative intensities of the lines and in particular the dominance of the lowest frequency satellite line has not been well understood. This has limited the use of CH radio emission as a tracer of the molecular interstellar medium. We present the first interferometric observations, with the Karl G. Jansky Very Large Array, of the CH 9 cm ground state HFS transitions at 3.264 GHz, 3.335 GHz, and 3.349 GHz toward four high mass star-forming regions (SFRs) Sgr B2 (M), G34.26+0.15, W49 (N), and W51. We investigate the nature of the (generally) weak CH ground state masers by employing synergies between the ground state HFS transitions themselves and with the far-infrared lines, near 149 μm (2 THz), that connect these levels to an also HFS split rotationally excited level. Employing recently calculated collisional rate coefficients, we perform statistical equilibrium calculations with the non-LTE radiative transfer code MOLPOP-CEP in order to model the excitation conditions traced by the ground state HFS lines of CH and to infer the physical conditions in the emitting regions while also accounting for the effects of far-infrared line overlap.

astro-ph.GA↗

arXiv2020

The magnetic field in the dense photodissociation region of DR 21

Measuring interstellar magnetic fields is extremely important for understanding their role in different evolutionary stages of interstellar clouds and of star formation. However, detecting the weak field is observationally challenging. We present measurements of the Zeeman effect in the 1665 and 1667~MHz (18~cm) lines of the hydroxyl radical (OH) lines toward the dense photodissociation region (PDR) associated with the compact H{\sc ii} region DR~21~(Main). From the OH 18~cm absorption, observed with the Karl G. Jansky Very Large Array, we find that the line of sight magnetic field in this region is $\sim 0.13$~mG. The same transitions in maser emission toward the neighboring DR~21(OH) and W~75S-FR1 regions also exhibit the Zeeman splitting. Along with the OH data, we use [C{\sc ii}] 158 $μ$m line and hydrogen radio recombination line data to constrain the physical conditions and the kinematics of the region. We find the OH column density to be $\sim 3.6\times10^{16}(T_{\rm ex}/25~{\rm K})~{\rm cm}^{-2}$, and that the 1665 and 1667 MHz absorption lines are originating from the gas where OH and C$^+$ are co-existing in the PDR. Under reasonable assumptions, we find the measured magnetic field strength for the PDR to be lower than the value expected from the commonly discussed density--magnetic field relation while the field strength values estimated from the maser emission are roughly consistent with the same. Finally, we compare the magnetic field energy density with the overall energetics of DR~21's PDR and find that, in its current evolutionary stage, the magnetic field is not dynamically important.

astro-ph.GA↗

arXiv2020

Extending the view of ArH+ chemistry in diffuse clouds

One of the surprises of the Herschel mission was the detection of ArH+ towards the Crab Nebula in emission and in absorption towards strong Galactic background sources. Although these detections were limited to the first quadrant of the Galaxy, the existing data suggest that ArH+ ubiquitously and exclusively probes the diffuse atomic regions of the ISM. In this study, we extend the coverage of ArH+ to other parts of the Galaxy with new observations of its J = 1-0 transition along seven Galactic sight lines towards bright sub-mm continuum sources. We aim to benchmark its efficiency as a tracer of purely atomic gas by evaluating its correlation (or lack there of) with other well-known atomic and molecular gas tracers. The observations of ArH+ near 617.5 GHz were made feasible with the new, sensitive SEPIA660 receiver on the APEX 12 m telescope. The two sidebands of this receiver allowed us to observe p-H2O+ transitions of at 607.227 GHz simultaneously with the ArH+ line. By analysing the steady state chemistry of OH+ and o-H2O+, we derive on average a cosmic-ray ionisation rate (CRIR), of 2.3e-16 s^-1 towards the sight lines studied in this work. Using the derived values of the CRIR and the observed ArH+ abundances we constrain the molecular fraction of the gas traced by ArH+ to lie below 2e-2 with a median value of 8.8e-4. Combined, our observations of ArH+, OH+, H2O+, and CH probe different regimes of the ISM, from diffuse atomic to diffuse and translucent molecular clouds. Over Galactic scales, we see that the distribution of N(ArH+) is associated with that of N(H), particularly in the inner Galaxy with potentially even contributions from the warm neutral medium phase of atomic gas at larger galactocentric distances. We derive an average o/p-ratio for H2O+ of 2.1, which corresponds to a nuclear spin temperature of 41 K, consistent with the typical gas temperatures of diffuse clouds.

astro-ph.GA↗

arXiv2020

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).

astro-ph.GA↗

arXiv2019

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

In this paper, we investigate the influence of hyperfine splitting on complex spectral lines, with the aim of evaluating canonical abundances by decomposing their dependence on hyperfine structures. This is achieved from first principles through deconvolution. We present high spectral resolution observations of the rotational ground state transitions of CH near 2 THz seen in absorption toward the strong FIR-continuum sources AGAL010.62$-$00.384, AGAL034.258+00.154, AGAL327.293$-$00.579, AGAL330.954$-$00.182, AGAL332.826$-$00.549, AGAL351.581$-$00.352 and SgrB2(M). These were observed with the GREAT instrument on board SOFIA. The observed line profiles of CH were deconvolved from the imprint left by the lines' hyperfine structures using the Wiener filter deconvolution, an optimised kernel acting on direct deconvolution. The quantitative analysis of the deconvolved spectra first entails the computation of CH column densities. Reliable N(CH) values are of importance owing to the status of CH as a powerful tracer for H$_2$ in the diffuse regions of the interstellar medium. The N(OH)/N(CH) column density ratio is found to vary within an order of magnitude with values ranging from one to 10, for the individual sources that are located outside the Galactic centre. Using CH as a surrogate for H$_2$, we determined the abundance of the OH molecule to be X(OH)=1.09$\times$10$^{-7}$ with respect to H$_2$. The radial distribution of CH column densities along the sightlines probed in this study, excluding SgrB2(M), showcase a dual peaked distribution peaking between 5 and 7 kpc. The similarity between the correspondingly derived column density profile of H$_2$ with that of the CO-dark H$_2$ gas traced by the cold neutral medium component of [CII] 158$~μ$m emission across the Galactic plane, further emphasises the use of CH as a tracer for H$_2$.

astro-ph.GA↗

arXiv2018

Unveiling the chemistry of interstellar CH: Spectroscopy of the 2 THz $N=2\leftarrow 1$ ground state line

The methylidyne radical CH is commonly used as a proxy for H$_2$ 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 H$_2$ 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 H$_2$. 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. The $N=2\leftarrow 1$ ground state transition of CH at $λ149\,μ$m has become accessible to high-resolution spectroscopy thanks to GREAT aboard SOFIA. Its unsaturated absorption and the absence of emission 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, far-infrared bright Galactic star forming regions. If combined with the sub-millimeter line of CH at $λ560\,μ$m, environments forming massive stars can be analyzed. For this we present a case study on the "proto-Trapezium" cluster W3 IRS5, and demonstrate that the sub-millimeter/far-infrared lines of CH reliably trace not only diffuse but also dense, molecular gas. While we confirm the global correlation between the column densities of HF and those of CH, clear signposts of an over-abundance of CH are observed towards lower densities. A quiescent ion-neutral chemistry alone cannot account for this over-abundance. Vortices forming in turbulent, diffuse gas may be the setting for an enhanced production path.

astro-ph.GA↗