Formed at an early stage of gas-phase ion-molecule chemistry, hydrides – molecules containing a heavy element covalently bonded to one or more hydrogen atoms – play an important role in interstellar ...chemistry as they are the progenitors of larger and more complex species in the interstellar medium. In recent years, the careful analysis of the spectral signatures of hydrides have led to their use as tracers of different constituents, and phases of the interstellar medium and in particular the more diffuse environments. Diffuse clouds form an essential link in the stellar gas life-cycle as they connect both the late and early stages of stellar evolution. As a result, diffuse clouds are continuously replenished by material which makes them reservoirs for heavy elements and hence ideal laboratories for the study of astrochemistry. This review will journey through a renaissance of hydride observations detailing puzzling hydride discoveries and chemical mysteries with special focus carbon-bearing hydrides to demonstrate the big impact of these small molecules and ending with remarks on the future of their studies.
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).
Context. CF + has been established as a valuable diagnostic tool for investigating photodissociation regions (PDRs) and fluorine abundances in the Milky Way. However, its role in extragalactic ...environments remains largely uncharted. Aims. Our objective is to explore the significance of CF + in the Large Magellanic Cloud (LMC) and assess its utility as a probe for examining C + and fluorine abundances in external galaxies. Methods. We performed pointed CF + observations toward an active star-forming region, N113 in the LMC, using the Atacama Pathfinder EXperiment 12 m submillimeter telescope. Results. We report the first discovery of CF + in the LMC through the successful detection of the CF + (2→1) and (3→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) × 10 4 cm −3 in N113. Our observations provide the first constraint on the fluorine abundance in molecular clouds in the LMC, ≲1.7 × 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. Conclusions. 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.
Context.
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 ...local thermodynamic equilibrium (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 are not well understood. This has limited the use of CH radio emission as a tracer of the molecular interstellar medium.
Aims.
We aim to investigate the nature of the (generally) weak CH ground-state masers by employing synergies between the ground-state HFS transitions themselves and the far-infrared lines near 149 μm (2 THz) that connect these levels to the first HFS-split, rotationally excited level of the
2
Π
1∕2
spin–orbital manifold.
Methods.
We present the first interferometric observations of the CH 9 cm ground-state HFS transitions at 3.264 GHz, 3.335 GHz, and 3.349 GHz towards the four high-mass star-forming regions (SFRs) Sgr B2 (M), G34.26+0.15, W49 (N), and W51 made with the
Karl G. Jansky
Very Large Array. We combine this data set with our high-spectral-resolution observations of the
N
,
J
= 2, 3∕2 → 1, 1∕2 transitions of CH near 149 μm observed towards the same sources made with the upGREAT receiver on SOFIA, which share common lower energy levels with the HFS transitions within the rotational ground state.
Results.
Towards all four sources, we observe the 3.264 GHz lower satellite line in enhanced emission with a higher relative intensity than is expected at LTE, by a factor of between 4 and 20. 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. The models account for effects of far-infrared line overlap with additional constraints provided by reliable column densities of CH estimated from the 149 μm lines.
Conclusions.
The derived gas densities indicate that the CH radio emission lines (and the far-infrared absorption) arise from the diffuse and translucent outer regions of the envelopes of the SFRs as well as in such clouds located along the lines of sight. We infer temperatures ranging from 50 to 125 K. These elevated temperatures, together with astrochemical considerations, may indicate that CH is formed in material heated by the dissipation of interstellar turbulence, which has been invoked for other molecules. The excitation conditions we derive reproduce the observed level inversion in all three of the ground-state HFS lines of CH over a wide range of gas densities with an excitation temperature of ~−0.3 K, consistent with previous theoretical predictions.
Abstract
We have revisited the chemistry of chlorine-bearing species in the diffuse interstellar medium with new observations of the HCl
+
molecular ion and new astrochemical models. Using the GREAT ...instrument on board SOFIA, we observed the
2
Π
3/2
J
= 5/2 − 3/2 transition of HCl
+
near 1444 GHz toward the bright THz continuum source W49N. We detected absorption by diffuse foreground gas unassociated with the background source, and were able to thereby measure the distribution of HCl
+
along the sight line. We interpreted the observational data using an updated version of an astrochemical model used previously in a theoretical study of Cl-bearing interstellar molecules. The abundance of HCl
+
was found to be almost constant relative to the related H
2
Cl
+
ion, but the observed
n
(H
2
Cl
+
)/
n
(HCl
+
) abundance ratio exceeds the predictions of our astrochemical model by an order of magnitude. This discrepancy suggests that the rate of the primary destruction process for H
2
Cl
+
, dissociative recombination, has been significantly overestimated. For HCl
+
, the model predictions can provide a satisfactory fit to the observed column densities along the W49N sight line while simultaneously accounting for the OH
+
and H
2
O
+
column densities.
Context. 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 redshift 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. Aims. 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. Methods. 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 = 11,0 − 10,1 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. Results. 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 and using statistical equilibrium calculations, we estimate the molecular fraction traced by ArH+ to be ∼10−3 and find that ArH+ resides in gas volumes with low electron densities. We further study the ortho-to-para ratio of H2O+ and find that the derived ratios do not significantly deviate from the equilibrium value of three with spin temperatures greater than 15 and 24 K.
Context. Despite being a commonly observed feature, the modification of the velocity structure in spectral line profiles by hyperfine structure complicates the interpretation of spectroscopic data. ...This is particularly true for observations of simple molecules such as CH and OH toward the inner Galaxy, which show a great deal of velocity crowding. Aims. 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. Methods. 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. Results. 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 H2 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 H2, we determined the abundance of the OH molecule to be X(OH) = 1.09 × 10−7 with respect to H2. 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 H2 with that of the CO-dark H2 gas traced by the cold neutral medium component of CII 158 μm emission across the Galactic plane, further emphasises the use of CH as a tracer for H2.
Context.
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 interstellar medium.
Aims.
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-millimetre continuum sources. We aim to benchmark its efficiency as a tracer of purely atomic gas by evaluating its correlation (or lack of correlation as suggested by chemical models) with other well-known atomic gas tracers such as OH
+
and H
2
O
+
and the molecular gas tracer CH.
Methods.
The observations of the
J
= 1−0 line of ArH
+
near 617.5 GHz were made feasible with the new, sensitive SEPIA660 receiver on the APEX 12 m telescope. Furthermore, the two sidebands of this receiver allowed us to observe the
N
K
a
K
c
= 1
1,0
−1
0,1
transitions of para-H
2
O
+
at 607.227 GHz simultaneously with the ArH
+
line.
Results.
We modelled the optically thin absorption spectra of the different species and subsequently derived their column densities. By analysing the steady state chemistry of OH
+
and o-H
2
O
+
, we derive on average a cosmic-ray ionisation rate,
ζ
p
(H), of (2.3 ± 0.3) × 10
−16
s
−1
towards the sight lines studied in this work. Using the derived values of
ζ
p
(H) and the observed ArH
+
abundances we constrain the molecular fraction of the gas traced by ArH
+
to lie below 2 × 10
−2
with a median value of 8.8 × 10
−4
. Combined, our observations of ArH
+
, OH
+
, H
2
O
+
, and CH probe different regimes of the interstellar medium, 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 (within 7 kpc of the Galactic centre) with potentially even contributions from the warm neutral medium phase of atomic gas at larger galactocentric distances. We derive an average ortho-to-para ratio for H
2
O
+
of 2.1 ± 1.0, which corresponds to a nuclear spin temperature of 41 K, consistent with the typical gas temperatures of diffuse clouds.
ABSTRACT
Measuring interstellar magnetic fields is extremely important for understanding their role in different evolutionary stages of interstellar clouds and 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 towards the dense photodissociation region (PDR) associated with the compact H 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 ∼0.13 mG. The same transitions in maser emission towards the neighbouring DR 21(OH) and W 75S-FR1 regions also exhibit the Zeeman splitting. Along with the OH data, we use C ii 158 μm 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 ∼3.6 × 1016(Tex/25 K) 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.
While the abundance of elemental deuterium is relatively low (D/H ~ a few ×10
−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
< 20 K), deuterium fractionation is driven by the H
2
D
+
ion, whereas at higher temperatures (
T
≥ 20–30 K) gas-phase deuteration is controlled by reactions with CH
2
D
+
and C
2
HD
+
. While the role of H
2
D
+
in driving cold interstellar deuterium chemistry is well understood, thanks to observational constraints from direct measurements of H
2
D
+
, deuteration stemming from CH
2
D
+
is far less understood as a result of the absence of direct observational constraints of its key ions. Therefore, making use of chemical surrogates is imperative in order to explore deuterium chemistry at intermediate temperatures. Formed at an early stage of ion-molecule chemistry directly from the dissociative recombination of CH
3
+
(CH
2
D
+
), CH (CD) is an ideal tracer for investigating deuterium substitution initiated by reactions with CH
2
D
+
. This paper reports the first detection of CD in the interstellar medium (ISM), carried out using the APEX 12 m telescope toward the widely studied low-mass protostellar system IRAS 16293–2422. Observed in absorption towards the envelope of the central protostar, the D/H ratio derived from the column densities of CD and CH is found to be 0.016 ± 0.003. This is an order of magnitude lower than the values found for other small molecules like C
2
H and H
2
CO observed in emission but whose formation, which is similar to that of CH, is also initiated via pathways involving warm deuterium chemistry. Gas-phase chemical models reproducing the CD/CH abundance ratio suggest that it reflects ‘warm deuterium chemistry’ (which ensues in moderately warm conditions of the ISM) and illustrates the potential use of the CD/CH ratio in constraining the gas temperatures of the envelope gas clouds it probes.
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