14 research outputs found
Small Molecules, Big Impact: Investigating Hydrides in the Interstellar Medium
No full textThis thesis investigates hydride molecules in the interstellar medium (ISM) and their use as diagnostics of different phases of the ISM. Particular emphasis is given to the central CH radical, a probe of diffuse and translucent molecular clouds, including those not traced by the otherwise common CO. We discuss our searches for the rare isotopologues of CH and address questions regarding the origin of another molecule, CH2, which despite being chemically associated with the ubiquitous CH, has largely remained elusive. In addition, the molecular ion, ArH+, a tracer for diffuse atomic gas is studied. Whilst the 9 cm ground state radio lines of CH are widely observed, their excitation has been poorly understood as their intensities have long been found to be inconsistent with the assumptions of local thermodynamic equilibrium (LTE). We present the first interferometric observations of these CH lines using the Karl G. Jansky Very Large Array (VLA) and provide constraints on the physical and excitation conditions traced by these lines, through non-LTE radiative transfer models which invoke the effects of far-infrared (FIR) line overlap. Furthermore, by taking advantage of the synergies between the FIR and radio transitions of CH we constrain our models using reliable column densities determined from the FIR lines of CH at 149 µm observed using the high spectral resolution upGREAT receiver on board the Stratospheric Observatory for Infrared Astronomy (SOFIA). Our results reveal that the physical conditions traced by CH are consistent with those found within the warm layers of photodissociation regions (PDRs), which along with astrochemical considerations suggest that CH maybe formed via the dissipation of turbulence, possible even in dark clouds. This analysis establishes the use of the widely observed ground state transitions of CH as a powerful tool to probe the diffuse and translucent ISM at radio wavelengths. By further exploiting the unique capabilities provided by SOFIA, we discuss our search for, and successful detection, of sup>13CH in the ISM for the first time. Combined with observations of its main isotopologue we highlight its use as a potentially unbiased tracer of the 12C/13C isotopic ratio, an important diagnostic tool for probing the nucleosynthesis history of the Galaxy. Based on early observations of high energy level transitions of CH2 between 68 and 71 GHz toward the hot cores in Orion and W51, astronomers associated the observed emission as arising from dense, hot environments. The subsequent non-detection of these lines toward the archetypal Orion-KL hot core in high resolution observations have raised questions regarding the molecule’s origin. To address this, we conducted new observations of these transitions toward the Orion region and other star-forming regions. Our data, with the aid of ancillary carbon recombination line data and radiative transfer models, establishes CH2’s association with warm but dilute PDR gas layers. Additionally, we find that for the physical conditions derived, the observed CH2 lines show level inversion and acts as a weak maser. Finally, we extend the view of the chemistry of argonium (ArH+) using observations carried out with the Atacama Pathfinder EXperiment (APEX) 12 m telescope. Combined with archival observations of OH+ and H2O+, we determined the properties of the gas traced by ArH+ by analysing the steady-state chemistry. Our results confirm the role of ArH+ as a tracer of atomic gas, probing regions with an extremely low average molecular fraction of 8.8×10-4. Furthermore, by comparing the abundance and molecular fraction traced by ArH+ and by atomic gas tracers OH+ and H2O+ and molecular gas tracers CH or HF, we study the transition from the diffuse atomic to the diffuse and translucent molecular clouds in the ISM
First detection of 13CH in the interstellar medium
No full textIn 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)
APEX survey of interstellar HCl
No full textContext. Despite being only the nineteenth 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 35Cl/37Cl ratio – shaped in supernovae and evolved stars – probes nucleosynthesis across the Galaxy. Yet, studies of Cl-bearing molecules have remained limited to a few sight lines due to observational challenges.
Aims. We systematically investigated the Galactic distribution of HCl and the [H35Cl]/[H37Cl] 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.
Methods. We surveyed the ground-state J = 1–0 transitions of H35Cl and H37Cl near 625 GHz towards 28 sources with the SEPIA660 receiver on the APEX 12 m sub-millimetre telescope. This survey more than doubles the number of sources with HCl detections and reveals HCl emission arising from both the background core and associated outflows. The spectra were modelled with XCLASS to derive column densities, isotopic ratios, and the kinematics of both the core and the outflow components.
Results. H35Cl was detected in all sources, H37Cl in all but two, with spectral line profiles ranging from those with only emission to complex emission–absorption mixtures. Column densities span from 2.3–22.8 × 1013 cm−2 for H35Cl and 0.6–12.5 × 1013 cm−2 for H37Cl, resulting in isotopic ratios between 1.6 and 3.5 in emission-only sources.
Conclusions. The derived [H35Cl]/[H37Cl] 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 synthesised during epochs of lower average metallicity in the Galaxy. Notably, we detect H35Cl 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
First detection of deuterated methylidyne (CD) in the interstellar medium
No full textWhile 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 H2D+ ion, whereas at higher temperatures (T ≥ 20–30 K) 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 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 (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 (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 C2H and H2CO 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
First detection of CF
No full textContext. 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 H2 number density of (0.5–7.9) × 104 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
Fingerprinting the effects of hyperfine structure on CH and OH far infrared spectra using Wiener filter deconvolution
No full textContext. 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
ArH
No full textContext. 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
Extending the view of ArH
No full textContext. 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 H2O+ 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 transitions of para-H2O+ 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-H2O+, we derive on average a cosmic-ray ionisation rate, ζp(H), of (2.3 ± 0.3) × 10−16 s−1 towards the sight lines studied in this work. Using the derived values of ζp(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+, H2O+, 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 H2O+ of 2.1 ± 1.0, which corresponds to a nuclear spin temperature of 41 K, consistent with the typical gas temperatures of diffuse clouds
Recommended from our members
Schlussbericht
No full textIm Vorhaben wurden Daten eines SOFIA Legacy Projekts, HyGAL, analysiert. HyGAL wurde zusammen mit Prof. Dr. David Neufeld (Johns-Hopkins-University, Baltimore, USA) durchgeführt und möchte durch die Beobachtung von diversen Hydriden das interstellare
Medium charakterisieren. Hydride sind sehr einfache Moleküle, die die Basis für komplexere Chemie bilden. Sie werden auch in diffusen Wolken gebildet, und erlauben die Charakterisierung von Phasen des interstellaren Mediums, die mit anderen Molekülen nicht
möglich ist. Das Projekt wurde durch die COVID bedingte Unterbrechung der Beobachtungen für ein Jahr, und dann die Stilllegung von SOFIA beeinträchtigt. Dennoch konnten bislang zwei Artikel publiziert werden, die Analyse der restlichen Daten geht weiter.
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The CH radical at radio wavelengths: revisiting emission in the 3.3 GHz ground-state lines
No full textContext. 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 2Π1∕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
