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At least 19 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2014

THE NITROGEN ISOTOPIC COMPOSITION OF METEORITIC HCN

HCN is ubiquitous in extraterrestrial environments and is central to current theories on the origin of early solar system organic compounds such as amino acids. These compounds, observed in carbonaceous meteorites, were likely important in the origin and/or evolution of early life. As part of our attempts to understand the origin(s) of meteoritic CN{sup –}, we have analyzed the {sup 15}N/{sup 14}N isotopic composition of HCN gas released from water extracts of the Murchison meteorite and found its value to be near those of the terrestrial atmosphere. The findings, when evaluated viz-a-viz molecular abundances and isotopic data of meteoritic organic compounds, suggest that HCN formation could have occurred during the protracted water alteration processes known to have affected the mineralogy of many asteroidal bodies during their solar residence. This was an active synthetic stage, which likely involved simple gasses, organic molecules, their presolar precursors, as well as mineral catalysts and would have lead to the formation of molecules of differing isotopic composition, including some with solar values.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Cyanide does more to inhibit heme enzymes, than merely serving as an active-site ligand

Highlights: • Cyanide (CN) is a well-studied toxic principle, known to inhibit heme-enzymes. • Inhibition is supposed to result from CN binding at the active site as a ligand. • Diverse heme enzymes’ CN inhibition profiles challenge prevailing mechanism. • Poor binding efficiency of CN at low enzyme concentrations and ligand pressures. • CN-based diffusible radicals cause ‘non-productive electron transfers’ (inhibition). - Abstract: The toxicity of cyanide is hitherto attributed to its ability to bind to heme proteins’ active site and thereby inhibit their activity. It is shown herein that the long-held interpretation is inadequate to explain several observations in heme-enzyme reaction systems. Generation of cyanide-based diffusible radicals in heme-enzyme reaction milieu could shunt electron transfers (by non-active site processes), and thus be detrimental to the efficiency of oxidative outcomes.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2017

AAV-mediated conversion of human pluripotent stem cell-derived pacemaker

Malfunction of nodal pacemaker (Pm) cardiomyocytes (CMs) due to diseases or aging leads to rhythm generation disorders, necessitating electronic Pm implantation. We functionally reprogrammed human pluripotent stem cell (hPSC) derived-ventricular (V) CMs into -PmCMs via recombinant adeno-associated virus serotype 9 (rAAV9)-mediated overexpression of engineered HCN1 channel (HCN1ΔΔΔ) whose S3-S4 linker has been strategically deleted by design to promote cardiac pacemaking. rAAV9-HCN1ΔΔΔ-reprogrammed hPSC-PmCMs converted from -VCMs showed automaticity and action potential parameters typical of native nodal PmCMs. Implantation of rAAV9-HCN1ΔΔΔ-based BPm in a preclinical porcine model of complete heart block significantly reduced the dependence on device-supported pacing and generated spontaneous heart rhythms from the BPm. Collectively, these results have further laid the groundwork on BPm for future translation.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2017

Dual expression of constitutively active Gαs-protein-coupled receptors differentially establishes the resting activity of the cAMP-gated HCN2 channel in a single compartment

Highlights: • Constitutively active GPCRs were dually expressed in a single compartment. • HCN2 channels were differentially activated depending on the GPCR pairs. • Basal actions of GPCRs were abolished by an adenylate cyclase inhibitor. • Basal cAMP pools were specifically established by constitutively active GPCR pairs. The hyperpolarization-activated cyclic nucleotide-gated 2 (HCN2) channel is a major subtype of the HCN channel family expressed in the nervous system that sets the membrane potential, regulates cell excitability and senses changes in the extracellular environment. Neurons express various Gα{sub s}-protein-coupled receptors (GPCRs), many of which show ligand-independent constitutive activity. These membrane-bound proteins are expressed in various subcellular compartments of neurons. Therefore, some proportion of HCN2 channels opens in response to the basal cAMP pool size produced by constitutively active GPCRs. Here, we employed an exogenous HEK293 expression system and voltage-clamp patch-clamp recordings to investigate basal HCN2 channel activity in the presence of two GPCRs with diverse basal activities in a single compartment. We utilized the β2-adrenoceptor (β2AR) together with odorant receptors (ORs), as both GPCR families are known to show strong basal activity. Consequently, β2AR alone strongly enhanced the activity of HCN2 channels, and co-expression of ORs further diversified the HCN2 channel activity, which was totally abolished by an adenylate cyclase inhibitor. Thus, we conclude that the dual expression of constitutively active GPCRs establishes the diverse range of the basal cAMP pool size in resting cells through mutual additive or suppressive interactions, even in the absence of external stimulation.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2014

Unconventional hydrogen bonding to organic ions in the gas phase: Stepwise association of hydrogen cyanide with the pyridine and pyrimidine radical cations and protonated pyridine

Equilibrium thermochemical measurements using the ion mobility drift cell technique have been utilized to investigate the binding energies and entropy changes for the stepwise association of HCN molecules with the pyridine and pyrimidine radical cations forming the C{sub 5}H{sub 5}N{sup +·}(HCN){sub n} and C{sub 4}H{sub 4}N{sub 2}{sup +·}(HCN){sub n} clusters, respectively, with n = 1–4. For comparison, the binding of 1–4 HCN molecules to the protonated pyridine C{sub 5}H{sub 5}NH{sup +}(HCN){sub n} has also been investigated. The binding energies of HCN to the pyridine and pyrimidine radical cations are nearly equal (11.4 and 12.0 kcal/mol, respectively) but weaker than the HCN binding to the protonated pyridine (14.0 kcal/mol). The pyridine and pyrimidine radical cations form unconventional carbon-based ionic hydrogen bonds with HCN (CH{sup δ+}⋯NCH). Protonated pyridine forms a stronger ionic hydrogen bond with HCN (NH{sup +}⋯NCH) which can be extended to a linear chain with the clustering of additional HCN molecules (NH{sup +}⋯NCH··NCH⋯NCH) leading to a rapid decrease in the bond strength as the length of the chain increases. The lowest energy structures of the pyridine and pyrimidine radical cation clusters containing 3-4 HCN molecules show a strong tendency for the internal solvation of the radical cation by the HCN molecules where bifurcated structures involving multiple hydrogen bonding sites with the ring hydrogen atoms are formed. The unconventional H-bonds (CH{sup δ+}⋯NCH) formed between the pyridine or the pyrimidine radical cations and HCN molecules (11–12 kcal/mol) are stronger than the similar (CH{sup δ+}⋯NCH) bonds formed between the benzene radical cation and HCN molecules (9 kcal/mol) indicating that the CH{sup δ+} centers in the pyridine and pyrimidine radical cations have more effective charges than in the benzene radical cation.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

Quantum treatment of protons with the reduced explicitly correlated Hartree-Fock approach

The nuclear-electronic orbital (NEO) approach treats select nuclei quantum mechanically on the same level as the electrons and includes nonadiabatic effects between the electrons and the quantum nuclei. The practical implementation of this approach is challenging due to the significance of electron-nucleus dynamical correlation. Herein, we present a general extension of the previously developed reduced NEO explicitly correlated Hartree-Fock (RXCHF) approach, in which only select electronic orbitals are explicitly correlated to each quantum nuclear orbital via Gaussian-type geminal functions. Approximations of the electronic exchange between the geminal-coupled electronic orbitals and the other electronic orbitals are also explored. This general approach enables computationally tractable yet accurate calculations on molecular systems with quantum protons. The RXCHF method is applied to the hydrogen cyanide (HCN) and FHF{sup −} systems, where the proton and all electrons are treated quantum mechanically. For the HCN system, only the two electronic orbitals associated with the CH covalent bond are geminal-coupled to the proton orbital. For the FHF{sup −} system, only the four electronic orbitals associated with the two FH covalent bonds are geminal-coupled to the proton orbital. For both systems, the RXCHF method produces qualitatively accurate nuclear densities, in contrast to mean field-based NEO approaches. The development and implementation of the RXCHF method provide the framework to perform calculations on systems such as proton-coupled electron transfer reactions, where electron-proton nonadiabatic effects are important.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

Nuclear-electronic orbital reduced explicitly correlated Hartree-Fock approach: Restricted basis sets and open-shell systems

The nuclear electronic orbital (NEO) reduced explicitly correlated Hartree-Fock (RXCHF) approach couples select electronic orbitals to the nuclear orbital via Gaussian-type geminal functions. This approach is extended to enable the use of a restricted basis set for the explicitly correlated electronic orbitals and an open-shell treatment for the other electronic orbitals. The working equations are derived and the implementation is discussed for both extensions. The RXCHF method with a restricted basis set is applied to HCN and FHF{sup −} and is shown to agree quantitatively with results from RXCHF calculations with a full basis set. The number of many-particle integrals that must be calculated for these two molecules is reduced by over an order of magnitude with essentially no loss in accuracy, and the reduction factor will increase substantially for larger systems. Typically, the computational cost of RXCHF calculations with restricted basis sets will scale in terms of the number of basis functions centered on the quantum nucleus and the covalently bonded neighbor(s). In addition, the RXCHF method with an odd number of electrons that are not explicitly correlated to the nuclear orbital is implemented using a restricted open-shell formalism for these electrons. This method is applied to HCN{sup +}, and the nuclear densities are in qualitative agreement with grid-based calculations. Future work will focus on the significance of nonadiabatic effects in molecular systems and the further enhancement of the NEO-RXCHF approach to accurately describe such effects.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

Dissociative electron attachment to the gas-phase nucleobase hypoxanthine

We present high-resolution measurements of the dissociative electron attachment (DEA) to isolated gas-phase hypoxanthine (C{sub 5}H{sub 4}N{sub 4}O, Hyp), a tRNA purine base. The anion mass spectra and individual ion efficiency curves from Hyp were measured as a function of electron energy below 9 eV. The mass spectra at 1 and 6 eV exhibit the highest anion yields, indicating possible common precursor ions that decay into the detectable anionic fragments. The (Hyp − H) anion (C{sub 5}H{sub 3}N{sub 4}O{sup −}) exhibits a sharp resonant peak at 1 eV, which we tentatively assign to a dipole-bound state of the keto-N1H,N9H tautomer in which dehydrogenation occurs at either the N1 or N9 position based upon our quantum chemical computations (B3LYP/6-311+G(d,p) and U(MP2-aug-cc-pVDZ+)) and prior studies with adenine. This closed-shell dehydrogenated anion is the dominant fragment formed upon electron attachment, as with other nucleobases. Seven other anions were also observed including (Hyp − NH){sup −}, C{sub 4}H{sub 3}N{sub 4}{sup −}/C{sub 4}HN{sub 3}O{sup −}, C{sub 4}H{sub 2}N{sub 3}{sup −}, C{sub 3}NO{sup −}/HC(HCN)CN{sup −}, OCN{sup −}, CN{sup −}, and O{sup −}. Most of these anions exhibit broad but weak resonances between 4 and 8 eV similar to many analogous anions from adenine. The DEA to Hyp involves significant fragmentation, which is relevant to understanding radiation damage of biomolecules.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

Photodissociation of HCN and HNC isomers in the 7-10 eV energy range

The ultraviolet photoabsorption spectra of the HCN and HNC isomers have been simulated in the 7-10 eV photon energy range. For this purpose, the three-dimensional adiabatic potential energy surfaces of the 7 lowest electronic states, and the corresponding transition dipole moments, have been calculated, at multireference configuration interaction level. The spectra are calculated with a quantum wave packet method on these adiabatic potential energy surfaces. The spectra for the 3 lower excited states, the dissociative electronic states, correspond essentially to predissociation peaks, most of them through tunneling on the same adiabatic state. The 3 higher electronic states are bound, hereafter electronic bound states, and their spectra consist of delta lines, in the adiabatic approximation. The radiative lifetime towards the ground electronic states of these bound states has been calculated, being longer than 10 ns in all cases, much longer that the characteristic predissociation lifetimes. The spectra of HCN is compared with the available experimental and previous theoretical simulations, while in the case of HNC there are no previous studies to our knowledge. The spectrum for HNC is considerably more intense than that of HCN in the 7-10 eV photon energy range, which points to a higher photodissociation rate for HNC, compared to HCN, in astrophysical environments illuminated by ultraviolet radiation.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

Multicomponent density functional theory embedding formulation

Multicomponent density functional theory (DFT) methods have been developed to treat two types of particles, such as electrons and nuclei, quantum mechanically at the same level. In the nuclear-electronic orbital (NEO) approach, all electrons and select nuclei, typically key protons, are treated quantum mechanically. For multicomponent DFT methods developed within the NEO framework, electron-proton correlation functionals based on explicitly correlated wavefunctions have been designed and used in conjunction with well-established electronic exchange-correlation functionals. Herein a general theory for multicomponent embedded DFT is developed to enable the accurate treatment of larger systems. In the general theory, the total electronic density is separated into two subsystem densities, denoted as regular and special, and different electron-proton correlation functionals are used for these two electronic densities. In the specific implementation, the special electron density is defined in terms of spatially localized Kohn-Sham electronic orbitals, and electron-proton correlation is included only for the special electron density. The electron-proton correlation functional depends on only the special electron density and the proton density, whereas the electronic exchange-correlation functional depends on the total electronic density. This scheme includes the essential electron-proton correlation, which is a relatively local effect, as well as the electronic exchange-correlation for the entire system. This multicomponent DFT-in-DFT embedding theory is applied to the HCN and FHF{sup −} molecules in conjunction with two different electron-proton correlation functionals and three different electronic exchange-correlation functionals. The results illustrate that this approach provides qualitatively accurate nuclear densities in a computationally tractable manner. The general theory is also easily extended to other types of partitioning schemes for multicomponent systems.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2014

Application of conical 90-degree reflectors for solving the problem of mirror alignment in terahertz-range lasers

We report a study of the conical mirrors with an apex angle of 90° in the resonator of the gas-discharge HCN laser with the radiation wavelength of 337 μm (0.89 THz). Experimental results have shown that such mirrors do not require precise alignment. This makes it possible to improve the radiation stability, significantly simplify the construction of laser and reduce the complexity of its maintenance. (laser applications and other topics in quantum electronics)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Transmission spectroscopy of the hot Jupiter WASP-12b from 0.7 to 5 μm

Since the first report of a potentially non-solar carbon-to-oxygen ratio (C/O) in its dayside atmosphere, the highly irradiated exoplanet WASP-12b has been under intense scrutiny and the subject of many follow-up observations. Additionally, the recent discovery of stellar binary companions ∼1'' from WASP-12 has obfuscated interpretation of the observational data. Here we present new ground-based multi-object transmission-spectroscopy observations of WASP-12b that we acquired over two consecutive nights in the red optical with Gemini-N/GMOS. After correcting for the influence of WASP-12's stellar companions, we find that these data rule out a cloud-free H{sub 2} atmosphere with no additional opacity sources. We detect features in the transmission spectrum that may be attributed to metal oxides (such as TiO and VO) for an O-rich atmosphere or to metal hydrides (such as TiH) for a C-rich atmosphere. We also reanalyzed NIR transit-spectroscopy observations of WASP-12b from HST/WFC3 and broadband transit photometry from Warm Spitzer. We attribute the broad spectral features in the WFC3 data to either H{sub 2}O or CH{sub 4} and HCN for an O-rich or C-rich atmosphere, respectively. The Spitzer data suggest shallower transit depths than the models predict at infrared wavelengths, albeit at low statistical significance. A multi-instrument, broad-wavelength analysis of WASP-12b suggests that the transmission spectrum is well approximated by a simple Rayleigh scattering model with a planet terminator temperature of 1870 ± 130 K. We conclude that additional high-precision data and isolated spectroscopic measurements of the companion stars are required to place definitive constraints on the composition of WASP-12b's atmosphere.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Alma observations of nearby luminous infrared galaxies with various agn energetic contributions using dense gas tracers

We present the results of our ALMA Cycle 0 observations, using HCN/HCO{sup +}/HNC J = 4-3 lines, of six nearby luminous infrared galaxies with various energetic contributions from active galactic nuclei (AGNs) estimated from previous infrared spectroscopy. These lines are very effective for probing the physical properties of high-density molecular gas around the hidden energy sources in the nuclear regions of these galaxies. We find that HCN to HCO{sup +} J = 4-3 flux ratios tend to be higher in AGN-important galaxies than in starburst-dominated regions, as was seen at the J = 1-0 transition, while there is no clear difference in the HCN-to-HNC J = 4-3 flux ratios among observed sources. A galaxy with a starburst-type infrared spectral shape and very large molecular line widths shows a high HCN-to-HCO{sup +} J = 4-3 flux ratio, which could be due to turbulence-induced heating. We propose that enhanced HCN J = 4-3 emission relative to HCO{sup +} J = 4-3 could be used to detect more energetic activity than normal starbursts, including deeply buried AGNs, in dusty galaxy populations.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

H ii REGION G46.5-0.2: THE INTERPLAY BETWEEN IONIZING RADIATION, MOLECULAR GAS, AND STAR FORMATION

H ii regions are particularly interesting because they can generate dense layers of gas and dust, elongated columns or pillars of gas pointing toward the ionizing sources, and cometary globules of dense gas where triggered star formation can occur. Understanding the interplay between the ionizing radiation and the dense surrounding gas is very important to explain the origin of these peculiar structures, and hence to characterize triggered star formation. G46.5-0.2 (G46), a poorly studied galactic H ii region located at about 4 kpc, is an excellent target for performing this kind of study. Using public molecular data extracted from the Galactic Ring Survey ({sup 13}CO J = 1–0) and from the James Clerk Maxwell Telescope data archive ({sup 12}CO, {sup 13}CO, C{sup 18}O J = 3–2, HCO{sup +}, and HCN J = 4–3), and infrared data from the GLIMPSE and MIPSGAL surveys, we perform a complete study of G46, its molecular environment, and the young stellar objects (YSOs) placed around it. We found that G46, probably excited by an O7V star, is located close to the edge of the GRSMC G046.34-00.21 molecular cloud. It presents a horse-shoe morphology opening in the direction of the cloud. We observed a filamentary structure in the molecular gas likely related to G46 and not considerable molecular emission toward its open border. We found that about 10′ to the southwest of G46 there are some pillar-like features, shining at 8 μm and pointing toward the H ii region open border. We propose that the pillar-like features were carved and sculpted by the ionizing flux from G46. We found several YSOs likely embedded in the molecular cloud grouped in two main concentrations: one, closer to the G46 open border consisting of Class II type sources, and another mostly composed of Class I type YSOs located just ahead of the pillar-like features, strongly suggesting an age gradient in the YSO distribution.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

The rest-frame submillimeter spectrum of high-redshift, dusty, star-forming galaxies

We present the average rest-frame spectrum of high-redshift dusty, star-forming galaxies from 250 to 770 GHz. This spectrum was constructed by stacking Atacama Large Millimeter/submillimeter Array (ALMA) 3 mm spectra of 22 such sources discovered by the South Pole Telescope and spanning z = 2.0-5.7. In addition to multiple bright spectral features of {sup 12}CO, [C I], and H{sub 2}O, we also detect several faint transitions of {sup 13}CO, HCN, HNC, HCO{sup +}, and CN, and use the observed line strengths to characterize the typical properties of the interstellar medium of these high-redshift starburst galaxies. We find that the {sup 13}CO brightness in these objects is comparable to that of the only other z > 2 star-forming galaxy in which {sup 13}CO has been observed. We show that the emission from the high-critical density molecules HCN, HNC, HCO{sup +}, and CN is consistent with a warm, dense medium with T {sub kin} ∼ 55 K and n{sub H{sub 2}}≳10{sup 5.5} cm{sup –3}. High molecular hydrogen densities are required to reproduce the observed line ratios, and we demonstrate that alternatives to purely collisional excitation are unlikely to be significant for the bulk of these systems. We quantify the average emission from several species with no individually detected transitions, and find emission from the hydride CH and the linear molecule CCH for the first time at high redshift, indicating that these molecules may be powerful probes of interstellar chemistry in high-redshift systems. These observations represent the first constraints on many molecular species with rest-frame transitions from 0.4 to 1.2 mm in star-forming systems at high redshift, and will be invaluable in making effective use of ALMA in full science operations.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Investigation of HNCO isomer formation in ice mantles by UV and thermal processing: An experimental approach

Current gas-phase models do not account for the abundances of HNCO isomers detected in various environments, suggesting their formation in icy grain mantles. We attempted to study a formation channel of HNCO and its possible isomers by vacuum-UV photoprocessing of interstellar ice analogs containing H{sub 2}O, NH{sub 3}, CO, HCN, CH{sub 3}OH, CH{sub 4}, and N{sub 2} followed by warm-up under astrophysically relevant conditions. Only the H{sub 2}O:NH{sub 3}:CO and H{sub 2}O:HCN ice mixtures led to the production of HNCO species. The possible isomerization of HNCO to its higher energy tautomers following irradiation or due to ice warm-up has been scrutinized. The photochemistry and thermal chemistry of H{sub 2}O:NH{sub 3}:CO and H{sub 2}O:HCN ices were simulated using the Interstellar Astrochemistry Chamber, a state-of-the-art ultra-high-vacuum setup. The ice was monitored in situ by Fourier transform mid-infrared spectroscopy in transmittance. A quadrupole mass spectrometer detected the desorption of the molecules in the gas phase. UV photoprocessing of H{sub 2}O:NH{sub 3}:CO and H{sub 2}O:HCN ices lead to the formation of OCN{sup –} as a main product in the solid state and a minor amount of HNCO. The second isomer HOCN has been tentatively identified. Despite its low efficiency, the formation of HNCO and the HOCN isomers by UV photoprocessing of realistic simulated ice mantles might explain the observed abundances of these species in photodissociation regions, hot cores, and dark clouds.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Spectral line survey toward the spiral arm of M51 in the 3 and 2 mm bands

We have conducted a spectral line survey in the 3 and 2 mm bands toward two positions in a spiral arm of M51 (NGC 5194) with the Institut de Radioastronomie Millimétrique 30 m telescope. In this survey, we have identified 13 molecular species, including CN, CCH, N{sub 2}H{sup +}, HNCO, and CH{sub 3}OH. Furthermore, six isotopologues of the major species have been detected. On the other hand, SiO, HC{sub 3}N, CH{sub 3}CN, and deuterated species such as DCN and DCO{sup +} were not detected. The deuterium fractionation ratios are evaluated to be less than 0.8% and 1.2% for DCN/HCN and DCO{sup +}/HCO{sup +}, respectively. By comparing the results of the two positions with different star formation activities, we have found that the observed chemical compositions do not strongly depend on star formation activities. They seem to reflect a chemical composition averaged over the 1 kpc scale region including many giant molecular clouds. Among the detected molecules CN, CCH, and CH{sub 3}OH are found to be abundant. High abundances of CN and CCH are consistent with the above picture of a widespread distribution of molecules because they can be produced by photodissociation. On the other hand, it seems likely that CH{sub 3}OH is liberated in the gas phase by shocks associated with large-scale phenomena such as cloud-cloud collisions and/or by nonthermal desorption processes such as photoevaporation due to cosmic-ray-induced UV photons. The present result demonstrates a characteristic chemical composition of a giant molecular cloud complex in the spiral arm, which can be used as a standard reference for studying chemistry in active galactic nuclei and starbursts.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

First extragalactic detection of submillimeter CH rotational lines from the Herschel space observatory

We present the first extragalactic detections of several CH rotational transitions in the far-infrared in four nearby galaxies, NGC 1068, Arp 220, M82, and NGC 253, using the Herschel Space Observatory. The CH lines in all four galaxies are a factor of 2-4 brighter than the adjacent HCN and HCO{sup +} J = 6-5 lines (also detected in the same spectra). In the star-formation-dominated galaxies, M82, NGC 253, and Arp 220, the CH/CO abundance ratio is low (∼10{sup –5}), implying that the CH is primarily arising in diffuse and translucent gas where the chemistry is driven by UV radiation as found in the Milky Way interstellar matter. In NGC 1068, which has a luminous active galactic nucleus (AGN), the CH/CO ratio is an order of magnitude higher, suggesting that CH formation is driven by an X-ray-dominated region (XDR). Our XDR models show that both the CH and CO abundances in NGC 1068 can be explained by an XDR-driven chemistry for gas densities and molecular hydrogen column densities that are well constrained by the CO observations. We conclude that the CH/CO ratio may a good indicator of the presence of AGN in galaxies. We also discuss the feasibility of detecting CH in intermediate- to high-z galaxies with ALMA.

79 ASTRONOMY AND ASTROPHYSICS↗