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Methyl isocyanate (CH3NCO) was recently found in hot cores and suggested to exist on comet 67P/CG. The incorporation of this molecule into astrochemical networks requires data on its formation and destruction. In this work, ices of pure CH3NCO and of CH3NCO(4%–5%)/H2O mixtures deposited at 20 K were irradiated with a UV D2 lamp (120–400 nm) and bombarded by 5 keV electrons to mimic the secondary electrons produced by cosmic rays (CRs). The destruction of CH3NCO was studied using IR spectroscopy. After processing, the νa–NCO band of CH3NCO disappeared and IR bands corresponding to CO, CO2, OCN−, and HCN/CN− appeared instead. The products of photon and electron processing were very similar. Destruction cross sections and half-life doses were derived from the measurements. Water ice provides a good shield against UV irradiation (half-life dose of ∼64 eV molecule−1 for CH3NCO in water ice), but is not so good against high-energy electrons (half-life dose ∼18 eV molecule−1). It was also found that CH3NCO does not react with H2O over the temperature range 20–200 K. These results indicate that hypothetical CH3NCO in the ices of dense clouds should be stable against UV photons and relatively stable against CRs over the lifetime of a cloud (∼107 yr), and could sublime in the hot core phase. On the surface of a Kuiper Belt object (the original location of comet 67P/CG) the molecule would be swiftly destroyed, by both photons and CRs, but embedded below just 10 μm of water ice, the molecule could survive for ∼109 yr. [ABSTRACT FROM AUTHOR]

We present new high angular resolution interferometer observations of the v = 0 J = 14–13 and 15–14 SiS lines toward IRC+10216, carried out with the Combined Array for Research in Millimeter-wave Astronomy and the Atacama Large Millimeter Array. The maps, with angular resolutions of ≃0.″25 and 0.″55, reveal (1) an extended, roughly uniform, and weak emission with a size of ≃0.″5; (2) a component elongated approximately along the east–west direction peaking at ≃0.″13 and 0.″17 at both sides of the central star; and (3) two blue- and redshifted compact components peaking around 0.″07 to the NW of the star. We have modeled the emission with a 3D radiation transfer code, finding that the observations cannot be explained only by thermal emission. Several maser clumps and one arc-shaped maser feature arranged from 5 to 20 from the central star, in addition to a thin shell-like maser structure at ≃13 , are required to explain the observations. This maser-emitting set of structures accounts for 75% of the total emission, while the other 25% is produced by thermally excited molecules. About 60% of the maser emission comes from the extended emission, and the rest comes from the set of clumps and the arc. The analysis of a time monitoring of these and other SiS and 29SiS lines carried out with the IRAM 30 m telescope from 2015 to present suggests that the intensity of some spectral components of the maser emission strongly depends on the stellar pulsation, while other components show a mild variability. This monitoring evidences a significant phase lag of ≃0.2 between the maser and near-IR light curves. [ABSTRACT FROM AUTHOR]

We present the detection of C4H2 for first time in the envelope of the C-rich AGB star IRC+10216, based on high spectral resolution mid-infrared observations carried out with the Texas Echelon-cross-Echelle Spectrograph mounted on the Infrared Telescope Facility. The obtained spectrum contains 24 narrow absorption features above the detection limit, identified as lines of the ro-vibrational C4H2 band . The analysis of these lines through a ro-vibrational diagram indicates that the column density of C4H2 is (2.4 ± 1.5) × 1016 cm−2. Diacetylene is distributed in two excitation populations accounting for 20% and 80% of the total column density and with rotational temperatures of 47 ± 7 and 420 ± 120 K, respectively. This two-folded rotational temperature suggests that the absorbing gas is located beyond ≃0.″4 ≃ 20 R⋆ from the star, with a noticeable cold contribution outwards from ≃10″ ≃ 500 R⋆. This outer shell matches up with the place where cyanoacetylenes and carbon chains are known to form due to the action of the Galactic dissociating radiation field on the neutral gas coming from the inner layers of the envelope. [ABSTRACT FROM AUTHOR]

The chemical kinetics of neutral–neutral gas-phase reactions at ultralow temperatures is a fascinating research subject with important implications on the chemistry of complex organic molecules in the interstellar medium (T ∼ 10–100 K). Scarce kinetic information is currently available for these kinds of reactions at T < 200 K. In this work, we use the Cinétique de Réaction en Ecoulement Supersonique Uniforme (CRESU; Reaction Kinetics in a Uniform Supersonic Flow) technique to measure for the first time the rate coefficients (k) of the gas-phase OH+H2CO reaction between 22 and 107 K. The k values greatly increase from 2.1 × 10−11 cm3 s−1 at 107 K to 1.2 × 10−10 cm3 s−1 at 22 K. This is also confirmed by quasi-classical trajectories (QCT) at collision energies down to 0.1 meV performed using a new full dimension and ab initio potential energy surface that generates highly accurate potential and includes long-range dipole–dipole interactions. QCT calculations indicate that at low temperatures HCO is the exclusive product for the OH+H2CO reaction. In order to revisit the chemistry of HCO in cold dense clouds, k is reasonably extrapolated from the experimental results at 10 K (2.6 × 10−10 cm3 s−1). The modeled abundances of HCO are in agreement with the observations in cold dark clouds for an evolving time of 105–106 yr. The different sources of production of HCO are presented and the uncertainties in the chemical networks are discussed. The present reaction is shown to account for a few percent of the total HCO production rate. This reaction can be expected to be a competitive process in the chemistry of prestellar cores. Extensions to photodissociation regions and diffuse cloud environments are also addressed. [ABSTRACT FROM AUTHOR]

The relative populations in rotational transitions of CO can be useful for inferring gas conditions and excitation mechanisms at work in the interstellar medium. We present CO emission lines from rotational transitions observed with Herschel/HIFI in the star-forming cores Orion S, Orion KL, Sgr B2(M), and W49N. Integrated line fluxes from these observations are combined with those from Herschel/PACS observations of the same sources to construct CO spectral line energy distributions (SLEDs) from 5 ≤ Ju ≤ 48. These CO SLEDs are compared to those reported in other galaxies, with the intention of empirically determining which mechanisms dominate excitation in such systems. We find that CO SLEDs in Galactic star-forming cores cannot be used to reproduce those observed in other galaxies, although the discrepancies arise primarily as a result of beam filling factors. The much larger regions sampled by the Herschel beams at distances of several megaparsecs contain significant amounts of cooler gas, which dominate the extragalactic CO SLEDs, in contrast to observations of Galactic star-forming regions, which are focused specifically on cores containing primarily hot molecular gas. [ABSTRACT FROM AUTHOR]

High spectral resolution mid-IR observations of ethylene () toward the AGB star IRC+10216 were obtained using the Texas Echelon Cross Echelle Spectrograph (TEXES) at the NASA Infrared Telescope Facility (IRTF). 80 ro-vibrational lines from the 10.5 μm vibrational mode with J ≲ 30 were detected in absorption. The observed lines are divided into two groups with rotational temperatures of 105 and 400 K (warm and hot lines). The warm lines peak at ≃ −14 km s−1 with respect to the systemic velocity, suggesting that they are mostly formed outwards from . The hot lines are centered at −10 km s−1 indicating that they come from a shell between 10 and . 35% of the observed lines are unblended and can be fitted with a code developed to model the emission of a spherically symmetric circumstellar envelope. The analysis of several scenarios reveals that the abundance relative to H2 in the range 5−20R⋆ is on average and it could be as high as 1.1 × 10−7. Beyond , it is 8.2 × 10−8. The total column density is (6.5 ± 3.0) × 1015 cm−2. is found to be rotationally under local thermodynamical equilibrium (LTE) and vibrationally out of LTE. One of the scenarios that best reproduce the observations suggests that up to 25% of the molecules at could condense onto dust grains. This possible depletion would not significantly influence the gas acceleration although it could play a role in the surface chemistry on the dust grains. [ABSTRACT FROM AUTHOR]

Previous detections of methyl and ethyl formate make other small substituted formates potential candidates for observation in the interstellar medium. Among them, vinyl formate is one of the simplest unsaturated carboxylic ester. The aim of this work is to provide direct experimental frequencies of the ground vibrational state of vinyl formate in a large spectral range for astrophysical use. The room-temperature rotational spectrum of vinyl formate has been measured from 80 to 360 GHz and analyzed in terms of Watson’s semirigid rotor Hamiltonian. Two thousand six hundred transitions within J = 3–88 and Ka = 0–28 were assigned to the most stable conformer of vinyl formate and a new set of spectroscopic constants was accurately determined. Spectral features of vinyl formate were then searched for in Orion KL, Sgr B2(N), B1-b, and TMC-1 molecular clouds. Upper limits to the column density of vinyl formate are provided. [ABSTRACT FROM AUTHOR]

We report laboratory spectroscopy for the first time of the J = 1–0 and J = 2–1 lines of Na35Cl and Na37Cl in several vibrational states. The hyperfine structure has been resolved in both transitions for all vibrational levels, which permit us to predict with high accuracy the hyperfine splitting of the rotational transitions of the two isotopologues at higher frequencies. The new data have been merged with all previous works at microwave, millimeter, and infrared wavelengths and fitted to a series of mass-independent Dunham parameters and to a potential energy function. The obtained parameters have been used to compute a new dipole moment function, from which the dipole moment for infrared transitions up to Δv = 8 has been derived. Frequency and intensity predictions are provided for all rovibrational transitions up to J = 150 and v = 8, from which the ALMA data of evolved stars can be modeled and interpreted. [ABSTRACT FROM AUTHOR]

In recent years, ultra-low temperature chemical kinetic experiments have demonstrated that some gas-phase reactions are much faster than was previously thought. One example is the reaction between OH and CH3OH, which has recently been found to be accelerated at low temperatures yielding CH3O as its main product. This finding raised the question of whether or not the CH3O observed in the dense core Barnard 1b could be formed by the gas-phase reaction of CH3OH and OH. Several chemical models including this reaction and grain-surface processes have been developed to explain the observed abundance of CH3O, but they have met with little success. Here, we report for the first time the rate coefficients for the gas-phase reaction of OH and CH3OH down to a temperature of 22 K, which is very close to the temperature in cold interstellar clouds. Two independent experimental set-ups based on the supersonic gas expansion technique coupled to the pulsed laser photolysis laser-induced fluorescence technique were used to determine the rate coefficients in the temperature range 22–64 K. The temperature dependence obtained in this work can be expressed as k(22–64 K) = cm3 molecule−1 s−1. Implementing this expression in a chemical model of a cold, dense cloud results in CH3O/CH3OH abundance ratios similar to or slightly lower than the value of ∼3 × 10−3 observed in Barnard 1b. This finding confirms that the gas-phase reaction between OH and CH3OH is an important contributor to the formation of interstellar CH3O. The role of grain-surface processes in the formation of CH3O, although it cannot be fully neglected, remains controversial. [ABSTRACT FROM AUTHOR]

The Atacama Large Millimeter/submillimeter Array is allowing us to study the innermost regions of the circumstellar envelopes of evolved stars with unprecedented precision and sensitivity. Key processes in the ejection of matter and dust from these objects occur in their inner zones. In this work, we present sub-arcsecond interferometric maps of transitions of metal-bearing molecules toward the prototypical C-rich evolved star IRC +10216. While Al-bearing molecules seem to be present as a roughly spherical shell, the molecular emission from the salts NaCl and KCl presents an elongation in the inner regions with a central minimum. In order to accurately analyze the emission from the NaCl rotational lines, we present new calculations of the collisional rates for this molecule based on new spectroscopic constants. The most plausible interpretation for the spatial distribution of the salts is a spiral with a NaCl mass of 0.08 . Alternatively, a torus of gas and dust would result in structures similar to those observed. From the torus scenario we derive a mass of ∼1.1 × 10−4. In both cases, the spiral and the torus, the NaCl structure presents an inner minimum of 27 AU. In the case of the torus, the outer radius is 73 AU. The kinematics of both the spiral and the torus suggests that they are slowly expanding and rotating. Alternative explanations for the presence of the elongation are explored. The presence of these features only in KCl and NaCl might be a result of their comparatively high dipole moment with respect to the Al-bearing species. [ABSTRACT FROM AUTHOR]

IRC +10216 is a circumstellar envelope around a carbon-rich evolved star which contains a large variety of molecules. According to interferometric observations, molecules are distributed either concentrated around the central star or as a hollow shell with a radius of ∼15″. We present ALMA Cycle 0 band 6 observations of the J = 14 – 13 rotational transition of CH3CN in IRC +10216, obtained with an angular resolution of 0.″76 × 0.″61. The bulk of the emission is distributed as a hollow shell located at just ∼2″ from the star, with a void of emission in the central region up to a radius of ∼1″. This spatial distribution is markedly different from those found to date in this source for other molecules. Our analysis indicates that methyl cyanide is not formed in either the stellar photosphere or far in the outer envelope, but at radial distances as short as 1″–2″, reaching a maximum abundance of ∼0.02 molecules cm−3 at 2″ from the star. Standard chemical models of IRC +10216 predict that the bulk of CH3CN molecules should be present at a radius of ∼15″ where other species such as polyyne radicals and cyanopolyynes are observed, with an additional inner component within 1″ from the star. The non-uniform structure of the circumstellar envelope and grain surface processes are discussed as possible causes of the peculiar distribution of methyl cyanide in IRC +10216. [ABSTRACT FROM AUTHOR]

We present the first ∼7.′5 × 11.′5 velocity-resolved (∼0.2 km s−1) map of the [C ii] 158 μm line toward the Orion molecular cloud 1 (OMC 1) taken with the Herschel/HIFI instrument. In combination with far-IR (FIR) photometric images and velocity-resolved maps of the H41α hydrogen recombination and CO J = 2–1 lines, this data set provides an unprecedented view of the intricate small-scale kinematics of the ionized/photodissociation region (PDR)/molecular gas interfaces and of the radiative feedback from massive stars. The main contribution to the [C ii] luminosity (∼85%) is from the extended, FUV-illuminated face of the cloud (G0 > 500, 5 × 103 cm−3) and from dense PDRs ( 104, 105 cm−3) at the interface between OMC 1 and the H ii region surrounding the Trapezium cluster. Around ∼15% of the [C ii] emission arises from a different gas component without a CO counterpart. The [C ii] excitation, PDR gas turbulence, line opacity (from [13C ii]), and role of the geometry of the illuminating stars with respect to the cloud are investigated. We construct maps of the L[C ii]/ and / ratios and show that L[C ii]/ decreases from the extended cloud component (∼10−2–10−3) to the more opaque star-forming cores (∼1010−4). The lowest values are reminiscent of the “[C ii] deficit” seen in local ultraluminous IR galaxies hosting vigorous star formation. Spatial correlation analysis shows that the decreasing L[C ii]/ ratio correlates better with the column density of dust through the molecular cloud than with /. We conclude that the [C ii]-emitting column relative to the total dust column along each line of sight is responsible for the observed L[C ii]/ variations through the cloud. [ABSTRACT FROM AUTHOR]

Silicon is present in interstellar dust grains, meteorites and asteroids, and to date thirteen silicon-bearing molecules have been detected in the gas-phase towards late-type stars or molecular clouds, including silane and silane derivatives. In this work, we have experimentally studied the interaction between atomic silicon and hydrogen under physical conditions mimicking those at the atmosphere of evolved stars. We have found that the chemistry of Si, H and H 2 efficiently produces silane (SiH 4 ), disilane (Si 2 H 6 ) and amorphous hydrogenated silicon (a-Si:H) grains. Silane has been definitely detected towards the carbon-rich star IRC+10216, while disilane has not been detected in space yet. Thus, based on our results, we propose that gas-phase reactions of atomic Si with H and H 2 are a plausible source of silane in C-rich AGBs, although its contribution to the total SiH 4 abundance may be low in comparison with the suggested formation route by catalytic reactions on the surface of dust grains. In addition, the produced a-Si:H dust analogs decompose into SiH 4 and Si 2 H 6 at temperatures above 500 K, suggesting an additional mechanism of formation of these species in envelopes around evolved stars. We have also found that the exposure of these dust analogs to water vapor leads to the incorporation of oxygen into Si-O-Si and Si-OH groups at the expense of SiH moieties, which implies that, if this type of grains are present in the interstellar medium, they will be probably processed into silicates through the interaction with water ices covering the surface of dust grains.

Interstellar carbonaceous dust is mainly formed in the innermost regions of circumstellar envelopes around carbon-rich asymptotic giant branch (AGB) stars. In these highly chemically stratified regions, atomic and diatomic carbon, along with acetylene are the most abundant species after H 2 and CO. In a previous study, we addressed the chemistry of carbon (C and C 2 ) with H 2 showing that acetylene and aliphatic species form efficiently in the dust formation region of carbon-rich AGBs whereas aromatics do not. Still, acetylene is known to be a key ingredient in the formation of linear polyacetylenic chains, benzene and polycyclic aromatic hydrocarbons (PAHs), as shown by previous experiments. However, these experiments have not considered the chemistry of carbon (C and C 2 ) with C 2 H 2 . In this work, by employing a sufficient amount of acetylene, we investigate its gas-phase interaction with atomic and diatomic carbon. We show that the chemistry involved produces linear polyacetylenic chains, benzene and other PAHs, which are observed with high abundances in the early evolutionary phase of planetary nebulae. More importantly, we have found a non-negligible amount of pure and hydrogenated carbon clusters as well as aromatics with aliphatic substitutions, both being a direct consequence of the addition of atomic carbon. The incorporation of alkyl substituents into aromatics can be rationalized by a mechanism involving hydrogen abstraction followed by methyl addition. All the species detected in gas phase are incorporated into the nanometric sized dust analogues, which consist of a complex mixture of sp, sp 2 and sp 3 hydrocarbons with amorphous morphology.

We aim at studying the properties of a particular type of evolved stars, the C-rich evolved stars with high expansion velocities. For this purpose we have focused on the two best studied objects within this group, IRC +10401 and AFGL 2233. We focused on determining their luminosity by studying their spectral energy distribution. Also we have obtained single-dish line profiles and interferometric maps of the CO J =1-0 and J =2-1emission lines for both objects. We have modeled this emission using a LVG radiative transfer code to determine the kinetic temperature and density profiles of the gas ejected by these stars. We have found that the luminosities obtained for these objects (log(L/ L ⊙ ) = 4.1 & 5.4) locate them in the domain of the massive Asymptotic Giant Branch stars (AGBs) and the Red Supergiant stars (RSGs). In addition, the mass-loss rates obtained (1.5 × 10 -5 - 6 × 10 -3 M ⊙ yr -1 ) suggest that while IRC+10401 might be an AGB star, AFGL 2233 could be a RSG star. All these results, together with those from previous works suggest that both objects are massive objects, IRC +10401 a massive evolved star with M init ~ 5 - 9 M ⊙ which could correspond to an AGB or a RSG and AFGL 2233 a RSG with M init ~ 20 M ⊙ , which would confirm the existence of massive C-rich evolved stars. Two scenarios are proposed to form these types of objects. The first one capable of producing high mass AGB stars up to ~ 8 M ⊙ and a second one capable of forming C-rich RSGs as AFGL 2233.

The presence of SiS in space seems to be restricted to a few selected types of astronomical environments. It is long known to be present in circumstellar envelopes around evolved stars and it has also been detected in a handful of star-forming regions with evidence of outflows, like Sgr B2, Orion KL and more recently L1157-B1. The kinetics of reactions involving SiS is very poorly known and here we revisit the chemistry of SiS in space by studying some potentially important reactions of formation and destruction of this molecule. We calculated ab initio potential energy surfaces of the SiOS system and computed rate coefficients in the temperature range 50-2500 K for the reaction of destruction of SiS, in collisions with atomic O, and of its formation, through the reaction between Si and SO. We find that both reactions are rapid, with rate coefficients of a few times 10 -10 cm 3 s -1 , almost independent of temperature. In the reaction between Si and SO, SiO production is 5-7 times more efficient than SiS formation. The reaction of SiS with O atoms can play an important role in destroying SiS in envelopes around evolved stars. We built a simple chemical model of a postshock gas to study the chemistry of SiS in protostellar outflows and we found that SiS forms with a lower abundance and later than SiO, that SiS is efficiently destroyed through reaction with O, and that the main SiS-forming reactions are Si + SO and Si + SO 2 .

We developed a new analytical experimental setup called AROMA (Astrochemistry Research of Organics with Molecular Analyzer) that combines laser desorption/ionization techniques with ion trap mass spectrometry. We report here on the ability of the apparatus to detect aromatic species in complex materials of astrophysical interests and characterize their structures. A limit of detection of 100 femto-grams has been achieved using pure polycyclic aromatic hydrocarbon (PAH) samples, which corresponds to 2x10 8 molecules in the case of coronene (C 24 H 12 ). We detected the PAH distribution in the Murchison meteorite, which is made of a complex mixture of extraterrestrial organic compounds. In addition, collision induced dissociation experiments were performed on selected species detected in Murchison, which led to the first firm identification of pyrene and its methylated derivatives in this sample.

The impact of the photodissociation of HCN and HNC isomers is analyzed in different astrophysical environments. For this purpose, the individual photodissociation cross section of HCN and HNC isomers have been calculated in the 7-13.6 eV photon energy range for a temperature of 10 K. These calculations are based on the ab initio calculation of three-dimensional adiabatic potential energy surfaces of the 21 lower electronic states. The cross sections are then obtained using a quantum wave packet calculation of the rotational transitions needed to simulate a rotational temperature of 10 K. The cross section calculated for HCN shows significant differences with respect to the experimental one, and this is attributed to the need of considering non-adiabatic transitions. Ratios between the photodissociation rates of HCN and HNC under different ultraviolet radiation fields have been computed by renormalizing the rates to the experimental one. It is found that HNC is photodissociated faster than HCN by a factor of 2.2, for the local interstellar radiation field, and 9.2, for the solar radiation field at 1 au. We conclude that to properly describe the HNC/HCN abundance ratio in astronomical environments illuminated by an intense ultraviolet radiation field it is necessary to use different photodissociation rates for each of the two isomers, obtained by integrating the product of the photodissociation cross sections and ultraviolet radiation field over the relevant wavelength range.

We report laboratory spectroscopy for the first time of the J = 1-0 and J = 2-1 lines of Na 35 Cl and Na 37 Cl in several vibrational states. The hyperfine structure has been resolved in both transitions for all vibrational levels, which permit us to predict with high accuracy the hyperfine splitting of the rotational transitions of the two isotopologues at higher frequencies. The new data have been merged with all previous works at microwave, millimeter, and infrared wavelengths and fitted to a series of mass-independent Dunham parameters and to a potential energy function. The obtained parameters have been used to compute a new dipole moment function, from which the dipole moment for infrared transitions up to Δ v = 8 has been derived. Frequency and intensity predictions are provided for all rovibrational transitions up to J = 150 and v = 8, from which the ALMA data of evolved stars can be modeled and interpreted.

We report on the detection of nine lines of the nu2 bending mode of triatomic carbon, C3, in the direction of Sagittarius B2. The R(4) and R(2) lines of C3 have been also detected in the carbon-rich star IRC +10216. The abundances of C3 in the direction of Sgr B2 and IRC +10216 are approximately 3x10-8 and approximately 10-6, respectively. In Sgr B2 we have also detected the 23-12 line of NH with an abundance of a few times 10-9. Polyatomic molecules will have a weak contribution from their pure rotational spectrum to the emission/absorption in the far-infrared. We suggest, however, that they could be, through their low-lying vibrational bending modes, the dominant carriers of emission/absorption in the spectrum of bright far-infrared sources.

We report on sensitive far-infrared observations of 12CO pure rotational transitions in the OMC-1 core of Orion. The lines were observed with the long-wavelength spectrometer in the grating mode on board the Infrared Space Observatory, covering the 43-197 µm wavelength range. The transitions from Jup=14 up to Jup=19 have been identified across the whole OMC-1 core, and lines up to Jup=43 have been detected toward the central region, KL/IRc2. In addition, we have taken high-quality spectra in the Fabry-Perot mode of some of the CO lines. In KL/IRc2, the lines are satisfactorily accounted for by a three-temperature model describing the plateau and ridge emission. The fluxes detected in the high-J transitions (Jup>34) reveal the presence of a very hot and dense gas component [T=1500-2500 K; N&parl0;CO&parr0;=2x1017 cm-2], probably originating from some of the embedded sources previously observed in the H2 near-infrared lines. At all other positions in the OMC-1 core, we estimate kinetic temperatures >/=80 K and as high as 150 K at some positions around IRc2, from a simple large-velocity gradient model.

We present Infrared Space Observatory long-wavelength spectrometer observations of the proto-planetary nebula CRL 618, a star evolving very fast toward the planetary nebula stage. In addition to the lines of 12CO, 13CO, HCN, and HNC, we report on the detection of H2O and OH emission together with the fine-structure lines of [O i] at 63 and 145 µm. The abundance of the latter three species relative to 12CO are 4x10-2, 8x10-4, and 4.5 (approximate value) in the regions where they are produced. We suggest that O-bearing species other than CO are produced in the innermost region of the circumstellar envelope. The UV photons from the central star photodissociate most of the molecular species produced in the asymptotic giant branch (AGB) phase and allow a chemistry dominated by standard ion-neutral reactions. They not only allow these reactions for the formation of O-bearing species but also modify the abundances of C-rich molecules like HCN and HNC for which we found an abundance ratio of approximately 1, which is much lower than in AGB stars. The molecular abundances in the different regions of the circumstellar envelope have been derived from radiative transfer models and from our knowledge of its physical structure.

We present the Infrared Space Observatory/Short-Wavelength Spectrometer full grating resolution spectrum of IRC +10216, which is dominated by strong absorption/emission bands of C2H2 and HCN. All C2H2 bands and the strong near-infrared stretching bands of HCN are observed in absorption, whereas the fundamental, hot, and combination bands of HCN involving the nu2 bending mode around 14 µm are observed in emission. Particularly strong is the HCN nu2=20-->nu2=11 vibrational transition at 14.3 µm. The most plausible mechanism for such emission is the radiative pumping of molecules from the ground to the nu2=20 state (7.1 µm) followed by radiative decay: nu2=20-->nu2=11. We present detailed models for HCN that verify the efficiency of the mentioned effect. The HCN abundance inferred from these models is &parl0;1.5-3&parr0;x10-5.