N-bearing Species Research Articles

A λ 3 mm and 1 mm line survey toward the yellow hypergiant IRC +10420: N-rich chemistry and IR flux variations.

Our knowledge of the chemical properties of the circumstellar ejecta of the most massive evolved stars is particularly poor. We aim to study the chemical characteristics of the prototypical yellow hypergiant star, IRC +10420. For this purpose, we obtained full line surveys at 1 and 3 mm atmospheric windows. We have identified 106 molecular emission lines from 22 molecular species. Approximately half of the molecules detected are N-bearing species, in particular HCN, HNC, CN, NO, NS, PN, and N2H+. We used rotational diagrams to derive the density and rotational temperature of the different molecular species detected. We introduced an iterative method that allows us to take moderate line opacities into account. We have found that IRC +10420 presents high abundances of the N-bearing molecules compared with O-rich evolved stars. This result supports the presence of a N-rich chemistry, expected for massive stars. Our analysis also suggests a decrease of the 12C/13C ratio from ≳ 7 to ~ 3.7 in the last 3800 years, which can be directly related to the nitrogen enrichment observed. In addition, we found that SiO emission presents a significant intensity decrease for high-J lines when compared with older observations. Radiative transfer modeling shows that this variation can be explained by a decrease in the infrared (IR) flux of the dust. The origin of this decrease might be an expansion of the dust shell or a lower stellar temperature due to the pulsation of the star.

Are infrared dark clouds really quiescent?

The dense, cold regions where high-mass stars form are poorly characterised, yet they represent an ideal opportunity to learn more about the initial conditions of high-mass star formation (HMSF), since high-mass starless cores (HMSCs) lack the violent feedback seen at later evolutionary stages. We present continuum maps obtained from the Submillimeter Array (SMA) interferometry at 1.1 mm for four infrared dark clouds (IRDCs, G28.34S, IRDC 18530, IRDC 18306, and IRDC 18308). We also present 1 mm/3 mm line surveys using IRAM 30 m single-dish observations. Our results are: (1) At a spatial resolution of 10^4 AU, the 1.1 mm SMA observations resolve each source into several fragments. The mass of each fragment is on average >10 Msun, which exceeds the predicted thermal Jeans mass of the whole clump by a factor of up to 30, indicating that thermal pressure does not dominate the fragmentation process. Our measured velocity dispersions in the 30 m lines imply that non-thermal motions provides the extra support against gravity in the fragments. (2) Both non-detection of high-J transitions and the hyperfine multiplet fit of N2H+(1-0), C2H(1-0), HCN(1-0), and H13CN(1-0) indicate that our sources are cold and young. However, obvious detection of SiO and the asymmetric line profile of HCO+(1-0) in G28.34S indicate a potential protostellar object and probable infall motion. (3) With a large number of N-bearing species, the existence of carbon rings and molecular ions, and the anti-correlated spatial distributions between N2H+/NH2D and CO, our large-scale high-mass clumps exhibit similar chemical features as small-scale low-mass prestellar objects. This study of a small sample of IRDCs illustrates that thermal Jeans instability alone cannot explain the fragmentation of the clump into cold (~15 K), dense (>10^5 cm-3) cores and that these IRDCs are not completely quiescent.

Resolving the chemical substructure of Orion-KL(Corrigendum)

The Kleinmann-Low nebula in Orion (Orion-KL) is the nearest example of a high-mass star-forming environment. For the first time, we complemented 1.3 mm Submillimeter Array (SMA) interferometric line survey with IRAM 30 m single-dish observations of the Orion-KL region. Covering a 4 GHz bandwidth in total, this survey contains over 160 emission lines from 20 species (25 isotopologues), including 11 complex organic molecules (COMs). At a spatial resolution of 1200 AU, the continuum substructures are resolved. Extracting the spectra from individual substructures and providing the intensity-integrated distribution map for each species, we studied the small-scale chemical variations in this region. Our main results are: (1) We identify lines from the low-abundance COMs CH3COCH3 and CH3CH2OH, as well as tentatively detect CH3CHO and long carbon-chains C6H and HC7N. (2) We find that while most COMs are segregated by type, peaking either towards the hot core (e.g., N-bearing species) or the compact ridge (e.g., O-bearing species like HCOOCH3 and CH3OCH3), while the distributions of others do not follow this segregated structure (e.g., CH3CH2OH, CH3OH, CH3COCH3). (3) We find a second velocity component of HNCO, SO2, 34SO2, and SO lines, which may be associated with a strong shock event in the low-velocity outflow. (4) Temperatures and molecular abundances show large gradients between central condensations and the outflow regions, illustrating a transition between hot molecular core and shock-chemistry dominated regimes. Our observations of spatially resolved chemical variations in Orion-KL provide the nearest reference source for hot molecular core and outflow chemistry, which will be an important example for interpreting the chemistry of more distant HMSFRs.

Gas and grain chemical composition in cold cores as predicted by the Nautilus three-phase model

We present an extended version of the two-phase gas–grain code nautilus to the three-phase modelling of gas and grain chemistry of cold cores. In this model, both the mantle and the surface are considered as chemically active. We also take into account the competition among reaction, diffusion and evaporation. The model predictions are confronted to ice observations in the envelope of low-mass and massive young stellar objects as well as towards background stars. Modelled gas-phase abundances are compared to species observed towards TMC-1 (CP) and L134N dark clouds. We find that our model successfully reproduces the observed ice species. It is found that the reaction–diffusion competition strongly enhances reactions with barriers and more specifically reactions with H2, which is abundant on grains. This finding highlights the importance having a good approach to determine the abundance of H2 on grains. Consequently, it is found that the major N-bearing species on grains go from NH3 to N2 and HCN when the reaction–diffusion competition is taken into account. In the gas phase and before a few 105 yr, we find that the three-phase model does not have a strong impact on the observed species compared to the two-phase model. After this time, the computed abundances dramatically decrease due to the strong accretion on dust, which is not counterbalanced by the desorption less efficient than in the two-phase model. This strongly constrains the chemical age of cold cores to be of the order of few 105 yr.

CONSTRAINING THE ABUNDANCES OF COMPLEX ORGANICS IN THE INNER REGIONS OF SOLAR-TYPE PROTOSTARS

The high abundances of Complex Organic Molecules (COMs) with respect to methanol, the most abundant COM, detected towards low-mass protostars, tend to be underpredicted by astrochemical models. This discrepancy might come from the large beam of the single-dish telescopes, encompassing several components of the studied protostar, commonly used to detect COMs. To address this issue, we have carried out multi-line observations of methanol and several COMs towards the two low-mass protostars NGC1333-IRAS2A and -IRAS4A with the Plateau de Bure interferometer at an angular resolution of 2 arcsec, resulting in the first multi-line detection of the O-bearing species glycolaldehyde and ethanol and of the N-bearing species ethyl cyanide towards low-mass protostars other than IRAS 16293. The high number of detected transitions from COMs (more than 40 methanol transitions for instance) allowed us to accurately derive the source size of their emission and the COMs column densities. The COMs abundances with respect to methanol derived towards IRAS2A and IRAS4A are slightly, but not substantitally, lower than those derived from previous single-dish observations. The COMs abundance ratios do not vary significantly with the protostellar luminosity, over five orders of magnitude, implying that low-mass hot corinos are quite chemically rich as high-mass hot cores. Astrochemical models still underpredict the abundances of key COMs, such as methyl formate or di-methyl ether, suggesting that our understanding of their formation remains incomplete.

New N-bearing species towards OH 231.8+4.2

OH231.8+4.2 is a well studied O-rich CSE around an intermediate-mass evolved star that displays bipolar molecular outflows accelerated up to 400 km/s. OH231 also presents an exceptional molecular richness probably due to shock-induced chemical processes. We report the first detection in this source of HNCO, HNCS, HC3N and NO, with the IRAM-30m telescope in a sensitive mm-wavelength survey towards this target. HNCO and HNCS are also first detections in CSEs. The observed line profiles show that the emission arises in the massive central component of the envelope and at the base of the fast lobes. The NO profiles are broader than those of HNCO, HNCS, and HC3N, and most importantly, broader than the line profiles of 13CO. This indicates that the NO abundance is enhanced in the fast lobes relative to the slow, central parts. From LTE and non-LTE excitation analysis, we estimate beam-average rotational temperatures of 15-30 K (and, maybe, up to 55 K for HC3N) and fractional abundances of X(HNCO)=[0.8-1]E-7, X(HNCS)=[0.9-1]E-8, X(HC3N)=[5-7]E-9 and X(NO)=[1-2]E-6. NO is, therefore, amongst the most abundant N-bearing species in OH231. We have performed thermodynamical chemical equilibrium and chemical kinetics models to investigate the formation of these N-bearing species in OH231. The model underestimates the observed abundances for HNCO, HNCS, and HC3N by several orders of magnitude, which indicates that these molecules can hardly be products of standard UV-photon and/or cosmic-ray induced chemistry in OH231, and that other processes (e.g. shocks) play a major role in their formation. For NO, the model abundance is compatible with the observed average value. The new detections presented in this work corroborate the particularly rich chemistry of OH231, which is likely profoundly influenced by shock-induced processes, as proposed in earlier works.

Photodissociation and chemistry of N2in the circumstellar envelope of carbon-rich AGB stars

The envelopes of AGB stars are irradiated externally by ultraviolet photons; hence, the chemistry is sensitive to the photodissociation of N$_2$ and CO, which are major reservoirs of nitrogen and carbon, respectively. The photodissociation of N$_2$ has recently been quantified by laboratory and theoretical studies. Improvements have also been made for CO photodissociation. For the first time, we use accurate N$_2$ and CO photodissociation rates and shielding functions in a model of the circumstellar envelope of the carbon-rich AGB star, IRC +10216. We use a state-of-the-art chemical model of an AGB envelope, the latest CO and N$_2$ photodissociation data, and a new method for implementing molecular shielding functions in full spherical geometry with isotropic incident radiation. We compare computed column densities and radial distributions of molecules with observations. The transition of N$_2$ $\to$ N (also, CO $\to$ C $\to$ C$^+$) is shifted towards the outer envelope relative to previous models. This leads to different column densities and radial distributions of N-bearing species, especially those species whose formation/destruction processes largely depend on the availability of atomic or molecular nitrogen, for example, C$_n$N ($n$=1, 3, 5), C$_n$N$^-$ ($n$=1, 3, 5), HC$_n$N ($n$=1, 3, 5, 7, 9), H$_2$CN and CH$_2$CN. The chemistry of many species is directly or indirectly affected by the photodissociation of N$_2$ and CO, especially in the outer shell of AGB stars where photodissociation is important. Thus, it is important to include N$_2$ and CO shielding in astrochemical models of AGB envelopes and other irradiated environments. In general, while differences remain between our model of IRC +10216 and the observed molecular column densities, better agreement is found between the calculated and observed radii of peak abundance.

Speciation of and D/H partitioning between fluids and melts in silicate-D-O-H-C-N systems determined in-situ at upper mantle temperatures, pressures, and redox conditions

Speciation of D-O-H-C-N volatiles in alkali aluminosilicate melts and of silicate in D-O-H-C-N fluid has been determined in situ to 800 °C and >2 GPa under reducing and oxidizing conditions by using an externally heated hydrothermal diamond-anvil cell with Raman spectroscopy as the structural probe. Reducing conditions were near those of the IW oxygen buffer, whereas oxidizing conditions were obtained by conducting the experiments with oxidized components only and with Pt as a catalyst. Raman bands assigned to C-H stretching in CH x D y isotopologues and CH 4 groups (including CH 3 ) were employed to determine the CH 4 /CH x D y ratio in fluids and melts. This ratio decreases from 1.5-2 at 500 °C to between 1.2 and 1 with 800 °C with Δ H -values of 13.6 ± 2.1 and 5.5 ± 1.1 kJ/mol for melt and fluid, respectively. The CH 4 /CH x D y fluid/melt partition coefficient ranges between ~ 16 and ~ 3 with Δ H = 33 ± 6 kJ/mol assuming no pressure effect. This behavior of deuterated and protonated complexes is ascribed to speciation of volatile and silicate components in fluids and melts in a manner that is conceptually similar to D/H partitioning among complexes and phases in brines and hydrous silicate systems. Molecular N 2 is the N-bearing species in fluids and melts under oxidizing conditions. Under reducing conditions, the dominant species are molecular NH 3 and ammine groups, NH 2 - . The NH 3 / NH 2 ratio varies between 0.15 and 0.75 in the 425-800 °C temperature range. The enthalpy change of the ammonia/ammine equilibrium, Δ H , derived from the temperature and assuming no pressure effect on the equilibrium, is 19 ± 8 and 61 ± 9 kJ/mol for melt and fluid, respectively. The fluid/melt partition coefficient, (NH 3 + NH 2 - ) fluid /(NH 3 + NH 2 - ) melt , ranges from 8 to 3 with Δ H = 45 ± 12 kJ/mol. For oxidized nitrogen, the fluid/melt partition coefficient is twice or more of those values for reduced nitrogen. Hydrogen bonding can be detected at 500 °C and below. This behavior resembles that of H 2 O. Deuterium-containing analogues of the (N+H)-species could not be detected with precision because these were in the frequency-range of the second-order Raman shift of diamond in the diamond-anvil cell itself and could not be isolated from the strong background generated by the Raman intensity from the diamond. These results imply that, unlike noble gases, degassing of N-bearing species from the mantle is redox dependent, and is also more efficient at lower temperatures (shallow depths). Reduced and oxidized C-O-H-N species exist fluids and melts in the modern mantle, whereas reduced species dominated in the young Earth. The f H2 -dependent speciation C-O-H-N volatile components result in f H2 -dependent thermodynamic and transport properties of fluids and melts in the interior of the Earth and terrestrial planets. In fluids, the solubility of nominally incompatible trace elements can increase by orders of magnitude upon its saturation with silicate components. Trace element and stable isotope partitioning between fluids and melt can change by >100% for the same reason.

Isotope selective photodissociation of N2by the interstellar radiation field and cosmic rays

Photodissociation of 14N2 and 14N15N occurs in interstellar clouds, circumstellar envelopes, protoplanetary discs, and other environments due to UV radiation from stellar sources and the presence of cosmic rays. This source of N atoms initiates the formation of complex N-bearing species and influences their isotopic composition. To study the photodissociation rates of 14N15N by UV continuum radiation and both isotopologues in a field of cosmic ray induced photons. To determine the effect of these on the isotopic composition of more complex molecules. High-resolution photodissociation cross sections of N2 are used from an accurate and comprehensive quantum- mechanical model of the molecule based on laboratory experiments. A similarly high-resolution spectrum of H2 emission following interactions with cosmic rays has been constructed. The spectroscopic data are used to calculate dissociation rates which are input into isotopically differentiated chemical models, describing an interstellar cloud and a protoplanetary disc. The dissociation rate of 14N15N in a Draine field assuming 30K excitation is 1.73x10-10s-1 and the rate due to cosmic rays assuming an H2 ionisation rate of 10-16s-1 is about 10-15s-1, with up to a factor of 10 difference between isotopologues. Shielding functions for 14N15N by 14N2, H2, and H are presented. Incorporating these into an interstellar cloud model, an enhancement of the atomic 15N/14N ratio is obtained due to the self-shielding of external radiation at an extinction of about 1.5 mag. This effect is larger where grain growth has reduced the opacity of dust to ultraviolet radiation. The transfer of photolytic isotopic fractionation N2 to other molecules is significant in a disc model, and is species dependent with 15N enhancement approaching a factor of 10 for HCN.

Simultaneous UV- and ion processing of astrophysically relevant ices

Context. Interstellar ices are known to be simultaneously processed by both cosmic-ray bombardment and UV photolysis. Our knowledge of the effects of energetic processing on relevant icy samples is mainly based on laboratory investigations. In the past 35 years many experiments have been performed to study these effects separately but, to the best of our knowledge, never simultaneously. Aims. The aim of this work is to study the effects of simultaneous processing of ices by both cosmic rays and UV photons to investigate to what extent the combined effect of ion bombardment and UV photolysis influences the chemical pathways. Methods. We carried out the simultaneous processing of CH3OH:N2 ice held at 16 K by 200 keV H + ions and Lyman-alpha 10.2 eV UV photons. The samples were analyzed by in situ transmission infrared spectroscopy. The un-combined processes of UV irradiation and bombardment by H + ions of CH3OH:N2 ice were also studied. This mixture was chosen because the effects of ion bombardment and UV photolysis on methanol and nitrogen have been extensively studied in previous investigations. This mixture enables one to investigate whether simultaneous processing (a) influences the destruction of original species; (b) influences the formation of new species; or (c) causes synergistic effects since Lyman-alpha photons have a very low efficiency in breaking the dinitrogen bond because N2 is almost transparent at Lyman-alpha wavelengths. Results. After processing a CH3OH:N2 sample, the intensity of the methanol bands was observed to decrease at the same rate in all cases. After ion bombardment, species such as CO2 ,C O, H2CO, CH4 ,N 2O, HNCO, and OCN − are formed in the ice mixture. After UV photolysis, species such as CO2 ,C O, H2CO, and CH4 are formed, but no N-bearing species are detected. Spectra of ices processed by both UV photons and ions were compared with spectra of ices bombarded only by ions. We find that there are no differences in the band area and profile of N-bearing species for the two types of experiment at the same ion fluence; therefore, the addition of UV irradiation to ion bombardment does not affect the abundance of N-bearing species. The initial formation rate of CH4, within the experimental uncertainties, is the same in all cases studied, while the saturation value of CH4 is higher for UV photolysis than for ion bombardment when they act separately. In the case of simultaneous processing, when the dose (eV/16u) given by UV photons is similar to the dose given during ion bombardment, the saturation value of CH4 reaches a value intermediate between the value obtained after UV photolysis and ion bombardment separately. Conclusions. Our results confirm that when UV photolysis and ion bombardment act separately, their effects are very similar from a qualitative point of view, while significant quantitative difference may exist. In the case of simultaneous processing we did not detect any synergistic effect, but in some instances the behavior of newly formed species (such as CH4) can significantly depend on the UV/ions dose ratio.

Acetone in Orion BN/KL

As one of the prime targets of interstellar chemistry study, Orion BN/KL clearly shows different molecular distributions between large nitrogen- (e.g., C2H5CN) and oxygen-bearing (e.g., HCOOCH3) molecules. However, acetone (CH3)2CO, a special complex O-bearing molecule, has been shown to have a very different distribution from other typical O-bearing molecules in the BN/KL region. We searched for acetone within our IRAM Plateau de Bure Interferometer 3 mm and 1.3 mm data sets. Twenty-two acetone lines were searched within these data sets. The angular resolution ranged from 1.8 X 0.8 to 6.0 X 2.3 arcsec^2, and the spectral resolution ranged from 0.4 to 1.9 km s-1. Nine of the acetone lines appear free of contamination. Three main acetone peaks (Ace-1, 2, and 3) are identified in Orion BN/KL. The new acetone source Ace-3 and the extended emission in the north of the hot core region have been found for the first time. An excitation temperature of about 150 K is determined toward Ace-1 and Ace-2, and the acetone column density is estimated to be 2-4 X 10^16 cm-2 with a relative abundance of 1-6 X 10^-8 toward these two peaks. Acetone is a few times less abundant toward the hot core and Ace-3 compared with Ace-1 and Ace-2. We find that the overall distribution of acetone in BN/KL is similar to that of N-bearing molecules, e.g., NH3 and C2H5CN, and very different from those of large O-bearing molecules, e.g., HCOOCH3 and (CH3)2O. Our findings show the acetone distribution is more extended than in previous studies and does not originate only in those areas where both N-bearing and O-bearing species are present. Moreover, because the N-bearing molecules may be associated with shocked gas in Orion BN/KL, this suggests that the formation and/or destruction of acetone may involve ammonia or large N-bearing molecules in a shocked-gas environment.

PHOSPHORUS CHEMISTRY IN THE SHOCKED REGION L1157 B1

We study the evolution of phosphorous-bearing species in one-dimensional C-shock models. We find that the abundances of P-bearing species depend sensitively on the elemental abundance of P in the gas phase and on the abundance of N atoms in the pre-shock gas. The observed abundance of PN and the non-detection of PO towards L1157 B1 are reproduced in C-shock models with shock velocity v=20km s-1 and pre-shock density n(H2) =10^4 - 10^5, if the elemental abundance of P in the gas phase is 10^-9 and the N-atom abundance is n(N)/nH - 10^-5 in the pre-shock gas. We also find that P-chemistry is sensitive to O- and N-chemistry, because N atoms are destroyed mainly by OH and NO. We identify the reactions of O-bearing and N-bearing species that significantly affect P chemistry.

Structure–property relationships of COHN-saturated silicate melt coexisting with COHN fluid: A review of in-situ, high-temperature, high-pressure experiments

The C–O–H–N solubility and solution mechanisms in silicate melts and C–O–H–N speciation in coexisting fluid to upper mantle temperatures and pressures and with redox conditions from the MH to the IW buffer are discussed. Focus is on in-situ structural characterization of coexisting melt and fluid. In fluid+melt-COH, fluid+melt-NOH, and fluid+melt-OH systems, volatiles are dissolved in molecular form (CO2, CH4, NH3, N2, H2O, H2) and as complexes that form chemical bonding with the silicate network (CO3, CH3, NH2, OH).In silicate-OH systems molecular H2O (H2O˚) and OH-groups exist in silicate- and aluminosilicate-saturated fluids and coexisting water-saturated melts above ~400°C and ~0.5GPa with their OH/H2O˚-ratio positively correlated with temperature. The extent of hydrogen bonding in both fluids and melts diminishes with temperature so that above ~400°C it cannot be detected. The ∆H of hydrogen bonding in aqueous fluid (22±1kJ/mol) is about twice that in silicate melts (10±2kJ/mol). Silicate speciation in silicate-saturated fluid and hydrous silicate melts comprises similar Q-species with ∆H of the solution reactions in silicate-saturated fluid, water-saturated melt, and supercritical fluid ~400kJ/mol.In COH-silicate systems methane solubility in melt increases from 0.2wt.% to ~0.5wt.% in the melt NBO/Si range from 0.4 to 1.0 at 1–2.5GPa and 1400°C. The solubility increases by ~150% between the redox conditions of the IW and MH buffers. At the NNO buffer conditions and more oxidizing, carbon exists as carbonate complexes in melts and as CO2 in fluid. Reduced (C+H)-bearing species in melts (CH3-groups and molecular CH4) are stable at fH2(MW) and more reducing conditions, whereas the species in coexisting fluid are CH4, H2, and H2O.In NOH-silicate systems, the N solubility in melt decreases from 0.98 to 0.28wt.% in the melt NBO/Si-range from 0.4 to 1.18 at the redox conditions of the IW buffer. The solubility decreases by about 50% between the redox conditions of the IW and MH buffers. At IW, nitrogen occurs in silicate melts amine groups, NH2, bonded to the silicate network, and as molecular NH3, whereas in coexisting NOH fluids the dominant species are NH3, N2, H2 and H2O. The NH2−/NH3 abundance ratio varies by ~55 between melt compositions with NBO/Si=1.18 and 0.4. In fluids and melts, decreasing hydrogen fugacity leads to oxidation of nitrogen to form molecular N2 so that at the MH redox conditions, the dominant N-bearing species is N2.The redox-dependent solution mechanisms of COHN volatile components in silicate melts affect their structure differently, which results in redox-dependent thermodynamic and transport properties of magmatic liquids in the interior of the Earth and terrestrial planets. These properties include mineral/melt minor and trace element partitioning, melt/fluid isotope fractionation, and transport and thermodynamic properties of melt saturated with variably-oxidized COHN volatile components.

A chemical model for the atmosphere of hot Jupiters

Our purpose is to release a chemical network, and the associated rate coefficients, developed for the temperature and pressure range relevant to hot Jupiters atmospheres. Using this network, we study the vertical atmospheric composition of the two hot Jupiters (HD209458b, HD189733b) with a model that includes photolyses and vertical mixing and we produce synthetic spectra. The chemical scheme is derived from applied combustion models that have been methodically validated over a range of temperatures and pressures typical of the atmospheric layers influencing the observations of hot Jupiters. We compare the predictions obtained from this scheme with equilibrium calculations, with different schemes available in the literature that contain N-bearing species and with previously published photochemical models. Compared to other chemical schemes that were not subjected to the same systematic validation, we find significant differences whenever non-equilibrium processes take place. The deviations from the equilibrium, and thus the sensitivity to the network, are more important for HD189733b, as we assume a cooler atmosphere than for HD209458b. We found that the abundances of NH3 and HCN can vary by two orders of magnitude depending on the network, demonstrating the importance of comprehensive experimental validation. A spectral feature of NH3 at 10.5$\mu$m is sensitive to these abundance variations and thus to the chemical scheme. Due to the influence of the kinetics, we recommend the use of a validated scheme to model the chemistry of exoplanet atmospheres. Our network is robust for temperatures within 300-2500K and pressures from 10mbar up to a few hundreds of bars, for species made of C,H,O,N. It is validated for species up to 2 carbon atoms and for the main nitrogen species.

Nitrogen oxides and carbon chain oxides formed after ion irradiation of CO:N2ice mixtures

Context. High CO depletion as well as depletion of N-bearing species is observed in dense pre-stellar cores. It is generally accepted that depleted species freeze out onto dust grains to form icy mantles and that these ices suffer energetic processing due to cosmic ion irradiation and ion-induced UV photons. Aims. The aim of this work is to study the chemical and structural effects induced by ion irradiation on different CO:N2 mixtures at low temperature (16 K) to simulate the effects of cosmic ion irradiation of icy mantles.Methods. Different CO:N2 mixtures and pure CO and pure N2 were irradiated with 200 keV H+ at 16 K. Infrared transmittance spectra of the samples were obtained in situ before and after irradiation. The samples were warmed up and spectra were taken at different temperatures. The residues left over on the substrate at room temperature were analysed ex situ by micro Raman spectroscopy. Results. Several new absorption features are present in the infrared spectra after irradiation, indicating that new species are formed. The most abundant are nitrogen oxides (such as NO, NO2 and N2 O), carbon chain oxides (such as C2 O, C3 O and C3 O2 ), carbon chains (such as C3 and C6 ), O3 and N3 . A refractory residue is also formed after ion irradiation and is clearly detected by Raman spectroscopy.Conclusions. We suggest that carbon chains and nitrogen oxides observed in the gas phase towards star-forming regions are formed in the solid phase after cosmic ion irradiation of icy grain mantles and are released into the gas phase after desorption of grain mantles. We expect that the Atacama Large Millimeter/submillimeter Array (ALMA), thanks to its high sensitivity and resolution, will increase the number of nitrogen oxides and carbon chain oxides detected towards star-forming regions.

Nitrogen hydrides and the H2ortho-to-para ratio in dark clouds

Nitrogen bearing species are common tracers of the physical conditions in a wide variety of objects, and most remarkably in dark clouds. The reservoir of gaseous nitrogen is expected to be atomic or molecular, but none of the two species are observable in the dark gas. Their abundances therefore derive indirectly from those of N-bearing species through chemical modelling. The recent years have accumulated data which stress our incomplete understanding of the nitrogen chemistry in dark cloud conditions. To tackle this problem of the nitrogen chemistry in cold gas, we have revised the formation of nitrogen hydrides, which is initiated by the key reaction \ce{N+ + H2 -> NH+ + H}. We propose a new rate for this reaction which depends on the ortho-to-para ratio of H2. This new rate allows to reproduce the abundance ratios of the three nitrogen hydrides, NH, \ce{NH2}, and \ce{NH3}, observed towards IRAS16293-2422, provided that the channel leading to NH from the dissociative recombination of \ce{N2H+} is not closed at low temperature. The ortho-to-para ratio of H2 is constrained to O/P=$10^{-3}$ by the abundance ratio NH:NH2, which provides a new method to measure O/P. This work stresses the need for reaction rates at the low temperatures of dark clouds, and for branching ratios of critical dissociative recombination reactions.

Nitrogen hydrides in the cold envelope of IRAS 16293-2422

Nitrogen is the fifth most abundant element in the Universe, yet the gas-phase chemistry of N-bearing species remains poorly understood. Nitrogen hydrides are key molecules of nitrogen chemistry. Their abundance ratios place strong constraints on the production pathways and reaction rates of nitrogen-bearing molecules. We observed the class 0 protostar IRAS 16293-2422 with the heterodyne instrument HIFI, covering most of the frequency range from 0.48 to 1.78 THz at high spectral resolution. The hyperfine structure of the amidogen radical o-NH<sub>2<sub/> is resolved and seen in absorption against the continuum of the protostar. Several transitions of ammonia from 1.2 to 1.8 THz are also seen in absorption. These lines trace the low-density envelope of the protostar. Column densities and abundances are estimated for each hydride. We find that NH:NH<sub>2<sub/>:NH<sub>3<sub/> <i>≈<i/> 5:1:300. Dark clouds chemical models predict steady-state abundances of NH<sub>2<sub/> and NH<sub>3<sub/> in reasonable agreement with the present observations, whilst that of NH is underpredicted by more than one order of magnitude, even using updated kinetic rates. Additional modelling of the nitrogen gas-phase chemistry in dark-cloud conditions is necessary before having recourse to heterogen processes.

THE c2dSPITZERSPECTROSCOPIC SURVEY OF ICES AROUND LOW-MASS YOUNG STELLAR OBJECTS. IV. NH3AND CH3OH

NH3 and CH3OH are key molecules in astrochemical networks leading to the formation of more complex N- and O-bearing molecules, such as CH3CN and HCOOCH3. Despite a number of recent studies, little is known about their abundances in the solid state. (...) In this work, we investigate the ~ 8-10 micron region in the Spitzer IRS (InfraRed Spectrograph) spectra of 41 low-mass young stellar objects (YSOs). These data are part of a survey of interstellar ices in a sample of low-mass YSOs studied in earlier papers in this series. We used both an empirical and a local continuum method to correct for the contribution from the 10 micron silicate absorption in the recorded spectra. In addition, we conducted a systematic laboratory study of NH3- and CH3OH-containing ices to help interpret the astronomical spectra. We clearly detect a feature at ~9 micron in 24 low-mass YSOs. Within the uncertainty in continuum determination, we identify this feature with the NH3 nu_2 umbrella mode, and derive abundances with respect to water between ~2 and 15%. Simultaneously, we also revisited the case of CH3OH ice by studying the nu_4 C-O stretch mode of this molecule at ~9.7 micron in 16 objects, yielding abundances consistent with those derived by Boogert et al. 2008 (hereafter paper I) based on a simultaneous 9.75 and 3.53 micron data analysis. Our study indicates that NH3 is present primarily in H2O-rich ices, but that in some cases, such ices are insufficient to explain the observed narrow FWHM. The laboratory data point to CH3OH being in an almost pure methanol ice, or mixed mainly with CO or CO2, consistent with its formation through hydrogenation on grains. Finally, we use our derived NH3 abundances in combination with previously published abundances of other solid N-bearing species to find that up to 10-20 % of nitrogen is locked up in known ices.

Chemistry in low-mass star forming regions

We review molecular evolution in low-mass star-forming regions and discuss what we can observe with ALMA. Recent observations have revealed chemical fractionation, i.e. spatial variation of molecular abundances, in dense prestellar cores. In the central regions of cold prestellar cores, CO is heavily depleted, while the depletion of N-bearing species are rare. Models show that CO is frozen onto grains, while N-bearing species survive because of the CO depletion and slow formation of N2 in the gas phase. CO depletion also enhances the molecular D/H ratio. Chemical fractionation and its variation among cores can be an indicator of evolutionary stage and/or accumulation process of cores.

Infrared and Raman spectroscopies of refractory residues left over after ion irradiation of nitrogen-bearing icy mixtures

Using infrared and Raman spectroscopies, we have studied the effects induced by ion irradiation on icy mixtures at low temperature ( T=12 K) and after warm up to room temperature. In particular, we have considered mixtures made of H 2O, CO, CH 4, and N 2. These mixtures have been irradiated with 30 keV He + and 60 keV Ar 2+ ions. After ion irradiation at low temperature, several new absorption features appear in the infrared spectra, some of which may be due to N-bearing molecular species. A refractory organic residue is left over after warm-up to room temperature. After further irradiation of the residue at room temperature, the intensity of all infrared absorption features decreases. Raman spectroscopy of similar mixtures has shown that ion irradiation causes a modification of the structure of the samples which evolve towards an amorphous carbon.