A KINETIC DATABASE FOR ASTROCHEMISTRY (KIDA) Wakelam, V; Herbst, E; Loison, J-C ...
The Astrophysical journal. Supplement series,
03/2012, Letnik:
199, Številka:
1
Journal Article
Recenzirano
Odprti dostop
We present a novel chemical database for gas-phase astrochemistry. Named the KInetic Database for Astrochemistry (KIDA), this database consists of gas-phase reactions with rate coefficients and ...uncertainties that will be vetted to the greatest extent possible. Submissions of measured and calculated rate coefficients are welcome, and will be studied by experts before inclusion into the database. Besides providing kinetic information for the interstellar medium, KIDA is planned to contain such data for planetary atmospheres and for circumstellar envelopes. each year, a subset of the reactions in the database (kida.uva) will be provided as a network for the simulation of the chemistry of dense interstellar clouds with temperatures between 10 K and 300 K. We also provide a code, named Nahoon, to study the time-dependent gas-phase chemistry of zero-dimensional and one-dimensional interstellar sources.
ABSTRACT Chemical models used to study the chemical composition of the gas and the ices in the interstellar medium are based on a network of chemical reactions and associated rate coefficients. These ...reactions and rate coefficients are partially compiled from data in the literature, when available. We present in this paper kida.uva.2014, a new updated version of the kida.uva public gas-phase network first released in 2012. In addition to a description of the many specific updates, we illustrate changes in the predicted abundances of molecules for cold dense cloud conditions as compared with the results of the previous version of our network, kida.uva.2011.
Context. The increased sensitivity and high spectral resolution of millimeter telescopes allow the detection of an increasing number of isotopically substituted molecules in the interstellar medium. ...The 14N/15N ratio is difficult to measure directly for molecules containing carbon. Aims. Using a time-dependent gas-phase chemical model, we check the underlying hypothesis that the 13C/12C ratio of nitriles and isonitriles is equal to the elemental value. Methods. We built a chemical network that contains D, 13C, and 15N molecular species after a careful check of the possible fractionation reactions at work in the gas phase. Results. Model results obtained for two different physical conditions that correspond to a moderately dense cloud in an early evolutionary stage and a dense, depleted prestellar core tend to show that ammonia and its singly deuterated form are somewhat enriched in 15N, which agrees with observations. The 14N/15N ratio in N2H+ is found to be close to the elemental value, in contrast to previous models that obtain a significant enrichment, because we found that the fractionation reaction between 15N and N2H+ has a barrier in the entrance channel. The high values of the N2H+/15NNH+ and N2H+/N15NH+ ratios derived in L1544 cannot be reproduced in our model. Finally, we find that nitriles and isonitriles are in fact significantly depleted in 13C, thereby challenging previous interpretations of observed C15N, HC15N, and H15NC abundances from 13C containing isotopologues.
We report the first experimental observation of single-photon ionization transitions of the SiC radical between 8.0 and 11.0 eV performed on the DESIRS beamline at the SOLEIL synchrotron facility. ...The SiC radical, very difficult to synthesize in the gas phase, was produced through chemical reactions between CH x ( x = 0–3) and SiH y ( y = 0–3) in a continuous microwave discharge flow tube, the CH x and SiH y species being formed by successive hydrogen-atom abstractions induced by fluorine atoms on methane and silane, respectively. Mass-selected ion yield and photoelectron spectra were recorded as a function of photon energy using a double imaging photoelectron/photoion coincidence spectrometer. The photoelectron spectrum enables the first direct experimental determinations of the X + 4 Σ − ← X 3 Π and 1 + 2 Π ← X 3 Π adiabatic ionization energies of SiC (8.978(10) eV and 10.216(24) eV, respectively). Calculated spectra based on Franck–Condon factors are compared with the experimental spectra. These spectra were obtained by solving the rovibrational Hamiltonian, using the potential energy curves calculated at the multireference single and double configuration interaction level with Davidson correction (MRCI + Q) and the aug-cc-pV5Z basis set. MRCI + Q calculations including the core and core–valence electron correlation were performed using the aug-cc-pCV6Z basis set to predict the spectroscopic properties of the six lowest electronic states of SiC + . Complete basis set extrapolations and relativistic energy corrections were also included in the determination of the energy differences characterizing the photoionization process. Using our experimental and theoretical results, we derived semi-experimental values for the five lowest ionization energies of SiC.
One of the major obstacles to accurately modeling the interstellar chemistry is inadequate knowledge of the binding energy (BE) of interstellar species with dust grains. In denser regions of ...molecular clouds, where very complex chemistry is active, interstellar dust is predominantly covered by H2O molecules, thus it is essential to know the interaction of gas-phase species with water ice to trace realistic physical and chemical processes. To this end, we consider water (cluster) ice to calculate the BE of several atoms, molecules, and radicals of astrochemical interest. Systematic studies have been carried out to come up with a relatively more accurate BE of astrophysically relevant species on water ice. We increase the size of the water cluster methodically to capture the realistic situation. Sequentially, one, three, four, five, and six water molecules are considered to represent water ice analogs in increasing order of complexity. We note that for most of the species considered here, as we increase the cluster size, our calculated BE value starts to converge toward the experimentally obtained value. More specifically, our computed results with the water c-pentamer (average deviation from experiment ∼ 15.8%) and c-hexamer (chair) (average deviation from experiment ∼ 16.7%) configurations are found to be nearer to an experimentally obtained value other than the value found for the water clusters we consider.
Context.
Under cold conditions in dense cores, gas-phase molecules and atoms are depleted from the gas-phase to the surface of interstellar grains. Considering the time scales and physical conditions ...within these cores, a portion of these molecules has to be brought back into the gas-phase to explain their observation by milimeter telescopes.
Aims.
We tested the respective efficiencies of the different mechanisms commonly included in the models (photo-desorption, chemical desorption, and cosmic-ray-induced whole-grain heating). We also tested the addition of sputtering of ice grain mantles via a collision with cosmic rays in the electronic stopping power regime, leading to a localized thermal spike desorption that was measured in the laboratory.
Methods.
The ice sputtering induced by cosmic rays has been added to the Nautilus gas-grain model while the other processes were already present. Each of these processes were tested on a 1D physical structure determined by observations in TMC1 cold cores. We focused the discussion on the main ice components, simple molecules usually observed in cold cores (CO, CN, CS, SO, HCN, HC
3
N, and HCO
+
), and complex organic molecules (COMs such as CH
3
OH, CH
3
CHO, CH
3
OCH
3
, and HCOOCH
3
). The resulting 1D chemical structure was also compared to methanol gas-phase abundances observed in these cores.
Results.
We found that all species are not sensitive in the same way to the non-thermal desorption mechanisms, and the sensitivity also depends on the physical conditions. Thus, it is mandatory to include all of them. Chemical desorption seems to be essential in reproducing the observations for H densities smaller than 4 × 10
4
cm
−3
, whereas sputtering is essential above this density. The models are, however, systematically below the observed methanol abundances. A more efficient chemical desorption and a more efficient sputtering could better reproduce the observations.
Conclusions.
In conclusion, the sputtering of ices by cosmic-rays collisions may be the most efficient desorption mechanism at high density (a few 10
4
cm
−3
under the conditions studied here) in cold cores, whereas chemical desorption is still required at smaller densities. Additional works are needed on both mechanisms to assess their efficiency with respect to the main ice composition.
•We developed a 1D model coupling neutrals with positive and negative ions.•We have updated the chemical scheme and absorption cross sections.•Agreement with observations is good for most neutrals ...and ions.•An uncertainty propagation study is performed to pinpoint key reactions.•The effect of the ion-neutral coupling is discussed.
Many models with different characteristics have been published so far to study the chemical processes at work in Titan’s atmosphere. Some models focus on neutral species in the stratosphere or ionic species in the ionosphere, but few of them couple all the species throughout the whole atmosphere. Very few of these emphasize the importance of uncertainties in the chemical scheme and study their propagation in the model.
We have developed a new 1D-photochemical model of Titan’s atmosphere coupling neutral species with positive and negative ions from the lower atmosphere up to the ionosphere and have compared our results with observations to have a comprehensive view of the chemical processes driving the composition of the stratosphere and ionosphere of Titan. We have updated the neutral, positive ion and negative ion chemistry and have improved the description of N2 photodissociation by introducing high resolution N2 absorption cross sections. We performed for the first time an uncertainty propagation study in a fully coupled ion-neutral model.
We determine how uncertainties on rate constants on both neutral and ionic reactions influence the model results and pinpoint the key reactions responsible for this behavior. We find very good agreement between our model results and observations in both the stratosphere and in the ionosphere for most neutral compounds. Our results are also in good agreement with an average INMS mass spectrum and specific flybys in the dayside suggesting that our chemical model (for both neutral and ions) provides a good approximation of Titan’s atmospheric chemistry as a whole. Our uncertainty propagation study highlights the difficulty to interpret the INMS mass spectra for masses 14, 31, 41 and we identified the key reactions responsible for these ambiguities.
Despite an overall improvement in the chemical model, disagreement for some specific compounds (HC3N, C2H5CN, C2H4) highlights the role that certain physical processes could play (meridional dynamics or sticking on aerosols). We find that some critical key reactions are important for many compounds including both neutrals and ions and should be studied in priority to lower the remaining model uncertainties. Extensive studies for some specific processes (including photolyses) are required.
Context.
Measuring isotopic ratios is a sensitive technique used to obtain information on stellar nucleosynthesis and chemical evolution.
Aims.
We present measurements of the carbon and sulphur ...abundances in the interstellar medium of the central region of our Galaxy. The selected targets are the +50 km s
−1
Cloud and several line-of-sight clouds towards Sgr B2(N).
Methods.
Towards the +50 km s
−1
Cloud, we observed the
J
= 2–1 rotational transitions of
12
C
32
S,
12
C
34
S,
13
C
32
S,
12
C
33
S, and
13
C
34
S, and the
J
= 3–2 transitions of
12
C
32
S and
12
C
34
S with the IRAM-30 m telescope, as well as the
J
= 6–5 transitions of
12
C
34
S and
13
C
32
S with the APEX 12 m telescope, all in emission. The
J
= 2–1 rotational transitions of
12
C
32
S,
12
C
34
S,
13
C
32
S, and
13
C
34
S were observed with ALMA in the envelope of Sgr B2(N), with those of
12
C
32
S and
12
C
34
S also observed in the line-of-sight clouds towards Sgr B2(N), all in absorption.
Results.
In the +50 km s
−1
Cloud we derive a
12
C/
13
C isotopic ratio of 22.1
−2.4
+3.3
, that leads, with the measured
13
C
32
S/
12
C
34
S line intensity ratio, to a
32
S/
34
S ratio of 16.3
−2.4
+3.0
. We also derive the
32
S/
34
S isotopic ratio more directly from the two isotopologues
13
C
32
S and
13
C
34
S, which leads to an independent
32
S/
34
S estimation of 16.3
−1.7
+2.1
and 17.9 ± 5.0 for the +50 km s
−1
Cloud and Sgr B2(N), respectively. We also obtain a
34
S/
33
S ratio of 4.3 ± 0.2 in the +50 km s
−1
Cloud.
Conclusions.
Previous studies observed a decreasing trend in the
32
S/
34
S isotopic ratios when approaching the Galactic centre. Our result indicates a termination of this tendency at least at a galactocentric distance of 130
−30
+60
pc. This is at variance with findings based on
12
C/
13
C,
14
N/
15
N, and
18
O/
17
O isotope ratios, where the above-mentioned trend is observed to continue right to the central molecular zone. This can indicate a drop in the production of massive stars at the Galactic centre, in the same line as recent metallicity gradient (Fe/H) studies, and opens the work towards a comparison with Galactic and stellar evolution models.
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