Dynamics Study of the Reaction S + O2 → SO + O and Its Reverse on a Single-Valued Double Many-Body Expansion Potential Energy Surface for Ground-State SO2

Ionization fraction and the enhanced sulfur chemistry in Barnard 1

CONTEXT

Barnard B1b has revealed as one of the most interesting globules from the chemical and dynamical point of view. It presents a rich molecular chemistry characterized by large abundances of deuterated and complex molecules. Furthermore, it hosts an extremely young Class 0 object and one candidate to First Hydrostatic Core (FHSC) proving the youth of this star forming region.

AIMS

Our aim is to determine the cosmic ray ionization rate, [Formula: see text], and the depletion factors in this extremely young star forming region. These parameteres determine the dynamical evolution of the core.

METHODS

We carried out a spectral survey towards Barnard 1b as part of the IRAM Large program ASAI using the IRAM 30-m telescope at Pico Veleta (Spain). This provided a very complete inventory of neutral and ionic C-, N- and S- bearing species with, up to our knowledge, the first secure detections of the deuterated ions DCS+ and DOCO+. We use a state-of-the-art pseudo-time-dependent gas-phase chemical model that includes the ortho and para forms of [Formula: see text] and [Formula: see text] to determine the local value of the cosmic ray ionization rate and the depletion factors.

RESULTS

Our model assumes n(H2)=105 cm-3 and T k =12 K, as derived from our previous works. The observational data are well fitted with ζH2 between 3×10-17 s-1 and 10-16 s-1, and the following elemental abundances: O/H=3 10-5, N/H=6.4-8 10-5, C/H=1.7 10-5 and S/H between 6.0 10-7 and 1.0 10-6. The large number of neutral/protonated species detected, allows us to derive the elemental abundances and cosmic ray ionization rate simultaneously. Elemental depletions are estimated to be ~10 for C and O, ~1 for N and ~25 for S.

CONCLUSIONS

Barnard B1b presents similar depletions of C and O than those measured in pre-stellar cores. The depletion of sulfur is higher than that of C and O but not as extreme as in cold cores. In fact, it is similar to the values found in some bipolar outflows, hot cores and photon-dominated regions. Several scenarios are discussed to account for these peculiar abundances. We propose that it is the consequence of the initial conditions (important outflows and enhanced UV fields in the surroundings) and a rapid collapse (~0.1 Myr) that permits to maintain most S- and N-bearing species in gas phase to great optical depths. The interaction of the compact outflow associated with B1b-S with the surrounding material could enhance the abundances of S-bearing molecules, as well.

Recalibrated Double Many-Body Expansion Potential Energy Surface and Dynamics Calculations for HN2

A single-sheeted double many-body expansion potential energy surface is reported for the lowest doublet state of HN2 by fitting additional multireference configuration interaction energies in the N...NH channel. A stratified analysis of the root-mean-squared error indicates an accuracy superior to that achieved for the previously reported form. Detailed dynamical tests are also performed for the N + NH reaction using both the quasi-classical trajectory method and the capture theory, and the results are compared with available empirical data. The vibrational resonances of the HN2 metastable radical are also calculated and compared with previous theoretical predictions.

New Double Many-Body Expansion Potential Energy Surface for Ground-State HCN from a Multiproperty Fit to Accurate ab Initio Energies and Rovibrational Calculations

An accurate single-sheeted double many-body expansion potential energy surface has been obtained for the ground electronic state of the hydrogen cyanide molecule via a multiproperty fit to ab initio energies and rovibrational data. This includes 106 rovibrational levels and 2313 discrete points, which are fit with a rmsd of 4 cm(-1) and 2.42 kcal mol(-1), respectively, and seven zero first-derivatives that are reproduced at three stationary points. Since the potential also describes accurately the appropriate asymptotic limits at the various dissociation channels, it is commended both for the simulation of rovibrational spectra and reaction dynamics.

Unimolecular and Bimolecular Calculations for HN2

Using a recently reported double many-body expansion potential energy surface, quasi-classical, statistical mechanics, and quantum resonance calculations have been performed for the HN(2) system by focusing on the determination of bimolecular (N + NH and H + N(2)) and unimolecular (decomposition of HN(2)) rate constants as well as the relevant equilibrium constants.

Experimental and theoretical investigations of rate coefficients of the reaction S([sup 3]P)+O[sub 2] in the temperature range 298–878 K

Rate coefficients of the reaction S+O(2) with Ar under 50 Torr in the temperature range 298-878 K were determined with the laser photolysis technique. S atoms were generated by photolysis of OCS with a KrF excimer laser at 248 nm; their concentration was monitored via resonance fluorescence excited by atomic emission of S produced from microwave-discharged SO(2). Our measurements show that k(298 K)=(1.92+/-0.29)x10(-12) cm(3) molecule(-1) s(-1), in satisfactory agreement with previous reports. New data determined for 505-878 K show non-Arrhenius behavior; combining our results with data reported at high temperatures, we derive an expression k(T)=(9.02+/-0.27)x10(-19)T(2.11+/-0.15) exp[(730+/-120)/T] cm(3) molecule(-1) s(-1) for 298< or =T< or =3460 K. Theoretical calculations at the G2M (RCC2) level, using geometries optimized with the B3LYP/6-311+G(3df) method, yield energies of transition states and products relative to those of the reactants. Rate coefficients predicted with multichannel RRKM calculations agree satisfactorily with experimental observations; the reaction channel via SOO(1A') dominates at T<500 K, whereas channels involving formation of SOO(3A") followed by isomerization to SO(2) before dissociation, and formation of SOO(1A") followed by direct dissociation, become important at high temperatures, accounting for the observed rapid increase in rate coefficient.