Models Of Interstellar Clouds Research Articles

Oscillations in gas-grain astrochemical kinetics

ABSTRACT We have studied gas-grain chemical models of interstellar clouds to search for non-linear dynamical evolution. A prescription is given for producing oscillatory solutions when a bistable solution exists in the gas-phase chemistry and we demonstrate the existence of limit cycle and relaxation oscillation solutions. As the autocatalytic chemical processes underlying these solutions are common to all models of interstellar chemistry, the occurrence of these solutions should be widespread. We briefly discuss the implications for interpreting molecular cloud composition with time-dependent models and some future directions for this approach.

Robert H. Rubin (1941–2013)

Rubin developed widely used physical models of interstellar clouds of gas ionized by nearby hot stars. His work provided evidence for the previously unsuspected presence of hot stars in the vicinity of the Galactic Center.

Kinetic Study of the Gas-Phase C(3P) + CH3CN Reaction at Low Temperatures: Rate Constants, H-Atom Product Yields, and Astrochemical Implications

Rate constants have been measured for the C(3P) + CH3CN reaction between 50 and 296 K using a continuous-flow supersonic reactor. C(3P) atoms were created by the in situ pulsed laser photolysis of CBr4 at 266 nm, while the kinetics of C(3P) atom loss were followed by direct vacuum ultraviolet laser-induced fluorescence at 115.8 nm. Secondary measurements of product H(2S) atom formation were also made, allowing absolute H-atom yields to be obtained by comparison with those obtained for the C(3P) + C2H4 reference reaction. In parallel, quantum chemical calculations were performed to obtain the various complexes, adducts, and transition states relevant to the title reaction over the 3A″ potential energy surface, allowing us to better understand the preferred reaction pathways. The reaction is seen to be very fast, with measured rate constants in the range of (3–4) × 10–10 cm3 s–1 with little or no observed temperature dependence. As the C + CH3CN reaction is not considered in the current astrochemical networks, we test its influence on interstellar methyl cyanide abundances using a gas-grain dense interstellar cloud model. Its inclusion leads to predicted CH3CN abundances that are significantly lower than the observed ones.

HIGH-RESOLUTION OSCILLATOR STRENGTH MEASUREMENTS OF THEv′ = 0,1 BANDS OF THEB-X,C-X, ANDE-XSYSTEMS IN FIVE ISOTOPOLOGUES OF CARBON MONOXIDE

We report oscillator strengths for six strong vibrational bands between 105.0 and 115.2 nm, associated with transitions from the v = 0 level of the X1Σ+ ground state to the v = 0 and 1 levels of the B1Σ+, C1Σ+, and E1Π states, in 12C16O, 12C17O, 12C18O, 13C16O, and 13C18O. These measurements extend the development of a comprehensive database of line positions, oscillator strengths, and linewidths of photodissociating transitions for all astrophysically relevant CO isotopologues. The E–X bands, in particular, play central roles in CO photodissociation and fractionation models of interstellar clouds and circumstellar disks including the early solar nebula. The resolving powers of the room-temperature measurements, R = 300,000–400,000, allow for the analysis of individual line strengths within bands; the measurements reveal J-dependences in the branch intensities of the C(v = 0,1)–X(0) and E(v = 0,1)–X(0) bands in all isotopologues. Minimal or no isotopologue dependence was found in the f-values of the C(v = 0,1)–X(0) and E(v = 0,1)–X(0) bands at a ∼5% uncertainty level. Revised dissociation branching ratios for the C(v = 0,1) and E(v = 0,1) levels are computed based on these f-values. The weak isotopologue dependence of the f-values presented here eliminates this mechanism as an explanation for the large 17O enrichments seen in recent laboratory photolysis experiments on CO at wavelengths from 105 to 108 nm.

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.

Photodissociation of interstellar N2

Molecular nitrogen is one of the key species in the chemistry of interstellar clouds and protoplanetary disks and the partitioning of nitrogen between N and N2 controls the formation of more complex prebiotic nitrogen-containing species. The aim of this work is to gain a better understanding of the interstellar N2 photodissociation processes based on recent detailed theoretical and experimental work and to provide accurate rates for use in chemical models. We simulated the full high-resolution line-by-line absorption + dissociation spectrum of N2 over the relevant 912-1000 \AA\ wavelength range, by using a quantum-mechanical model which solves the coupled-channels Schr\"odinger equation. The simulated N2 spectra were compared with the absorption spectra of H2, H, CO, and dust to compute photodissociation rates in various radiation fields and shielding functions. The effects of the new rates in interstellar cloud models were illustrated for diffuse and translucent clouds, a dense photon dominated region and a protoplanetary disk.

Significant nonadiabatic effects in the C + CH reaction dynamics

Rigorous quantum nonadiabatic calculations are carried out on the two coupled electronic states (1(2)A' and 2(2)A') for the C + CH reaction. For all calculations, the initial wave packet was started from the entrance channel of the 1(2)A' state and the initial state of the CH reactant was kept in its ground rovibrational state. Reaction probabilities for total angular momenta J from 0 to 160 are calculated to obtain the integral cross section over an energy range from 0.005 to 0.8 eV collision energy. Significant nonadiabatic effects are found in the reaction dynamics. The branching ratio of the ground state and excited state of C(2) produced is around 0.6, varying slightly with the collision energy. Also, a value of 2.52 × 10(-11) cm(3) molecule(-1) s(-1) for the state selected rate constant k (v = 0, j = 0) at 300 K is obtained, which may be seen as a reference in the future chemical models of interstellar clouds.

ORFEUS OBSERVATIONS OF ULTRAVIOLET EXCITED HIGH-J MOLECULAR HYDROGEN

We present measurements of diuse interstellar H 2 absorption lines in the continuum spectra of 10 early-type stars. The data were observed with the Berkeley Extreme and Far-Ultraviolet Spectrometer (BEFS) of the ORFEUS telescope on board the ORFEUS-SPAS I and II space-shuttle missions in 1993 and 1996, respectively. The spectra extend from the interstellar cuto at 912 ˚ to about 1200 ˚ with a resolution of » 3000 and statistical signal-to-noise ratios between 10 and 65. Adopting Doppler broadening velocities from high-resolution optical observations, we obtain the H2 column densities of rotational levels J 00 = 0 through 5 for each line of sight. The kinetic temperatures derived from J 00 = 0 and 1 states show a small variation around the mean value of 80 K, except for the component toward HD 219188, which has a temperature of 211 K. Based on a synthetic interstellar cloud model described in our previous work, we derive the incident UV intensity IUV and the hydrogen density nH of the observed components to be i0:4 • log IUV • 2:2 and 6:3 • nH • 2500 cm i3 , respectively.

Formation of biomolecule precursors in space

Alcohols and nitriles not only play an important role as templates for synthesis of larger molecules in the interstellar medium and planetary atmospheres, but they can also be regarded as precursors for biomolecules. Alcohols can form carbohydrates through reaction with HCO and nitriles can be hydrolysed to amino acids in aqueous solutions, which is the final step of the well-known Strecker synthesis. Therefore the question of the pathways of formation of alcohols and nitriles and the efficiency and the product distribution of their subsequent degradation reactions in the above-mentioned astrophysical environments is of great interest. In both processes dissociative recombination reactions of protonated nitriles and alcohols may play a major role and are included in models of interstellar clouds and planetary atmospheres. However, the reaction rate coefficients and product branching ratios for the majority of these processes are so far still unknown, which adversely affects the quality of predictions of model calculations. In this Contribution, we therefore present branching ratios and rate constants of the dissociative recombination of protonated methanol (CH3OH2), as well as protonated acetonitrile (CH3CNH+), acrylonitrile (C2H3CNH+) and cyanoacetylene (HC3NH+). The impact of the obtained new data on model calculations of abundances of important interstellar molecules in dark clouds is discussed.

Ultraviolet Excited High‐JMolecular Hydrogen in Photodissociation Regions

We have calculated synthetic interstellar cloud models to investigate the formation and destruction of high-J molecular hydrogen in photodissociation regions. The effects of five physical parameters (the incident ultraviolet [UV] intensity, H2 column density, cloud temperature, total density, and H2 formation rate coefficient) on the populations of H2 rotational levels are explored. We have found that N(4)/N(0) is proportional to the incident UV intensity IUV and the H2 molecular fraction f is simply related to the ratio of IUV and the hydrogen density nH, implying a new method to derive IUV and nH with the observational parameters N(4)/N(0) and f, assuming an H2 formation rate R. High-resolution FUSE spectra of H2 toward three translucent sight lines (HD 110432, HD 192639, and HD 185418) in the Milky Way and 24 diffuse sight lines in the SMC and LMC are referenced to obtain N(4)/N(0) and f. Using our method, we are able to derive IUV ~ 10-40 for the translucent sight lines, when R is assumed to be 10 times higher than the Galactic value. Synthetic models for the Magellanic sight lines suggest that IUV ≥ 10, nH ≤ 100 cm-3, and R ~ 3 × 10-16 for the low H2 column density sight lines (N[H2] ≤ 1018 cm-2) and IUV ≥ 10, nH ≤ 1000 cm-3, and R ~ 3 × 10-17 for the high H2 column density sight lines (N[H2] ≥ 1019 cm-2).

Reaction of cyanoacetylene HCCCN(XΣ+1) with ground-state carbon atoms C(P3) in cold molecular clouds

The reaction of the simplest cyanopolyyne, cyanoacetylene [HCCCN(X (1)Sigma(+))], with ground-state atomic carbon C((3)P) is investigated theoretically to explore the probable routes for the depletion of the famed interstellar molecule HCCCN, and the formation of carbon-nitrogen-bearing species in extraterrestrial environments particularly of ultralow temperature. Six collision complexes (c1-c6) without entrance barrier as a result of the carbon atom addition to the pi systems of HCCCN are located. The optimized geometries and harmonic frequencies of the intermediates, transition states, and products along the isomerization and dissociation pathways of each collision complex are obtained by utilizing the unrestricted B3YLP6-311G(d,p) level of theory, and the corresponding CCSD(T)/cc-pVTZ energies are calculated. Subsequently, with the facilitation of Rice-Ramsperger-Kassel-Marcus (RRKM) and variational RRKM rate constants at collision energy of 0-10 kcal/mol, the most probable paths for the titled reaction are determined, and the product yields are estimated. Five collision complexes (c1-c3, c5, and c6) are predicted to give the same products, a chained CCCCN (p2)+H, via the linear and most stable intermediate, HCCCCN (i2), while collision complex c4 is likely to dissociate back to C+HCCCN. The study suggests that this class of reaction is an important route to the destruction of cyanoacetylene and cyanopolyynes in general, and to the synthesis of linear carbon-chained nitriles at the temperature as low as 10 K to be incorporated in future chemical models of interstellar clouds.

MODEL CALCULATIONS OF THE UV - EXCITED MOLECULAR HYDROGEN IN INTERSTELLAR CLOUDS

We have calculated 2448 interstellar cloud models to investigate the formation and destruction of high rotational level <TEX>$H_2$</TEX> according to the combinations of five physical conditions: the input UV intensity, the <TEX>$H_2$</TEX> column density, cloud temperature, total density, and the <TEX>$H_2$</TEX> formation rate efficiency. The models include the populations of all the accessible states of <TEX>$H_2$</TEX> with the rotational quantum number J < 16 as a function of depth through the model clouds, and assume that the abundance of <TEX>$H_2$</TEX> is in a steady state governed primarily by the rate of formation on the grain surfaces and the rates of destruction by spontaneous fluorescent dissociation following absorption in the Lyman and Werner band systems. The high rotational levels J = 4 and J = 5 are both populated by direct formation into these levels of newly created molecules, and by pumping from J = 0 and J = 1, respectively The model results show that the high rotational level ratio N(4)/N(0) is proportional to the incident UV intensity, and is inversely proportional to the <TEX>$H_2$</TEX> molecular fraction, as predicted in theory.

A theoretical study for the reaction of vinyl cyanide C2H3CN(XA′1) with the ground state carbon atom C(P3) in cold molecular clouds

The reaction of the ground state atomic carbon, C(3P), with simple unsaturated nitrile, C2H3CN(X1A' (vinyl cyanide), is investigated theoretically to explore the probable routes for the formation of carbon-nitrogen-bearing species in extraterrestrial environments particularly of ultralow temperature. Five collision complexes without entrance barrier as a result of the carbon atom addition to the pi systems of C2H3CN are characterized. The B3YLP/6-311G(d,p) level of theory is utilized in obtaining the optimized geometries, harmonic frequencies, and energies of the intermediates, transition states, and products along the isomerization and dissociation pathways of each collision complex. Subsequently, with the facilitation of computed RRKM rate constants at collision energy of 0-10 kcal/mol, the most probable paths for each collision complexes are determined, of which the CCSD(T)/6-311G(d,p) energies are calculated. The major products predicted are exclusively due to the hydrogen atom dissociations, while the products of H2, CN, and CH2 decompositions are found negligible. Among many possible H-elimination products, cyano propargyl (p4) and 3-cyano propargyl (p5) are the most probable, in which p5 can be formed via two intermediates, cyano allene (i8) and cyano vinylmethylene (i6), while p4 is yielded from i8. The study suggests this class of reaction is an important route to the synthesis of unsaturated nitriles at the temperature as low as 10 K, and the results are valuable for future chemical models of interstellar clouds.

An Ab Initio Investigation of Reactions of Carbon Atoms, C(3Pj), with C2H4and C3H6in the Interstellar Medium

The reactions of ground state atomic carbon C(3Pj) with the two simplest olefines ethylene (C2H4) and propylene (C3H6) are investigated computationally to explore the formation of carbon-bearing radicals in extraterrestrial environments. The calculations revealed that the reactions proceed on the triplet surface and are initiated by a barrier-less addition of the carbon atom to the π-bond forming three-membered ring adducts cyclopropylidene and methylcyclopropylidene. Both complexes reside in deep potential energy wells and ring-open to allene and cis/trans 1,2-butadiene, respectively. The decomposition of these intermediates is dominated by an H atom loss to form the propargyl radical (C(3Pj) /C2H4) as well as 1-, 3-methylpropargyl, and 1,3-butadienyl-2 radicals (C(3Pj)/C3H6). Additionally, triplet 1,2-butadiene can lose a CH3 group to give a propargyl radical. To a minor amount, triplet allene was found to undergo a hydrogen migration followed by fragmentation of the H2CCHCH complex to acetylene (C2H2) and triplet carbene (CH2). All reaction channels have no entrance barrier, show exit barriers well below the energy of the reactant molecules, and are strongly exothermic. Therefore, binary collisions of carbon atoms with ethylene and propylene present compelling candidates to form (substituted) propargyl radicals even in cold molecular clouds. Since these radicals are suggested to play an important role in the formation of polycyclic aromatic hydrocarbons, all isomers should be incorporated in future chemical models of interstellar clouds and circumstellar envelopes around carbon stars.

The Sensitivity of Gas‐Phase Chemical Models of Interstellar Clouds to C and O Elemental Abundances and to a New Formation Mechanism for Ammonia

The Sensitivity of Gas‐Phase Chemical Models of Interstellar Clouds to C and O Elemental Abundances and to a New Formation Mechanism for Ammonia

Grain Surface Chemistry: Modified Models

The rate equation approach to the chemistry occurring on grain surfaces in interstellar clouds has been criticized for not taking the discrete nature of grains into account. Indeed, investigations of simple models show that results obtained from rate equations can be significantly different from results obtained by a Monte Carlo procedure. Some modifications of the rate equations have been proposed that have the effect of eliminating most of the differences with the Monte Carlo procedure for simplified models of interstellar clouds at temperatures of 10 K and slightly above. In this study we investigate the use of the modified rate equations in more realistic chemical models of dark interstellar clouds with complex gas-grain interactions. Our results show some discrepancies between the results of models with unmodified and modified rate equations; these discrepancies are highly dependent, however, on the initial form of hydrogen chosen. If the initial form is mainly molecular, at early stages of cloud evolution there are some significant differences in calculated molecular abundances on grains, but at late times the two sets of results tend to converge for the main components of the grain mantles. If the initial form is atomic hydrogen, there are essentially no differences in results between models based on the unmodified rate equations and those based on the modified rate equations, except for the abundances on grains of some minor complex molecules. Thus, the major results of previous gas-grain models of cold, dark interstellar clouds remain at least partially intact.

The Sensitivity of Gas‐Phase Chemical Models of Interstellar Clouds to C and O Elemental Abundances and to a New Formation Mechanism for Ammonia

The effects of variations in the gas-phase carbon-to-oxygen elemental abundance ratio (0.42 ? C/O ? 1.2) and the absolute gas-phase carbon and oxygen elemental abundances on calculated molecular concentrations have been studied for three gas-phase chemical models of dense interstellar clouds. Both the C and O elemental abundances were varied from their low metal values, in which C/O = 0.42. The results were compared with observations of the dark interstellar clouds TMC-1 and L134N, the latter being chosen because TMC-1, with its singularly rich component of large hydrocarbons and cyanopolyynes, may not represent dense cores universally. In general, variations in the gas-phase C and O elemental abundances have a large and time-dependent effect on calculated molecular concentrations for all three models. For the new standard model, which does not contain many rapid neutral-neutral reactions, excellent early-time agreement with TMC-1 occurs for a variety of C/O ratios obtained by depleting the low metal O abundance, but the time of best agreement tends to increase with increasing C/O ratio. At these early times, ? 80% of the calculated abundances are within an order of magnitude of the observed values. Agreement at this level also occurs at steady state if the C and O abundances are first depleted by a factor of 5 and then O is additionally depleted so that C/O ? 0.80. In general, a factor of 5 depletion of both C and O increases the production efficiency of large molecules. When the new standard model is applied to L134N, the early-time agreement is not as good as for TMC-1 unless both C and O are first depleted by factors of 5 from their low metal values and the C/O ratio is then maintained at a value less than 0.80. Under these conditions, the steady state results are only slightly worse. The other two models, containing fast neutral-neutral reactions, have their best agreement with TMC-1 when C/O ? 1, although the level of agreement is typically worse than with the new standard model, and factor of 5 depletions have little effect. For L134N, on the other hand, the early-time agreement with these latter two models for a wide range of C/O values is almost as good as with the new standard model if factor of 5 depletions in C and O are utilized and is actually superior for most cases when C/O ? 1. In general, the negative conclusions concerning models with rapid neutral-neutral reactions may therefore be overly harsh. When the newly studied rapid reaction H -->+3+N?NH -->+2 + H is included in our model calculations, the abundances of some N-containing species are in better agreement with observed values, but this effect decreases as C/O is increased.

A Proposed Modification of the Rate Equations for Reactions on Grain Surfaces

The rate equations currently utilized for grain reactions in gas-grain chemical models of interstellar clouds are inappropriate under certain conditions because of the discrete nature of grain particles. Appropriate modifications are suggested, and calculated surface abundances for various molecules are tested against Monte Carlo calculations for three simple systems. In the first system, the gas is comprised of only O and H atoms that accrete on grain surfaces and subsequently react to form only the diatomic molecules H2, OH, and O2. In the second system, N atoms are also present in the gas, and the grain chemistry leads to more molecules, including the polyatomic species NH3 and H2O. In the third and final system, the gas contains H2, and the contribution of this species to grain chemistry is considered. The modifications to the rate equations lead to reasonable agreement with the results of Monte Carlo simulations for all three systems. These modifications can then be used in time-dependent gas-grain chemical models until a more detailed but usable theory becomes available.

Neutral‐Neutral Reactions in the Interstellar Medium. I. Formation of Carbon Hydride Radicals via Reaction of Carbon Atoms with Unsaturated Hydrocarbons

The reactions of ground-state atomic carbon with acetylene, C2H2 (1), methylacetylene, CH3CCH (2), ethylene, C2H4 (3), and propylene, C3H6 (4), are investigated at relative collision energies between 8.8 and 45 kJ mol-1 in crossed-beam experiments to elucidate the reaction products and chemical dynamics of atom-neutral encounters relevant to the formation of carbon-bearing molecules in the interstellar medium (ISM). Reactive scattering signal is found for C3H (1), as well as the hitherto unobserved interstellar radicals C4H3 (2), C3H3 (3), and C4H5 (4). All reactions proceed on the triplet surface via addition of the carbon atom to the molecular π-bond. The initial collision complexes undergo hydrogen migration (1/2) or ring opening (3/4) and decompose via C-H-bond rupture to l/c-C3H (1), n-C4H3 (2), propargyl (3), and methylpropargyl (4). The explicit identification of the carbon-hydrogen exchange channel under single collision conditions identifies this class of reaction as a potential pathway to carbon-bearing species in the ISM. Especially, the formation of l/c-C3H correlates with actual astronomical observations and explains a higher [c-C3H]/[l-C3H] ratio in the dark cloud TMC-1 as compared to the carbon star IRC +10216. Our findings strongly demand the incorporation of distinct structural isomers in prospective chemical models of interstellar clouds, hot cores, and circumstellar envelopes around carbon stars.

Application of Principal Component Analysis to Large‐Scale Spectral Line Imaging Studies of the Interstellar Medium

The multivariate statistical technique of principal component analysis (PCA) is described and demonstrated to be a valuable tool to consolidate the large amount of information obtained with spectroscopic imaging observations of the interstellar medium. Simple interstellar cloud models with varying degrees of complexity and Gaussian noise are constructed and analyzed to demonstrate the ability of PCA to statistically extract physical features and phenomena from the data and to gauge the effects of random noise upon the analysis. Principal components are calculated for high spatial dynamic range 12CO and 13CO data cubes of the Sh 155 (Cep OB3) cloud complex. These identify the three major emission components within the cloud and the spatial differences between 12CO and 13CO emissions. Higher order eigenimages identify small velocity fluctuations and therefore provide spatial information to the turbulent velocity field within the cloud. A size line width relationship δv ~ Rα is derived from spatial and kinematic characterizations of the principal components of 12CO emission from the Sh 155, Sh 235, Sh 140, and Gem OB1 cloud complexes. The power-law indices for these clouds range from 0.42 to 0.55 and are similar to those derived from an ensemble of clouds within the Galaxy found by Larson (1981) and Solomon et al. (1987). The size-line width relationship within a given cloud provides an important diagnostic to the variation of kinetic energy with size scale within turbulent flows of the interstellar medium.