The mass of molecular gas in an interstellar cloud is often measured using line emission from low rotational levels of CO, which are sensitive to the CO mass, and then scaling to the assumed molecular hydrogen H2 mass. However, a significant H2 mass may lie outside the CO region, in the outer regions of the molecular cloud where the gas-phase carbon resides in C or C+. Here, H2 self-shields or is shielded by dust from UV photodissociation, whereas CO is photodissociated. This H2 gas is "dark" in molecular transitions because of the absence of CO and other trace molecules, and because H2 emits so weakly at temperatures 10 K <T < 100 K typical of this molecular component. This component has been indirectly observed through other tracers of mass such as gamma rays produced in cosmic-ray collisions with the gas and far-infrared/submillimeter wavelength dust continuum radiation. In this paper, we theoretically model this dark mass and find that the fraction of the molecular mass in this dark component is remarkably constant (~0.3 for average visual extinction through the cloud ) and insensitive to the incident ultraviolet radiation field strength, the internal density distribution, and the mass of the molecular cloud as long as , or equivalently, the product of the average hydrogen nucleus column and the metallicity through the cloud, is constant. We also find that the dark mass fraction increases with decreasing , since relatively more molecular H2 material lies outside the CO region in this case.

https://iopscience.iop.org/article/10.1088/0004-637X/716/2/1191/pdf

The Dark Molecular Gas

Aims. Photodissociation by UV light is an important destruction mechanism for carbon monoxide (CO) in many astrophysical environments, ranging from interstellar clouds to protoplanetary disks. The aim of this work is to gain a better understanding of the depth dependence and isotope-selective nature of this process.Methods. We present a photodissociation model based on recent spectroscopic data from the literature, which allows us to compute depth-dependent and isotope-selective photodissociation rates at higher accuracy than in previous work. The model includes self-shielding, mutual shielding and shielding by atomic and molecular hydrogen, and it is the first such model to include the rare isotopologues C17 O and 13 C17 O. We couple it to a simple chemical network to analyse CO abundances in diffuse and translucent clouds, photon-dominated regions, and circumstellar disks.Results. The photodissociation rate in the unattenuated interstellar radiation field is 2.6 10-10  s-1 , 30% higher than currently adopted values. Increasing the excitation temperature or the Doppler width can reduce the photodissociation rates and the isotopic selectivity by as much as a factor of three for temperatures above 100 K. The model reproduces column densities observed towards diffuse clouds and PDRs, and it offers an explanation for both the enhanced and the reduced N (12 CO)/N (13 CO) ratios seen in diffuse clouds. The photodissociation of C17 O and 13 C17 O shows almost exactly the same depth dependence as that of C18 O and 13 C18 O, respectively, so 17 O and 18 O are equally fractionated with respect to 16 O. This supports the recent hypothesis that CO photodissociation in the solar nebula is responsible for the anomalous 17 O and 18 O abundances in meteorites. Grain growth in circumstellar disks can enhance the N (12 CO)/N (C17 O) and N (12 CO)/N (C18 O) ratios by a factor of ten relative to the initial isotopic abundances.

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The photodissociation and chemistry of CO isotopologues: applications to interstellar clouds and circumstellar disks

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.

/pdf/a-kinetic-database-for-astrochemistry-kida-21via2ms84.pdf

A KInetic Database for Astrochemistry (KIDA)

In the past twenty years, the reconnaissance of 12 CO and 13 CO emission in the Milky Way by single-dish millimeter-wave telescopes has expanded our view and understanding of interstellar molecular gas. We enumerate the major surveys of CO emission along the Galactic plane and summarize the various approaches that leverage these data to determine the large-scale distribution of molecular gas: its radial and vertical distributions, its concentration into clouds, and its relationship to spiral structure. The integrated properties of molecular clouds are compiled from catalogs derived from the CO surveys using uniform assumptions regarding the Galactic rotation curve, solar radius, and the CO-to-H2 conversion factor. We discuss the radial variations of cloud surface brightness, the distributions of cloud mass and size, and scaling relations between velocity dispersion, cloud size, and surface density that affirm that the larger clouds are gravitationally bound. Measures of density structure and gas kinematics within nearby, well-resolved clouds are examined and attributed to the effects of magnetohydrodynamic turbulence. We review the arguments for short, intermediate, and long molecular lifetimes based on the observational record. The review concludes with questions that shall require further observational attention.

Molecular Clouds in the Milky Way

Rate coefficients for state-to-state rotational transitions in CO induced by both para- and ortho-H2 collisions are presented. The results were obtained using the close-coupling method and the coupled-states approximation, with the CO-H2 interaction potential of Jankowski & Szalewicz. Rate coefficients are presented for temperatures between 1 and 3000 K, and for CO(v = 0, j) quenching from j = 1-40 to all lower j' levels. Comparisons with previous calculations using an earlier potential show some discrepancies, especially at low temperatures and for rotational transitions involving large |Δj|. The differences in the well depths of the van der Waals interactions and the anisotropy of the two potential surfaces lead to different resonance structures in the energy dependence of the cross sections which influence the low temperature rate coefficients. Applications to far infrared observations of astrophysical environments are briefly discussed.

https://iopscience.iop.org/article/10.1088/0004-637X/718/2/1062/pdf

Rotational quenching of co due to h2 collisions

We present the revised "Meudon" model of photon-dominated region (PDR) code, available on the Web under the GNU Public License. General organization of the code is described down to a level that should allow most observers to use it as an interpretation tool with minimal help from our part. Two grids of models, one for low-excitation diffuse clouds and one for dense highly illuminated clouds, are discussed, and some new results on PDR modelization highlighted.

https://hal.archives-ouvertes.fr/hal-00018656/document

A model for atomic and molecular interstellar gas: the meudon pdr code

Aims. We model a diffuse molecular cloud present along the line of sight to the star HD 102065. We compare our modeling with observations to test our understanding of physical conditions and chemistry in diffuse molecular clouds. Methods. We analyze an extensive set of spectroscopic observations which characterize the diffuse molecular cloud observed toward HD 102065. Absorption observations provide the extinction curve, H2, C I, CO, CH, and CH+ column densities and excitation. These data are complemented by observations of CII, CO and dust emission. Physical conditions are determined using the Meudon PDR model of UV illuminated gas. Results. We find that all observational results, except column densities of CH, CH+ and H2 in its excited (J > 2) levels, are consistent with a cloud model implying a Galactic radiation field (G~0.4 in Draine's unit), a density of 80 cm-3 and a temperature (60-80 K) set by the equilibrium between heating and cooling processes. To account for excited (J >2) H2 levels column densities, an additional component of warm (~ 250K) and dense (nH>10^4 cm-3) gas within 0.03 pc of the star would be required. This solution reproduces the observations only if the ortho-to-para H2 ratio at formation is 1. In view of the extreme physical conditions and the unsupported requirement on the ortho-to-para ratio, we conclude that H2 excitation is most likely to be accounted for by the presence of warm molecular gas within the diffuse cloud heated by the local dissipation of turbulent kinetic energy. This warm H2 is required to account for the CH+ column density. It could also contribute to the CH abundance and explain the inhomogeneity of the CO abundance indicated by the comparison of absorption and emission spectra.

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Modeling of diffuse molecular gas applied to HD 102065 observations

Context. Nonlinear behavior in interstellar chemical models has been recognized for 25 years now. Different mechanisms account for the possibility of multiple fixed-points at steady-state, characterized by the ionization degree of the gas.Aims. Chemical oscillations are also a natural behavior of nonlinear chemical models. We study under which conditions spontaneous sustained chemical oscillations are possible, and what kind of bifurcations lead to, or quench, the occurrence of such oscillations.Methods. The well-known ordinary differential equations (ODE) integrator VODE was used to explore initial conditions and parameter space in a gas phase chemical model of a dark interstellar cloud.Results. We recall that the time evolution of the various chemical abundances under fixed temperature conditions depends on the density over cosmic ionization rate n H ∕ζ ratio. We also report the occurrence of naturally sustained oscillations for a limited but well-defined range of control parameters. The period of oscillations is within the range of characteristic timescales of interstellar processes and could lead to spectacular resonances in time-dependent models. Reservoir species (C, CO, NH3 , ...) oscillation amplitudes are generally less than a factor two. However, these amplitudes reach a factor ten to thousand for low abundance species, e.g. HCN, ND3 , that may play a key role for diagnostic purposes. The mechanism responsible for oscillations is tightly linked to the chemistry of nitrogen, and requires long chains of reactions such as found in multi-deuteration processes.

/pdf/sustained-oscillations-in-interstellar-chemistry-models-4jtk2m5sr6.pdf

Sustained oscillations in interstellar chemistry models

Non-linear behavior in interstellar chemical models has been recognized for 25 years now. Different mechanisms account for the possibility of multiple fixed-points at steady state, characterized by the ionization degree of the gas. Chemical oscillations are also a natural behaviour of non-linear chemical models. We study under which conditions spontaneous sustained chemical oscillations are possible, and what kind of biffurcations lead to, or quench, the occurrence of such oscillations. Methods. The well known Ordinary Differential Equations (ODE) integrator VODE is used to explore initial conditions and parameter space in a gas phase chemical model of a dark interstellar cloud. We recall that the time evolution of the various chemical abundances under fixed temperature conditions depends on the density over cosmic ionization rate nH/{\zeta} ratio. We also report the occurrence of naturally sustained oscillations for a limited but well defined range of control parameters. The period of oscillations is within the range of characteristic time scales of interstellar processes and could lead to spectacular resonances in time dependent models. Reservoir species (C, CO, NH3, ...) oscillation amplitudes are generally less than a factor two. However, these amplitudes reach a factor ten to thousand for low abundance species, e.g. HCN, ND3, that may play a key role for diagnostic purposes.The mechanism responsible for oscillations is tightly linked to the chemistry of nitrogen, and requires long chains of reactions such as found in multi-deuteration processes.

/pdf/sustained-oscillations-in-interstellar-chemistry-models-801ues8u54.pdf

Jacques Le Bourlot

Papers

A model for atomic and molecular interstellar gas: the meudon pdr code

Modeling of diffuse molecular gas applied to HD 102065 observations

Sustained oscillations in interstellar chemistry models

Sustained oscillations in Interstellar chemistry models