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1982Apjs. . .48. .32 IG the Astrophysical Journal Supplement IG .32 The Astrophysical Journal Supplement Series, 48:321-368, 1982 March .48. © 1982. The American Astronomical Society. All rights reserved. Printed in U.S.A. 1982ApJS. THE KINETIC CHEMISTRY OF DENSE INTERSTELLAR CLOUDS T. E. Graedel Bell Laboratories William D. Langer Crawford Hill Laboratory, Bell Laboratories and Princeton University AND M. A. Frerking1 Crawford Hill Laboratory, Bell Laboratories Received 1980 November 17; accepted 1981 August 27 ABSTRACT A detailed model of the time-dependent chemistry of dense interstellar clouds has been developed to study the dominant chemical processes in carbon and oxygen isotope fractionation, formation of nitrogen-containing molecules, evolution of product molecules as a function of cloud density and temperature, and other topics of interest. The full computation involves 328 individual reactions (expanded to 1067 to study carbon and oxygen isotope chemistry); photodegradation processes are unimportant in these dense clouds and are excluded. In this paper, the first in a series of three, we describe the formulation of the chemical model and the calculation of the abundances of the dominant isotopes of the carbon- and oxygen-bearing molecules. Special attention is paid to comparison of the results with observations and to the implications of the results for future observational work. These goals are pursued with calculations that cover a wide range of densities and times; the extensive results are presented compactly for species of particular observational interest by three-dimensional graphical displays. Among the results of note are the following: i) The chemical abundances are quite sensitive to the electron concentration, since the latter determines the ratio of H J to He+ (the two ions which govern the vigor of the molecular chemistry). ii) The electron density is strongly influenced by the metals abundance, a parameter poorly constrained at present by observations, and by the reactions of ions with metals. Results for calculations performed at different abundances suggest lower abundances for S, Si, and M (Fe, Mg, etc.) than are commonly assumed for the dense portions of dark clouds. 4 iii) For typical metal abundances (as determined in f Oph) and for cloud (H2) densities ^10 molecules cm-3 nearly all carbon exists as CO at late cloud ages. At lower densities, a significant fraction of the carbon is neutral and uncombined. For lower metal abundances (reduced by 102) 2 -3 nearly all carbon exists as CO, even for rc(H2) as low as 5X10 cm . iv) At high cloud densities many aspects of the chemistry are strongly time dependent. For example, within the cloud age window from 105 to 107 yr, many relative abundances change by as much as an order of magnitude. Among the species showing wide abundance variations with time are C, CH, CN, and C2H. Most of the carbon-containing molecules have reached their equilibrium values by ~5X106 yr. Many of the nitrogen-containing molecules equilibrate somewhat later. The evolutionary times are inversely proportional to the cosmic ray flux (a parameter uncertain to perhaps a factor of 5), and the time for the chemistry to reach equilibrium could thus be considerably longer than stated above. v) In general, the model calculations agree reasonably well with the abundances deduced from observations of molecular line emission in cold dense clouds. Model calculations for conditions corresponding to low electron abundances are able to predict the abundances of a number of species, + including CO, H2CO, HCO , CS, CH, CN, OH, HCN, and C2H. Model calculations for conditions -7 corresponding to high electron abundances [«(c)/«(H2)>fewX 10 ] predict much different values: in particular, much smaller abundances of HCO+ and CO and much larger abundances of C. These results are in good agreement with recent observations of the three species in the dense regions of giant molecular clouds in which high electron abundances are determined. Subject headings: atomic processes — interstellar: abundances — interstellar: matter — molecular processes ^ow at Jet Propulsion Laboratory, Pasadena, CA. 321 © American Astronomical Society • Provided by the NASA Astrophysics Data System IG .32 322 GRAEDEL, LANGER, AND FRERKING Vol. 48 .48. I. INTRODUCTION sequences involving N and S. The result of these consid- Interstellar clouds of gas and dust are prolific genera- erations and of connecting observed molecules with tors of molecules and molecular ions. In molecular plausible intermediate species is that we explicitly treat 63 different species (126 when carbon and oxygen iso- 1982ApJS. clouds with sufficient dust screening so that photochem- ical processes can be ignored, the chemical reactions topic variants are included) in our calculations. occur in thermally stable regimes of low temperatures The basic aspects of the ion-molecule carbon and (10-50 K) and densities in the range 5X102< oxygen chemistry in our formulation are similar to those H2(molecules cm-3) < 4X105. Theoretical studies of that have evolved from the early work of Herbst and the chemistry of molecular clouds have yielded signifi- Klemperer (1973) and Watson (1973). The chemistry of cant information over the past several years, as well as silicon, sulfur, and nitrogen, however, differs in many suggesting areas requiring further investigation. A prin- important respects from previous calculations. The most cipal motivation for the present work is to investigate significant difference is the omission of reactions involv- theoretically the processes of time-dependent carbon ing the transfer of a molecular hydrogen ion to nitrogen and oxygen isotope fractionation in all observable and sulfur atoms. These aspects are discussed in more species. A second motivation is to study the time-depen- detail below. dent chemistry of nitrogen-containing compounds in dense clouds. Neither of these areas of interest can be h) Initial Conditions investigated without a detailed description of interstellar cloud chemistry; as a result, the computational formu- A convenient reference for the abundances of the lation we have devised has broad application to inves- individual atoms relative to that of hydrogen are those tigations of the time-dependent chemistry of molecular listed by Mitchell, Ginsburg, and Kuntz (1978) for clouds. f Oph (see Table 1). The abundances for all molecules Our work is to be presented in three parts, for pur- except H2 are initially set to zero in our calculations. We poses of both convenience and tractability. In the pre- assume C, Si, S, and M ( = Mg+Fe) to be initially sent paper we describe the model in sufficient detail so ionized and N and O to be initially neutral. The fraction that our results can be individually assessed and go on of helium that is ionized is established by the ratio of the + to present selected results and conclusions of interest. principal He generation reaction: The emphasis in the latter is on the time-dependent + characteristics of the most abundant molecules, particu- cosmic ray(CR) + He ^ He + e + CR, larly with respect to sensitivity studies in which metal + abundance, temperature, cosmic-ray flux, initial ioniza- and the principal He destruction reaction at / = 0: tion state, and initial cloud density are varied. In a He+ +H ->He+H+H+. companion paper (Langer et al. 1982; hereafter Paper 2 II) we discuss in more detail the portions of the calcula- tion pertaining to carbon and oxygen isotope fractiona- tion. A second companion paper (Frerking, Langer, and TABLE 1 a Graedel 1982; hereafter Paper III) presents our results Elemental Abundances (relative to H2) for the nitrogen-containing molecules, together with the results of related observational studies. Element Abundance He 0.28 160 3.52 X10-4 II. THE CHEMICAL MODEL 180 7.04X1CT7 12C+ 1.46X10-4 a) Introduction 13C+ 2.24 X 10~6 N 4.28 X10-5 The complexity of a chemical kinetic calculation is S+ 1.66 X10-5 dictated almost entirely by the number and type of Si+ 1.66 X10-6 M+ 2.58 X HT6 species that are included. The study of time-dependent 4 carbon and oxygen isotope fractionation in dense clouds e 1.69XHT requires that an extensive reaction sequence leading a from C+ to H CO be included. This sequence must be Values are those given by Mitchell et al. 2 1978 for f Oph, except that 13C+ and 180 more than doubled to permit individual assessment of are set by the choice of the carbon and the 12C and 13C and 160 and 180 isotopic forms of each oxygen isotope ratios and He+ is calcu- compound, as well as to examine molecules with mixed lated as described in the text. The electron carbon and oxygen isotopes. The study of the time- abundance is set equal to the sum of the positive ion abundances. The initial ioniza- dependent evolution of nitrogen- and sulfur-containing tion states are indicated here and dis- molecules and ions requires the formulation of reaction cussed in the text. © American Astronomical Society • Provided by the NASA Astrophysics Data System IG .32 No. 3, 1982 DENSE INTERSTELLAR CLOUD CHEMISTRY 323 .48. The electron abundance is set to the sum of the positive mitting abundances and rate parameters to dictate the ion abundances. branching ratios for competing channels. For the prin- The assumption that all hydrogen is initially in molec- cipal chemical species, our selections introduce little ular form requires some discussion because clouds con- error when compared with the results of calculations 1982ApJS. tain primarily atomic hydrogen when they first form. including more reactions (see § III), yet they are suffi- Molecular hydrogen cannot be made efficiently in the ciently tractable to permit us to explore the effects of a gas phase and is formed almost exclusively on grains. wide range of choices of important physical parameters. Observational studies of H2 abundances in diffuse clouds The ordering of Table 2 follows the approach of are in excellent agreement with theoretical models Prasad and Huntress (1980a), who begin with atomic (Spitzer and Jenkins 1975) and provide strong support reactions and proceed to reactions of molecules com- for this mechanism.
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