Interstellar clouds, chemical models

With so many molecules now being observed in interstellar clouds, chemical reaction models which can explain how these molecules are produced and destroyed are becoming increasingly more valuable. The most modern chemical reaction networks that have been proposed involve following the concentration of several hundred atomic and molecular species as a function of time, and reliable temperature-dependent rate coefficients for several thousand reactions are a vital requirement in such simulations. The role of ion-molecule reactions has been shown to be of particular Importance in these networks as these reactions can have very large rate coefficients at the low temperatures of interstellar clouds [2]. Furthermore, a more limited number of neutral species, particularly radicals and open-shell atoms, can have large rate coefficients at low temperatures [3]. Since only a relatively small number of reactions have been studied in the laboratory at the temperatures relevant to Interstellar chemistry, theory plays an Important role in producing many of the required rate coefficients. 

The first question to ask about the formation of interstellar molecules is where the formation occurs. There are two possibilities the molecules are formed within the clouds themselves or they are formed elsewhere. As an alternative to local formation, one possibility is that the molecules are synthesized in the expanding envelopes of old stars, previously referred to as circumstellar clouds. Both molecules and dust particles are known to form in such objects, and molecular development is especially efficient in those objects that are carbon-rich (elemental C > elemental O) such as the well-studied source IRC+10216.12 Chemical models of carbon-rich envelopes show that acetylene is produced under high-temperature thermodynamic equilibrium conditions and that as the material cools and flows out of the star, a chemistry somewhat akin to an acetylene discharge takes place, perhaps even forming molecules as complex as PAHs.13,14 As to the contribution of such chemistry to the interstellar medium, however, all but the very large species will be photodissociated rapidly by the radiation field present in interstellar space once the molecules are blown out of the protective cocoon of the stellar envelope in which they are formed. Consequently, the material flowing out into space will consist mainly of atoms, dust particles, and possibly PAHs that are relatively immune to radiation because of their size and stability. It is therefore necessary for the observed interstellar molecules to be produced locally. 

Although most of the ion-molecule reactions used in large chemical models of interstellar clouds have not been studied in the laboratory, a few classes of reactions... 

Figure 2. The new species added to our chemical models of interstellar clouds. The species range in complexity from 10-64 carbon atoms and comprise the following groups of molecules linear carbon chains, monocyclic rings, tricyclic rings, and fullerenes. The synthetic pathways are also indicated. See ref. 83. Reproduced from the International Journal of Mass Spectrometry and Ion Processes, vol. 149/150, R.P.A. Bettens, Eric Herbst "The interstellar gas phase production of highly complex hydrocarbons construction of a model", pp 321-343 (1995) with kind permission from Elsevier Science-NL, Sara Burgerhartstraat 25,1055 KV, Amsterdam, The Netherlands.

Although there is no substitute for accurate measurements, it is unlikely that experiments will provide rate constants for all the reactions that may be important in chemical models of interstellar clouds. Already, however, the measured values provide a fertile testing ground for theories of the lands propounded by Clary [10] and Troe [6]. Until such time as either experiment or detailed theory provides a reliable kinetic data base, modellers are likely to appreciate guidelines as to how they might estimate approximate rate constants for neutral-neutral reactions between 10 and 50 K, since it is now clear that the rate of any neutral-neutral reaction which has a rate constant within an order of magnitude of the collisional value at room temperature may have a rate constant which is determined by capture [21] and which may increase as the temperature is lowered. 

We have developed recently a comprehensive model of the f Oph diffuse interstellar cloud (Viala et al, 1987) where the populations of the first ten rotational levels of and HD have been calculated by taking into account UV pumping to excited electronic states followed by fluorescence and radiative decay inside the ground state, collisional processes, and chemical... 

The fractionation of deuterium in interstellar molecules continues to excite considerable interest. Cosmologists identify the cosmic D/H ratio as a parameter critical to the assessment of cosmological models. Astrophysicists can use the isotopic ratio of species found in interstellar clouds as a probe of the conditions in those clouds. Isotopic abundances can help ion chemists to map synthetic pathways for forming interstellar molecules. Rnally to chemical kineticists, interested in the formation of interstellar molecules at temperatures approaching absolute zero, isotope effects offer a unique challenge — what is a minor perturbation at 300 K must exercise a profound influence at 10 K. Thus the equilibrium constant for the reaction... 

Much of our understanding of the chemistry of dense interstellar clouds derives from chemical models of these sources. A chemical model of an interstellar cloud is simply the solution of coupled differential equations that contain the time derivatives of the concentrations of individual molecular species. Each individual differential equation relates the time dependence of a specific molecular concentration to its formation and depletion rates. For example, consider the formation and depletion of a molecular ion labelled C" " ... 

The most complex type of model is a time-dependent approach in which physical as well as chemical conditions evolve. Time-dependent methods involving both diffuse and dense clouds have been reviewed by Prasad et al. (1987) within the last year. Since no major models have appeared since the time of that review, the subject of time-dependent models will not be raised at length here. A brief history of die method is in order, however, especially as regards calculations involving dense interstellar clouds. TTie simplest approach is to allow the cloud initially under diffuse conditions to undergo a free-fall or delayed free-fall collapse concomitandy with chemical evolution. Some early treatments of this variety include those of Kiguchi et al. (1974) and Suzuki et al. (1976). 

Chemical models of dense interstellar clouds have grown in complexity in recent years (see Herbst, this volume). However, since molecules are widely observed in the gas phase, many modellers have chosen to ignore the effects of accretion. Gas phase atoms and radicals, upon collision with cold dust grains present in the interstellar medium, are expected to stick to the dust forming an icy mantle and thus leaving the gas phase. This is thought to occur on a timescale comparable to the expected lifetime of the clouds. 

ABSTRACT. The physics of interstellar clouds affects the chemistry within them one must consider the likely physical conditions before specifying the chemistry in detail. In many dark cloudSi both observations and theory indicate that material is continually recycled between dense and tenuous states and that the dynamics of this recycling drastically affects the chemical evolution. In diffuse clouds, recent studies of interstellar grains indicate that there is a continual interaction between gas and dust which is limited by intermittent shocks. Thus, in both situations the timescales available for chemistry are relatively brief and so constrain the variety and nature of the chemical reactions to be included in chemical models. 

We begin this section by discussing time scales appropriate for the physical and chemical evolution of cold cores, also known as dark interstellar clouds, before discussing various models for describing the gas-phase chemistry and the uncertainty associated with such models. It should be mentioned that kinetic treatments of the chemistry are needed because the time scales to reach chemical equilibrium are far longer than the lifetimes of the clouds, and because the chemistry is powered by an external energy source. 

Herbst E, Leung CM. (1986) Effects of Large Rate Coefficients for Ion-Polar Neutral Reactions on Chemical Models of Dense Interstellar Clouds. Astrophys. J. 310 378-382. 

Herbst E, Lee HH, Howe DA, Millar TJ (1994) The effect of rapid neutral-neutral reactions on chemical-models of dense interstellar clouds. Mon Not R Astron Soc 268 335-344... 

Hasegawa n, Herbst E (1993) New gas-grain chemical models of quiescent dense interstellar clouds - the effects of H2 tunnelling reactions and cosmic ray induced desorption. Mon Not Roy Astron Soc 261 83-102... 

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