The chemistry of the Interstellar Medium


Chemical models of interstellar environmnents

As a general rule, the purpose of a chemical model of interstellar regions is essentially to identify the main chemical routes of formation and destruction o f the particular observed species. At the same time, a chemical model is a valuable tool to obtain informations on the physical environment in which molecule s form and interact. Usually two kinds of approaches exist in the field of chemical modelling of diffuse clouds.

On one hand, one can build detailed models of a given single line of sight which could explain a large range of observed atoms and molecules. This methods has been used pioneristically by Black & Dalgarno (1977) for the well-known zeta Oph, and by Black, Hartquist & Dalgarno (1978) for zeta Per. These comprehensive models have been revisited, in the classical works by van Dishoeck & Black (1986) and Viala, Roueff and Abgrall (1988) and more recently by Wagenblast & Williams (1996). This method has the advantage to infer the more realistic possible conditions for a given line of sight, but obviously these conditions are not easily ``exportable'' to the physics of diffuse clouds in general, since the may depend on peculiarity of the region modelled.

An alternative possibility consists in starting from a as much larger as possible number of observational data of a given molecule towards many diffuse lines of sight, and try to build a grid of simple global chemical models which can explain the abundance of that molecule in terms of the physical conditions. This approach has been used, for example, by Federman and co-workers for CH and CN (Federman, Danks and Lambert 1984; Danks, Federman and Lambert 1984; Gredel, Pineau des Forets and Federman 2002) and more recently by Turner et al. (1999, 2000) . This method is in principle the opposite of the previous and has the natural advantage of its ``globality''. The natural disadvantage is that it requires a s trong effort in collecting data and in compute a statistically significative grid of models. This work follows the latter approach.

Steady state chemical models

Despite the long history of gas-phase chemical modelling (see Watson, 1976; van Dishoeck and Black 1986; Zsargò and Federman, 2003 and references therein) and the large amount of observational work, some long-standing enigmas and a few recent failures push for a re-visitation of commonly accepted schemes and hint that diffuse clouds are much more complex than usually assumed. Among these enigmas, observations show that complex molecules may achieve dark clouds abundances in the DISM (Lucas and Liszt, 2002, and references therein). The interpretation of such a phenomenon is still lacking a robust observational scenario and it is doubtful whether present theoretical models may be reconciled with observed column densities.

To assess this uncertainty, we are exploring the applicability of the standard gas-phase chemistry to the physical interpretation of observations in diffuse and translucent clouds. We are currently computing an extensive grid (~4500) of depth-dependent, quiescent, steady-state chemical models, exploring the dependences of the predicted column densities on a wide range of several physical parameters (see Table 1)
Models grid
0.1 25 3.1 1 10
1.0 50 4.0 2 30
10. 100 5.0 3 50
200 4 70
5 100
6 300
representative of the common definition of "diffuse molecular" medium. The final goal of this line of research is the creation of a large database of chemical results (photodissociation rates, volume and column densities, etc..) for a very large range of physical conditions. The final database will be available online in this web page and in ARS Italian Network web page). .

Our adopted chemical network has been constructed from 170 species consisting of the elements H, He, O, C, S and N. We selected from the UMIST data file (Millar et al. 1997) all the relevant reactions which couple the adopted species. The standard UMIST reaction network has been modified by incorporating the branching ratios for dissociative recombination given by Andersen et al. (1996), Vejby-Christensen et al. (1997), Larson et al. (1998) and Derkatch et al. (1999).

We assumed a plane parallel geometry for all the modelled clouds. The model cloud is illuminated by a normally incident UV radiation field, whose intensity is given in Draine (1978). A scaling factor IUV has been used to vary this intensity. The cosmic-ray ionization rate was assumed to be zeta=3x10-17s-1.

The radiative transfer problem has been solved by means of the SHM (Spherical Harmonic Method), Case & Zweifel 1967; Flannery, Roberge & Rybicki 1980). We describe the depth-dependent H2 and CO photodissociation rates by shielding functions (van Dishoeck & Black, 1988; Sternberg & Dalgarno 1995).

We computed the column density of each species included in the network for all the possible combinations of physical parameters. As an example, Fig. 1 shows the expected correlation between N(CO) and N(HCO+)(left panel) and between N(HCN) and N(CS) (right panel) for a subset of physical parameters (see caption): the results are compared with the observed correlation (Lucas and Liszt, 2002).


Fig. 1: Left panel: correlation between HCO+ and CO column densities for different values of gas density and visual extinction for RV = 3.1; Rigth panel: the same for N(HCN) and N(CS). The meaning of each symbol is reported in the legend. Black filled triangles show observational data as reported by Lucas and Liszt 2002. Green: T = 25 K; red: T = 50 K; blue: T = 100 K.(Casu & Cecchi-Pestellini, in preparation)

The comparison between model results and observations suggests that standard chemical routes produce too low values of the HCO+ column density and fail miserably to reproduce the observed column density of CS. Similar considerations can be made for other molecular species, the main result being that standard chemical models in the physical conditions usually considered as typical of diffuse clouds (T ~ 100 K, nH ~ 100 cm-3, AV<2 mag) are unable to meet the observational constraints. More specifically, there is no possible combinations of plausible physical parameters which may explain the observed column densities in a global scenario.

Silvia Casu
Last modified: Thu Sep 23 13:33:18 CEST 2004