1 Introduction

It has often been asserted that water and molecular oxygen are important reservoirs of the elemental oxygen in the ISM. It is also frequently assumed that they provide major sources of cooling. Atmospheric absorption impedes ground based observations of both of these species and it was not until the advent of SWAS (Submillimeter Wave Astronomy Satellite) that direct detection of likely components of the gas phase oxygen reservoir in dense cores could be made. SWAS has been used in pointed observations of the ground state ortho-H₂O and low energy O₂ transitions towards regions of high and low mass star formation and several clouds with little or no star formation. These observations have given severe constraints for theoretical models of interstellar clouds (see the special issue of ApJL, Vol. 539, Part II). On the other hand, ISO - SWS observations (Vandenbussche et al. 1999) have provided other constraints on the abundance of solid O₂ which accounts less than 6% of the total oxygen budget in the dense interstellar clouds.

SWAS data show that in the Orion Molecular Cloud, gaseous water and O₂ are not the main reservoirs of elemental oxygen. Their abundances account for much less than 1% of the total abundance of oxygen; the total is constrained by observations of diffuse clouds where the fractional abundance of all the oxygen not locked in grains (in silicates) is of the order of $\sim$3.2 10^-4. In dense regions of interstellar space, however, a large portion of the oxygen is found on grains as water ice.

Contrary to model predictions (e.g. Maréchal et al. 1997), the SWAS results also indicate that O₂ and H₂O are not the major coolants in general for molecular clouds, although in some localized regions, such as shocked, hot regions, ISO has shown that the gas-phase fractional abundance of water can be as high as $\sim$10^-4 (Harwit et al. 1998; Nisini et al. 2000).

Bergin et al. (2000) summarized the constraints yielded by SWAS and ISO as follows:

(1)

regardless of the line of sight, the fractional abundance relative to hydrogen nuclei of gaseous molecular oxygen, X(O₂), in dense molecular clouds is less than $\sim$10^-6;

(2)

in starless cores (such as core B in TMC-1) the fractional abundance of H₂O, X(H₂O), is less than 7 10^-8;

(3)

in low density regions, such as those along the line of sight towards Sgr B2, gaseous water is relatively abundant (X(H₂O) $\sim$10^-6) while O₂ is not detected; X(O₂) < 10^-6. The fractional abundance of water found by SWAS along the line of sight towards Sgr B2 also confirms previous ISO results (Moneti et al. 2001). If low temperature gas phase chemistry is responsible for the gaseous H₂O, this result is puzzling as molecular oxygen and water formation are coupled in the standard gas-phase chemistry: once H₃⁺ reacts with neutral oxygen, rapid reactions will lead to the formation of the ion H₃O⁺. The latter dissociatively recombines with electrons to form, among other species, OH and H₂O. OH then reacts with atomic oxygen to form molecular oxygen; if H₂O is abundant, one would expect O₂ to be abundant also;

(4)

in high density star formation regions gaseous X(H₂O) is $\sim$10^-8;

(5)

the fractional abundance of water ice is measured to be $\sim$10^-4 in dense cores (10³ $\le$ $n_{\rm H}$ cm^-3$\le$ 10⁶ where $n_{\rm H}$ is the number density of hydrogen nuclei; $T_{\rm gas}$ < 50 K) (Shutte 1999).

Bergin et al. remarked that the low abundance of water in star formation regions cannot be accounted for by rapid photodissociation by intense UV radiation fields as other species, such as NH₃, would also be destroyed. Such species are however observed to be abundant.

The constraints imposed by SWAS (and ISO) represent a challenge for theorists. Why do simple models predict large abundances at steady-state for gaseous H₂O and O₂? Where is the oxygen in quiescent clouds, and in regions of low-mass and high-mass star formation? Chemical models must self-consistently reproduce: (i) the gas-phase molecular oxygen and water abundances observed with SWAS; (ii) the water ice abundance measured with ISO; (iii) the abundances of simple and complex oxygen and carbon bearing species observed with ground based telescopes, and (iv) the observed abundances in a variety of different environments such as regions of high and low mass star formation and starless cores. We note, however, that the SWAS beam is as large as 4 arcmin and that the SWAS observations inevitably encompass a variety of interstellar and circumstellar regions; in effect, the SWAS results give averages over a variety of conditions.

In this paper we explore two theoretical approaches which may provide solutions to the constraints imposed by SWAS while still satisfying the constraints of observations of other simple and complex species. Section 2 is dedicated to the first approach: we investigate time-dependent models where gas phase species are allowed to deplete on to the grains. Bergin et al. (2000) showed that the SWAS data can be explained with low temperature gas phase models only if the observed clumps are short-lived, and we explore this point in detail. We show that "transient" states in which the chemistry satisfies constraints arising from SWAS and other observations are very long-lived in models where most species freeze-out efficiently on to dust grains except for CO and N₂ which are returned promptly to the gas phase. Section 3 concerns the applicability of bistability solutions to the interpretation of the SWAS results. The gas phase chemistries discussed in Sects. 2 and 3 are identical. Section 4 presents a discussion of our findings.