Oxygen, which is cosmically the most abundant element after H and He, plays an important role in interstellar chemistry and in the energy balance of interstellar clouds. Knowledge of its major reservoirs, both in the gas and on grains, is therefore essential. The principal oxygen-bearing species in diffuse and dense clouds have been the subject of considerable discussion (see van Dishoeck & Blake 1998 for a review). In some regions, up to 50% of the oxygen is unaccounted for if solar abundances are assumed.
Recently, a revised O0 abundance has been determined for the local diffuse interstellar medium using the Hubble Space Telescope. Measurements of the OI] line at 1356 Å toward several stars indicate gaseous atomic oxygen abundances of 319 14 per 106 nH (Meyer et al. 1998). Together with an estimated abundance of O in silicates of 180 per 106 nH, this comes close to the oxygen abundance in B stars. At the same time, ground-based, balloon-borne and ISO satellite data yielded abundances of several other oxygen-bearing species, in both the gas phase (including O0, O2 and H2O) and the solid state (including H2O, CO2 and CO) (see discussion in Sect. 4). A re-evaluation of the oxygen budget in dense clouds is therefore warranted.
Theoretical models predict that oxygen could be accreted onto grains from the gas in the form of solid O2 (Tielens & Hagen 1982), mixed with CO ice in apolar ices. O2 is an infrared inactive molecule, which does not have any signature in the infrared and radio range and is therefore difficult to observe. Different methods to detect solid O2 on interstellar grains have been discussed by Ehrenfreund & van Dishoeck (1998), and the most direct opportunity is the search for the weak fundamental O2 transition at 6.45 µm (Ehrenfreund et al. 1992). In the solid state, this transition becomes weakly infrared active due to interactions with neighboring atoms, and laboratory results indicate that the band strength of molecular oxygen depends on the ice matrix. Other constraints on the O2 abundance come from the analyses of solid CO profiles and from searches for photoproducts of O2 (such as O3, CO3, etc.). Strazzulla et al. (1997) searched for a means of indirectly detecting O2 and N2 through changes induced in the CO absorption profile as a result of ion irradiation and the products formed during the radiolysis. Here, we present new ISO-SWS and ground based data in order to constrain the solid O2 abundance in dense clouds.
© European Southern Observatory (ESO) 1999
Online publication: June 17, 1999