THE ABUNDANCES OF SOLID AND GASEOUS IN INTERSTELLAR DENSEN2 CO2 MOLECULAR CLOUDS

We present 2338È2322 cm~1 (4.277È4.307 km) infrared spectra of a number of mixedN2-containingmolecular ices and demonstrate that the strength of the infrared forbidden ÏÏ band due to the NN stretch near 2328 cm~1 (4.295 km) is extremely sensitive to the composition of the ice. The strength of the 2328 cm~1 fundamental is signiÐcantly enhanced relative to that of pure ice whenN2 N2 NH3, H2O,or are present, but it is largely unaected by the presence of CO, or We use the labor-CO2 CH4, O2.atory data in conjunction with Infrared Space Observatory (ISO) data that probe several lines of sight through dense molecular clouds to place limits on the abundance of interstellar solid phase and theN2composition of the ices. Deriving upper limits is complicated by the presence of overlapping absorptions due to gas in the clouds and, in some cases, to photospheric CO in the background star. TheseCO2upper limits are just beginning to be low enough to constrain interstellar grain models and the composi-tion of possible interstellar ices. We outline the search criteria that will need to be met ifN2-bearingsolid interstellar is to be detected in the future. We also discuss some of the implications of theN2presence of warm gas along the lines of sight to embedded protostars and demonstrate that itsCO2presence may help resolve certain puzzles associated with the previously derived gas/solid ratiosCO2and the relative abundances of polar and nonpolar ices toward these objects. Finally, we brieÑy comment on the possible implications of these results for the interpretation of detections on outerN2solar system bodies