Solid carbonyl sulfide (OCS) in dense molecular clouds

An absorption feature at 2040 cm(-1) (4.90 mu m), previously observed only in W33A, has been detected toward two deeply embedded young stellar objects (YSOs), AFGL 989 and Mon R2 IRS 2. Upper limits are reported for several other YSOs and for one object located behind the Taurus Dark Cloud. We attribute this interstellar feature to solid carbonyl sulfide (OCS) embedded in icy grain mantles along the line of sight. As in the case of W33A, the best match of the newly observed features with laboratory spectra of astrophysically relevant mixtures is obtained for traces of OCS in a methanol-rich matrix. This, again, suggests the presence of independent grain mantle populations and, in particular, of a minor fraction of methanol-rich icy grain mantles in which OCS is embedded. From the strength of the absorption feature we deduce OCS column densities and OCS/H2O ratios toward the observed objects. Taking into consideration sulfur chemistry and the origins of solid OCS, we conclude that a major fraction of the elemental sulfur is presently unaccounted for in dense molecular clouds.