EVIDENCE FOR CHEMICAL-PROCESSING OF PRECOMETARY ICY GRAINS IN CIRCUMSTELLAR ENVIRONMENTS OF PRE-MAIN-SEQUENCE STARS

We report the detection of a broad absorption feature near 2166 cm(-1) in the spectrum of the Taurus cloud source Elias 18. This pre-main-se quence source is the second in Taurus, the third in our survey, and th e fifth known in the sky to show the broad 2166 cm(-1) absorption feat ure. Of equal importance, this feature is not seen toward several othe r embedded sources in our survey, nor is it seen toward the source Eli as 16, located behind the Taurus cloud. Laboratory experiments with in terstellar ice analogs show that such a feature is associated with a c omplex C=N containing compound [called X(C=N)] that results from high- energy processing (ultraviolet irradiation or ion bombardment) of simp le ice components into more complex, organic components. We find a non linear anticorrelation between the abundance of X(C=N) and frozen CO i n nonpolar lattices. We find no correlation between the abundance of X (C=N) and frozen CO in polar lattices. Because the abundances of froze n CO and H2O are strongly correlated with each other and with visual e xtinction toward sources embedded in and located behind the Taurus mol ecular cloud, these ice components usually are associated with intracl oud material. Our results indicate that X(C=N) molecules result from c hemical processing of dust grains dominated by nonpolar icy mantles in the local environments of pre-main-sequence stars. Such processing of icy grains in the early solar system may be an important source of or ganic compounds observed in minor solar system bodies. The delivery of these organic compounds to the surface of the primitive Earth through comet impacts may have provided the raw materials for prebiotic chemi stry.