PROCESSING OF ICY MANTLES IN PROTOSTELLAR ENVELOPES

We have obtained CO absorption profiles of several young stellar objec ts (YSOs), spanning a range of mass and luminosity, in order to invest igate their ice mantle composition. We present the first detection of CO toward the class I YSO L1489 IRS in the Taurus dark cloud. In gener al, the CO profiles for YSOs show evidence for both processed and pris tine ices in the same line of sight; strong indirect evidence for CO, is suggested in R CrA IRS 7, L1489 IRS, Elias 18, and GL 961E. Toward other sources (R CrA IRS 1, IRS 2, W33A, NGC 7538 IRS 9, Mon R2 IRS 2) CO is present in (nearly) pure form. We propose an evolutionary scena rio to explain the chemical diversity of the icy mantles toward these objects.