Direct measurement of the CO depletion in the Elias 18 circumstellar disk

In this Letter we present low-resolution 2.3 mum and high-resolution 4.6 mu m spectra of Elias 18 (IRAS 04369+2539), a low-mass young stellar object (Y SO) in the Taurus molecular cloud. The 2.3 mm spectrum shows CO band head e mission (v = 2-0) indicating a circumstellar disk. The 4.6 mum spectrum sho ws very weak CO absorption in the lowest J levels. Comparing preexisting CO ice observations to our data indicates that there is at least 30 times les s gas-phase CO than solid-state CO in the outer, cold (T < 20 K) regions of the disk. High depletions of CO in YSO disks have been inferred from milli meter-wave CO emission and detailed disk models, but Elias 18 is the first case for which both gas- and ice-phase data have been directly compared.