A Direct Measurement of the CO Depletion in the Elias 18 Circumstellar Disk

Abstract

In this Letter we present low-resolution 2.3 μm and high-resolution 4.6 μm spectra of Elias 18 (IRAS 04369+2539), a low-mass young stellar object (YSO) in the Taurus molecular cloud. The 2.3 μm spectrum shows CO band head emission (v = 2-0) indicating a circumstellar disk. The 4.6 μm spectrum shows 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 less 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 millimeter-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.