3 MICRON ICE-BAND FEATURES IN THE RHO-OPHIUCHI SOURCES

Low-resolution spectra in 2.4-3.8 μm were obtained for 31 sources of various kinds in the ρ Oph dark cloud. The 3 μm absorption was detected in almost all the embedded sources: protostars, emission-line stars, and early-type stars. For the seven protostars, which have H-K magnitude larger than 2 mag, the optical depth at 3 μm due to ice-band absorption, τice, increases linearly with H-K and can be expressed by τice = 0.68 (H-K - 1.26). This relation indicates the critical visual extinction, ,AVC, of 10-15 mag, which is larger than 3 mag reported for the Taurus dark cloud. The larger Avc for the ρ Oph cloud implies the stronger UV radiation field. One of the remarkable results is that the emission-line stars are present with the optical depth of ≃0.2 at 3 μm in spite of the fact that their Aυ's are smaller than Avc. This indicates that the ice mantle has still remained within their circumstellar disks in an environment with strong UV radiation, if the absorption is assumed to be attributed to H2O ice. The existence of ice-mantled grains around such stars strongly suggests that the key process that controls formation of H2O ice in dark clouds is (a) formation of H2O vapor but not condensation of H2O vapor onto grains, in the case that H2O is formed in a gas phase, and (b) adsorption of radicals and atoms on grain surfaces but not H2O formation from the radicals, in the case that H2O is formed on grains. Furthermore, it is concluded that almost all H2O molecules in dark clouds exist as grain mantles; i.e., in a solid phase, not in a gas phase. The "shoulder" component of the absorption profile at longer wavelength is deeper for emission-line stars than for protostars, suggesting that the mantle ice has been altered during the evolution from protostars to emission-line stars. The background M giants show a distinct absorption profile peaked at ∼2.95 μm, which cannot be explained by only H2O ice and H2O vapor.