The nature of CO and H2O ices in the Corona Australis molecular cloud

Abstract

Low-resolution spectra at 2.0 to 3.8 micrometers were obtained for 11 sources in the R CrA molecular cloud. The 3 micrometers absorption due to H2O ice was detected in three types of sources including protostars, T Tauri-like stars, and background field stars. There is a tendency that the protostars with large H-K have large tau_{3 micrometers}, but no clear linear correlation can be seen between tau_{3 micrometers} and H-K as is seen in the Taurus and the rho Ophiuchi dark clouds. This scattered distribution suggests the significant contribution of the circumstellar hot dust to the H-K color and/or sublimation of H2O ice at the inner region of the circumstellar disk due to heating by protostars. Medium-resolution spectra at 4.7 micrometers were also obtained for nine sources. The deep absorption at 4.67 micrometers due to solid CO was detected in four protostars and marginally in one background field star, but none in the T Tauri-like stars. Among them, the optical depth of CO ice in IRS 2 is the largest so far detected. It is found that the absorption feature of each source has almost the same central frequency and FWHM which coincide with the calculated values of small grains with dominant CO mantle. On the other hand, the column density of CO ice is found to be substantially smaller than that of H2O ice. This apparent contradiction suggests that the grain mantle has a layered structure of H2O and CO ices possibly due to the difference in the conditions of condensation of these molecules onto grain surfaces. Fr om an analysis of previous observations we find that the column density ratio of CO ice to H2O ice is 0.25 for background field stars and suggest that approximately 25% of CO exists in the grain mantles in dark clouds.

The nature of CO and H2O ices in the Corona Australis molecular cloud

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Abstract

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