A New Study of Carbon Monoxide Depletion: Gas versus Ice

CO is a vital molecule in astrochemistry. It is the second most common molecule in interstellar space after molecular hydrogen (which is much harder to detect), and millimeter-wave observations of gaseous CO are used extensively to map molecular material. CO is also a major repository for the biologically important chemical elements carbon and oxygen in the clouds, and its ability to participate in chemical reactions is the key to synthesis of complex organic molecules that might accumulate into new planetary systems and provide a resource for the origin of life.

In a recent paper published in Astrophysical Journal, we have studied the distribution of CO between the gaseous and solid phases of an interstellar cloud. It has long been known that CO tends to freeze out onto dust grains in the coldest regions of the interstellar medium, but the degree of depletion (i.e., the fraction of CO frozen onto the dust relative to the total amount of CO) has been difficult to quantify. In a collaboration between teams at  RPI and NASA’s Jet Propulsion Laboratory, we have addressed this problem by comparing observations of ices made at infrared wavelengths with observations of gas made at radio wavelengths in the same lines of sight toward the Taurus Dark Cloud. We find that the depletion increases rapidly with the total quantity of dust, and exceeds 60% toward the densest cores. We show that it is plausible for such high levels of depletion to be reached in dense cores on timescales of about 6oo,ooo years, comparable with their expected lifetimes. Dispersal of cores during star formation may be effective in maintaining observable levels of gaseous CO on the longer timescales (a few million years) estimated for the age of the cloud.

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