Probing the surfaces of interstellar dust grains: the adsorption of CO at bare grain surfaces

A solid-state feature was detected at around 2175 cm[-1] towards 30 embedded young stellar objects in spectra obtained using the Infrared Spectrometer and Array Camera at the European Southern Observatory Very Large Telescope. We present results from laboratory studies of CO adsorbed at the surface of zeolite wafers, where absorption bands were detected at 2177 and 2168 cm[-1] (corresponding to CO chemisorbed at the zeolite surface) and 2130 cm[-1] (corresponding to CO physisorbed at the zeolite surface), providing an excellent match to the observational data. We propose that the main carrier of the 2175-band is CO chemisorbed at bare surfaces of dust grains in the interstellar medium. This result provides the first direct evidence that gas-surface interactions do not have to result in the formation of ice mantles on interstellar dust. The strength of the 2175-band is estimated to be ∼4 x 10[-19] cm molecule[-1]. The abundance of CO adsorbed at bare grain surfaces ranges from 0.06 to 0.16 relative to H[2]O ice, which is, at most, half of the abundance (relative to H[2]O ice) of CO residing in H[2]O-dominated ice environments. These findings imply that interstellar grains have a large (catalytically active) surface area, providing a refuge for interstellar species. Consequently, the potential exists for heterogeneous chemistry to occur involving CO molecules in unique surface chemistry pathways not currently considered in gas grain models of the interstellar medium.