CO is one of the main components of ice mantles covering interstellar dust grains. The observed solid state CO profile at around 2140 cm-1 was previously decomposed into three components, the red, middle and blue components. In most of the observed sources, the middle component at ~2139.9 cm-1 is the dominant one. It was attributed to CO in the "pure" form with a CDE grain shape correction. Consequently, a layered model was proposed in which the bottom layer of the ice mantle consists of a mixture of CO2 and H2O, and the top layer consists of "pure" solid CO. Through laboratory measurement, we found that CO on the surface of porous water ice shows an absorption peak between 2139 and 2140 cm-1, which matches the position of the middle component. Through a fitting of the VLT-ISAAC and NIRESPEC spectra toward star formation regions, we found that solid CO profile in many sources can be fit well using one Gaussian component at around 2140 cm-1 due to CO interacting with water surface, and another Gaussian component at 2135-2136 cm-1 due to CO trapped inside water. We conclude that in at least some of the sources the 2140 cm-1 component is primarily due to CO interacting with water surface, instead of CO in the "pure" solid state form. We also suggest that the 2152 cm-1 component observed in the laboratory but not in space can be explained by the blocking of dangling OH sites by CO2.

Is carbon monoxide in the pure solid form in the ice mantle?

254th National Meeting (2017)

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