2011ApJ...727...36S

We present Spitzer Space Telescope spectroscopy of the CO₂ ice absorption feature at 15.2 µm toward 41 high-mass young stellar objects in the Large Magellanic Cloud (LMC). As the shape of the CO₂ absorption profile is a measure of both the composition and thermal history of the ice, we have performed a decomposition of the spectral profiles to determine the nature of the CO₂ ice. We fit the absorption profiles to laboratory analogues of ice spectra with two different methods: (1) a five-component fit with polar and apolar ices and (2) a two-component fit with a polar and an annealed H₂O:CH₃OH:CO₂ ice mixture. Many of the LMC sources have a pronounced double peak in their CO₂ feature profiles analogous to that seen from pure CO₂or annealed CO₂ laboratory ice mixtures; these represent the first direct detections of the characteristic double peak in an extragalactic environment. Fits to annealed laboratory ices suggest that the ices around massive LMC young stellar objects (YSOs) have been warmed and thermally processed. We find that a majority of the CO₂ is embedded in a polar ice matrix; however, the observations suggest that a lower fraction of CO₂ is locked in polar ices in the LMC compared to the Milky Way, which is in agreement with the proposed lower LMC abundance of water ice. In addition, we find that the ices are best fit with laboratory ice mixtures composed of less than 50% methanol, and most absorption spectra can be fit by ices with no methanol. Finally, we corroborate mounting evidence of an enhanced CO₂ ice abundance in the LMC relative to the Milky Way, and determine a CO₂/H₂ O ratio of 0.33±0.01 by combining the column densities of these observations with those in the literature.