2013ApJ...766..117P

We report the detection of a unique CO₂ ice band toward the deeply embedded, low-mass protostar HOPS-68. Our spectrum, obtained with the Infrared Spectrograph on board the Spitzer Space Telescope, reveals a 15.2 µm CO₂ ice bending mode profile that cannot be modeled with the same ice structure typically found toward other protostars. We develop a modified CO₂ice profile decomposition, including the addition of new high-quality laboratory spectra of pure, crystalline CO₂ice. Using this model, we find that 87%-92% of the CO₂ is sequestered as spherical, CO₂-rich mantles, while typical interstellar ices show evidence of irregularly shaped, hydrogen-rich mantles. We propose that (1) the nearly complete absence of unprocessed ices along the line of sight is due to the flattened envelope structure of HOPS-68, which lacks cold absorbing material in its outer envelope, and possesses an extreme concentration of material within its inner (10 AU) envelope region and (2) an energetic event led to the evaporation of inner envelope ices, followed by cooling and re-condensation, explaining the sequestration of spherical, CO₂ ice mantles in a hydrogen-poor mixture. The mechanism responsible for the sublimation could be either a transient accretion event or shocks in the interaction region between the protostellar outflow and envelope. The proposed scenario is consistent with the rarity of the observed CO₂ ice profile, the formation of nearly pure CO₂ice, and the production of spherical ice mantles. HOPS-68 may therefore provide a unique window into the protostellar feedback process, as outflows and heating shape the physical and chemical structure of protostellar envelopes and molecular clouds.