We report the detection of a unique CO$_{2}$ ice band toward the deeply embedded, low-mass protostar HOPS-68. Our spectrum, obtained with the Infrared Spectrograph on board the Spitzer Space... Show moreWe report the detection of a unique CO$_{2}$ 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 {$μ$}m CO$_{2}$ ice bending mode profile that cannot be modeled with the same ice structure typically found toward other protostars. We develop a modified CO$_{2}$ ice profile decomposition, including the addition of new high-quality laboratory spectra of pure, crystalline CO$_{2}$ ice. Using this model, we find that 87%-92% of the CO$_{2}$ is sequestered as spherical, CO$_{2}$-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, CO$_{2}$ 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 CO$_{2}$ ice profile, the formation of nearly pure CO$_{2}$ ice, 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. Show less
Wang, K.; Bourke, T.; Hogerheijde, M.R.; Tak, F.F.S. van der; Benz, A.; Megeath, S.; Wilson, T. 2013
Context. Two competing models describe the formation of massive stars in objects like the Orion Trapezium. In the turbulent core accretion model, the resulting stellar masses are directly related... Show moreContext. Two competing models describe the formation of massive stars in objects like the Orion Trapezium. In the turbulent core accretion model, the resulting stellar masses are directly related to the mass distribution of the cloud condensations. In the competitive accretion model, the gravitational potential of the protocluster captures gas from the surrounding cloud for which the individual cluster members compete. Aims: With high resolution submillimeter observations of the structure, kinematics, and chemistry of the proto-Trapezium cluster W3 IRS5, we aim to determine which mode of star formation dominates. Methods: We present 354 GHz Submillimeter Array observations at resolutions of 1{Prime}-3{Prime} (1800-5400 AU) of W3 IRS5. The dust continuum traces the compact source structure and masses of the individual cores, while molecular lines of CS, SO, SO$_{2}$, HCN, H$_{2}$CS, HNCO, and CH$_{3}$OH (and isotopologues) reveal the gas kinematics, density, and temperature. Results: The observations show five emission peaks (SMM1-5). SMM1 and SMM2 contain massive embedded stars (~{}20 M$_{⊙}$); SMM3-5 are starless or contain low-mass stars ({lt}8 M$_{⊙}$). The inferred densities are high, {ge}10$^{7}$ cm$^{-3}$, but the core masses are small, 0.2-0.6 M$_{⊙}$. The detected molecular emission reveals four different chemical zones. Abundant (X ~{} few 10$^{-7}$ to 10$^{-6}$) SO and SO$_{2}$ are associated with SMM1 and SMM2, indicating active sulfur chemistry. A low abundance (5 { imes} 10$^{-8}$) of CH$_{3}$OH concentrated on SMM3/4 suggest the presence of a hot core that is only just turning on, possibly by external feedback from SMM1/2. The gas kinematics are complex with contributions from a near pole-on outflow traced by CS, SO, and HCN; rotation in SO$_{2}$, and a jet in vibrationally excited HCN. Conclusions: The proto-Trapezium cluster W3 IRS5 is an ideal test case to discriminate between models of massive star formation. Either the massive stars accrete locally from their local cores; in this case the small core masses imply that W3 IRS5 is at the very end stages (1000 yr) of infall and accretion, or the stars are accreting from the global collapse of a massive, cluster forming core. We find that the observed masses, densities and line widths observed toward W3 IRS 5 and the surrounding cluster forming core are consistent with the competitive accretion of gas at rates of {.M} ~{} 10$^{-4}$M$_{⊙}$ yr$^{-1}$ by the massive young forming stars. Future mapping of the gas kinematics from large to small scales will determine whether large-scale gas inflow occurs and how the cluster members compete to accrete this material. Show less
