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The Boomerang Nebula is the coldest known object in the Universe, and anextreme member of the class of Pre-Planetary Nebulae, objects which represent ashort-lived transitional phase between the AGB and Planetary Nebulaevolutionary stages. Previous single-dish CO (J=1-0) observations (with a 45arcsec beam) showed that the high-speed outflow in this object has cooled to atemperature significantly below the temperature of the cosmic backgroundradiation. Here we report the first observations of the Boomerang with ALMA inthe CO J=2-1 and J=1-0 lines to resolve the structure of this ultra-coldnebula. We find a central hourglass-shaped nebula surrounded by a patchy, butroughly round, cold high-velocity outflow. We compare the ALMA data withvisible-light images obtained with HST and confirm that the limb-brightenedbipolar lobes seen in these data represent hollow cavities with dense walls ofmolecular gas and dust producing both the molecular-emission-line andscattered-light structures seen at millimeter and visible wavelengths. Thelarge diffuse biconical shape of the nebula seen in the visible is likely dueto preferential illumination of the cold high-velocity outflow. We find acompact source of millimeter-wave continuum in the nebular waist -- these data,together with sensitive upper limits on the radio continuum using observationswith ATCA, indicate the presence of a substantial mass of very large (mm-sized)grains in the waist of the nebula. Another unanticipated result is thedetection of CO emission regions beyond the ultracold region which indicate there-warming of the cold gas, most likely due to photoelectric grain heating.

ALMA Observations of the Coldest Place in the Universe: The Boomerang Nebula