The Coldest Place in the Universe: Probing the Ultra-cold Outflow and Dusty Disk in the Boomerang Nebula

Abstract

Our Cycle 0 ALMA observations confirmed that the Boomerang Nebula is the coldest known object in the universe, with a massive high-speed outflow that has cooled significantly below the cosmic background temperature. Our new CO 1–0 data reveal heretofore unseen distant regions of this ultra-cold outflow, out to ≳120,000 au. We find that in the ultra-cold outflow, the mass-loss rate ( ) increases with radius, similar to its expansion velocity ( V )—taking , we find . The mass in the ultra-cold outflow is M ⊙ , and the Boomerang’s main-sequence progenitor mass is M ⊙ . Our high angular resolution ( ) CO J  = 3–2 map shows the inner bipolar nebula’s precise, highly collimated shape, and a dense central waist of size (FWHM) ∼1740 au × 275 au. The molecular gas and the dust as seen in scattered light via optical Hubble Space Telescope imaging show a detailed correspondence. The waist shows a compact core in thermal dust emission at 0.87–3.3 mm, which harbors M ⊙  of very large (∼millimeter-to-centimeter sized), cold ( K) grains. The central waist (assuming its outer regions to be expanding) and fast bipolar outflow have expansion ages of and : the “jet-lag” (i.e., torus age minus the fast-outflow age) in the Boomerang supports models in which the primary star interacts directly with a binary companion. We argue that this interaction resulted in a common-envelope configuration, while the Boomerang’s primary was an RGB or early-AGB star, with the companion finally merging into the primary’s core, and ejecting the primary’s envelope that now forms the ultra-cold outflow.