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 (\dot{M}) increases with radius, similar to its expansion velocity (V)—taking V\propto r, we find \dot{M}\propto {r}^0.9{--2.2}. The mass in the ultra-cold outflow is ≳ 3.3 M _⊙, and the Boomerang’s main-sequence progenitor mass is ≳ 4 M _⊙. Our high angular resolution (∼ 0\buildrel{\prime\prime}\over{.} 3) 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 (4{--}7)× {10}^-4 M _⊙ of very large (∼millimeter-to-centimeter sized), cold (∼ 20{--}30 K) grains. The central waist (assuming its outer regions to be expanding) and fast bipolar outflow have expansion ages of ≲ 1925 {years} and ≤slant 1050 {years}: 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.