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

TL;DR

This paper presents high-resolution ALMA observations of the Boomerang Nebula, revealing its ultra-cold structure, morphology, and evidence of re-warming, providing new insights into the properties of the coldest known object in the universe.

Contribution

First ALMA imaging of the Boomerang Nebula's structure, revealing detailed morphology, cold high-velocity outflows, and evidence of re-warming of the gas.

Findings

Resolved the nebula's hourglass shape and cold outflows.

Detected large millimeter-sized grains in the nebula's waist.

Observed signs of re-warming in the cold gas regions.

Abstract

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