Thackeray’s globules in IC 2944: the rocket effect revisited by ALMA

Abstract

The prominent Thackeray's globules are a collection of cloudlets seen in silhouette against the bright emission of the IC 2944 H II region, ionized by the Collinder 249 cluster of early-type stars (placed at 2331 $\pm$ 30 pc, derived from a Gaia DR3 analysis of the parallaxes of 11 massive stars). Here we present the analysis of Band 3 ALMA data that reveals the cold emission (continuum and molecular) associated with the neutral gas and its kinematic behaviour. Many of the globules follow a linear velocity gradient that can be explained as the result of an acceleration process due to the rocket effect, where freshly ionized material streams away from the globule, compressing and accelerating it. We identified 46 globules (12 of which are new detections), measured their kinematics, and estimated their densities and masses. At least five of them are associated with emission of dense molecular tracers and/or millimetre continuum sources and have indications of possible gravitational collapse. We applied a simple model for the acceleration of the globules, which accounts for the observed kinematics. In this scenario only the most massive of the globules will be able to gravitationally collapse before being completely destroyed, in the process reaching speeds of tens km s$^{-1}$, and potentially becoming low-mass walkaway/runaway protostars.