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

TL;DR

This study uses ALMA data to analyze Thackeray's globules in IC 2944, revealing their kinematics, densities, and potential for star formation, and revisiting the rocket effect as a key process in their evolution.

Contribution

First detailed ALMA analysis of Thackeray's globules, quantifying their kinematics, masses, and evidence of gravitational collapse, and modeling their acceleration due to the rocket effect.

Findings

46 globules identified, including 12 new detections

Many globules show velocity gradients consistent with the rocket effect

Some globules exhibit signs of possible gravitational collapse

Abstract

The prominent Thackeray's globules are a collection of cloudlets seen in silhouette against the bright emission of the IC 2944 HII 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 behavior. 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 5 of them are associated with emission of dense molecular tracers and/or millimeter 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, and potentially becoming low-mass walkaway/runaway protostars.