Stephenson 2 DFK 52: Discovery of an exotic red supergiant in the massive stellar cluster RSGC2

TL;DR

This paper reports the discovery and detailed characterization of an extremely large and complex circumstellar environment around the red supergiant DFK 52 in the RSGC2 cluster, revealing unprecedented mass-loss features.

Contribution

It provides the first detailed analysis of DFK 52's circumstellar environment, highlighting its unique outflow size, morphology, and mass-loss history among red supergiants.

Findings

DFK 52 has an outflow up to 50,000 au in radius with complex morphology.

The star experienced a dramatic mass-loss event approximately 4000 years ago.

The current mass-loss rate is estimated at about 3×10⁻⁶ M☉/yr.

Abstract

Atacama Large Millimeter/submillimeter Array (ALMA) observations at 1.3mm have recently revealed surprising complexity in the circumstellar environment of DFK 52, a red supergiant (RSG) located in the Stephenson 2 massive open cluster. We provide an initial characterisation of the star's mass-loss properties by studying its circumstellar emission in continuum, $^{12}$CO, $^{13}$CO, and SiO rotational lines. We find that DFK 52 is surrounded by an extremely large outflow (up to 50,000 au in radius) that shows complex morphologies in both its molecular and dust emission. The size of the circumstellar medium is unprecedented, even when compared with other known extreme RSGs, and its lower luminosity indicates that its mass ejection mechanism may be unique among this population. The molecular emission can be partially reproduced by a two-component model consisting of a fast (27 km/s) detached equatorial component with $M{\sim}0.05$ $M_{\odot}$ and a slow (10 km/s) spherical envelope with $\dot{M}\sim3\times10^{-6}$ $M_{\odot}$ yr$^{-1}$. This suggests that DFK 52 underwent a dramatic mass-loss event $\sim$4000 years ago, but has since transitioned into having a slower more symmetric mass loss. We conservatively estimate a total mass of $0.1-1$ $M_{\odot}$ in the complex extended regions of the outflow. The uncertain nature of the dramatic mass loss warrants extensive follow-up of this likely supernova progenitor.