The highly ionized, high velocity gas in NGC 6231

TL;DR

This study investigates the ionized, high-velocity gas in NGC 6231, revealing how stellar winds and cluster environment interactions contribute to galactic feedback and cluster evolution.

Contribution

It demonstrates that broad C IV absorption features originate from cluster gas enriched by a WC star, offering insights into cluster wind dynamics and feedback processes.

Findings

Detection of high-velocity C IV absorption linked to cluster gas

Enrichment of cluster gas by WC star WR 79's wind

Implication of either a wind from WR 79 or a collective cluster wind

Abstract

It is well known that clusters of massive stars are influenced by the presence of strong winds, that they are sources of diffuse X-rays from shocked gas, and that this gas can be vented into the surrounding region or the halo through the champagne effect. However, the details of how these different environments interact and evolve are far from complete. This paper attributes the broad C IV 1500 absorption features (extending to -1900 \kms) that are seen in the spectra of main sequence B stars in NGC 6231 to gas in the cluster environment and not the B stars themselves. It is shown that the presence of a WC star, WR 79, in the cluster makes this gas detectable because its wind enriches the cluster gas with carbon. Given the available data, it is not clear whether the absorbing gas is simply the far wind of WR 79 or a collective cluster wind enriched by carbon from the wind of WR 79. If it is simply due to the wind, then this wind must flow, unimpeded for more than 2 pc, suggesting that the inner region of the cluster is nearly devoid of obstructing material. If it is actually a collective wind from the cluster, then we could be witnessing an important stage of galactic feedback. In either case, the observations provide a unique and significant piece to the puzzle of how massive, open clusters evolve.