Using $^{26}$Al to detect ongoing self-enrichment in young massive star clusters

TL;DR

Detecting radioactive $^{26}$Al in young massive star clusters could confirm ongoing self-enrichment processes that lead to chemical anomalies in globular clusters, with upcoming instruments improving detection prospects.

Contribution

This study evaluates the feasibility of detecting $^{26}$Al in young star clusters as evidence for self-enrichment, highlighting observational strategies and promising targets like R136 in the Large Magellanic Cloud.

Findings

Detection of $^{26}$AlF with ALMA is feasible in some clusters.

The Large Magellanic Cloud's R136 is a prime candidate for $^{26}$Al detection.

Upcoming instruments like COSI can observe the $^{26}$Al decay line within 15 months.

Abstract

Self-enrichment is one of the leading explanations for chemical anomalies in globular clusters. In this scenario, various candidate polluter stars have been proposed to eject gas with altered chemical composition during the self-enrichment process. Most of the proposed polluters will also eject radioactive $^{26}$Al into the surroundings. Hence, any detection of $^{26}$Al in young massive star clusters (YMCs) would support the self-enrichment scenario if YMCs were indeed the progenitors of globular clusters. Observations of gamma-ray data from COMPTEL and INTEGRAL, as well as detections of $^{26}$AlF molecules by the Atacama Large Millimeter-submillimeter Array (ALMA), indicate the maturing of $^{26}$Al detection methods. Detection possibilities will be enhanced in the short- to mid-term by the upcoming launch of the Compton Spectrometer and Imager (COSI). The Square Kilometer Array (SKA) could in principle also detect radio recombination lines of the positronium formed from the decay products of $^{26}$Al. Here, we show for a sample of YMCs in the nearby Universe, where self-enrichment could plausibly take place. For some nearby galaxies, this could enhance $^{26}$Al by an order of one magnitude. Detecting $^{26}$AlF with ALMA appears feasible for many candidate self-enrichment clusters, although significant challenges remain with other detection methods. The Large Magellanic Cloud, with its YMC R136, stands out as the most promising candidate. Detecting a 1.8~MeV radioactive decay line of $^{26}$Al here would require at least 15 months of targeted observation with COSI, assuming ongoing self-enrichment in R136.