Spectroscopic Confirmation: Fast rotators in the young clusters NGC 1856 and NGC 1953

TL;DR

This study spectroscopically investigates young clusters NGC 1856 and NGC 1953, revealing differences in Be star populations that suggest cluster density influences stellar rotation and impacts interpretations of multiple stellar populations.

Contribution

It provides the first spectroscopic comparison of Be star fractions in these clusters, highlighting the role of cluster density in stellar rotation and refining photometric detection methods.

Findings

High Be star fraction in NGC 1856 with broad Hα emission.

Only one Be star candidate detected in NGC 1953.

Photometric studies may underestimate Hα emitter fractions.

Abstract

We present the results of a spectroscopic investigation of two Large Magellanic Cloud globular clusters, NGC 1953 and NGC 1856. Both clusters have similar ages (250 and 300 Myr, respectively). Spectra were recorded with the Michigan/Magellan Fiber System located on the Magellan-Clay 6.5m telescope. Spectra were visually inspected to assess the presence of stellar H$\alpha$ emission lines attributed to B stars rotating close to breakup velocity (Be stars). High fractions of Be stars in the cluster typically indicate the presence of a large population of fast rotating stars, predicted by some models to explain the observed split and extended main sequence. There are numerous Be star candidates in NGC 1856, exhibiting weak but broad H$\alpha$ emission. However, only one such target was detected in NGC 1953. This stark contrast between the observed populations for NGC 1856 and NGC 1953 may suggest that cluster density plays a key role in determining the fraction of Be stars. These results provide essential constraints for the different scenarios attempting to explain the bimodal distribution of rotational velocities and the multiple populations of stars observed in globular clusters. The impact of stellar radial velocity and nebular emission on photometric measures is assessed through simulations relying on the spectra. These simulations suggest that photometric studies can under-estimate the fraction of H$\alpha$ emitters in a cluster, in particular for stars with relatively weak emission features. The results also show that nebular emission has minimal impact on the photometric H$\alpha$ excesses.