NGC 1866: First Spectroscopic Detection of Fast Rotating Stars in a Young LMC Cluster

TL;DR

This study provides the first spectroscopic evidence of fast-rotating stars in a young LMC cluster, confirming their presence and characteristics through high-resolution spectra.

Contribution

It is the first to directly detect rapidly rotating stars in NGC 1866 using spectroscopy, confirming photometric inferences with concrete spectral evidence.

Findings

Detection of H-alpha emission indicating Be-star disks

Identification of broad H-alpha absorption in fast rotators

Spectral evidence for two stellar populations with different rotation speeds

Abstract

High-resolution spectroscopic observations were taken of 29 extended main sequence turn-off (eMSTO) stars in the young ($\sim$200 Myr) LMC cluster, NGC 1866 using the Michigan/Magellan Fiber System and MSpec spectrograph on the Magellan-Clay 6.5-m telescope. These spectra reveal the first direct detection of rapidly rotating stars whose presence has only been inferred from photometric studies. The eMSTO stars exhibit H-alpha emission (indicative of Be-star decretion disks), others have shallow broad H-alpha absorption (consistent with rotation $\gtrsim $150 km s$^{-1}$), or deep H-alpha core absorption signaling lower rotation velocities ($ \lesssim $150 km s$^{-1}$ ). The spectra appear consistent with two populations of stars - one rapidly rotating, and the other, younger and slowly rotating.