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

High-resolution spectroscopic observations were taken of 29 extended main-sequence turnoff (eMSTO) stars in the young (∼200 Myr) Large Magellanic Cloud (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 Hα emission (indicative of Be-star decretion disks), others have shallow broad Hα absorption (consistent with rotation ≳150 km s−1), or deep Hα core absorption signaling lower rotation velocities (≲150 km s−1). The spectra appear consistent with two populations of stars—one rapidly rotating, and the other, younger and slowly rotating.