NGC 1866: First Spectroscopic Detection of Fast Rotating Stars in a Young LMC Cluster
Abstract
High-resolution spectroscopic observations were taken of 29 extended main sequence turn-off (eMSTO) stars in the young (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 150 km s-1), or deep H-alpha 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.