The Origins of Young Stars in the Direction of the Leading Arm of the Magellanic Stream: Abundances, Kinematics, and Orbits

Abstract

We explore the origins of the young B-type stars found by Casetti-Dinescu et al. at the outskirts of the Milky Way disk in the sky region of the Leading Arm of the Magellanic Stream. High-resolution spectroscopic observations made with the MIKE instrument on the Magellan Clay 6.5 m telescope for nine stars are added to the previous sample analyzed by Zhang et al. We compile a sample of 15 young stars with well-determined stellar types, ages, abundances, and kinematics. With proper motions from Gaia DR2 we also derive orbits in a realistic Milky Way potential.We find that our previous radial-velocity-selected Leading Arm candidates have substantial orbital angular momentum. The substantial amount of rotational component for these stars is in contrast with the near-polar Magellanic orbit, thus making it unlikely that these stars are members of the Leading Arm. There are four large-orbital-energy stars in our sample. The star