2019ApJ...871...99Z

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 with the highest orbital energy has an age shorter than the time to disk crossing, with a birthplace z = 2.5 kpc and R_GC ∼ 28 kpc. Therefore, the origin of this star is uncertain. The remaining three stars have disk runaway origins with birthplaces between 12 and 25 kpc from the Galactic center. Also, the most energetic stars are more metal-poor ([Mg/H] = -0.50 ± 0.07) and have larger He scatter (σ_[He/H] = 0.72) than the inner-disk ones ([Mg/H] = 0.12 ± 0.36, σ_[He/H] = 0.15). While the former group's abundance is compatible with that of the Large Magellanic Cloud, it could also reflect the metallicity gradient of the Milky Way disk and the group;s runaway status via different runaway mechanisms.