The Origins of Young Stars in the Direction of the Leading Arm of the
  Magellanic Stream: Abundances, Kinematics, and Orbits
  We explore the origins of the young B-type stars found by Casetti-Dinescu et
al.(2014) at the outskirts of the Milky-Way disk in the sky region of Leading
Arm of the Magellanic Stream. High-resolution spectroscopic observations made
with the MIKE instrument on the Magellan Clay 6.5m telescope for nine stars are
added to the previous sample analyzed by Zhang et al. (2017). We compile a
sample of fifteen 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 LA 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 rendering these stars
unlikely members of the LA. There are four large orbital-energy stars in our
sample. The highest orbital-energy one has an age shorter than the time to disk
crossing, with a birthplace $z=2.5$~kpc and $R_{\rm GC}\sim 28$~kpc. Therefore,
the origin of this star is uncertain. The remaining three stars have disk
runaway origin with birthplaces between 12 and 25 kpc from the Galactic center.
Also, the most energetic stars are more metal poor ([Mg/H] =$-0.50\pm0.07$) and
with larger He scatter ($\sigma_{\rm [He/H]} = 0.72$) than the inner disk ones
([Mg/H] $=0.12\pm0.36$, $\sigma_{\rm [He/H]} = 0.15$). While the former group's
abundance is compatible with that of the Large Magellanic Cloud, it could also
reflect the metallicity gradient of the MW disk and their runaway status via
different runaway mechanisms.
