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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 GC ~ 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 +/- 0.07) and with larger He scatter (sigma [He/H] = 0.72) than the inner disk ones ([Mg/H] = 0.12 +/- 0.36, sigma [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.

4 citations (Crossref) [2023-10-31] Place: United States Zhang, Lan. Key Lab of Optical Astronomy, National Astronomical Observatories, CAS, 20A Datun Road, Chaoyang District, 100012 Beijing, China. Casetti-Dinescu, Dana I. Department of Physics, Southern Connecticut State University, 501 Crescent St., New Haven, CT 06515, USA. Casetti-Dinescu, Dana I. Astronomical Institute of the Romanian Academy, str. Cutitul de Argint 5, Bucharest, Romania. Casetti-Dinescu, Dana I. Radiology and Biomedical Imaging, Yale School of Medicine, 300 Cedar Street, New Haven 06519, USA. Bidin, Christian Moni. Instituto de Astronomia, Universidad Catolica del Norte, Av. Angomos 0610, Antofagasta, Chile. Mendez, Rene A. Departamento de Astronomia, Universidad de Chile, Casilla 36-D, Santiago, Chile. Girard, Terrence M. 14 Dunn Rd, Hamden, Connecticut, CT 06518, USA. Vieira, Katherine. Centro de Investigaciones de Astronomia, Apartado Postal 264, Merida 5101-A, Venezuela. Korchagin, Vladimir I. Institute of Physics, Southern Federal University, Stachki st/194, 344090, Rostov-on-Don, Russia. van Altena, William F. Astronomy Department, Yale University, 260 Whitney Ave., New Haven, CT 06511, USA. Zhao, Gang. Key Lab of Optical Astronomy, National Astronomical Observatories, CAS, 20A Datun Road, Chaoyang District, 100012 Beijing, China.