Zhang Lan, Casetti-Dinescu Dana I, Bidin Christian Moni, Méndez Réne A, Girard Terrence M, Vieira Katherine, Korchagin Vladimir I, van Altena William F, Zhao Gang
Key Lab of Optical Astronomy, National Astronomical Observatories, CAS, 20A Datun Road, Chaoyang District, 100012 Beijing, China.
Department of Physics, Southern Connecticut State University, 501 Crescent St., New Haven, CT 06515, USA.
Astrophys J. 2019;871(1). doi: 10.3847/1538-4357/aaf560. Epub 2019 Jan 24.
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 = 2.5 kpc and ~ 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 ( = 0.72) than the inner disk ones ([Mg/H] = 0.12 ± 0.36, = 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.
我们探究了卡塞蒂 - 迪内斯库等人（2014年）在麦哲伦流主臂天空区域的银河系盘边缘发现的年轻B型恒星的起源。利用麦哲伦克莱6.5米望远镜上的MIKE仪器对九颗恒星进行的高分辨率光谱观测被添加到张等人（2017年）之前分析的样本中。我们编制了一个包含十五颗年轻恒星的样本，这些恒星具有确定的恒星类型、年龄、丰度和运动学特征。利用盖亚DR2的自行数据，我们还在一个真实的银河系引力势中推导了轨道。我们发现，我们之前通过径向速度选择的麦哲伦流候选恒星具有相当大的轨道角动量。这些恒星大量的旋转分量与近乎极地的麦哲伦轨道形成对比，因此这些恒星不太可能是麦哲伦流的成员。我们的样本中有四颗具有高轨道能量的恒星。轨道能量最高的那颗恒星的年龄短于穿越盘所需的时间，其诞生地为2.5千秒差距，距离约为28千秒差距。因此，这颗恒星的起源尚不确定。其余三颗恒星具有盘逃逸起源，诞生地距离银河系中心在12至25千秒差距之间。此外，能量最高的恒星比内盘恒星的金属丰度更低（[Mg/H] = -0.50 ± 0.07），氦含量的散射更大（σ = 0.72）（内盘恒星[Mg/H] = 0.12 ± 0.36，σ = 0.15）。虽然前一组的丰度与大麦哲伦星系的丰度相符，但它也可能通过不同的逃逸机制反映了银河系盘的金属丰度梯度及其逃逸状态。
