[论文解读] A Population of Accreted SMC Stars in the LMC

Knut Olsen, Dennis Zaritsky+3 more|arXiv (Cornell University)|May 31, 2011

Stellar, planetary, and galactic studies参考文献 48被引用 57

一句话总结

本研究识别出大麦哲伦云（LMC）中约5%的恒星，其运动学特征与LMC盘面旋转不一致，表明这些恒星可能来自小麦哲伦云（SMC）的吸积。通过径向速度、Ca二价离子谱线的金属丰度测量以及空间分布分析，作者发现这些恒星呈逆行旋转或处于高度倾斜的盘面中，与此前被认为是由潮汐作用形成的H I臂相关联，提示这些臂可能实际上是来自SMC的正在下落的物质。

ABSTRACT

We present an analysis of the stellar kinematics of the Large Magellanic Cloud based on ~5900 new and existing velocities of massive red supergiants, oxygen-rich and carbon-rich AGB stars, and other giants. After correcting the line-of-sight velocities for the LMC's space motion and accounting for asymmetric drift in the AGB population, we derive a rotation curve that is consistent with all of the tracers used, as well as that of published HI data. The amplitude of the rotation curve is v_0=87+/-5 km s^-1 beyond a radius R_0=2.4+/-0.1 kpc, and has a position angle of the kinematic line of nodes of theta=142 degrees +/-5 degrees. By examining the outliers from our fits, we identify a population of 376 stars, or >~5% of our sample, that have line-of-sight velocities that apparently oppose the sense of rotation of the LMC disk. We find that these kinematically distinct stars are either counter-rotating in a plane closely aligned with the LMC disk, or rotating in the same sense as the LMC disk, but in a plane that is inclined by 54 degrees +/- 2 degrees to the LMC. Their kinematics clearly link them to two known HI arms, which have previously been interpreted as being pulled out from the LMC. We measure metallicities from the Ca triplet lines of ~1000 LMC field stars and 30 stars in the kinematically distinct population. For the LMC field, we find a median [Fe/H]=-0.56 +/- 0.02 with dispersion of 0.5 dex, while for the kinematically distinct stars the median [Fe/H] is -1.25 +/- 0.13 with a dispersion of 0.7 dex. The metallicity differences provide strong evidence that the kinematically distinct population originated in the SMC. This interpretation has the consequence that the HI arms kinematically associated with the stars are likely falling into the LMC, instead of being pulled out.

研究动机与目标

• 利用大质量红超巨星和AGB星的径向速度，研究大麦哲伦云（LMC）的内部运动学。
• 识别并表征LMC内一个运动学上独特的恒星种群，以判断其是否可能来自小麦哲伦云（SMC）的吸积。
• 通过将H I臂的运动学与恒星种群关联，确定其起源。
• 通过评估异常种群的金属丰度和空间分布，检验其SMC起源的假设。

提出的方法

• 利用位于4米布兰科望远镜上的Hydra-CTIO多光纤光谱仪，获取了约5900个红超巨星、AGB星及其他巨星的径向速度，数据来自新近和已有的光谱观测。
• 对LMC的整体空间运动进行视线速度修正，并对AGB星种群应用非对称速度漂移修正，以获得一致的旋转曲线。
• 通过分析旋转曲线拟合中的速度异常值，结合空间和运动学聚类，识别出运动学上独特的恒星，定义为异常种群。
• 对约1000颗LMC场星和30颗异常种群中的恒星，测量其Ca II三重线吸收线的金属丰度，以与SMC的金属丰度进行比较。
• 将异常种群的色-星等图与外层SMC的色-星等图进行比较，以评估其光度一致性。
• 对异常种群的运动学几何结构进行建模，以判断其是相对于LMC平面逆行旋转，还是在相对于LMC平面倾角为54° ± 2°的高倾角盘面中旋转。

实验结果

研究问题

• RQ1 LMC中运动学独特的恒星种群的起源是什么？其特征是否与来自SMC的吸积一致？
• RQ2 异常恒星的金属丰度与LMC和SMC的金属丰度相比如何？这对其起源有何启示？
• RQ3 与异常恒星相关的H I臂是否真的是潮汐剥离的残余物，还是更可能是SMC的下落物质？
• RQ4 异常种群的空间和运动学结构是什么？其与正常LMC盘面有何不同？
• RQ5 运动学和金属丰度数据是否支持将H I臂重新解释为下落物质而非潮汐剥离的产物？

主要发现

• 基于红超巨星、AGB星和H I数据的LMC旋转曲线在不同示踪体之间保持一致，旋转速度为v₀ = 87 ± 5 km s⁻¹，适用于R₀ = 2.4 ± 0.1 kpc以外的区域。
• 共识别出376颗恒星，占样本的约5%，其运动学特征与LMC盘面旋转不一致，表明它们可能在LMC平面内逆行旋转，或在相对于LMC平面倾角为54° ± 2°的盘面中旋转。
• 运动学上独特的恒星主要为AGB星，其空间位置偏离LMC棒状结构，避开正常AGB星密集的中心区域。
• 异常种群的中位金属丰度为[Fe/H] = -1.25 ± 0.13 dex，标准差为0.7 dex，显著低于LMC场星的中位金属丰度[Fe/H] = -0.56 ± 0.02 dex，与SMC起源一致。
• 将异常种群的J - [3.6], [3.6]色-星等图与外层SMC的图进行距离校正后匹配，支持其SMC起源。
• 与异常恒星相关的H I臂很可能并非来自LMC的潮汐剥离，而是来自SMC的下落气体，暗示需对大麦哲伦流和领头臂的形成机制进行重新建模。