We conduct a line-by-line differential analysis of a sample of 125 co-moving pairs of stars (dwarfs and subgiants near solar metallicity). We obtain high precision stellar parameters with average uncertainties in effective temperature, surface gravity and metallicity of 16.5 K, 0.033 dex and 0.014 dex, respectively. We classify the co-moving pairs of stars into two groups, chemically homogeneous (conatal; |Delta[Fe/H]| $\le$ 0.04 dex) and inhomogeneous (non-conatal), and examine the fraction of chemically homogeneous pairs as a function of separation and effective temperature. The four main conclusions from this study are: (1) A spatial separation of \ds = 10$^6$ AU is an approximate boundary between homogeneous and inhomogeneous pairs of stars, and we restrict our conclusions to only consider the 91 pairs with \ds $\le$ 10$^6$ AU; (2) There is no trend between velocity separation and the fraction of chemically homogeneous pairs in the range \dv $\le$ 4 \kms; (3) We confirm that the fraction of chemically inhomogeneous pairs increases with increasing \teff\ and the trend matches a toy model of that expected from planet ingestion; (4) Atomic diffusion is not the main cause of the chemical inhomogeneity. A major outcome from this study is a sample of 56 bright co-moving pairs of stars with chemical abundance differences $\leq$ 0.02 dex (5\%) which is a level of chemical homogeneity comparable to that of the Hyades open cluster. These important objects can be used, in conjunction with star clusters and the \gaia\ ``benchmark'' stars, to calibrate stellar abundances from large-scale spectroscopic surveys.
HST astrometry in the Orion Nebula Cluster: census of low-mass runaways
我们提出了一个目录的猎户座星云团（ONC）的高精度自行，根据财政部计划的观测与哈勃太空望远镜（HST）的ACS/WFC相机。我们的星表包含2，454个在$14.2<m_{\rm F775W}<24.7$星等范围内的天体，因此探测了ONC的恒星质量，从$\sim$ 0.4 $M_\odot$到$\sim$ 0.02 $M_\odot$，面积为$\sim$ 550弧分$^2$。我们提供了一些内部的速度色散估计的ONC表明一个弱的恒星位置和质量的依赖。尽管几乎所有的速度色散估计（包括我们在$σ_{v,x}=0.94$和$σ_{v,y}=1.25$ mas yr $^{-1}$上的估计）都可能受到猎户座A年轻恒星星族重叠的影响，但它们与已发表的速度色散估计非常一致。我们根据HST和Gaia DR 2数据确定了4个新的ONC候选逃逸体，它们的质量都小于$\sim$ 1 $M_\odot$。已知的候选逃逸源的总数是10个-这是在银河系任何疏散星团中发现的最大样本之一。令人惊讶的是，它们中没有一个的切向速度超过20 km s $^{-1}$。如果它们中的大多数确实起源于ONC，那么可能会迫使重新检查非常年轻的星团中的动力学过程。看起来ONC的质量功能并不受丢失的失控物的显著影响。
Spectroscopy of Very Low Mass Stars and Brown Dwarfs in the Lambda Orionis Star Forming Region. I. Enlarging the census down to the planetary mass domain in Collinder 69
虽然对于低质量恒星的形成有一个普遍接受的演化方案，但当质量下降到棕矮星时，类似的过程还没有得到很好的理解。在这第一篇论文中，我们试图建立最完整和公正的光谱证实的人口普查的科林斯69，中央集群的猎户座λ恒星形成区域，作为第一步，在解决问题的褐矮星和行星质量的物体如何形成。我们已经研究了年龄相关的功能，在光学和近红外光谱的候选成员的集群（如碱线和吸积相关的指标）。此外，我们还对其他青年指标进行了分析，如X射线发射或中红外线过量。我们已经确认了\sim 90个测光候选成员中的Collinder 69的成员资格。作为一个副产品，我们已经确定了年轻的M，非常低质量的恒星和褐矮星的温度范围。我们已经组装了一个最完整的初始质量函数从0.016至20 M \odot。最后，我们研究了已确认成员的空间分布对棕矮星形成机制的影响。
ACRONYM III: Radial Velocities for 336 Candidate Young Low-Mass Stars in the Solar Neighborhood, Including 78 Newly Confirmed Young Moving Group Members
