再论未确认的双星对超微弱恒星系统中速度色散测量的影响
Revisiting the influence of unidentified binaries on velocity dispersion measurements in ultra-faint stellar systems
摘要：
用$M_V\gtrsim-7$对最近发现的银河系卫星进行速度色散测量，表明它们具有很高的质光比。由于重子质量，预期的速度色散为$\sim0.2$ \，km\，s $^{-1}$，但测量值为$\gtrsim3$ \，km\，s $^{-1}$。我们进行蒙特卡罗模拟的模拟径向速度测量这些系统假设他们有类似的球状星团的质光比和不明身份的双星人口，以确定这些恒星是否可以提高速度色散的观测值。我们发现，这一假设是不可能产生的色散远远超过$\sim 4.5$ \，km\，s $^{-1}$，与以前的工作。然而，对于具有潜在的最小速度色散的系统，在我们的模拟中，超过$f_{bin}(P\le10$ \，yrs $)\sim5%$的二元分数的5-40%中产生与观测一致的值。这个样本包括在$M_V-r_h$空间中最接近经典球状星团的矮星系候选者。作为一个群体，不太可能所有这些矮星系候选者都具有典型的球状星团的质光比，但是对于几个单独的矮星系候选者来说，从接近零的值可以排除观测到的色散的提升。鉴于获得最微弱卫星的精确速度弥散和动力质量的重要性，显然需要直接排除双星对这些系统的可能影响。这就需要多历元径向速度测量，其单个不确定度为$\lesssim$ 1\，km\，s $^{-1}$，以识别轨道速度为观测速度色散量级的光谱双星。
英文摘要：
Velocity dispersion measurements of recently discovered Milky Way satellites with $M_V\gtrsim-7$ imply they posses high mass-to-light ratios. The expected velocity dispersions due to their baryonic mass are $\sim0.2$\,km\,s$^{-1}$, but values $\gtrsim3$\,km\,s$^{-1}$ are measured. We perform Monte Carlo simulations of mock radial velocity measurements of these systems assuming they have mass-to-light ratios similar to globular clusters and posses an unidentified binary star population, to determine if these stars could boost the velocity dispersion to the observed values. We find that this hypothesis is unlikely to produce dispersions much in excess of $\sim 4.5$\,km\,s$^{-1}$, in agreement with previous work. However, for the systems with potentially the smallest velocity dispersions, values consistent with observations are produced in 5-40% of our simulations for binary fractions in excess of $f_{bin}(P\le10$\,yrs$)\sim5%$. This sample includes the dwarf galaxy candidates that lie closest to classical globular clusters in $M_V-r_h$ space. Considered as a population, it is unlikely that all of these dwarf galaxy candidates have mass-to-light ratios typical of globular clusters, but boosting of the observed dispersion by binaries from near-zero values cannot be ruled out at high confidence for several individual dwarf galaxy candidates. Given the importance of obtaining accurate velocity dispersions and dynamical masses for the faintest satellites, it is clearly desirable to exclude directly the possible effect of binaries on these systems. This requires multi-epoch radial velocity measurements with individual uncertainties of $\lesssim$1\,km\,s$^{-1}$ to identify spectroscopic binaries with orbital velocities of order the observed velocity dispersion.
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