帕洛玛13球状星团的结构和动力学
Structure and Dynamics of the Globular Cluster Palomar 13
摘要：
本文介绍了银河系球状星团Palomar 13的Keck/DEIMOS光谱和CFHT/MegaCam测光。我们将光谱确认的成员数量增加了两倍，包括许多重复的速度测量。Palomar 13是唯一已知的球状星团，可能有暗物质存在的证据，基于Keck/HIRES 21恒星速度色散为2.2+/-0.4 km/s。我们重现了这种测量，但证明它是由未解决的双星膨胀。对于我们的61颗恒星样本，速度色散为sigma=0.7（+0.6/-0.5）km/s。结合我们的DEIMOS数据与文献值，我们的最终速度色散是sigma=0.4（+0.4/-0.3）km/s。我们确定了[Fe/H]=-1.6+/-0.1 dex的光谱金属丰度，将σ_[Fe/H]~0.2 dex的1 σ上限置于任何内部金属丰度分布上。我们确定Palomar 13的总光度为M_V=-2.8+/-0.4，使其成为已知最不明亮的球状星团之一。光度学等照度线在半光半径范围内是规则的，在此半径范围外是轻微不规则的。外表面亮度剖面斜率比典型的球状星团要浅（eta=-2.8+/-0.3）。因此，在大半径，潮汐碎片可能会影响帕洛玛13的外观。结合我们的光度和内禀速度色散，我们得到了M_1/2=1.3（+2.7/-1.3 x10 ^3）M_sun的动力学质量和M/L_V=2.4（+5.0/-2.4）M_sun/L_sun的质光比。在我们的测量误差范围内，质光比与单个恒星星族的理论预测一致。我们的结论是，虽然有一些证据，潮汐剥离在大半径，帕洛玛13的动力学质量是一致的，它的恒星质量和既不显着的暗物质，也不极端的潮汐加热，是需要解释集群动态。
英文摘要：
We present Keck/DEIMOS spectroscopy and CFHT/MegaCam photometry for the Milky Way globular cluster Palomar 13. We triple the number of spectroscopically confirmed members, including many repeat velocity measurements. Palomar 13 is the only known globular cluster with possible evidence for dark matter, based on a Keck/HIRES 21 star velocity dispersion of sigma=2.2+/-0.4 km/s. We reproduce this measurement, but demonstrate that it is inflated by unresolved binary stars. For our sample of 61 stars, the velocity dispersion is sigma=0.7(+0.6/-0.5) km/s. Combining our DEIMOS data with literature values, our final velocity dispersion is sigma=0.4(+0.4/-0.3) km/s. We determine a spectroscopic metallicity of [Fe/H]=-1.6+/-0.1 dex, placing a 1-sigma upper limit of sigma_[Fe/H]~0.2 dex on any internal metallicity spread. We determine Palomar 13's total luminosity to be M_V=-2.8+/-0.4, making it among the least luminous known globular clusters. The photometric isophotes are regular out to the half-light radius and mildly irregular outside this radius. The outer surface brightness profile slope is shallower than typical globular clusters (eta=-2.8+/-0.3). Thus at large radius, tidal debris is likely affecting the appearance of Palomar 13. Combining our luminosity with the intrinsic velocity dispersion, we find a dynamical mass of of M_1/2=1.3(+2.7/-1.3)x10^3 M_sun and a mass-to-light ratio of M/L_V=2.4(+5.0/-2.4) M_sun/L_sun. Within our measurement errors, the mass-to-light ratio agrees with the theoretical predictions for a single stellar population. We conclude that, while there is some evidence for tidal stripping at large radius, the dynamical mass of Palomar 13 is consistent with its stellar mass and neither significant dark matter, nor extreme tidal heating, is required to explain the cluster dynamics.
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