A High-precision Trigonometric Parallax to an Ancient Metal-poor Globular Cluster

作者：Brown T M; Casertano S; Strader J; Riess A; VandenBerg D A; Soderblom D R; Kalirai J; Salinas R

来源：Astrophysical Journal Letters, 2018, 856(1): L6.

DOI：10.3847/2041-8213/aab55a

• astrometry
• globular clusters: general
• globular clusters: individual (NGC 6397)
• stars: distances
• stars: evolution

Using the Wide Field Camera 3 (WFC3) on the Hubble Space Telescope (HST), we have obtained a direct trigonometric parallax for the nearest metal-poor globular cluster, NGC 6397. Although trigonometric parallaxes have been previously measured for many nearby open clusters, this is the first parallax for an ancient metal-poor population-one that is used as a fundamental template in many stellar population studies. This high-precision measurement was enabled by the HST/WFC3 spatial-scanning mode, providing hundreds of astrometric measurements for dozens of stars in the cluster and also for Galactic field stars along the same sightline. We find a parallax of 0.418 +/- 0.013 +/- 0.018 mas (statistical, systematic), corresponding to a true distance modulus of 11.89 +/- 0.07 +/- 0.09 mag (2.39 +/- 0.07 +/- 0.10 kpc). The V luminosity at the stellar main-sequence turnoff implies an absolute cluster age of 13.4 +/- 0.7 +/- 1.2 Gyr.

• 出版日期2018-3-20