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arXiv · 1803.02927

A High-Precision Trigonometric Parallax to an Ancient Metal-Poor Globular Cluster

T. M. Brown·S. Casertano·J. Strader·A. Riess·D. A. VandenBerg·D. R. Soderblom·J. Kalirai·R. Salinas
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Abstract

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.

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T. M. Brown, S. Casertano, J. Strader, A. Riess, D. A. VandenBerg, D. R. Soderblom, J. Kalirai, R. Salinas. 2018-03-16. A High-Precision Trigonometric Parallax to an Ancient Metal-Poor Globular Cluster. https://doi.org/10.3847/2041-8213%2Faab55a

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