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

TL;DR

This paper reports a precise trigonometric parallax measurement of the ancient, metal-poor globular cluster NGC 6397 using HST/WFC3, providing a fundamental distance and age estimate crucial for stellar population studies.

Contribution

First direct parallax measurement for an ancient metal-poor globular cluster, enabling improved distance and age determinations for such populations.

Findings

Parallax of 0.418 mas with high precision.

Distance of approximately 2.39 kpc.

Cluster age estimated at 13.4 Gyr.

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.