Astrometric Membership Tests for the Zinn-Newell-Gibson "UV-Bright" Stars in Galactic Globular Clusters

TL;DR

This study re-identifies UV-bright stars in Galactic globular clusters from the 1972 ZNG list using Gaia data, determining cluster membership and evolutionary stages, and highlights stars of particular interest for future research.

Contribution

The paper provides the first precise astrometric identification and membership analysis of the ZNG UV-bright stars using Gaia EDR3 data, linking historical observations to modern measurements.

Findings

45% of ZNG candidates are probable cluster members.

Identified evolutionary stages for the UV-bright stars.

Highlighted several stars of special interest for further study.

Abstract

In 1972, Zinn, Newell, & Gibson (ZNG) published a list of 156 candidate "UV-bright" stars they had found in 27 Galactic globular clusters (GCs), based on photographs in the U and V bands. UV-bright stars lie above the horizontal branch (HB) and blueward of the asymptotic-giant branch (AGB) and red-giant branch in the clusters' color-magnitude diagrams. They are in rapid evolutionary phases--if they are members and not unrelated bright foreground stars. The ZNG list has inspired numerous follow-up studies, aimed at understanding late stages of stellar evolution. However, the ZNG candidates were presented only in finding charts, and celestial coordinates were not given. Using my own collection of CCD frames in u and V, I have identified all of the ZNG objects, and have assembled their coordinates, parallaxes, and proper motions from the recent Gaia Early Data Release 3 (EDR3). Based on the Gaia astrometry, I have determined which objects are probable cluster members (45% of the sample). For the members, using photometry from EDR3, I have assigned the stars to various evolutionary stages, including luminous post-AGB stars, and stars above the HB. I point out several ZNG stars of special interest that have still, to my knowledge, never been studied in detail. This study is an adjunct to a forthcoming survey of the Galactic GCs in the uBVI photometric system, designed for detection of low-gravity stars with large Balmer discontinuities.