A Census of Blue Post-Horizontal-Branch Stars in Galactic Globular Clusters

Globular clusters are some of the oldest astronomical objects in the universe. They contain within them, tens of thousands of stars, and are a gateway to understanding and determining the age of the universe. Despite their frequent occurrence, there are still parts of globular clusters that are yet to be completely understood or surveyed. One such aspect is the presence of blue, post-horizontal-branch (HB) stars.

These stars are rare, tend to emit most of their light in the UV band, and have properties that can be useful in further understanding how globular clusters develop. The literature surrounding globular clusters has determined that yellow post-asymptotic-giant-branch (AGB) stars can be used as standard candles to verify distance measurements. Our study looks to extend this to blue post-HB stars as well.

As part of our analysis, we took a look at 109 globular clusters present in the Milky Way and the Magellanic Clouds, using data collected by the Cerro-Telolo Inter-American Observatory (CTIO) and the Kitt Peak National Observatory(KPNO). These data consisted of photometric measurements of the stars in the Thuan-Gunn u, and Johnson B, V and I bands. We constructed color-magnitude and color-color diagrams of these clusters, using which we were able to create a census of such blue (having a (B-V)0 <-0.05) post-HB stars. We used a mix of positional and photometric data from the Gaia Data Release 3 and a density-based spatial clustering algorithm (DBSCAN) to verify cluster membership of these stars and eliminate field stars present in our original photometric data. Our final analysis will allow us to create the largest sample of extremely blue post-HB (EHB )stars ever compiled, trace the various evolutionary scenarios of EHB stars, and better test the hypothesis that post-AGB stars in old stellar populations are excellent standard candles for extragalactic distance measurements.