Two Luminous Post-AGB Stars in the Galactic Globular Cluster M19

TL;DR

This paper reports the discovery of two luminous post-AGB stars in the globular cluster M19, demonstrating their potential as reliable Population II standard candles for extragalactic distance measurements.

Contribution

The study identifies and characterizes two luminous post-AGB stars in M19, establishing their consistent luminosities and potential as standard candles, which is a novel application in this context.

Findings

Two luminous PAGB stars confirmed in M19.

Their luminosities are consistent with other GCs, supporting their use as standard candles.

PAGB stars are brighter than RR Lyrae and less affected by extinction.

Abstract

We report the discovery of a luminous "yellow" post-asymptotic-giant-branch (PAGB) star in the globular cluster (GC) M19 (NGC 6273), identified during our uBVI survey of Galactic GCs. The uBVI photometric system is optimized to detect stars with large Balmer discontinuities, indicating very low surface gravities and high luminosities. The spectral-energy distribution (SED) of the star is consistent with an effective temperature of about 6250 K and a surface gravity of $\log g=0.5$. We use Gaia data to show that the star's proper motion and radial velocity are consistent with cluster membership. One aim of our program is to test yellow PAGB stars as candidate Population II standard candles for determining extragalactic distances. We derive a visual absolute magnitude of $M_V=-3.39\pm0.09$ for the M19 star. This is in close agreement with the $M_V$ values found for yellow PAGB stars in the GCs omega Cen, NGC 5986, and M79, indicating a very narrow luminosity function. These objects are four magnitudes brighter than RR Lyrae variables, and they can largely avoid the issues of interstellar extinction that are a problem for Population I distance indicators. We also identified a second luminous PAGB object in M19, this one a hotter "UV-bright" star. Its SED is consistent with an effective temperature of about 11,750 K and $\log g=2.0$. The two objects have nearly identical bolometric luminosities, $\log L/L_\odot=3.24$ and 3.22, respectively.