Yellow Post-Asymptotic-Giant-Branch Stars as Standard Candles. I. Calibration of the Luminosity Function in Galactic Globular Clusters

Abstract

We use results of a survey for low-surface-gravity stars in Galactic (and LMC) globular clusters to show that "yellow" post-asymptotic-giant-branch (yPAGB) stars are likely to be excellent extragalactic standard candles, capable of producing distances to early-type galaxies that are accurate to a few percent. We show that the mean bolometric magnitude of the 10 known yPAGB stars in globular clusters is M bol = −3.38 ± 0.03, a value that is ∼0.2 mag brighter than that predicted from the latest post-horizontal-branch evolutionary tracks. More importantly, we show that the observed dispersion in the distribution is only 0.10 mag, i.e., smaller than the scatter for individual Cepheids. We describe the physics that can produce such a small dispersion, and show that, if one restricts surveys to the color range 0.0 (B − V ) 0 0.5, then samples of non-variable yPAGB stars can be identified quite easily with a minimum of contamination. The bright absolute V magnitudes of these stars ( M V = −3.37) make them, by far, the visually brightest objects in old stellar populations and ideal Population II standard candles for measurements out to ∼10 Mpc with current instrumentation. A Hubble Space Telescope survey in the halos of galaxies in the M81 and Sculptor groups could therefore serve as an effective cross-check on both the Cepheid and TRGB distance scales.

Spectroscopic Confirmation of Two Luminous Post-AGB Stars in the Globular Cluster M19

“…Evolving at nearly constant bolometric luminosity, PAGB stars attain their brightest visual magnitudes as they pass through spectral types F and A, where the bolometric correction is smallest. These luminous and non-variable "yellow" PAGB (yPAGB) stars have been proposed as potentially useful Population II standard candles for measuring extragalactic distances, based on their expected narrow luminosity function in old pop-ulations, and ease of detection because of their large Balmer discontinuities (Bond 1997a,b;Ciardullo et al 2022).…”