Population II PAGB Stars. I. Methodology, Initial Results, and the First Luminosity Function
- 1The Pennsylvania State University
Abstract
Post-asymptotic giant branch (PAGB) stars share many advantages with the tip of the red giant branch (TRGB) for Population II distance measurements, including minimal internal reddening, easy identification, a solid theoretical foundation, and an expected weak metallicity dependence. At visual wavelengths, PAGB stars are the brightest members in old populations, appearing up to ∼2 mag brighter than the TRGB and ∼4 mag brighter than RR Lyrae stars. Stellar evolution theory predicts that these objects occupy a narrow luminosity strip across the top of the Hertzsprung–Russell (HR) diagram, making them excellent standard candles. Observations in Galactic globular clusters (GCs) support this prediction, but a comprehensive survey is required to define their luminosity function and test for systematic errors. In this work, we construct a multiwavelength catalog, spanning the ultraviolet (UV) to infrared (IR), for PAGB stars in 39 Galactic GCs. We validate cluster membership with Gaia astrometry, identify 21 objects across 16 GCs using ground-based uBVI and space-based UV photometry, and perform Bayesian spectral energy distribution (SED) fitting with stellar atmosphere models to estimate their effective temperatures, luminosities, and radii. We find that PAGB stars occupy a narrow luminosity range centered at log ( L bol / L ⊙ ) = 3.25 ± 0.03 , with dispersion σ = 0.12 dex, supporting their use as reliable Population II distance indicators. Comparisons with theoretical tracks show excellent agreement with evolutionary models. We confirm that PAGB stars occur preferentially in metal-poor clusters with blue horizontal branches. Our results demonstrate that PAGB stars provide a powerful probe of late stellar evolution and a promising Population II standard candle.
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BibTeX
@article{feeser2026population,
title = {{Population II PAGB Stars. I. Methodology, Initial Results, and the First Luminosity Function}},
author = {S. Jean Feeser and Robin Ciardullo and Michael H. Siegel and C. J. Hapich and Akshat S. Chaturvedi},
journal = {The Astrophysical Journal},
year = {2026},
month = oct,
doi = {10.3847/1538-4357/aeae23},
url = {https://iopscience.iop.org/article/10.3847/1538-4357/aeae23},
note = {CC BY 4.0},
}
RIS
TY - JOUR
TI - Population II PAGB Stars. I. Methodology, Initial Results, and the First Luminosity Function
AU - S. Jean Feeser
AU - Robin Ciardullo
AU - Michael H. Siegel
AU - C. J. Hapich
AU - Akshat S. Chaturvedi
PY - 2026
DA - 2026/10/29
JO - The Astrophysical Journal
DO - 10.3847/1538-4357/aeae23
UR - https://iopscience.iop.org/article/10.3847/1538-4357/aeae23
AB - Post-asymptotic giant branch (PAGB) stars share many advantages with the tip of the red giant branch (TRGB) for Population II distance measurements, including minimal internal reddening, easy identification, a solid theoretical foundation, and an expected weak metallicity dependence. At visual wavelengths, PAGB stars are the brightest members in old populations, appearing up to ∼2 mag brighter than the TRGB and ∼4 mag brighter than RR Lyrae stars. Stellar evolution theory predicts that these objects occupy a narrow luminosity strip across the top of the Hertzsprung–Russell (HR) diagram, making them excellent standard candles. Observations in Galactic globular clusters (GCs) support this prediction, but a comprehensive survey is required to define their luminosity function and test for systematic errors. In this work, we construct a multiwavelength catalog, spanning the ultraviolet (UV) to infrared (IR), for PAGB stars in 39 Galactic GCs. We validate cluster membership with Gaia astrometry, identify 21 objects across 16 GCs using ground-based uBVI and space-based UV photometry, and perform Bayesian spectral energy distribution (SED) fitting with stellar atmosphere models to estimate their effective temperatures, luminosities, and radii. We find that PAGB stars occupy a narrow luminosity range centered at log ( L bol / L ⊙ ) = 3.25 ± 0.03 , with dispersion σ = 0.12 dex, supporting their use as reliable Population II distance indicators. Comparisons with theoretical tracks show excellent agreement with evolutionary models. We confirm that PAGB stars occur preferentially in metal-poor clusters with blue horizontal branches. Our results demonstrate that PAGB stars provide a powerful probe of late stellar evolution and a promising Population II standard candle.
SN - 1538-4357
SN - 0004-637X
ER -