CHASE Spectral Survey I: Measuring Contamination in the Color-selected J-region Asymptotic Giant Branch Distance Indicator
Abstract
The J-region asymptotic giant branch (JAGB) method for distance measurements uses the near uniform luminosity of carbon-rich AGB stars as a standard candle. Its application rests on an assumed but rarely examined premise: the stars contained in the JAGB color-magnitude selection box are primarily carbon-rich AGB stars. We investigate the purity of the photometric selection in the Milky Way with an optical spectroscopic survey (~3590-7150 mathringA) called CHASE (Carbon stars, Hubble tension, And Spectroscopic Exploration of JAGB populations) using the Shane 3 m telescope at Lick Observatory. These stars are classified using molecular bandhead step indices, which recover 100% of known classifications. We report the observations of 76 stars (54 within the standard JAGB selection box of 1.5 < J - K < 2.0 mag) and find only 13 of 54 stars are carbon-rich, a 76±6 oxygen-rich contamination under blind photometric selection. This contamination remains high (71%) even within ±0.4 mag of the JAGB peak itself, showing it is not confined to the edges of the selection box. This inhomogeneity may explain why recent literature find field-to-field and methodological variations of 0.1-0.2 mag that now dominate its systematic error budget. These variations could be caused, in part, by inhomogeneous stellar populations that consist of differing spectral types, and hence differing mean magnitudes.
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BibTeX
@misc{li2026chase,
title = {{CHASE Spectral Survey I: Measuring Contamination in the Color-selected J-region Asymptotic Giant Branch Distance Indicator}},
author = {Siyang Li and Alexei V. Filippenko and Arief Ahmad and Adam G. Riess and Stefano Casertano and Thomas G. Brink and Richard Anderson and Caroline D. Huang and Louise Breuval and Noriyuki Matsunaga and Gracelynn Jost and Yash Mehta and Elaina Sadler and Julian Shapiro},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2609.25412},
eprint = {2609.25412},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2609.25412},
}
RIS
TY - UNPB
TI - CHASE Spectral Survey I: Measuring Contamination in the Color-selected J-region Asymptotic Giant Branch Distance Indicator
AU - Siyang Li
AU - Alexei V. Filippenko
AU - Arief Ahmad
AU - Adam G. Riess
AU - Stefano Casertano
AU - Thomas G. Brink
AU - Richard Anderson
AU - Caroline D. Huang
AU - Louise Breuval
AU - Noriyuki Matsunaga
AU - Gracelynn Jost
AU - Yash Mehta
AU - Elaina Sadler
AU - Julian Shapiro
PY - 2026
DA - 2026/09/21
PB - arXiv
DO - 10.48550/arxiv.2609.25412
UR - https://arxiv.org/abs/2609.25412
AB - The J-region asymptotic giant branch (JAGB) method for distance measurements uses the near uniform luminosity of carbon-rich AGB stars as a standard candle. Its application rests on an assumed but rarely examined premise: the stars contained in the JAGB color-magnitude selection box are primarily carbon-rich AGB stars. We investigate the purity of the photometric selection in the Milky Way with an optical spectroscopic survey (~3590-7150 mathringA) called CHASE (Carbon stars, Hubble tension, And Spectroscopic Exploration of JAGB populations) using the Shane 3 m telescope at Lick Observatory. These stars are classified using molecular bandhead step indices, which recover 100% of known classifications. We report the observations of 76 stars (54 within the standard JAGB selection box of 1.5 < J - K < 2.0 mag) and find only 13 of 54 stars are carbon-rich, a 76±6 oxygen-rich contamination under blind photometric selection. This contamination remains high (71%) even within ±0.4 mag of the JAGB peak itself, showing it is not confined to the edges of the selection box. This inhomogeneity may explain why recent literature find field-to-field and methodological variations of 0.1-0.2 mag that now dominate its systematic error budget. These variations could be caused, in part, by inhomogeneous stellar populations that consist of differing spectral types, and hence differing mean magnitudes.
ER -