Breaking the AGB Color Degeneracy with Variability and H2O: A Consistent Picture of AGB Chemistry, Dust, and the J-Region Across Environments
Abstract
Resolved asymptotic giant branch (AGB) populations present many observational puzzles. We consider three often treated separately: photometric C/O classification boundaries must be redrawn empirically in new environments; hydrostatic model grids are too blue in bands sampling H2O absorption; and J-region (JAGB) luminosities can differ between fields of a single galaxy. Pulsation connects them into a coherent picture. Using SPHEREx spectroscopy of AGB stars in the Large Magellanic Cloud (LMC) with JWST/NIRCam photometry and HST time-series of the SN Ia host M101, placed on a common scale by synthetic photometry, we investigate how AGB variability, surface chemistry, and circumstellar dust depend on environment. We develop an F182M index sensitive to 1.9μm H2O absorption that chemically classifies variable AGB stars and reveals oxygen-rich (O-rich) stars degenerate with carbon-rich (C-rich) stars in broadband colors. Because these features arise in extended atmospheric layers, hydrostatic grids struggle to reproduce them. Pulsation and circumstellar dust are asymmetrically linked: nearly all stars with substantial infrared excess are variable, while many variables show little dust. This supports mass loss initiated by pulsation and enhanced by dust formation. Variable O-rich AGB stars in M101 are up to ~0.7 mag redder than their LMC counterparts in dust-sensitive colors, consistent with more efficient silicate-dust production at higher metallicity, while C-rich populations depend less on environment. Color windows that select C-rich stars cleanly in the LMC are therefore 33-68% O-rich in the metal-rich M101. This challenges the J-region's universality as a standard candle while evading standard self-consistency checks. We recommend combining variability, or the infrared excess that implies it, with photometry of molecular features to improve AGB classification at 0.75-5.0μm.
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BibTeX
@misc{huang2026breaking,
title = {{Breaking the AGB Color Degeneracy with Variability and H$_2$O: A Consistent Picture of AGB Chemistry, Dust, and the J-Region Across Environments}},
author = {Caroline D. Huang and Massimo Marengo and Antonio Capodagli and Adam G. Riess and Louise Breuval and Stefano Casertano and Patricia A. Whitelock and Gagandeep S. Anand and Randall K. Smith and Wenlong Yuan},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2609.30551},
eprint = {2609.30551},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2609.30551},
}
RIS
TY - UNPB
TI - Breaking the AGB Color Degeneracy with Variability and H_2O: A Consistent Picture of AGB Chemistry, Dust, and the J-Region Across Environments
AU - Caroline D. Huang
AU - Massimo Marengo
AU - Antonio Capodagli
AU - Adam G. Riess
AU - Louise Breuval
AU - Stefano Casertano
AU - Patricia A. Whitelock
AU - Gagandeep S. Anand
AU - Randall K. Smith
AU - Wenlong Yuan
PY - 2026
DA - 2026/09/24
PB - arXiv
DO - 10.48550/arxiv.2609.30551
UR - https://arxiv.org/abs/2609.30551
AB - Resolved asymptotic giant branch (AGB) populations present many observational puzzles. We consider three often treated separately: photometric C/O classification boundaries must be redrawn empirically in new environments; hydrostatic model grids are too blue in bands sampling H2O absorption; and J-region (JAGB) luminosities can differ between fields of a single galaxy. Pulsation connects them into a coherent picture. Using SPHEREx spectroscopy of AGB stars in the Large Magellanic Cloud (LMC) with JWST/NIRCam photometry and HST time-series of the SN Ia host M101, placed on a common scale by synthetic photometry, we investigate how AGB variability, surface chemistry, and circumstellar dust depend on environment. We develop an F182M index sensitive to 1.9μm H2O absorption that chemically classifies variable AGB stars and reveals oxygen-rich (O-rich) stars degenerate with carbon-rich (C-rich) stars in broadband colors. Because these features arise in extended atmospheric layers, hydrostatic grids struggle to reproduce them. Pulsation and circumstellar dust are asymmetrically linked: nearly all stars with substantial infrared excess are variable, while many variables show little dust. This supports mass loss initiated by pulsation and enhanced by dust formation. Variable O-rich AGB stars in M101 are up to ~0.7 mag redder than their LMC counterparts in dust-sensitive colors, consistent with more efficient silicate-dust production at higher metallicity, while C-rich populations depend less on environment. Color windows that select C-rich stars cleanly in the LMC are therefore 33-68% O-rich in the metal-rich M101. This challenges the J-region's universality as a standard candle while evading standard self-consistency checks. We recommend combining variability, or the infrared excess that implies it, with photometry of molecular features to improve AGB classification at 0.75-5.0μm.
ER -