Enabling Metallicity Measurements of M-dwarf Microlensing Lenses out to the Galactic Bulge
Abstract
We demonstrate that metallicity becomes identifiable for M-dwarf microlensing lenses out to bulge distances when multi-band lens photometry is combined with the angular Einstein radius, thereby defining the photometric--microlensing metallicity method. The cool atmospheres of M dwarfs contain abundant molecules, whose broad absorption bands make their positions in an optical--NIR color--absolute magnitude diagram sensitive to metallicity. The angular Einstein radius provides the mass--distance constraint needed to infer the lens absolute magnitude, while the intrinsic M-dwarf locus and reddening vector are non-parallel in color--color space, allowing the lens extinction and intrinsic color to be inferred simultaneously from its multi-band photometry, thereby recovering the lens intrinsic position in the metallicity-sensitive color--absolute magnitude diagram. The method requires three-band lens photometry spanning roughly the R, Z, and K bands, making it particularly well suited to Roman through its high-resolution F062, F087, and F213 imaging. With the planned Roman Galactic Bulge Time-Domain Survey (GBTDS) observations plus an additional ∼8 hr of F062 imaging per field (∼40 hr in total) roughly a decade after Roman launch, host metallicities could be measured for ∼150 planetary systems under the GBTDS yield forecast, with a typical 1σ precision of ∼0.25 dex from our mock-recovery analysis. Applied homogeneously to GBTDS microlensing events with and without detected planets, the method would enable the first measurement of the occurrence--metallicity relation for cold low-mass planets beyond the snow line, constrain the low-metallicity cutoff for their formation, and extend occurrence--metallicity studies into the inner Galaxy.
Related
- The Galactic Dynamics of Free-Floating Planets: From Ejection Kicks to Microlensing
- Cepheid Metallicity in the Leavitt Law (C-MetaLL) survey: X First spectroscopic characterization of Galactic Type II Cepheids and calibration of the Period-Wesenheit-Metallicity relations
- Neglecting Dynamical Microlensing Can Bias Accretion Disk Size Measurements at the Population Level
- An Analytic Model for Stellar Metallicity Gradient Residuals in Cold, Phase-Mixed Galactic Disks
- Constraints on the Metallicity-dependent Explodability of Massive Stars from Galactic Chemical Evolution: Toward Alleviating the Red Supergiant Problem
BibTeX
@misc{zhang2026enabling,
title = {{Enabling Metallicity Measurements of M-dwarf Microlensing Lenses out to the Galactic Bulge}},
author = {Jiyuan Zhang and Gregory Green and Weicheng Zang and Hongjing Yang and Shude Mao},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2609.05265},
eprint = {2609.05265},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2609.05265},
}
RIS
TY - UNPB
TI - Enabling Metallicity Measurements of M-dwarf Microlensing Lenses out to the Galactic Bulge
AU - Jiyuan Zhang
AU - Gregory Green
AU - Weicheng Zang
AU - Hongjing Yang
AU - Shude Mao
PY - 2026
DA - 2026/09/16
PB - arXiv
DO - 10.48550/arxiv.2609.05265
UR - https://arxiv.org/abs/2609.05265
AB - We demonstrate that metallicity becomes identifiable for M-dwarf microlensing lenses out to bulge distances when multi-band lens photometry is combined with the angular Einstein radius, thereby defining the photometric--microlensing metallicity method. The cool atmospheres of M dwarfs contain abundant molecules, whose broad absorption bands make their positions in an optical--NIR color--absolute magnitude diagram sensitive to metallicity. The angular Einstein radius provides the mass--distance constraint needed to infer the lens absolute magnitude, while the intrinsic M-dwarf locus and reddening vector are non-parallel in color--color space, allowing the lens extinction and intrinsic color to be inferred simultaneously from its multi-band photometry, thereby recovering the lens intrinsic position in the metallicity-sensitive color--absolute magnitude diagram. The method requires three-band lens photometry spanning roughly the R, Z, and K bands, making it particularly well suited to Roman through its high-resolution F062, F087, and F213 imaging. With the planned Roman Galactic Bulge Time-Domain Survey (GBTDS) observations plus an additional ∼8 hr of F062 imaging per field (∼40 hr in total) roughly a decade after Roman launch, host metallicities could be measured for ∼150 planetary systems under the GBTDS yield forecast, with a typical 1σ precision of ∼0.25 dex from our mock-recovery analysis. Applied homogeneously to GBTDS microlensing events with and without detected planets, the method would enable the first measurement of the occurrence--metallicity relation for cold low-mass planets beyond the snow line, constrain the low-metallicity cutoff for their formation, and extend occurrence--metallicity studies into the inner Galaxy.
ER -