Double Trouble: How Unresolved Spectroscopic Binary Stars Skew GALAH DR4 Stellar Parameters
Abstract
Unresolved binary stars introduce systematic biases in stellar parameter estimates when analysed assuming a single star. Using ∼917{,}000 spectra from the Galactic Archaeology with HERMES (GALAH) Data Release 4 (DR4), we develop a binary spectral model that fits each observed spectrum as the superposition of two synthetic spectra generated from the same model architecture used by GALAH DR4. These generative spectral models take as input: effective temperature (Teff), surface gravity (log g), iron abundance ([Fe/H]), projected rotation velocity (v sin i), microturbulence velocity (vmic), and radial velocity (vrad). In our binary model, Teff and log g are not fitted directly but are instead interpolated from PARSEC isochrones based on stellar mass and age, under the assumption that both components formed contemporaneously with the same compositions. We fit for the primary and secondary masses, a common age and [Fe/H], as well as individual v sin i, vmic, and vrad for each component. After iterative optimization and conservative quality cuts to ensure robust detections, we identify ∼12{,}911 candidate binary systems. We quantify typical biases introduced by unresolved binarity: Teff shifts of −142 to +200 K, log g shifts of −0.070 to +0.35 dex, and [Fe/H] shifts of −0.055 to +0.13 dex. Despite our conservative approach reducing the number of detections, we recover known trends of increasing binary fraction with higher primary mass and lower metallicity, thus reflecting the broad trends of binary populations. Accounting for unresolved binaries is essential for accurate stellar population modelling and for interpreting spectroscopic surveys--particularly in low-metallicity systems or populations with more massive stars.
Versions
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BibTeX
@misc{lach2026double,
title = {{Double Trouble: How Unresolved Spectroscopic Binary Stars Skew GALAH DR4 Stellar Parameters}},
author = {Yani Lach and Sven Buder and Gregor Traven},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2610.22707},
eprint = {2610.22707},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2610.22707},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Double Trouble: How Unresolved Spectroscopic Binary Stars Skew GALAH DR4 Stellar Parameters
AU - Yani Lach
AU - Sven Buder
AU - Gregor Traven
PY - 2026
DA - 2026/10/15
PB - arXiv
DO - 10.48550/arxiv.2610.22707
UR - https://arxiv.org/abs/2610.22707
AB - Unresolved binary stars introduce systematic biases in stellar parameter estimates when analysed assuming a single star. Using ∼917{,}000 spectra from the Galactic Archaeology with HERMES (GALAH) Data Release 4 (DR4), we develop a binary spectral model that fits each observed spectrum as the superposition of two synthetic spectra generated from the same model architecture used by GALAH DR4. These generative spectral models take as input: effective temperature (T_eff), surface gravity (log g), iron abundance ([Fe/H]), projected rotation velocity (v sin i), microturbulence velocity (v_mic), and radial velocity (v_rad). In our binary model, T_eff and log g are not fitted directly but are instead interpolated from PARSEC isochrones based on stellar mass and age, under the assumption that both components formed contemporaneously with the same compositions. We fit for the primary and secondary masses, a common age and [Fe/H], as well as individual v sin i, v_mic, and v_rad for each component. After iterative optimization and conservative quality cuts to ensure robust detections, we identify ∼12{,}911 candidate binary systems. We quantify typical biases introduced by unresolved binarity: T_eff shifts of −142 to +200 K, log g shifts of −0.070 to +0.35 dex, and [Fe/H] shifts of −0.055 to +0.13 dex. Despite our conservative approach reducing the number of detections, we recover known trends of increasing binary fraction with higher primary mass and lower metallicity, thus reflecting the broad trends of binary populations. Accounting for unresolved binaries is essential for accurate stellar population modelling and for interpreting spectroscopic surveys--particularly in low-metallicity systems or populations with more massive stars.
ER -