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arXiv · 2610.30346

Optical and near-infrared s-process abundance scales in the Galactic disc

Andrés E. Piatti
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Abstract

A discrepancy has recently emerged between optical and near-infrared (NIR) s-process abundance scales, particularly for Neodymium (Nd). We investigated this issue using a direct cross-match between the optical Gaia-ESO Survey (DR5) and NIR SDSS-V/MWM (DR19). While atmospheric parameters and Cerium (Ce) abundances exhibit robust 1:1 correlations, Nd displays a systematic X-shaped vertical stretching. Our analysis reveals that the SDSS-V pipeline expands intrinsic Nd variations by a scaling factor of 5.54 relative to the stable optical baseline. This phenomenon is driven by baseline compression in the APOGEE H-band, where the growth of the molecular forest (CN and CO) at high metallicities depresses the local continuum. This forces the chi^2 minimisation engine in the NIR abundance determination to inflate derived abundances to reach mathematical convergence. We provide an empirical calibration framework to reconcile these disparate regimes. This correction effectively eliminates the contradictory negative radial [Nd/Fe] gradient of -0.052 dex/kpc reported in recent NIR surveys, bringing them into alignment with the rising trends established by the optical high-resolution Gaia-ESO survey. These results demonstrate that the observed scale expansion is a spectroscopic artefact of line-blanketing rather than a physical Galactic trend.

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Andrés E. Piatti. 2026-10-21. Optical and near-infrared s-process abundance scales in the Galactic disc. https://arxiv.org/abs/2610.30346

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