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Large surveys are providing a diversity of spectroscopic observations withGaia alone set to deliver millions of Ca-triplet-region spectra across theGalaxy. We aim to understand the dimensionality of the chemical abundanceinformation in the Gaia-RVS data to inform galactic archaeology pursuits. Wefit a quadratic model of four primary sources of variability, described bylabels of $T_{\rm eff}$, $\log g$, [Fe/H], and [$\alpha$/Fe], to the normalizedflux of 10,802 red-clump stars from the Gaia-RVS-like ARGOS survey. We examinethe residuals between ARGOS spectra and the models and find that the modelscapture the flux variability across $85\%$ of the wavelength region. Theremaining residual variance is concentrated to the Ca-triplet features, at anamplitude up to $12\%$ of the normalized flux. We use principal componentanalysis on the residuals and find orthogonal correlations in the Ca-tripletcore and wings. This variability, not captured by our model, presumably marksdepartures from the completeness of the 1D-LTE label description. To test theindication of low-dimensionality, we turn to abundance-space to infer how wellwe can predict measured [Si/H], [O/H], [Ca/H], [Ni/H], and [Al/H] abundancesfrom the Gaia-RVS-like RAVE survey with models of $T_{\rm eff}$, $\log g$,[Fe/H], and [Mg/Fe]. We find that we can near-entirely predict theseabundances. Using high-precision APOGEE abundances, we determine that ameasurement uncertainty of $<$ 0.03 dex is required to capture additionalinformation from these elements. This indicates that a four-label modelsufficiently describes chemical abundance variance for $\approx$ S/N $<$ 200per pixel, in Gaia-RVS spectra.

The Astrophysical Variance in Gaia-RVS Spectra