The LBT Y$_{\rm p}$ Project VI: Gas-Phase Chemical Abundance Patterns of Metal-Poor Nebulae

Noah S. J. Rogers; Evan D. Skillman; Richard W. Pogge; Erik Aver; Miqaela K. Weller; Danielle A. Berg; John J. Salzer; John H. Miller Jr; Jayde Speigel; Allison L. Strom

arXiv · 2610.30289

The LBT Y$_{\rm p}$ Project VI: Gas-Phase Chemical Abundance Patterns of Metal-Poor Nebulae

Noah S. J. Rogers·Evan D. Skillman·Richard W. Pogge·Erik Aver·Miqaela K. Weller·Danielle A. Berg·John J. Salzer·John H. Miller Jr·Jayde Speigel·Allison L. Strom

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Abstract

Through a combined rest-optical and NIR observing campaign, the Large Binocular Telescope Primordial He Abundance ($Y_{\rm p}$) Project has delivered a precision measurement of $Y_{\rm p}$. Given the stringent requirements to measure gas-phase He abundances, the optical spectra contain emission features from a plethora of metals in the Interstellar Medium (ISM). We present gas-phase N, O, Ne, S, Cl, Ar, K and Fe abundance patterns in 62 low-metallicity nebulae. We assess ionization correction factors for each element and select those that reproduce the observed ionic abundance trends. The Ne/O, S/O, and Ar/O abundances do not evolve with metallicity and agree with the solar ratios. Given the Type Ia enrichment of S and Ar, the solar S/O and Ar/O suggest extended star formation histories in local galaxies relative to high-$z$ galaxies at fixed O/H. We report the first ISM K abundances in low-metallicity extragalactic nebulae; both Cl/O and K/O are constant with O/H, indicating the production of these odd-Z elements follows that of $α$ elements. Consistent with prior analyses, gas-phase Fe/O is observed to decrease with O/H due to dust depletion; correcting for this produces an average [Fe/O]$=$$-$0.80, implying Fe-deficient stars capable of producing hard ionizing spectra. Finally, the metallicity-insensitive log(N/O) plateau from 50 nebulae is $-$1.43$\pm$0.10, consistent with previous works. Five galaxies have N/O significantly enhanced relative to the primary plateau, and we discuss their properties, mechanisms that could bias measurements of N/O, and nucleosynthetic progenitors that can produce enhanced N at low metallicity.

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Noah S. J. Rogers, Evan D. Skillman, Richard W. Pogge, Erik Aver, Miqaela K. Weller, Danielle A. Berg, John J. Salzer, John H. Miller Jr, Jayde Speigel, Allison L. Strom. 2026-10-21. The LBT Y$_{\rm p}$ Project VI: Gas-Phase Chemical Abundance Patterns of Metal-Poor Nebulae. https://arxiv.org/abs/2610.30289

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