A comprehensive chemical abundance analysis of the extremely metal poor\n Leoncino Dwarf galaxy (AGC 198691)

Abstract

We re-examine the extremely metal-poor (XMP) dwarf galaxy AGC 198691 using a\nhigh quality spectrum obtained by the LBT's MODS instrument. Previous spectral\nobservations obtained from KOSMOS on the Mayall 4-m and the Blue Channel\nspectrograph on the MMT 6.5-m telescope did not allow for the determination of\nsulfur, argon, or helium abundances. We report an updated and full chemical\nabundance analysis for AGC 198691, including confirmation of the extremely low\n"direct" oxygen abundance with a value of 12 + log(O/H) = 7.06 $\\pm$ 0.03. AGC\n198691's low metallicity potentially makes it a high value target for helping\ndetermine the primordial helium abundance ($Y_p$). Though complicated by a Na I\nnight sky line partially overlaying the He I $\\lambda$5876 emission line, the\nLBT/MODS spectrum proved adequate for determining AGC 198691's helium\nabundance. We employ the recently expanded and improved model of Aver et al.\n(2021), incorporating higher Balmer and Paschen lines, augmented by the\nobservation of the infrared helium emission line He I $\\lambda$10830 obtained\nby Hsyu et al. (2020). Applying our full model produced a reliable helium\nabundance determination, consistent with the expectation for its metallicity.\nAlthough this is the lowest metallicity object with a detailed helium\nabundance, unfortunately, due to its faintness (EW(H$\\beta$) $<$ 100 AA) and\nthe compromised He I $\\lambda$5876, the resultant uncertainty on the helium\nabundance is too large to allow a significant improvement on the measurement of\n$Y_p$.\n