Stellar Models are Reliable at Low Metallicity: An Asteroseismic Age for the Ancient Very Metal-poor Star KIC 8144907
Li, Yaguang; Huber, Daniel; Ong, J. M. Joel; Bedding, Timothy R.; Hirano, Teruyuki; Stello, Dennis; Serenelli, Aldo; Hon, Marc; Pinsonneault, Marc H.; Kirby, Evan N.; Grunblatt, Samuel K.; Joyce, Meridith; Greklek-McKeon, Michael; Alencastro Puls, Arthur; Berger, Travis; Slumstrup, Ditte; Børsen-Koch, Victor Aguirre; Zinn, Joel
United States, Australia, Chile, Denmark, Hungary, Spain, Japan, Germany
Abstract
Very-metal-poor stars ([Fe/H] < ‑2) are important laboratories for testing stellar models and reconstructing the formation history of our galaxy. Asteroseismology is a powerful tool to probe stellar interiors and measure ages, but few asteroseismic detections are known in very-metal-poor stars and none have allowed detailed modeling of oscillation frequencies. We report the discovery of a low-luminosity Kepler red giant (KIC 8144907) with high signal-to-noise ratio oscillations, [Fe/H] = ‑2.66 ± 0.08 and [α/Fe] = 0.38 ± 0.06, making it by far the most metal-poor star to date for which detailed asteroseismic modeling is possible. By combining the oscillation spectrum from Kepler with high-resolution spectroscopy, we measure an asteroseismic mass and age of 0.79 ± 0.02(ran) ± 0.01(sys) M ⊙ and 12.0 ± 0.6(ran) ± 0.4(sys) Gyr, with remarkable agreement across different codes and input physics, demonstrating that stellar models and asteroseismology are reliable for very-metal-poor stars when individual frequencies are used. The results also provide a direct age anchor for the early formation of the Milky Way, implying that substantial star formation did not commence until redshift z ≈ 3 (if the star formed in situ) or that the Milky Way has undergone merger events for at least ≈12 Gyr (if the star was accreted by a dwarf satellite merger such as Gaia-Enceladus).