Discovery of a Metal-poor Red Giant Star with the Highest Ultralithium Enhancement

Abstract

We present the discovery of 2MASS J05241392−0336543 (hereafter J0524−0336), a very metal-poor ([Fe/H] = −2.43 ± 0.16), highly r-process-enhanced ([Eu/Fe] = +1.34 ± 0.10) Milky Way halo field red giant star, with an ultrahigh Li abundance of A(Li, 3D, NLTE) = 6.15 ± 0.25 and [Li/Fe] = +7.64 ± 0.25, respectively. This makes J0524−0336 the most lithium-enhanced giant star discovered to date. We present a detailed analysis of the star’s atmospheric stellar parameters and chemical abundance determinations. Additionally, we detect indications of infrared excess, as well as observe variable emission in the wings of the Hα absorption line across multiple epochs, indicative of a potential enhanced mass-loss event with possible outflows. Our analysis reveals that J0524−0336 lies either between the bump and the tip of the red giant branch (RGB), or on the early asymptotic giant branch (e-AGB). We investigate the possible sources of lithium enrichment in J0524−0336, including both internal and external sources. Based on current models and on the observational evidence we have collected, our study shows that J0524−0336 may be undergoing the so-called lithium flash that is expected to occur in low-mass stars when they reach the RGB bump and/or the e-AGB.