Lithium detected in an ancient star gives new clues for Big Bang nucleosynthesis
Researchers from the Instituto de Astrofísica de Canarias (Spain) and the University of Cambridge (UK) have detected lithium (Li) in the ancient star J0023+0307, a main-sequence extremely iron-poor dwarf star about 9,450 light years away in the galactic halo.
The study of the most ancient stars in the Milky Way allows us to infer the early properties of the galaxy, its chemical composition, and its assembly history. Metal-poor stars are invaluable messengers that carry information from early epochs, and are an important key to understand the primordial production of Li and the processes responsible for the possible "meltdown" of the Li plateau (a typical Li abundance of a metal-poor dwarf star which is related to the primordial lithium abundance). All stars with low metallicities and low Li abundances, significantly below A(Li)~2.2, are considered to have been likely affected by destruction of the Li in the stars.
New or poorly measured nuclear reaction resonances could affect the Li production predicted by the standard Big Bang nucleosynthesis (SBBN). Processes injecting neutrons at the relevant temperatures of the primordial plasma can also alter the primordial Li abundance. In addition, time-varying fundamental constants may lead to a significant Li lower value. Li observations in stars at the lowest metallicities are especially important to bring an insight into the processes of potential Li depletion in stars and, ultimately, to establish if any non-standard physics may have played a role during or after SBBN.
Stars that formed in the first or second generation are extremely rare objects, and only a few are known. The lack of metals in the gas available in the mini-halos, where the first stars formed, limits radiative cooling, increasing the Jeans mass and shifting the initial mass function to large masses, to the point that perhaps no low-mass stars were formed in the first generation. This picture has been challenged in recent years by the discovery of low-mass stars which show extremely low metallicity and low carbon and nitrogen abundances, suggesting that low-mass stars can form even at such low metallicities.
A year ago, astronomers using the ISIS spectrograph at the William Herschel Telescope (WHT) discovered the star J0023+0307, one of the most metal-poor stars known, with about a million times less iron than the sun. J0023+0307 also shows very little carbon, an important element for the formation of low-mass stars in the low metallicity regime.
New data obtained using UVES, a high-resolution spectrograph at the Very Large Telescope (VLT) in Paranal Observatory (Chile), revealed a Li abundance with values consistent with the extended Li plateau at these low metallicities. However, the predicted Li abundance from the SBBN theory remains a factor of three higher than that of the Li plateau.
The presence of Li in this extremely iron-poor star has implications for the production of Li at the Big Bang, and strongly constrains any theory aiming at explaining the cosmological Li problem. The fact that no star in this large low-metallicity regime has been detected showing a Li abundance between that inferred from SBBN and the Li plateau, makes this upper boundary of Li abundance at low metallicities difficult to explain by destruction in the stars, and may support a lower primordial Li production, driven by non-standard nucleosynthesis processes.
David S. Aguado et al. Back to the Lithium Plateau with the [Fe/H] < −6 Star J0023+0307, The Astrophysical Journal (2019). DOI: 10.3847/2041-8213/ab1076