Chemo-dynamical abundance analysis of the very metal-poor halo star HE 2315-4240

Abstract

Learning about the chemical evolution of the universe is crucial to understanding the formation of the first stars and structure formation in the early universe. To find out how elements are produced via nucleosynthesis and how their relative amounts have evolved with time, abundance trends of stars with different metallicities can be established and compared. In this study, we present a spectrum of a very metal-poor star, HE 2315$-$4240, with [Fe/H] = $-$2.89 based on a Magellan/MIKE high-resolution visual light spectrum. The star has a radial velocity of +41.9 km s$^{-1}$, an effective temperature of 5181 K, a surface gravity of 2.24 dex, and a microturbulence of 1.61 km s$^{-1}$. The $\alpha$-elements and the iron peak elements agree well with the abundance trend. The low abundance of [Sr/Ba] and [C/Fe] suggests that HE 2315$-$4240 is accreted and formed in a dwarf galaxy. The value of [Ba/Eu] suggests the operation of a limited r-process. The abundance pattern of [Mg/Fe] and [Si/Fe] in HE 2315$-$4240 and its metallicity indicated that the star is formed from the gas enriched by a Type II supernova of a massive Pop III star. The abundance pattern fits Population III supernova yields moderately. The star's kinematic behavior shows that the star has a retrograde orbit and is moving away from the galactic center and out of the galactic disk to the south, and although the star is located in the halo of the Milky Way, it didn't form in the Milky Way but was rather formed in a small dwarf galaxy that was later absorbed by the Milky Way.