Sodium Abundances in Very Metal-poor Stars

Abstract

Chemical composition of very metal-poor (VMP) stars can provide observational constraints on current models of nucleosynthesis and the chemical evolution of the Galaxy. It has been found that the scatter of [Na/Fe] versus [Fe/H] in VMP stars is very large in contrast with most other elements. Moreover, a negative slope in [Na/Fe] versus [Fe/H] was found for giants, which is very unlikely according to the theory of nucleosynthesis. For the sample of 93 VMP stars in the metallicity range −4.25 < [Fe/H] < −1.64 we obtained NLTE sodium abundances using the line profile fitting method by employing accurate atmospheric parameters determined when taking into account NLTE line formation for both Fe i and Fe ii. Originally selected from the LAMOST low-resolution spectral database, the spectra of stars were obtained with the High Dispersion Spectrograph of the Subaru Telescope. For 57 turn-off stars in metallicity domain −3.04 < [Fe/H] < −1.64, we obtained mean [Na/Fe] = −0.29 ± 0.14 and positive slope 0.09 ± 0.06. For 21 giants distributed over metallicity −3.59 < [Fe/H] < −2.19, we found mean [Na/Fe] = −0.35 ± 0.1 and positive slope 0.07 ± 0.07. Our [Na/Fe] trend is lower by ∼0.2 dex, compared to the modern GCE model. We believe the GCE model should be adjusted, by considering the associated scatter. Twelve stars in our sample are found to be outliers, with too low or too high Na abundances.