LAUSR

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 had been found that the scatter of [Na/Fe] versus [Fe/H] in VMP stars was very large in contrast with most of the other elements. Moreover, it was found negative slope [Na/Fe] vs. [Fe/H] for giants, which is very unlikely according to theory of nucleosynthesis. For the sample of 93 VMP stars in metallicity range -4.25 < [Fe/H] < -1.64 we obtained NLTE sodium abundances using line profile fitting method by employing accurate atmospheric parameters determined with taking into account NLTE line formation for both Fe\ione\ and Fe\ii. Originally selected from LAMOST low-resolution spectra 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$\pm$0.14 and positive slope 0.09$\pm$0.06. For 21 giants distributed over metallicity -3.59 < [Fe/H] < -2.19, we obtained mean [Na/Fe]=-0.35$\pm$0.1 and positive slope 0.07$\pm$0.07. Our [Na/Fe] trend are lower by $\sim$0.2~dex, compared to modern GCE model. We believe the GCE model should be adjusted, including considering the scatter. Twelve stars in our sample are found to be outliers, with too low or too high Na abundances.