Four-hundred Very Metal-poor Stars Studied with LAMOST and Subaru. II. Elemental Abundances

We present homogeneous abundance analysis of over 20 elements for 385 very metal-poor (VMP) stars based on the LAMOST survey and follow-up observations with the Subaru Telescope. It is the largest high-resolution VMP sample (including 363 new objects) studied by a single program, and the first attempt to accurately determine evolutionary stages for such a large sample based on Gaia parallaxes. The sample covers a wide metallicity range from [Fe/H] ≲ -1.7 down to [Fe/H] ~ -4.3, including over 110 objects with [Fe/H] ≤ -3.0. The expanded coverage in evolutionary status makes it possible to define the abundance trends respectively for giants and turnoff stars. The newly obtained abundance data confirm most abundance trends found by previous studies, but also provide useful updates and new samples of outliers. The Li plateau is seen in main-sequence turnoff stars with -2.5 < [Fe/H] < -1.7 in our sample, whereas the average Li abundance is clearly lower at lower metallicity. Mg, Si, and Ca are overabundant with respect to Fe, showing decreasing trend with increasing metallicity. Comparisons with chemical evolution models indicate that the overabundance of Ti, Sc, and Co are not well reproduced by current theoretical predictions. Correlations are seen between Sc and α-elements, while Zn shows a detectable correlation only with Ti but not with other α-elements. The fraction of carbon-enhanced stars ([C/Fe] > 0.7) is in the range of 20%-30% for turnoff stars depending on the treatment of objects for which C abundance is not determined, which is much higher than that in giants (~8%). Twelve Mg-poor stars ([Mg/Fe] < 0.0) have been identified in a wide metallicity range from [Fe/H] ~ -3.8 through -1.7. Twelve Eu-rich stars ([Eu/Fe] > 1.0) have been discovered in -3.4 < [Fe/H] < -2.0, enlarging the sample of r-process-enhanced stars with relatively high metallicity.