Searching for Carbon-enhanced Metal-poor Main-sequence Turnoff Stars and the Abundances Analysis

Decades of studies on metal-poor stars reveal that there are large fraction of metal-poor stars showing enhancement in carbon. Carbon-enhanced metal-poor (CEMP) stars are believed to be the trace of the first nucleosynthesis process and the chemical composition of the interstellar medium at the early stage of the universe. Different abundance patterns of neutron-capture elements reflect different origins of CEMP stars. Main-sequence turnoff stars are believed to preserve primordial material in its surface atmosphere since its birth date and place, as they have not suffered from dilution, whereas giants should have already gone through at least the first dredge-up. We report high-resolution abundances analysis of 12 CEMP turnoff stars which are selected from LAMOST survey and follow-up observed with Subaru/HDS. Among them, ten are analyzed for the first time. Compared with previous works, our sample includes larger fractions of CEMP-no (indicating carbon enrichment, but no enhancement in neutron-capture elements) stars, proving important new data for moderately C-enriched metal-poor turnoff stars. Especially, this sample provides Li abundance measurements for six CEMP-no stars including three extremely metal-poor ones whose current sample is still limited. The new sample confirmed that, when it comes to regions aroundFe/H~-3.0, CEMP-no stars show normal lithium abundances while CEMP-s stars are usually depleted in lithium.