The Origin of Carbon Enhancement and the Initial Mass Function of Extremely Metal‐poor Stars in the Galactic Halo

Abstract

It is known that the carbon-enhanced, extremely metal-poor (CEMP) stars\nconstitute a substantial proportion in the extremely metal-poor (EMP) stars of\nthe Galactic Halo, by far larger than CH stars in Population II stars. We\ninvestigate their origin with taking into account an additional evolutionary\npath to the surface carbon-enrichment, triggered by hydrogen engulfment by the\nhelium flash convection, in EMP stars of $[Fe/H] \\lesssim -2.5$. This process\nis distinct from the third dredge-up operating in more metal-rich stars and\nalso in EMP stars. In binary systems of EMP stars, the secondary stars become\nCEMP stars through mass transfer from the primary stars of low and intermediate\nmasses, which have developed the surface carbon-enhancement. Our binary\nscenario can predict the variations in the abundances not only for carbon but\nalso for nitrogen and s-process elements and reasonably explain the observed\nproperties such as the stellar distributions with respect to the carbon\nabundances, the binary periods, and the evolutionary stages. Furthermore, from\nthe observed frequencies of CEMP stars with and without s-process element\nenhancement, we demonstrate that the initial mass function of EMP stars need to\ngive the mean mass $~10\\msun$ under the reasonable assumptions on the\ndistributions of orbital separations and mass ratio of binary components. This\nalso indicates that the currently observed EMP stars were exclusively born as\nthe secondary members of binaries, making up $\\sim 10%$ remnants of EMP binary\nsystems of mass $~10^8\\msun$ in total; in addition to CEMP stars with white\ndwarf companions, a significant fraction of them have experienced supernova\nexplosions of their companions. We discuss the implications of the present\nresults in relation to the formation of Galactic halo.\n