出版社：SISSA, Scuola Internazionale Superiore di Studi Avanzati
摘要：We discuss the characteristics of known extremely metal-poor (EMP) stars in the Galaxy using the Stellar Abundances for Galactic Archaeology (SAGA) database∗. We focus on the characteristics of carbon-enhanced EMP (CEMP) stars and the possible changeover of the initial mass function (IMF) above [Fe/H]& −2.5. This peak in the IMF at high mass (∼10M⊙) is supported by studies of the statistics of CEMP stars and the chemical enrichment history of the Galactic halo with respect to the resultant metallicity distribution function. An analysis of the carbon-enhanced stars in our sample suggests that the contribution of AGB stars to the total carbon enrichment differs at metallicities above or below [Fe/H]∼−2.5. The transition at [Fe/H]∼−2.5 can be interpreted as being due to a change in the nucleosynthesis processes in EMP stars, where at metallicities lower than [Fe/H] ∼ −2.5 hydrogen ingestion into the helium flash convective zones is more prevalent or there is a decline in the 13C pocket efficiency. A statistical test was performed to determine the Zn and Ba abundance trends as a function of metallicity. A subsequent Welch’s test revealed two abundance breaks at [Fe/H] ∼ −2.8 and −1.9. The former break corresponds either to the contribution of different supernovae or to the detection limit of spectral lines. The latter one can be interpreted as a result of the changeover of the IMF from a 10M⊙ peaked IMF to a Salpeter type IMF at [Fe/H] ∼ −1.9.
