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 constitute a substantial proportion of the extremely metal-poor (EMP) stars of the Galactic halo, and a by far larger proportion than CH stars among Population II stars. We investigate their origin by taking into account an additional evolutionary path to the surface carbon enrichment, triggered by hydrogen engulfment by the helium flash convection, in EMP stars with ½Fe/H P À2:5. This process is distinct from the third dredge-up operating in more metal-rich stars and in EMP stars. In binary systems of EMP stars, the secondary stars become CEMP stars through mass transfer from the low-and intermediatemass primary stars that have developed the surface carbon enhancement. Our binary scenario can predict the variations in the abundances not only for carbon but also for nitrogen and s-process elements and can reasonably explain the observed properties such as the stellar distributions of the carbon abundances, the binary periods, and the evolutionary stages. Furthermore, from the observed frequencies of CEMP stars with and without s-process element enhancement, we demonstrate that the initial mass function of EMP stars needed gives the mean mass $ 10 M under the reasonable assumptions for the distributions of orbital separations and mass ratios of the binary components. This also indicates that the currently observed EMP stars were exclusively born as the secondary members of binaries, making up $10% of EMP binary systems, with mass $ 10 8 M in total; in addition to CEMP stars with white dwarf companions, a significant fraction of them have experienced supernova explosions of their companions. We discuss the implications of the present results for the formation of the Galactic halo.

Hierarchical Formation of the Galactic Halo and the Origin of Hyper Metal-Poor Stars

“…Massive stars have already ended their lives, and yet, may have left their imprints on the low-mass survivors. On the basis of the observations, Komiya et al (2007Komiya et al ( , 2009, hereafter referred to as Papers I and II, respectively) discuss the characteristics of the stars with [Fe/H] −2.5, formed in early universe as "EMP population." They show that a high-mass initial mass function (IMF) of typical mass ∼10 M , in combination with the binary formation and evolution, can provide an account of the statistics of carbon-enhanced metalpoor stars (both with and without the enrichment of s-process elements), the chemical evolution of the Galactic halo, and the total number of observed EMP survivors.…”

The IMF of Extremely Metal‐Poor Stars as Constrained by Data from the Milky Way

“…In this case, however, the enrichment of sprocess elements, as suggested from the observations is not necessarily accompanied [8]. Accordingly, the secondary stars with primary star in this range may show up as CEMP-nos stars after the mass transfer; the upper limit of the primary mass range is imposed by the hot bottom burning in the envelope, which converts the dredged carbon into nitrogen.…”

The Puzzling Frequencies of CEMP and NEMP Stars

“…The NEMP/CEMP ratio is too large to be compatible with observations. This effect is much more extreme when we assume the IMF suggested by Komiya et al (2007) which has a median mass of 10 M . Although this gives rise to a substantial CEMP fraction, close to being compatible with observations without the need to assume enhanced dredge-up (Model 1E), the CEMP stars are outnumbered by NEMP stars by a factor of two.…”

“…In recent studies (Lucatello et al 2005b;Komiya et al 2007) it has been argued that a different initial mass function (IMF) is therefore needed at low metallicity, weighted towards intermediate-mass stars. If true, this in turn has important consequences for the chemical evolution of the halo and of other galaxies.…”