TOPoS: II. On the bimodality of carbon abundance in CEMP stars.
  Implications on the early chemical evolution of galaxies
  In the course of the TOPoS (Turn Off Primordial Stars) survey, aimed at
discovering the lowest metallicity stars, we have found several carbon-enhanced
metal-poor (CEMP) stars. We here present our analysis of six CEMP stars.
Calcium and carbon are the only elements that can be measured in all six stars.
The range is -5.0<=[Ca/H]< -2.1 and 7.12<=A(C)<=8.65. For star SDSS J1742+2531
we were able to detect three FeI lines from which we deduced [Fe/H]=-4.80, from
four CaII lines we derived [Ca/H]=-4.56, and from synthesis of the G-band we
derived A(C)=7.26. For SDSS J1035+0641 we were not able to detect any iron
lines, yet we could place a robust (3sigma) upper limit of [Fe/H]< -5.0 and
measure the Ca abundance, with [Ca/H]=-5.0, and carbon, A(C)=6.90. No lithium
is detected in the spectrum of SDSS J1742+2531 or SDSS J1035+0641, which
implies a robust upper limit of A(Li)<1.8 for both stars. Our measured carbon
abundances confirm the bimodal distribution of carbon in CEMP stars,
identifying a high-carbon band and a low-carbon band. We propose an
interpretation of this bimodality according to which the stars on the
high-carbon band are the result of mass transfer from an AGB companion, while
the stars on the low-carbon band are genuine fossil records of a gas cloud that
has also been enriched by a faint supernova (SN) providing carbon and the
lighter elements. (Abridged)
