出版社：SISSA, Scuola Internazionale Superiore di Studi Avanzati
摘要：Approximately 20% of very metal-poor stars ([Fe/H] < –2.0) are strongly enhanced in carbon
([C/Fe] > +1.0). Such stars are referred to as carbon-enhanced metal-poor (CEMP) stars. A variety
of abundance patterns are found among CEMP stars. Strong overabundances of nitrogen are
common, and overabundances of neutron capture elements are often, however not always, present.
The variety of abundance patterns among CEMP stars strongly suggests that this population of
stars comprises several astrophysical origins.
We are conducting a high-resolution follow-up of candidate EMP stars extracted from the Sloan
Digital Sky Survey (SDSS; York et al. 2000) using UVES at the VLT. Three of the programme
stars, SDSS J0912+0216, SDSS J1036+1212 and SDSS J1349-0229, where deliberately targetted
as CEMP stars since a strong G band was evident from the SDSS spectra and the weakness of the
Ca II K line testified their very low metallicity. The UVES high resolution follow-up confirmed
the original findings ([Fe/H]<−2.50 ) and allowed amore detailed investigation of their chemical
composition.
We determined the carbon abundance from molecular lines which form in the outer layers of the
stellar atmosphere. It is known that convection in metal-poor stars induces very low temperatures
which are not predicted by classical 1D stellar atmospheres. To obtain the correct temperature
structure, one needs full 3D hydrodynamical models. 3D carbon abundances were determined
for all three stars, using CO5BOLD 3D hydrodynamical model atmospheres. 3D effects on the
carbon abundance are found to be quite significant for these stars, with 3D corrections of up to
–0.7 dex.
Two of the stars, SDSS J0912+0216 and SDSS J1349-0229 exhibit an overabundance of neutron
capture elements which classifies them as CEMP-s. Star SDSS J1036+1212, instead belongs to
the elusive class of CEMP-no/s stars, with enhanced Ba, but deficient Sr, of which it is the third
member discovered to date.
