A high-resolution spectral analysis of three carbon-enhanced metal-poor stars*

We present results of an analysis of high-resolution spectra (R � 50000), obtained with the Subaru Telescope High Dispersion Spectrograph, of two CarbonEnhanced Metal-Poor (CEMP) stars selected from the Hamburg/ESO prism survey, HE 1305+0007 and HE 1152 0355, and of the classical CH star HD 5223. All of these stars have relatively low effective temperatures (4000–4750K) and high carbon abundances, which results in the presence of very strong molecular carbon bands in their spectra. The stellar atmospheric parameters for these stars indicate that they all have surface gravities consistent with a present location on the red-giant branch, and metallicities of [Fe/H] = 2.0 (HE 1305+0007, HD 5223) and [Fe/H] = 1.3 (HE 1152 0355). In addition to their large enhancements of carbon ([C/Fe] = +1.8, +1.6 and +0.6. respectively), all three stars exhibit strong enhancements of the sprocess elements relative to iron. HE 1305+0007 exhibits a large enhancement of the 3rd-peak s-process element lead, with [Pb/Fe] = +2.37, as well as a high abundance of the r-process element europium, [Eu/Fe] = +1.97. The 2nd-peak s-process elements Ba, La, Ce, Nd, and Sm are found to be more enhanced than the 1st-peak s-process elements Zr, Sr and Y. Thus, HE 1305+0007 joins the growing class of the so-called “Lead Stars”, and also the class of objects that exhibit the presence of both r-process and s-process elements, the CEMP-r/s stars. The large enhancement of n-capture elements exhibited by HE 1152 0355 and HD 5223 are more consistent with the abundance patterns generally noticed in CH stars, essentially arising from pure s-process nucleosynthesis. The elemental abundance distributions observed in these stars are discussed in the light of existing theories of CH star formation, as well as the suggested formation scenarios of the CEMP-r/s group.