Subaru/HDS Study of the Extremely Metal-Poor Star CS 29498-043: Abundance Analysis Details and Comparison with Other Carbon-Rich Objects

We report on the details of an abundance study using the Subaru Telescope High Dispersion Spectrograph for the extremely metal-poor ([Fe/H] = 3.75) star CS 29498-043, whose overabundances of C, N, Mg, and Si were reported in our previous work. The effect of the large overabundance of Mg found in this object on the stellar atmosphere is included in the abundance analysis. We also performed a similar analysis for the other eight carbon-rich, metal-poor (-2.7 < [Fe/H] < -1.9) stars with excesses of s-process elements, and for the carbon-rich object CS 22957-027 which shows no excess of neutron-capture elements. Neutron-capture elements in CS 29498-043, as well as CS 22949-037 which is also known to have large excesses of C, N, Mg, and Si, do not show any overabundance. This fact clearly distinguishes these two stars from other carbon-rich objects with excesses of s-process elements. The abundances of Mg, Al, and Si in CS 29498-043 and CS 22949-037 are significantly higher than those in other carbon-rich objects and non-carbon-rich stars with similar metallicities. The abundance nature of the se two stars suggests the existence of a class of extremely metal-poor stars with large excesses of C, N, Mg, Al, and Si, which presumably originate from supernovae in which relatively little material escaped from the iron core.