Chemical Composition of Carbon-Rich, Very Metal-Poor Subgiant LP 625-44 Observed with the Subaru/HDS

We have obtained a high resolution (R~90,000) spectrum of the carbon- ands-process-element-rich, very metal-poor ([Fe/H]=-2.7) subgiant LP625-44, aswell as that of HD140283 (a metal-poor subgiant with normal abundance ratio)for comparison, with the High Dispersion Spectrograph (HDS) for the SubaruTelescope for detailed abundance study. The oxygen abundance derived from the OI triplet around 7770A is uncertain, but the excess of oxygen in LP625-44 seemsremarkable (perhaps by nearly a factor 10), in comparison with that of HD140283derived from the same lines. The Na enhancement in LP625-44 is by about afactor 50, suggesting hydrogen burning in the 22Ne-rich layer in an asymptoticgiant branch star which produces the abundance pattern of this object. In ournew spectrum of LP625-44, the Pb I lambda 3683A line has been detected, as wellas the Pb I lambda 4057A line which has already been studied, confirming the Pbabundance (log epsilon(Pb)~1.9) derived by the previous work. The abundanceratio of s-process elements at the second peak (e.g., La, Ce and Nd) to that atthe third peak (Pb) in LP 625-44 is significantly higher (by a factor 5) thanthat in other three s-process element-rich objects recently studied by van Ecket al.. Recent theoretical works have modeled the s-process nucleosynthesis inthe radiative layer of asymptotic giant branch stars in the inter-pulse phase,and the above results means that these processes produced a large scatter inthe abundance ratios. Another possiblity is that different processes (e.g.,s-process nucleosynthesis during thermal pulses) have contributed to heavyelements in the early Galaxy.Comment: 65 pages, 11 figures, to appear in PAS