Chemical composition of carbon-rich, very metal-poor subgiant LP 625-44 observed with the Subaru/HDS

Abstract

We have obtained high-resolution (R ∼ 90000) spectra of the carbon- and s-process-element-rich, very metal-poor ([Fe/H] = -2.7) subgiant LP 625-44, as well as those of HD 140283 (a metal-poor subgiant with normal abundance ratio) for a comparison, with the High Dispersion Spectrograph (HDS) for the Subaru Telescope for detailed abundance study. The excess of oxygen in LP 625-44 seems to be remarkable (perhaps by nearly a factor 10) compared with that of HD 140283 derived from the O I triplet around 7770 Å, though the oxygen abundance derived from these lines is uncertain. The Na enhancement in LP 625-44 is by about a factor 50, suggesting hydrogen burning in the 22Ne-rich layer in an asymptotic giant branch star which produced the abundance pattern of this object. In our new spectrum of LP 625-44, the Pb I λ3683 Å line has been detected, confirming the Pb abundance [log ∈ (Pb) ∼ 1.9] derived from the Pb I λ4057 Å line by previous work. The abundance ratio of s-process elements at the second peak (e.g., La, Ce, and Nd) to that at the third peak (Pb) in LP 625-44 is significantly higher (by a factor 5) than that in other three s-process element-rich objects recently studied by van Eck et al. Recent theoretical studies have modeled s-process nucleosynthesis in the radiative layer of asymptotic giant branch stars in the inter-pulse phase. The above results mean that these processes produced a large scatter in the abundance ratios, or different processes (e.g., s-process nucleosynthesis during thermal pulses) contributed to heavy elements in the early Galaxy.