Chemical Analysis of Two Extremely Metal-poor Stars HE 2148-2039 and HE 2155-2043

Abstract

We present elemental abundance results for HE 2148−2039 and HE 2155−2043 based on a detailed high-resolution spectroscopic analysis. The high-resolution Subaru/High Dispersion Spectrograph (HDS) spectra used for our analysis have a resolution of R ~ 60,000. Although limited information based on photometry and low-resolution spectroscopy is available, we present for the first time an abundance analysis based on high-resolution spectra for both objects. Our analysis shows that the two objects are extremely metal-poor with [Fe/H] < −3. Among the neutron-capture elements, abundances of only Sr and Ba could be determined in our program stars. For both the objects [Ba/Fe] is found to be <0. While strontium is underabundant in HE 2148−2039 with [Sr/Fe] ~ −2.02, Sr is near solar in HE 2155−2043. The locations of the program stars in the absolute carbon abundance, A(C) versus [Fe/H] diagram, show that HE 2148−2039 is a carbon-enhanced metal-poor (CEMP)-no Group II object and HE 2155−2043 is a CEMP-no Group III object. Observed [Sr/Ba] ratios are characteristics of a fast rotating massive star progenitor for HE 2155−2043 and a metal-poor asymptotic giant branch (AGB) star for HE 2148−2039. The estimated [Sc/Mn] as well as [C/Cr] ratios in HE 2155−2043 show that the surface chemical composition of this object is mono-enriched. The surface chemical composition of HE 2148−2039 is also found to be mono-enriched based on the [Mg/C] ratio. With respect to their locations in the [C/N] versus Teff diagram, HE 2148−2039 shows signatures of mixing, and HE 2155−2043 falls in the unmixed region of the [C/N] versus Teff plot. Kinematic analysis shows that both objects belong to the Galactic halo population.