A new type of extremely metal-poor star

We present an abundance analysis for the extremely metal-poor (EMP) star HE 1424-0241 based on high-dispersion spectra from HIRES at Keck. This star is a giant on the lower red giant branch with [Fe/H] ∼ -4.0 dex. Relative to Fe, HE 1424-0241 has normal Mg, but it shows a very large deficiency of Si, with ε(Si)/ε(Fe) ∼ 1/10 and ε(Si)/ε(Mg) ∼ 1/25 that of all previously known EMP giants or dwarfs. It also has a moderately large deficiency of Ca and a smaller deficit of Ti, combined with enhanced Mn and Co and normal or low C. We suggest that in HE 1424-0241 we see the effect of a very small number of contributing supernovae, and that the SNe II contributing to the chemical inventory of HE 1424-0241 were biased in progenitor mass or in explosion characteristics so as to reproduce its abnormal extremely low Si/Mg ratio. HE 1424-0241 shows a deficiency of the explosive α-burning elements Si, Ca, and Ti coupled with a ratio [Mg/Fe] normal for EMP stars; Mg is produced via hydrostatic α-burning. The latest models of nucleosynthesis in SNe II fail to reproduce the abundance ratios seen in HE 1424-0241 for any combination of the parameter space of core-collapse explosions they explore.