First Stars II. Elemental abundances in the extremely metal-poor star CS 22949--037: A diagnostic of early massive supernovae

CS 22949–037 is one of the most metal-poor giants known ([Fe/H]$\approx -4.0$), and it exhibits large overabundances of carbon and nitrogen (Norris et al.). Using VLT-UVES spectra of unprecedented quality, regarding resolution and S/N ratio, covering a wide wavelength range (from $\lambda = 350$ to 900 nm), we have determined abundances for 21 elements in this star over a wide range of atomic mass. The major new discovery is an exceptionally large oxygen enhancement, [O/Fe] $=1.97\pm 0.1$, as measured from the [OI] line at 630.0 nm. We find an enhancement of [N/Fe] of $2.56\pm 0.2$, and a milder one of [C/Fe] $=1.17\pm$0.1, similar to those already reported in the literature. This implies $Z_{\star }=0.01Z_{\odot }$. We also find carbon isotopic ratios ${}^{12}$C/${}^{13}$C$=4\pm 2.0$ and ${}^{13}$C/${}^{14}$N$ =0.03 ^{+0.035}_{-0.015}$, close to the equilibrium value of the CN cycle. Lithium is not detected. Na is strongly enhanced ([Na/Fe] $=+2.1\pm 0.2$), while S and K are not detected. The silicon-burning elements Cr and Mn are underabundant, while Co and Zn are overabundant ([Zn/Fe]$=+0.7$). Zn is measured for the first time in such an extremely metal-poor star. The abundances of the neutron-capture elements Sr, Y, and Ba are strongly decreasing with the atomic number of the element: [Sr/Fe] $\approx +0.3$, [Y/Fe] $\approx -0.1$, and [Ba/Fe] $\approx -0.6$. Among possible progenitors of CS 22949–037, we discuss the pair-instability supernovae. Such very massive objects indeed produce large amounts of oxygen, and have been found to be possible sources of primary nitrogen. Other scenarios are also discussed. A 30-40$M_{\odot }$ supernova, with fallback, seems the most likely progenitor for CS 22949–037.