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]≈-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 $λ= 350$ 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±0.1, as measured from the [OI] line at 630.0 nm. We find an enhancement of [N/Fe] of 2.56± 0.2, and a milder one of [C/Fe] = 1.17±0.1, similar to those already reported in the literature. This implies Z_=0.01 Z_. We also find carbon isotopic ratios ¹²C/¹³C=4±2.0 and ¹³C/¹⁴N=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 ± 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] ≈ +0.3, [Y/Fe] ≈ -0.1, and [Ba/Fe] ≈ -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 30-40M_ supernova, with fallback, seems the most likely progenitor for CS 22949--037.