M. Pignatari

Nucleosynthesis in the early galaxy

Recent observations of r-process-enriched metal-poor star abundances reveal a nonuniform abundance pattern for elements -->Z ≤ 47. Based on noncorrelation trends between elemental abundances as a function of Eu richness in a large sample of metal-poor stars, it is shown that the mixing of a consistent and robust light element primary process (LEPP) and the r-process pattern found in r-II metal-poor stars explains such apparent nonuniformity. Furthermore, we derive the abundance pattern of the LEPP from observation and show that it is consistent with a missing component in the solar abundances when using a recent s-process model. As the astrophysical site of the LEPP is not known, we explore the possibility of a neutron-capture process within a site-independent approach. It is suggested that scenarios with neutron densities -->nn ≤ 1013 cm−3 or in the range -->nn ≥ 1024 cm−3 best explain the observations.

Lanthanum: An s- and r-Process Indicator

The neutron capture cross section of 139La has been measured at a thermal energy of kT = 5 keV by means of the activation technique. Three irradiations were performed in a quasi-stellar neutron spectrum at the Karlsruhe 3.7 MV pulsed Van de Graaff accelerator, and the induced activities were measured by means of HPGe clover detectors. The final cross section of 113.7 ± 4.0 mbarn was found to be 10% higher than that previously assumed. Together with an earlier measurement at kT = 25 keV, Maxwellian-averaged neutron capture cross sections at the relevant thermal energies of the main s-process component, i.e., at kT = 8 and 23 keV, could be reliably interpolated. The s-abundances obtained on the basis of these data showed that the r-process contribution to solar lanthanum, Nr = N☉ - Ns, is 30%. This is in good agreement with the lanthanum abundance of extremely metal-poor and very r-process-rich stars.