Molybdenum, Ruthenium, and the Heavy r-process Elements in Moderately
  Metal-Poor Main-Sequence Turnoff Stars
  The ratios of elemental abundances observed in metal-poor stars of the
Galactic halo provide a unique present-day record of the nucleosynthesis
products of its earliest stars. While the heaviest elements were synthesized by
the r- and s-processes, dominant production mechanisms of light trans-ironic
elements were obscure until recently. This work investigates further our 2011
conclusion that the low-entropy regime of a high-entropy wind (HEW) produced
molybdenum and ruthenium in two moderately metal-poor turnoff stars that showed
extreme overabundances of those elements with respect to iron. Only a few, rare
nucleosynthesis events may have been involved.
  Here we determine abundances for Mo, Ru, and other trans-Fe elements for 28
similar stars by matching spectral calculations to well-exposed near-UV Keck
HIRES spectra obtained for beryllium abundances. In each of the 26 turnoff
stars with Mo or Ru line detections and no evidence for s-process production
(therefore old), we find Mo and Ru to be three to six times overabundant. In
contrast, the maximum overabundance is reduced to factors of three and two for
the neighboring elements zirconium and palladium. Since the overproduction
peaks sharply at Mo and Ru, a low-entropy HEW is confirmed as its origin.
  The overabundance level of the heavy r-process elements varies significantly,
from none to a factor of four, but is uncorrelated with Mo and Ru
overabundances. Despite their moderate metallicity, stars in this group trace
the products of different nucleosynthetic events: possibly very few events,
possibly events whose output depended on environment, metallicity, or time.
