Local associations and the barium puzzle
  We have observed high-dispersion echelle spectra of main-sequence stars in
five nearby young associations -- Argus, Carina-Near, Hercules-Lyra, Orion and
Subgroup B4 -- and derived abundances for elements ranging from Na to Eu. These
are the first chemical abundance measurements for two of the five associations,
while the remaining three associations are analysed more extensively in our
study. Our results support the presence of chemical homogeneity among
association members with a typical star-to-star abundance scatter of about 0.06
dex or less over many elements. The five associations show log$\epsilon$(Li)
consistent with their age and share a solar chemical composition for all
elements with the exception of Ba. We find that all the heavy elements (Y, Zr,
La, Ce, Nd, Sm and Eu) exhibit solar ratios, i.e., [X/Fe] $\simeq$ 0, while Ba
is overabundant by about 0.2-0.3 dex. The origin of the overabundance of Ba is
a puzzle. Within the formulation of the s-process, it is difficult to create a
higher Ba abundance without a similar increase in the s-process contributions
to other heavy elements (La-Sm). Given that Ba is represented by strong lines
of Ba II and La-Sm are represented by rather weak ionized lines, the
suggestion, as previously made by other studies, is that the Ba abundance may
be systematically overestimated by standard methods of abundance analysis
perhaps because the upper reaches of the stellar atmospheres are poorly
represented by standard model atmospheres. A novel attempt to analyse the Ba I
line at 5535 \AA\ gives a solar Ba abundance for stars with effective
temperatures hotter than about 5800 K but increasingly subsolar Ba abundances
for cooler stars with apparent Ba deficiencies of 0.5 dex at 5100 K. This trend
with temperature may signal a serious non-LTE effect on the Ba I line.
