Elemental Abundance Ratios in Stars of the Outer Galactic Disk. IV. A
  New Sample of Open Clusters
  We present radial velocities and chemical abundances for nine stars in the
old, distant open clusters Be 18, Be 21, Be 22, Be 32, and PWM 4. For Be 18 and
PWM 4, these are the first chemical abundance measurements. Combining our data
with literature results produces a compilation of some 68 chemical abundance
measurements in 49 unique clusters. For this combined sample, we study the
chemical abundances of open clusters as a function of distance, age, and
metallicity. We confirm that the metallicity gradient in the outer disk is
flatter than the gradient in the vicinity of the solar neighborhood. We also
confirm that the open clusters in the outer disk are metal-poor with
enhancements in the ratios [alpha/Fe] and perhaps [Eu/Fe]. All elements show
negligible or small trends between [X/Fe] and distance (< 0.02 dex/kpc), but
for some elements, there is a hint that the local (RGC < 13 kpc) and distant
(RGC > 13 kpc) samples may have different trends with distance. There is no
evidence for significant abundance trends versus age (< 0.04 dex/Gyr). We
measure the linear relation between [X/Fe] and metallicity, [Fe/H], and find
that the scatter about the mean trend is comparable to the measurement
uncertainties. Comparison with solar neighborhood field giants shows that the
open clusters share similar abundance ratios [X/Fe] at a given metallicity.
While the flattening of the metallicity gradient and enhanced [alpha/Fe] ratios
in the outer disk suggest a different chemical enrichment history to the solar
neighborhood, we echo the sentiments expressed by Friel et al. that definitive
conclusions await homogeneous analyses of larger samples of stars in larger
numbers of clusters. Arguably, our understanding of the evolution of the outer
disk from open clusters is currently limited by systematic abundance
differences between various studies.
