Trans-iron Ge, As, Se, and heavier elements in the dwarf metal-poor
  stars, HD~19445, HD~84937, HD~94028, HD~140283, and HD~160617
  Spectra of unevolved metal-poor halo stars uniquely reflect the elemental
abundances incorporated during the earliest Galactic epoch. Their heavy-element
content is well understood as the products of neutron capture on iron-peak
elements. However, the lightest elements just past the iron peak, with atomic
number 30<Z<52, show striking abundance patterns open to several
interpretations. Understanding their nature may illuminate the diverse halo,
thick disk, or extragalactic origins of metal-poor stars. For five metal-poor
dwarfs, we analyzed high-resolution echelle UV spectra from the Hubble Space
Telescope Imaging Spectrograph, as well as archival optical echelle spectra.
The goal was to derive reliable halo dwarf abundances and uncertainties for six
trans-iron elements from UV spectra, and optical abundances for four additional
trans-Fe elements and two well-understood heavier elements. Our two independent
analyses showed that the largest source of discrepancy is UV continuum
placement. Once rectified, the internal results agree to 0.2 dex for
moderately-unblended, moderately strong lines. Our results similarly agree with
previous work, except where new data and unidentified Fe I lines are important.
We show that these heavily congregate blueward of 2000A and redward of 2600A.
Our exclusion of trans-Fe lines blended by such lines proved critical for
arsenic. A metallicity-dependent odd-even effect is uncovered among trans-Fe
elements: an odd-Z element abundance is depressed relative to those of adjacent
even-Z elements, especially at low metallicity. This is supported by previous
studies of Sr-Y-Zr, and also appears in some theoretical calculations. To date,
no calculations predict the high Mo/Ge ratio, independent of Mo/Fe, that we
find in all five stars.
