Germanium, Arsenic, and Selenium Abundances in Metal-Poor Stars
  The elements germanium (Ge, Z=32), arsenic (As, Z=33), and selenium (Se,
Z=34) span the transition from charged-particle or explosive synthesis of the
iron-group elements to neutron-capture synthesis of heavier elements. Among
these three elements, only the chemical evolution of germanium has been studied
previously. Here we use archive observations made with the Space Telescope
Imaging Spectrograph on board the Hubble Space Telescope and observations from
several ground-based facilities to study the chemical enrichment histories of
seven stars with metallicities -2.6 < [Fe/H] < -0.4. We perform a standard
abundance analysis of germanium, arsenic, selenium, and several other elements
produced by neutron-capture reactions. When combined with previous derivations
of germanium abundances in metal-poor stars, our sample reveals an increase in
the [Ge/Fe] ratios at higher metallicities. This could mark the onset of the
weak s-process contribution to germanium. In contrast, the [As/Fe] and [Se/Fe]
ratios remain roughly constant. These data do not directly indicate the origin
of germanium, arsenic, and selenium at low metallicity, but they suggest that
the weak and main components of the s-process are not likely sources.
