The Diverse Origins of Neutron-Capture Elements in the Metal-Poor Star
  HD 94028: Possible Detection of Products of i-process Nucleosynthesis
  We present a detailed analysis of the composition and nucleosynthetic origins
of the heavy elements in the metal-poor ([Fe/H]=-1.62+/-0.09) star HD94028.
Previous studies revealed that this star is mildly enhanced in elements
produced by the slow neutron-capture process (s-process; e.g.,
[Pb/Fe]=+0.79+/-0.32) and rapid neutron-capture process (r-process; e.g.,
[Eu/Fe]=+0.22+/-0.12), including unusually large molybdenum
([Mo/Fe]=+0.97+/-0.16) and ruthenium ([Ru/Fe]=+0.69+/-0.17) enhancements.
However, this star is not enhanced in carbon ([C/Fe]=-0.06+/-0.19). We analyze
an archival near-ultraviolet spectrum of HD94028, collected using the Space
Telescope Imaging Spectrograph on board the Hubble Space Telescope, and other
archival optical spectra collected from ground-based telescopes. We report
abundances or upper limits derived from 64 species of 56 elements. We compare
these observations with s-process yields from low-metallicity AGB evolution and
nucleosynthesis models. No combination of s- and r-process patterns can
adequately reproduce the observed abundances, including the super-solar [As/Ge]
ratio (+0.99+/-0.23) and the enhanced [Mo/Fe] and [Ru/Fe] ratios. We can fit
these features when including an additional contribution from the intermediate
neutron-capture process (i process), which perhaps operated by the ingestion of
H in He-burning convective regions in massive stars, super-AGB stars, or
low-mass AGB stars. Currently, only the i process appears capable of
consistently producing the super-solar [As/Ge] ratios and ratios among
neighboring heavy elements found in HD94028. Other metal-poor stars also show
enhanced [As/Ge] ratios, hinting that operation of the i process may have been
common in the early Galaxy.
