2020A&A...638A..64P

Context. The 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, for the lightest trans-iron elements with atomic number 30 < Z < 52, they show striking abundance patterns that defy model predictions. Understanding their sources may illuminate the diverse halo, thick disk, or extragalactic origins of metal-poor stars.
Aims. The primary goal is the derivation of halo dwarf abundances and their uncertainties for six trans-iron elements from UV spectra, plus optical abundances for four additional trans-Fe elements and two well-understood heavier elements.
Methods. For five metal-poor dwarfs, we analyzed high-resolution UV spectra from the Hubble Space TelescopeImaging Spectrograph, supplemented by archival optical echelle spectra. Two independent analyses adopted different programs, models, and line lists, clarifying systematic errors.
Results. The results from the separate UV analyses are in good agreement. The largest source of discrepancy is the placement of the UV continuum. Once rectified, the separate results agree to 0.2 dex for moderately unblended, moderately strong lines. Similar agreement is found with previous works, except where new data and line selection become important, notably our exclusion of trans-Fe lines blended by newly identifed FeI lines.
Conclusions.Improved line lists lead to low As/Ge ratios that no longer require an early arsenic enhancement. All five stars exhibit a high Mo/Ge abundance ratio, independent of Mo/Fe. The trans-Fe elements show an odd-even effect: an odd-Z element abundance is depressed relative to those of adjacent even-Z elements. Its suggested metallicity dependence is supported by previous studies of Sr-Y-Zr. Some theoretical yields show a metallicity-dependent odd-even effect, but none have predicted a constant Mo/Ge abundance ratio. Our work thus highlights the complexity of predicting the production of light trans-Fe elements in metal-poor stars.