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Molybdenum in metal poor stars from ultraviolet spectra near 2000Å

Ruth Peterson (UCO, Lick Obs)

Our recent analysis of high-resolution Hubble Space Telescope spectra near
2000Å in five halo metal-poor turnoff stars has revealed two with extreme
overabundances of the light trans-ironic element molybdenum (Z = 42).
In both cases, the neighboring elements Zr and Ru (Z = 40, 44) are less
enhanced, as are the heavy elements.

Of the several nucleosynthesis scenarios that can produce light trans-ironic
nuclei in the oldest stars, a high-entropy wind acting in a core-collapse
supernova seems uniquely capable of a factor-of-ten overproduction confined
to a limited mass range. That this occurs only in a narrow range of physical
conditions suggests that very few individual nucleosynthesis events were
responsible for the synthesis of such elements in these stars, even though
both are only moderately metal-poor.

We discuss the prospects for constraining both the production and the
incorporation of light trans-ironic elements in very old metal-poor stars,
using new ultraviolet spectra to derive abundances for Mo and additional light trans-ironic elements in a few stars, and existing optical spectra to survey Zr, Mo, and Ru in many more cool metal-poor stars.