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.