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Stars formed with initial mass over 50 Msun are very rare today, but they are
thought to be more common in the early universe. The fates of those early,
metal-poor, massive stars are highly uncertain. Most are expected to directly
collapse to black holes, while some may explode as a result of rotationally
powered engines or the pair-creation instability. We present the chemical
abundances of J0931+0038, a nearby low-mass star identified in early followup
of SDSS-V Milky Way Mapper, which preserves the signature of unusual
nucleosynthesis from a massive star in the early universe. J0931+0038 has
relatively high metallicity ([Fe/H] = -1.76 +/- 0.13) but an extreme odd-even
abundance pattern, with some of the lowest known abundance ratios of [N/Fe],
[Na/Fe], [K/Fe], [Sc/Fe], and [Ba/Fe]. The implication is that a majority of
its metals originated in a single extremely metal-poor nucleosynthetic source.
An extensive search through nucleosynthesis predictions finds a clear
preference for progenitors with initial mass > 50 Msun, making J0931+0038 one
of the first observational constraints on nucleosynthesis in this mass range.
However the full abundance pattern is not matched by any models in the
literature. J0931+0038 thus presents a challenge for the next generation of
nucleosynthesis models and motivates study of high-mass progenitor stars
impacted by convection, rotation, jets, and/or binary companions. Though rare,
more examples of unusual early nucleosynthesis in metal-poor stars should be
found in upcoming large spectroscopic surveys.