The Sc, Ti, and V Abundance Discrepancy: Testing High-mass Initial Mass Function Variation and Massive-star Rotation

Scandium, titanium, and vanadium can be synthesized primarily in massive stars. Yet many of the current Galactic chemical evolution (GCE) models underproduce these elements at early epochs. Motivated by evidence that the initial mass function (IMF) varied in the past on the Galactic disk, we examine how assumptions about massive-star rotation and the IMF affect the inferred evolution of Sc, Ti, and V. We compute a grid of one-zone GCE models that varies the initial rotational velocity (IRV) of massive stars and the high-mass slope of the IMF. We compare the resulting [X/Fe] versus [Fe/H] for X = Sc, Ti, and V tracks and cross-element correlations with Galactic abundance data. We find that adopting rotating massive-star yields with an IRV of 300 km s −1 brings the model trends closer to metal-poor observations, especially for halo stars ([Fe/H] < −2), and improves the joint behavior of Sc, Ti, and V. Variations of the high-mass slope of the IMF produce a secondary modulation. The remaining tensions, most apparent at solar to supersolar metallicities, motivate future work with a more complete treatment of the enrichment physics and model uncertainties.