The Sc, Ti, and V Abundance Discrepancy: Testing High-Mass IMF Variation and Massive-Star Rotation

Abstract

Scandium, titanium, and vanadium can be synthesized primarily in massive stars. Yet many of the current Galactic chemical evolution models under-produce these elements at early epochs. Motivated by evidence that the initial mass function varied in the past on the Galactic disc, we examine how assumptions about massive-star rotation and the initial mass function affect the inferred evolution of Sc, Ti, and V. We compute a grid of one-zone Galactic chemical evolution models that varies the initial rotational velocity of massive stars and the high-mass slope of the initial mass function. We compare the resulting [X/Fe] vs [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 initial rotational velocity of 300 km/s 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 initial mass function produce a secondary modulation. The remaining tensions, most apparent at solar to super-solar metallicities, motivate future work with a more complete treatment of the enrichment physics and model uncertainties.