Protostellar evolution of intermediate-mass stars: implications for the survival of fossil magnetic fields

Abstract

The detection of strong magnetic fields in radiative stars, particularly chemically peculiar Ap/Bp stars, has motivated various scenarios regarding their origin, including the fossil field hypothesis. Because field survival depends strongly on stellar internal structure, determining when protostars transition from convective to radiative states is essential to assess whether fossil fields persist. We determine the characteristic mass at which intermediate-mass protostars transition from convective to radiative interiors, exploring how this transition favors magnetic field survival. Using the 1D stellar evolution code MESA, we model the pre-main-sequence evolution of intermediate-mass protostars from convective initial seeds across different metallicities, average accretion rates, and accretion histories. The transition mass from predominantly convective to radiative states depends on both accretion rate and metallicity, occurring between $\sim 1.3\,M_\odot$ and $2.8\,M_\odot$ for $1\,M_\odot$ reference models (and up to $\sim 4\,M_\odot$ for other initial masses at high accretion rates). Accretion history details play a secondary role compared to the average rate and metallicity. Lower metallicities shift the transition to lower masses. For $Z \ge 0.01$ and accretion rates above $4 \times 10^{-6}\,M_\odot\,\mathrm{yr}^{-1}$, models retain convective envelopes up to the end of accretion, potentially dissipating accreted magnetic flux. Because this transition typically occurs above $\sim 1.3\,M_\odot$, our results help explain the preferential survival of fossil fields in intermediate-mass stars, provided the radiative structure persists post-accretion. We also predict an increased incidence of strongly magnetized radiative stars at metallicities below $Z=0.01$.