Separating Photospheric and Starspot Magnetic Fields in Pre-main-sequence Stars Using IGRINS Spectroscopy

Abstract Magnetic fields in pre-main-sequence (PMS) stars regulate angular momentum evolution, drive magnetic activity, and modify stellar structure, yet their surface distributions remain poorly constrained. Traditional single-component Zeeman broadening analyses typically yield mean field strengths of 2–4 kG, sometimes exceeding the photospheric equipartition limit, and assume complete magnetic coverage. These assumptions conflict with evidence that strong fields are concentrated in cool starspots. Here, we present the first systematic separation of photospheric and starspot magnetic field strengths in PMS stars, using high-resolution ( R ≈ 45,000) H - and K -band spectra from the Raw and Reduced IGRINS Archive. By modeling temperature and magnetic field strength simultaneously for a vetted sample of 33 classes II–III young stellar objects, we find median photospheric field strengths of 1.2 kG and median spot field strengths over 2 times stronger at 2.56 kG, resolving the apparent superequipartition tension and removing the need for a unity magnetic filling factor. Our results show that PMS surfaces are permeated by concentrated, kilogauss-strength spot fields covering 27%–83% of the visible hemisphere. This two-component framework offers a physically motivated means to reconcile spectroscopic and imaging-based magnetic diagnostics and enables large-scale magnetic population studies across young clusters and star-forming regions.