Age Spreads and Systematics in λ Orionis with Gaia DR2 and the SPOTS tracks

In young pre-main sequence clusters, observations typically reveal a large luminosity spread between cluster members, which may be the result of observational errors or evolutionary processes. In this work, we attempt to constrain observational uncertainties through improved membership, extinction, empirical colors, and independent T_eff information. The recent success of magnetic and spotted models in explaining stellar parameter discrepancies for eclipsing binaries and pre-main sequence stars motivates a comparison with classical models in a young cluster. We apply our open cluster-calibrated, spectral energy distribution fitting technique to pre-MS stars on a sample of selected literature and APOGEE Net T_eff's. We develop a Gaussian Mixture Model for determining cluster membership with Gaia DR2 data, and apply a highly selective cut in parallax and proper motion space. After obtaining stellar luminosity and temperature for our members, we provide isochronal ages with classical stellar models as well as spotted and magnetic models. Our luminosities imply a significant luminosity spread even with star-by-star extinction. We suggest that the ages of Class II stars in our cluster appear systematically younger than the Class III stars after accounting for star-by-star extinction. We find an age of Lambda Orionis of 3.9±0.2 Myr; with empirical intrinsic colors, classical models may underestimate stellar ages by as much as 50% at these early ages, if accounting for low mass stars. With multiple families of isochrones, we suggest that the inconsistency in isochrone ages between stars of high and low mass with classical models may be improved through the introduction of spots or magnetic effects in stellar evolution.