An Improved Hertzsprung–Russell Diagram for the Orion Trapezium Cluster

Abstract We present a study of the Trapezium cluster in Orion. analyze flux-calibrated Very Large Telescope/Multi-Unit Spectroscopic Explorer spectra 361 stars to simultaneously measure spectral types, reddening, and optical veiling due accretion. find that extinction law from Cardelli et al. with total-to-selective value R V = 5.5 is more suitable for this cluster. For 68% sample new types are consistent literature within two subclasses but, as expected, we derive systematically later than by one sources significant accretion levels. Here an improved Hertzsprung–Russell (H-R) diagram cluster, which contamination on stellar luminosities has been properly removed. A comparison locations H-R non-magnetic magnetic pre-main-sequence evolutionary tracks indicates age 1–2 Myr. The can better explain low-mass stars. In diagram, exhibits large luminosity spread ( σ (Log L ⋆ / ⊙ ) ∼ 0.3). By collecting 14 clusters/groups different ages, tends be constant 0.2–0.25) after 2 Myr, suggests not main cause spread. There ∼0.1 dex larger spreads younger clusters, e.g., older explained starspots, history, circumstellar disk orientations.