X-ray emission from early-type stars in the Orion Nebula Cluster
  The X-ray properties of twenty ~1 Myr old O, B, and A stars of the Orion Trapezium are examined with data from the Chandra Orion Ultradeep Project (COUP). On the basis of simple theories for X-ray emission, we define two classes separated at spectral type B4: hotter stars have strong winds that may give rise to X-ray emission in small- or large-scale wind shocks, and cooler stars that should be X-ray dark due to their weaker winds and absence of outer convection zones where dynamos can generate magnetic fields.   Only two of the massive stars show exclusively the constant soft-spectrum emission expected from the standard model for X-ray emission from hot stars involving many small shocks in an unmagnetized radiatively accelerated wind. Most of the other massive O7-B3 stars exhibit some combination of soft-spectrum wind emission, hard-spectrum flaring, and/or rotational modulation indicating large-scale inhomogeneity, suggesting magnetic confinement of winds with large-scale shocks.   Most of the stars in the weak-wind class exhibit X-ray flares and have luminosities that are consistent with magnetic activity from known or unseen low-mass companions. All non-detections belong to the weak-wind class. 
B. Stelzer, E. Flaccomio, T. Montmerle, G. Micela, S. Sciortino, F.   Favata, Th. Preibisch, E. D. Feigelson