X-ray emission from early-type stars in the Orion Nebula Cluster

The X-ray properties of twenty similar to 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. Emission by late-type magnetically active companions may be present in either class. Sixteen of the 20 stars are detected with a wide range of X-ray luminosities, log L-X (ergs s(-1)) similar to 29-33, and X-ray efficiencies, log (L-X/L-bol) similar to - 4 to - 8. Only two stars, theta(1) Ori D (B0.5) and NU Ori (B1), show exclusively the constant soft-spectrum emission at log (L-X/L-bol) similar to - 7 expected from the standard model 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. Magnetic confinement of winds with large-scale shocks can be invoked to explain these phenomena. This is supported in some cases by nonthermal radio emission and/or chemical peculiarities, or direct detection of the magnetic field (theta(1) Ori C). Most of the stars in the weak-wind class exhibit X-ray flares and log L-X < 31 ergs s(-1), consistent with magnetic activity from known or unseen low-mass companions. In most cases, the X-ray spectra can be interpreted in terms of a two-temperature plasma model with a soft component of 3 - 10 MK and a hard component up to 40 MK. All nondetections belong to the weak-wind class. A group of stars exhibit hybrid properties - flarelike behavior superimposed on a constant component with log L-X similar to 32 ergs s(-1) - which suggest both magnetic activity and wind emission.