Astrophys. J., Suppl. Ser., 160, 557-581 (2005/October-0)
X-ray emission from early-type stars in the Orion nebula cluster.
STELZER B., FLACCOMIO E., MONTMERLE T., MICELA G., SCIORTINO S., FAVATA F., PREIBISCH T. and FEIGELSON E.D.
Abstract (from CDS):
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. 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, logLXergs/s∼29-33, and X-ray efficiencies, log(LX/Lbol)~-4 to -8. Only two stars, θ1 Ori D (B0.5) and NU Ori (B1), show exclusively the constant soft-spectrum emission at log(LX/Lbol)~-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 (θ1 Ori C). Most of the stars in the weak-wind class exhibit X-ray flares and logLX<31 ergs/s, 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 logLX∼32 ergs/s–which suggest both magnetic activity and wind emission.
Stars: Binaries: General - Galaxy: Open Clusters and Associations: Individual: Name: Orion Nebula Cluster - Stars: Early-Type - Stars: Pre-Main-Sequence - Stars: Winds, Outflows - X-Rays: Stars
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