Beate Stelzer, Ettore Flaccomio, Thierry Montmerle, Giusi Micela,
Salvatore Sciortino, Fabio Favata, Thomas Preibisch, Eric D. Feigelson
X-ray emission from early-type stars in the Orion Nebula Cluster
Astrophysical Journal Supplements, COUP Special Issue, 160, 557-581 (2005)
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
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.
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