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)
Abstract.
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.
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