The Orion Nebula is illuminated by the bright Trapezium O and B stars with masses ranging up to ~45 M_o.  X-rays from such stars were thought to be generated in a myriad of tiny shocks in their powerful winds.  But this model predicts that the X-ray spectrum will be soft and the emission will be constant.  COUP, and other recent studies of OB stars, show that the spectrum often has a hard component and can exhibit rapid flares.  


Of the 9 Orion Nebula Cluster stars with spectral types O7-B3, only three show the expected signature of many small wind shocks (lightcurve above).   Most show flares (lightcurve below) and/or hard spectral components.  These suggest that the winds of OB stars are, at least in part, trapped by magnetic fields.  The `magnetically confined wind shock' model suggests that the magnetic field deflects the wind into a giant and unstable shock region which produces the hard, flaring X-rays.  As with all COUP lightcurves, these are plots of the X-ray count rate (vertical axis) against time (13-day observation with gaps due to Chandra's orbit). 


We also confirm that the X-ray emission from intermediate-mass stars with spectral types B5-A9 is attributable to lower mass companions rather than the intermediate-mass star itself.  This finding agrees with theory that such stars should be X-ray-silent. COUP scientists have prepared additional Web material on why the X-ray properties of intermediate-mass stars differ from lower-mass stars here.

The full paper on COUP massive stars is:
X-ray emission from early-type stars in the Orion Nebula Cluster
B. Stelzer, E. Flaccomio, T. Montmerle, G. Micela, S. Sciortino, F. Favata, T. Preibisch & E. D. Feigelson

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