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Session 119 - Active Stars.
Oral session, Thursday, January 16
Ten ROSAT HRI snapshots of the Trapezium cluster taken over the course of 21 days show that the count rate of the O7 V star \theta^1 Orionis C varies from 0.26 to 0.41 counts s^-1 with a clear 15-day period. The soft X-ray variations have the same phase and period as H\alpha and He II \lambda 4686 variations reported by Stahl et al., and are in anti-phase with the C IV and Si IV ultraviolet absorption features. We consider five mechanisms which might explain the amplitude, phase, and periodicity of the X-ray variations: colliding-wind emission with the wind of an unseen binary companion, (2) coronal emission from an unseen late-type pre-main--sequence star, (3) periodic density fluctuations, (4) absorption of magnetospheric X-rays in a corotating wind, and (5) magnetosphere eclipses. The ROSAT data rule out the first three scenarios, but cannot rule out either of the latter two which require the presence of an extended magnetosphere, consistent with the suggestion of Stahl et al. that \theta^1 Ori C is an oblique magnetic rotator. We present preliminary models of X-ray emission and absorption from the magnetosphere and wind of an O-type oblique magnetic rotator.
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Program listing for Thursday