New Challenges for Wind Shock Models: The<i>Chandra</i>Spectrum of the Hot Star δ Orionis

The Chandra spectrum of Ori A shows emission lines from hydrogen- and helium-like states of Si, Mg, Ne, and O, along with N vii Ly and lines from ions in the range Fe xvii–Fe xxi. In contrast to the broad lines seen in Pup and Ori (850 40 and 1000 240 km s 1 half-width at half-maximum [HWHM], respectively), these lines are broadened to only 430 60 km s 1 HWHM. This is much lower than the measured wind terminal velocity of 2000 km s 1. The forbidden, intercombination, and resonance ( fir) lines from He-like ions indicate that the majority of the X-ray line emission does not originate at the base of the wind, in agreement with the standard wind shock models for these objects. However, in that model the X-ray emission is distributed throughout an expanding, X-ray–absorbing wind, and it is therefore surprising that the emission lines appear relatively narrow, unshifted, and symmetric. We compare the observed line profiles to recent detailed models for X-ray line profile generation in hot stars, but none of them offers a fully satisfactory explanation for the observed line profiles.