Profile Shapes for Optically Thick X‐Ray Emission Lines from Stellar Winds

Abstract

We consider the consequences of appreciable line optical depth for the profile shape of X-ray emission lines formed in stellar winds. The hot gas is thought to arise in distributed wind shocks, and the line formation is predominantly via collisional excitation followed by radiative decay. Such lines are often modeled as optically thin, but the theory has difficulty matching resolved X-ray line profiles. We suggest that for strong lines of abundant metals, newly created photons may undergo resonance scattering, modifying the emergent profile. Using Sobolev theory in a spherically symmetric wind, we show that thick-line resonance scattering leads to emission profiles that still have blueshifted centroids, like the thin lines, but are considerably less asymmetric in appearance. We focus on winds in the constant-expansion domain and derive an analytic form for the profile shape in the limit of large line and photoabsorptive optical depths. In this limit the emission profile reduces to a universal shape and has a centroid shift of -0.24v∞, with a half-width at half-maximum (HWHM) of 0.63v∞. Using published data for Chandra observations of five emission lines from the O star ζ Pup, we find that the observed HWHMs are somewhat smaller than predicted by our theory; however, the centroid shifts of all five lines are consistent with our theoretical result. These optical depth effects can potentially explain the more nearly symmetric emission lines observed in ζ Ori, θ1 Ori C, and δ Ori by Chandra, although an alternative explanation is required to account for the unshifted peak line emission. We also consider enhanced reabsorption by continuous opacity as line photons multiply scatter within an optically thick line, and find, for lines with optical depths of a few, that such reabsorption can further reduce the line asymmetry. It also reduces the line equivalent width, but probably not enough to alleviate the problem of subsolar metallicities inferred from O star X-ray spectra by ASCA, unless the width of the resonance regions are superthermally enhanced.