Understanding the High‐Resolution X‐Ray Spectra of Early‐Type Stars

Abstract

Fig. 1.—Chandra Medium Energy Grating spectra of the six target stars, showing emission from Hand He-like S, Si, Ne, Mg, and O, along with a number of Fe ions (principally Fe xvii). Note the variety of emission line profile shapes. The jump in X-ray spectral resolution made possible by the orbiting Chandra X-Ray Observatory has inspired a host of new techniques for determining the locations, motions, and temperature structures of the X-ray–emitting plasma of hot stars. Although steady-state models of stellar winds are relatively well developed, X-rays offer a unique window on small-scale inhomogeneous structures in hot star winds. Currently, the most viable theory of X-ray generation in hot stars relies on small instabilities growing into X-ray–emitting shocks distributed throughout the wind. The advent of high-resolution X-ray data allows many new ways to test this paradigm that are applied here. In a detailed study of the O4f star z Pup (J. P. Cassinelli et al. 2001, ApJ, 554, L55), X-ray–emitting gas is found extremely close to the photosphere, a region where it would be difficult to generate strong shocks. The X-ray line profiles of z Pup do conform well to what would be expected from an Xray source embedded in an expanding, absorbing wind: they are broad because of the wind expansion, and their centroids are blueshifted because of wind absorption of X-rays originating on the far side of the wind. Achieving an understanding of the line profiles of the O9 III star d Ori is not as straightforward, however. The emission lines are surprisingly narrow and do not show the expected blueshift (N. A. Miller et al. 2002, ApJ, 577, 951). Detailed comparisons to theories of line profile generation are not able to resolve this discrepancy. A study of the six normal hot stars observed with Chandra (see Fig. 1) reveals that the good agreement found between the line profiles of z Pup and theoretical expectations is the exception, not the rule. Perhaps most puzzling, the X-ray line profiles of the two B stars studied are broadened by less than a couple of hundred kilometers per second, making it difficult to imagine how the X-ray–emitting gas can be related to the rapid outflow of the wind as a whole. A recently proposed infalling clump model for the B star t Sco (J. C. Howk et al. 2000, ApJ, 534, 348) can explain nearly all of its X-ray properties. However, this model probably does not explain all B star X-ray emission, for the other B star studied (b Cru) is a much dimmer and softer X-ray emitter.