This was the first paper to hypothesize the sort of phenomena which we are studying. Granted, they deal only with chemically-peculiar stars, but even so it is still applicable. The paper consists mostly of data on the stars that they studied, but there is still a lot of good information regarding the physical origin of the detection of MCWS X-ray emission.

The chemical peculiarity of these stars is what causes the existence of a surface magnetic field, which in turn produces an anisotropic stellar wind. Previous authors have hypothesized that hydrogen Balmer emission comes from a trapped, corotating magnetosphere near the magnetic equator, and proceeded to model line profiles for such a situation. So trapped plasma can be used to model line variations in chemically peculiar stars.

The authors used observations obtained from the International Utraviolet Explorer satellite (IUE). The reason that this range of wavelengths was studied is as follows. Previous studies of helium-strong stars at centimeter wavelengths have shown stronger- than-usual radio emissions. This was attributed to nonthermal radiation from trapped magnetospheric electrons which were emitting gyrosyncrotron radiation. Using the Very Long Base Interferometry (VLBI) telescope, the magnetosphere of some of the stars exhibiting this behavior could be discerned, confirming the above hypothesis. These results then suggest that UV spectroscopic variations of all helium-strong stars could be the result of magnetically trapped circumstellar plasma as well.

UV line variations typically fall into one of three categories: variable, but always absorption; constant, strong absorption; and constant, asymmetric, redward-shaded emission. The variable profiles are attributed to rapid rotators, while the constant profiles are attributed to slow-rotators. The paper goes on to derive from a vast archive of ultraviolet spectra a comprehensive model of UV line profiles and their relation to helium-srong stars. The unified model explains the formation of UV resonance lines by means of circumstellar material trapped in the magnetosphere about the magnetic equator as well as jetlike outflows from the magnetic poles.

One interesting conclusion from this paper is that there are no slow-rotating helium- strong stars on the upper main-sequence. Any that were thought to be slow-rotators were really just being seen pole-on. Using C IV and Si IV variations (or actually lack thereof) in helium-strong stars, the existence of magnetospherically trapped circumstellar material was confirmed. In addition, none of the stars studied were found to exhibit any kind of compelling evidence for jetlike outflows at the magnetic poles.