Bowen mechanism in HZ Herculis

Abstract

Most X-ray star companions show λ4640 emission due to an uncertain but probably selective emission process. We have obtained a series of high-dispersion (~14 Å mm^(-1)) blue and ultraviolet spectra of HZ Her, the companion of the X-ray pulsar Her X-l, using the Mayall 4 m telescope. In addition to the N iii λ4634-4641 emission reported by previous observers, the spectra reveal for the first time the presence of O iii λ3444.1 emission. This provides conclusive evidence that the Bowen fluorescence mechanism is active in HZ Her/Her X-l, in agreement with theoretical predictions by McClintock et al. and Hatchett et al., and suggests that this is also the correct explanation for the λ4640 emission in many such X-ray systems. The observed strengths of the emission features are below those reported in previous lower-dispersion investigations, in some cases very substantially so. The velocities provide no clear resolution of the question of which of the two stars, HZ Her or Her X-l, is associated with the emitting region.

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