2016MNRAS.456.1346H

We examine the little-known phenomenon of orbitally modulated Sii emission at λ3905.523Å and λ4102.936Å in composite-spectrum binaries, with specific reference to ζ Aurigae (K4 Ib + B5 V). The emission is detected in the isolated spectrum of the B-type dwarf secondary, and while λ4102Å is heavily blended with Hδ, λ3905Å falls in the B-star's featureless continuum. The narrowness of the emission (vturb ≃ 6 km/s) demonstrates that it originates in the upper photosphere or deep chromosphere of the K star primary. We propose that photoexcitation by the hot star's UV continuum, followed by recombination and cascades, leads to resonant scattering and subsequent pumping of lower opacity transitions in the singlet and triplet systems of Sii. This process channels the UV continuum into select narrow emission lines. We have also identified weaker photoexcited emission of Feii at λ3938.289Å. The strengths, positions, and widths of the λ3905Å emission line vary with orbital phase owing to changes in the dilution of the irradiating flux and in the geometrical aspect of the irradiated hemisphere. Utilizing the inherent spatial resolution provided by the illuminated patch, and assuming that the K star is spherical with isotropic emission, yields vsini ∼ 5.7 km/s. Evidence of tidal distortion was deduced from the timing of the rapidly rising phase of the emission just after periastron. Increasing the diagnostic potential requires radiative transfer modelling of the formation and centre-to-limb variation of the emission.