Orbitally modulated photoexcited Si I emission in the eclipsing composite-spectrum binary [zeta] Aurigae

Abstract

We examine the little-known phenomenon of orbitally modulated Si I emission at [lambda] 3905.523 [angstrom] and [lambda] 4102.936 [angstrom] in composite-spectrum binaries, with specific reference to [zeta] Aurigae (K4 Ib + B5 V). The emission is detected in the isolated spectrum of the B-type dwarf secondary, and while [lambda] 4102 [angstrom] is heavily blended with H[delta], [lambda] 3905 [angstrom] falls in the B-star’s featureless continuum. The narrowness of the emission (v [subscript turb] [similar or equal to] 6 km s[superscript-1]) 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 Si I. This process channels the UV continuum into select narrow emission lines. We have also identified weaker photoexcited emission of Fe II at [lambda] 3938.289 [angstrom]. The strengths, positions, and widths of the [lambda] 3905 [angstrom] 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 vsin i [similar to] 5.7 km s[superscript-1]. 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.