Orbitally modulated photoexcited Si i emission in the eclipsing composite-spectrum binary ζ Aurigae

Abstract

We examine the little-known phenomenon of orbitally modulated Si I emission at λ 3905.523 Å and λ 4102.936 Å in composite-spectrum binaries, with specific reference to ζ Aurigae (K4 Ib + B5 V). The emission is detected in the isolated spectrum of the Btype dwarf secondary, and while λ 4102 Å is heavily blended with Hδ, λ 3905 Å falls in the B-star's featureless continuum. The narrowness of the emission (v turb 6 km s −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 λ 3938.289 Å. The strengths, positions, and widths of the λ 3905 Å 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 ∼ 5.7 km s −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.

SNAPSHOT: Connections between Internal and Surface Properties of Massive Stars

“…Our results about the behavior of the stellar radius as a function of envelope mass are also relevant for RSGs in binary systems, of which there are many observations (e.g., Hansen & M 1944;McLaughlin 1950;Wright 1970;Stencel et al 1984;Hagen Bauer & Bennett 2014;Harper et al 2016;Neugent et al 2018Neugent et al , 2019.When a RSG undergoes RLOF, it often results in non-conservative mass- transfer. As the star loses mass via RLOF, the radius of the star increases faster than the Roche-Lobe increases (Eggleton 2006).…”

HST STIS Observations of ζ Aurigae A's Irradiated Atmosphere

“…This emission has also been detected in HR 2030 (K1 II + B8.5 V) and 32 Cyg (K5 Ib + B6 V) (Griffin & Griffin 2000). Other chromospheric irradiation-driven emission that has been detected in ζ Aur are Ca II H & K (Walter 1937) and Fe II (Harper et al 2016), see also Eaton et al (2008). Since the photospheric spectrum of ζ Aur's B star has been accurately constrained by a combination of UV flux-calibrated spectra and synthetic spectra from photospheric models (Bennett et al 1995), and the orbital elements (Griffin 2005;Eaton et al 2008) and stellar parameters (Bennett et al 1996) are well known, ζ Aur A is a prime target for quantitative spectroscopy of cool stellar atmospheres.…”

“…The Harper et al (2016) study also found that the intrinsic Si I 3905 Å line width (turbulent most probable velocity, hereafter mpv) to be v turb = 6 km s −1 . The Doppler shifts of the narrow profiles during the orbit also revealed a v i sin 5.7 1   km s −1 , assuming isotropic emission.…”

“…A detailed empirical study of Si I 3905 Å and 4102 Å emission was presented by Harper et al (2016), and Figure 1 shows the Si I 3905 Å emission equivalent width (EEW) data (red circles) and a theoretical relative isotropic emissivity curve (gold) from that paper. The EEWs are the additional K-star emission measured with respect to the B star's photospheric spectrum as a function of orbital phase, as measured from periastron.…”

“…Since the solution of the full line-formation problem, where the B-star irradiation of overlapping continua of neutral species and hydrogen Rayleigh scattering combined with a large Si I atomic model is nontrivial, a simple angle-averaged escape probability photon-counting model was developed. The predictions from this analysis (Harper et al 2016) are summarized in Figure 3. The salient features are: (1) the flux ratio of Si I 4102 Å to Si I 3905 Å line is less than 1.4; (2) the 2987.6/3905 flux ratio will be less than ∼1.5; and (3) the 2881.6 Å emission will be negligible unless the column density of formation is very much smaller than expected.…”