Be discs in binary systems – I. Coplanar orbits

Abstract

Be stars are surrounded by outflowing circumstellar matter structured in the form of decretion discs. They are often members of binary systems, where it is expected that the decretion disc interacts both radiatively and gravitationally with the companion. In this work we study how various orbital (period, mass ratio and eccentricity) and disc (viscosity) parameters affect the disc structure in coplanar systems. We simulate such binaries with the use of a smoothed particle hydrodynamics code. The main effects of the secondary on the disc are its truncation and the accumulation of material inwards of truncation. We find two cases with respect to the effects of eccentricity: (i) In circular or nearly circular prograde orbits, the disc maintains a rotating, constant in shape, configuration, which is locked to the orbital phase. The disc is smaller in size, more elongated and more massive for low viscosity parameter, small orbital separation and/or high mass ratio. (ii) Highly eccentric orbits are more complex, with the disc structure and total mass strongly dependent on the orbital phase and the distance to the secondary. We also study the effects of binarity in the disc continuum emission. Since the infrared and radio SED are sensitive to the disc size and density slope, the truncation and matter accumulation result in considerable modifications in the emergent spectrum. We conclude that binarity can serve as an explanation for the variability exhibited in observations of Be stars, and that our model can be used to detect invisible companions.

A Spectroscopic Orbit for the Late-type Be Star β CMi

“…The images (Fig. 4 of Panoglou et al 2016) produced in these models suggest the largest density perturbation occurs at the sub-companion point in the disk. Our V /R measurements (Fig.…”

“…Lastly, we return to the models of Be systems with companions in coplanar orbits to the Be disk (Panoglou et al 2016). The images (Fig.…”

“…This point is typically at a distance of few 100 stellar radii, much larger than the disk in the β CMi system. In a binary system, hydrodynamical simulations show that the companion star truncates the disk with a steep fall-off in density somewhere between the 3:1 and 2:1 resonance radius (the exact locations depends on the viscosity parameter and on the binary mass ratio; Okazaki et al 2002;Panoglou et al 2016). …”

“…For example, Panoglou et al (2016) showed how the companion can create standing density waves in the disk that are phase-locked to the binary, when the orbit is circular or of moderate eccentricity. Consequently, the weak V /R variations with a 183 d timescale reported by Folsom et al (2016) are another hint of the possible presence of the binary companion.…”

“…Their results indicated a periodicity of the V /R variations with a period of 183 d, similar to our measurement of the orbital period (P = 170.36 d). Since current modeling of Be binaries shows disks with density waves driven by the tidal influence of the secondary (Panoglou et al 2016), it is worth looking for structure in the Hα V /R variations as a function of orbital phase. Since the violet (V ) and red (R) peak heights sample the density in the approaching and receding portions of the disk, respectively, we should expect to see a periodic V /R variation as the tidally induced density wave moves around the disk synchronously with the binary.…”