Hydrodynamical models of the β Lyr A circumstellar disc

¹ Charles University, Faculty of Mathematics and Physics, Institute of Astronomy, V Holešovičkách 2, 18000 Prague, Czech Republic
² Heidelberger Institut für Theoretische Studien, Schloss-Wolfsbrunnenweg 35, 69118 Heidelberg, Germany

^★ Corresponding author: krivit97@gmail.com

Received: 11 January 2024
Accepted: 8 September 2025

Abstract

Aims. We aim to study dynamics of circumstellar discs, with a focus on the β Lyræ A binary system. This system with ongoing mass transfer has been extensively observed using photometry, spectroscopy, and interferometry. All these observations were recently interpreted using a radiation-transfer kinematic model.

Methods. We modified the analytical Shakura-Sunyaev models for a general opacity prescription, and derived radial profiles of various quantities. These profiles were computed for the fixed accretion rate, Ṁ = 2 × 10⁻⁵ M_⊙ yr⁻¹, inferred from the observed rate of change of the binary period. More general models were computed numerically, using one-dimensional radiative hydrodynamics, accounting for viscous, radiative, and irradiation terms. The initial conditions were taken from the analytical models.

Results. To achieve the accretion rate, the surface density, Σ, must be much higher (of the order of 10⁴ kg m⁻² for the viscosity parameter α = 0.1) than in the kinematic model. Viscous dissipation and radiative cooling in the optically thick regime lead to a high midplane temperature, T (up to 10⁵ K). The accretion disc is still gas-pressure-dominated, with the opacity close to that of Kramers’. To reconcile temperature profiles with observations, we had to distinguish three different temperatures: midplane, atmospheric, and irradiation. The latter two are comparable to observations (30 000-12 000 K). We demonstrate that the aspect ratio, H, of 0.08 can be achieved in a hydrostatic equilibrium, as opposed to previous works considering the disc to be vertically unstable.