Revisiting Viscous Transonic Decretion Disks of Be Stars

TL;DR

This paper revisits the viscous transonic decretion disk model of Be stars, analyzing the effects of viscosity and radiative forces on disk structure and sonic point location, offering insights into observed spectral features.

Contribution

It provides a detailed topological analysis and numerical solutions for the transonic disk model, highlighting the role of viscosity and radiative forces in disk truncation.

Findings

Viscosity influences the topology of disk solutions but not the sonic point location.

Two nearly indistinguishable Nodal solutions are identified.

Disk truncation can occur at less than 50 stellar radii, explaining spectral features.

Abstract

In the context of Be stars, we restudied the viscous transonic decretion disk model of these stars. This model is driven by a radiative force due to an ensemble of optically-thin lines and viscosity considering the Shakura Sunyaev prescription. The non-linear equation of motion presents a singularity (sonic point) and an eigenvalue, which is also the initial condition at the stellar surface. Then, to obtain this eigenvalue, we set it as a radial quantity and perform a detailed topological analysis. Thereafter, we describe a numerical method for solving either Nodal and Saddle transonic solutions. The value of the viscosity,"alpha", barely determine the location of the sonic point, but it determines the topology of the solution. We found two Nodal solutions, which are almost indistinguishable between them. Saddle solutions are founded for lower values of "alpha" than the required of the Nodal solutions. In addition, rotational velocity do not play a determine role in the velocity (and density) profile, because viscosity effects collapse all the solutions to almost a unique one in a small region above the stellar surface. A suitable combination of line force parameters and/or disk temperature, give location of the sonic point lower than 50 stellar radii, describing a truncated disk. This could explain the SED turndown observed in Be stars without needing a binary companion.