Revealing the structure of the outer disks of Be stars

Abstract

Context. The structure of the inner parts of Be star disks ( 20 stellar radii) is well explained by the viscous decretion disk (VDD) model, which is able to reproduce the observable properties of most of the objects studied so far. The outer parts, on the other hand, are not observationally well-explored, as they are observable only at radio wavelengths. A steepening of the spectral slope somewhere between infrared and radio wavelengths was reported for several Be stars that were previously detected in the radio, but a convincing physical explanation for this trend has not yet been provided. Aims. We test the VDD model predictions for the extended parts of a sample of six Be disks that have been observed in the radio to address the question of whether the observed turndown in the spectral energy distribution (SED) can be explained in the framework of the VDD model, including recent theoretical development for truncated Be disks in binary systems. Methods. We combine new multi-wavelength radio observations from the Karl. G. Jansky Very Large Array (JVLA) and Atacama Pathfinder Experiment (APEX) with previously published radio data and archival SED measurements at ultraviolet, visual, and infrared wavelengths. The density structure of the disks, including their outer parts, is constrained by radiative transfer modeling of the observed spectrum using VDD model predictions. In the VDD model we include the presumed effects of possible tidal influence from faint binary companions.Results. For 5 out of 6 studied stars, the observed SED shows strong signs of SED turndown between far-IR and radio wavelengths. A VDD model that extends to large distances closely reproduces the observed SEDs up to far IR wavelengths, but fails to reproduce the radio SED. Using a truncated VDD model improves the fit, leading to a successful explanation of the SED turndown observed for the stars in our sample. The slope of the observed SEDs in the radio is however not well reproduced by disks that are simply cut off at a certain distance. Rather, some matter seems to extend beyond the truncation radius, where it still contributes to the observed SEDs, making the spectral slope in the radio shallower. This finding is in agreement with our current understanding of binary truncation from hydrodynamical simulations, in which the disk does extend past the truncation radius. Therefore, the most probable cause for the SED turndown is the presence of binary companions that remain undetected for most of our sources.

Two-dimensional modeling of density and thermal structure of dense circumstellar outflowing disks

“…For the less massive disks witḣ M ≤ 10 −8 M yr −1 the value of the same slope parameter within the same region is approximately 3.125. This agrees with observational results of Klement et al (2017), who determine the values of the disk midplane density slope parameter between approximately 3.0 and 3.5 for the Be stars' disks that are in a steady state. They also predict that disks with this parameter higher than 3.5 should correspond to disks that are in the process of formation, and that disks with this parameter lower than 3 are in the process of dissipation; the variations of the values of the slope parameter in the case of very massive disks indicate that these processes occur up to a certain distance (see Sect.…”

Be and Bn stars: Balmer discontinuity and stellar-class relationship

“…Finally, we comment on the disk structure near the central star that is suggested by some results of the present paper. Detailed accounts of successful representations of the observed emission in the Hα line of Be stars and of the continuum energy distribution from the far-UV to the IR, as well as of the polarization in the visible spectral range, are given in a series of rather recent papers and references therein (Araya et al 2017;Arcos et al 2017;Klement et al 2017;Marr et al 2018). In all these studies, the physical structure of the CE of Be stars is studied in the frame of an axisymmetric quasi-Keplerian disk in hydrostatic equilibrium in the vertical axis, where the scale height H is proportional to R 3/2 T 1/2 and R represents the equatorial radius of the disk and T is its local temperature.…”

The life cycles of Be viscous decretion discs: The case of ω CMa

“…where Λ = 1/(1 −r − 1 2 out ), withr out = R out /R eq , is a number usually just slightly larger than 1, as R out R eq . The steadystate AM loss rate, therefore, depends very little (through the factor Λ) on the outer radius of the disc, whose value is poorly known for a few Be stars, and completely unknown for most (Klement et al 2017). In the simulations presented in this work, we have setr out = 1000.…”