The life cycles of Be viscous decretion discs: time-dependent modelling of infrared continuum observations

TL;DR

This study applies the viscous decretion disc model to infrared observations of 80 Be stars, revealing their disc structures, mass decretion rates, and evolutionary patterns, and establishing a link between disc density and density gradient.

Contribution

It provides a comprehensive analysis of Be star disc properties using time-dependent modelling, highlighting the evolutionary connection between disc density and structure.

Findings

Disc density and gradient are correlated.

Early-type stars have denser discs than late-type stars.

Disc growth timescale is shorter than dissipation timescale.

Abstract

We apply the viscous decretion disc (VDD) model to interpret the infrared disc continuum emission of 80 Be stars observed in different epochs. In this way, we determined 169 specific disc structures, namely their density scale, $\rho_0$, and exponent, $n$. We found that the $n$ values range mainly between $1.5$ and $3.5$, and $\rho_0$ varies between $10^{-12}$ and $10^{-10}\,\mathrm{g\,cm^{-3}}$, with a peak close to the lower value. Our large sample also allowed us to firmly establish that the discs around early-type stars are denser than in late-type stars. Additionally, we estimated the disc mass decretion rates and found that they range between $10^{-12}$ and $10^{-9}\,\mathrm{M_{\odot}\,yr^{-1}}$. These values are compatible with recent stellar evolution models of fast-rotating stars. One of the main findings of this work is a correlation between the $\rho_0$ and $n$ values. In order to find out whether these relations can be traced back to the evolution of discs or have some other origin, we used the VDD model to calculate temporal sequences under different assumptions for the time profile of the disc mass injection. The results support the hypothesis that the observed distribution of disc properties is due to a common evolutionary path. In particular, our results suggest that the timescale for disc growth, during which the disc is being actively fed by mass injection episodes, is shorter than the timescale for disc dissipation, when the disc is no longer fed by the star and dissipates as a result of the viscous diffusion of the disc material.