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

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, ρ 0 , and exponent, n. We found that the n values range mainly between 1.5 and 3.5, and ρ 0 varies between 10 −12 and 10 −10 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 M ⊙ 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 ρ 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.

Stellar parameters of Be stars observed with X-shooter

“…Because the emission is weak and the Balmer decrement too steep, the disks found by X-shooter are too tenuous to allow a reliable measurement of the Balmer decrement. This is in agreement with A108, page 7 of 43 A&A 609, A108 (2018) Vieira et al (2017), who found that disks of late-type Be stars are less dense than those of early-type stars. As mentioned above, there is also a general agreement that late-type Be stars show less variability than early-type stars.…”

The Disk Physical Conditions of 48 Persei Constrained by Contemporaneous Hα Spectroscopy and Interferometry

“…(2010) and Vieira et al (2017). Our best-fitting combined model from Hα spectroscopy, Hα interferometry and SED fitting has a power law density fall off, n, of 2.3 and an initial density at the stellar surface of ρ 0 = 1.0 × 10 −11 g · cm −3 with a inclination constrained by Hα spectroscopy and interferometry of 45 • ± 5 • .…”

“…Vieira et al (2015) studied the continuum emission of this system using pseudo-photosphere model and obtained n of 2.5 and log ρ 0 of ∼ −11.48. Most recently, Vieira et al (2017) used IRAS, Japanese Aerospace infrared satellite (AKARI) and Wide-field Infrared Survey Explorer (WISE) observations to constrain their viscous decretion disk model. Vieira et al (2017) find n of 2.9, 2.8 and 2.7 and log ρ 0 of -11.4, -11.4 and -11.5, using IRAS, AKARI and WISE data, respectively.…”

“…Most recently, Vieira et al (2017) used IRAS, Japanese Aerospace infrared satellite (AKARI) and Wide-field Infrared Survey Explorer (WISE) observations to constrain their viscous decretion disk model. Vieira et al (2017) find n of 2.9, 2.8 and 2.7 and log ρ 0 of -11.4, -11.4 and -11.5, using IRAS, AKARI and WISE data, respectively. Despite the differences in the models and adopted stellar parameters, the values obtained for 48 Per with Waters et al (1987), Vieira et al (2015) and Vieira et al (2017) are remarkably similar.…”

“…Spectral energy distributions, SEDs, were also computed with beray for wavelengths between 0.4 and 60 microns for comparison to the reported observations of Touhami et al (2010) (optical and near-IR) and Vieira et al (2017) (IR). The model SEDs were computed for the same range of density parameters and disk sizes as described in Section 3.3.…”