The life cycles of Be viscous decretion discs: fundamental disc parameters of 54 SMC Be stars
Leandro Rocha Rímulo, Alex Cavaliéri Carciofi, Rodrigo Georgetti Vieira, Thomas Rivinius, Daniel Moser Faes, André Luiz Figueiredo, Jon Eric Bjorkman, Cyril Geo… 展开作者
Leandro Rocha Rímulo, Alex Cavaliéri Carciofi, Rodrigo Georgetti Vieira, Thomas Rivinius, Daniel Moser Faes, André Luiz Figueiredo, Jon Eric Bjorkman, Cyril Georgy, Mohammad Reza Ghoreyshi, Igor Soszyński
Be stars are main-sequence massive stars with emission features in their spectrum, which originates in circumstellar gaseous discs. Even though the viscous decretion disc (VDD) model can satisfactorily explain most observations, two important physical ingredients, namely the magnitude of the viscosity ($α$) and the disk mass injection rate, remain poorly constrained. The light curves of Be stars that undergo events of disc formation and dissipation offer an opportunity to constrain these quantities. A pipeline was developed to model these events that uses a grid of synthetic light curves, computed from coupled hydrodynamic and radiative transfer calculations. A sample of 54 Be stars from the OGLE survey of the Small Magellanic Cloud (SMC) was selected for this study. Because of the way our sample was selected (bright stars with clear disc events), it likely represents the densest discs in the SMC. Like their siblings in the Galaxy, the mass of the disc in the SMC increases with the stellar mass. The typical mass and angular momentum loss rates associated with the disk events are of the order of $\sim$$10^{-10}\, M_\odot\,\mathrm{yr^{-1}}$ and $\sim$$5\times 10^{36}\, \mathrm{g\, cm^{2}\, s^{-2}}$, respectively. The values of $α$ found in this work are typically of a few tenths, consistent with recent results in the literature and with the ones found in dwarf novae, but larger than current theory predicts. Considering the sample as a whole, the viscosity parameter is roughly two times larger at build-up ($\left\langleα_\mathrm{bu}\right\rangle = 0.63$) than at dissipation ($\left\langleα_\mathrm{d}\right\rangle = 0.26$). Further work is necessary to verify whether this trend is real or a result of some of the model assumptions.
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 solutio…
Circumstellar discs around Be stars are formed by the material ejected by the central star. This process removes excess angular momentum from the star as viscosity facilitates the mass and angular momentum transfer within the disc and its growth. The angular momentum loss rates (AMLR) of Be stars is a subject of debate in the literature. Through the modelling of the disc formation and dissipation phases observed from Be stars, their average AMLR can be determined and this is the goal of this work. We use the viscous decretion disc (VDD) model to provide a range of the average AMLR for Be stars…
We explore the pulsationally driven orbital mass ejection mechanism for Be star disc formation using isothermal, 3D magnetohydrodynamic (MHD) and hydrodynamic simulations. Non-radial pulsations are added to a star rotating at 95\% of critical as an inner boundary condition that feeds gas into the domain. In MHD, the initial magnetic field within the star is weak. The hydrodynamics simulation has limited angular momentum transport, resulting in repeating cycles of mass accumulation into a rotationally-supported disc at small radii followed by fall-back onto the star. The MHD simulation, convers…
The birth of Be star disks III. SPH models of localised mass ejections
Instituto de Astronomia, Geofísica e Ciências Atmosféricas, Universidade de São Paulo（圣保罗大学地球物理与大气科学天文研究所）；European Organisation for Astronomical Research in the Southern Hemisphere (ESO)（欧洲南方天文台）；DTU Space, Technical University of Denmark（丹麦技术大学空间中心）；LIRA, Paris Observatory, CNRS, PSL University, Sorbonne University, Université Paris Cité, CY Cergy Paris University（巴黎天文台LIRA实验室）；Department of Physics and Astronomy, Western University（韦仕敦大学物理与天文学系）；Max-Planck-Institut für Astrophysik（马克斯·普朗克天体物理学研究所）
The observed emission lines of Be stars originate from a circumstellar Keplerian disk that are generally well explained by the Viscous Decretion Disk model. In an earlier work we performed the modeling of the full light curve of the bright Be star $ω$ CMa (Ghoreyshi et al. 2018) with the 1-D time-dependent hydrodynamics code SINGLEBE and the Monte Carlo radiative-transfer code HDUST. We used the V -band light curve that probes the inner disk through four disk formation and dissipation cycles. This new study compares predictions of the same set of model parameters with time-resolved photometry …
