Structure of a Be binary Structure of a Be binary B-type, non-supergiant stars that have the highest rotation rates amongst non- degenerate objects. They are non-radial pulsators and undergo episodes of mass ejection which lead to the formation of a decretion disk around the star. The disk is viscous and very sismilar to accretion disks, except that it builds from inside out. Observationally, Be stars are known for their emission lines in Hydrogen (Hα in particular), linear polarisation, and IR excess. i n n e r B e d i s k spiral dominated disk bridge circumsecondary region circumsecondary region circumbinary region Tightly wound one armed structure Matter that escapes from behind the Be disk (away from the companion), meets up with the main arm Radio emission? A flow of matter is flung out of the system after it has been focused by the companion Characterized by: Observationally: Decretion and Accretion Decretion and Accretion in be star binary systems in be star binary systems Amanda C. Rubio, Alex C. Carciofi, J. E. Bjorkman Universidade de São Paulo; ESO Be stars Be stars Smoothed particle hydrodynamics (SPH) is a mathematical technique created to describe the dynamics of fluids in a discrete fashion, using particles. As Be disks are gaseous and dustless, mostly controlled by viscosity, SPH can describe them well. In our code, we have implemented particle splitting, which allows us to explore less dense regions of the disk [1, 2, 3]. Our goal is to characterise Be stars in binary systems. The presence of the companion tidally affects the Be disk in many ways, leading to profound changes in its structure. We ran a grid of SPH models for binary Be stars with different periods and different values for the viscosity of the disk, α. SPH simulations SPH simulations Inner disk Circular orbits of the particles Azimuthal symmetry Accumulation of matter in the inner disk Larger scale height (puffier disk) More emission in lines that are formed closer to the star, such as He Stronger V band magnitude This region is the closest to the central Be star, and the most oblivious to the presence of the companion, as it is dominated by the gravity field of the primary and strong viscous torque. Characterized by: But it is not the same as an isolated Be disk! Observationally, the consequences would be: The formation of Be stars is still not understood. How do they obtain their fast rotation rates? Currently, the most favoured scenario is that they are a product of binary star evolution: the future Be star gains mass and angular momentum from a donor star that evolves faster. The fact that many Be stars are found in systems with evolved objects (such as sub- dwarfs and neutron stars [4, 5]) is an indicator that this is a viable route. Why do we care about Why do we care about binary Be stars? binary Be stars? Fig. 1: Snapshot of one of our SPH simulations. The two back dots represent the Be star and the companion, while the colours represent the density of the gas around them. The 5 regions we could discern in our work are marked. Conclusion Conclusion spiral dominated disk Elliptic orbits of the particles Two armed density waves that travel through the disk Deviations from the VDD in velocities and scale height V/R variations in Hα The companion makes itself known by messing up the disk as it affects it tidally Characterized by: Observationally: Fig. 2: Snapshot of one of our SPH simulations. The back dot represent the Be star, while the colours represent the radial velocity of the gas around them. The density arms are the regions marked by the highest velocities. Fig. 3: Example of Hα profiles of a real Be binary (pi Aqr). The strength of the violet and red peaks (V/R) changes depending on the movement of the density waves in the disk. circumsecondary region Density depends strongly on viscosity and period Radial velocities have similar amplitudes, but azimuthal velocities are much larger for lower viscosities Part of the matter is accreted