The impact of separation on binary star evolution

In this specific post, I discuss the impact of separation on the formation of fall back disk around binary stars.

  1. Model Description

In this simulation, there are two stars. One is assumed to be an AGB star, it has two states - spherical wind state and equatorial outflow state and the other is a companion.

The equatorial outflow state eject high density but low speed wind (20 km/s) into the space. It has open angle. Open angle means, compared to earth, the latitude. For example, a pi/6 open angle means the outflow is being ejected from pi/6 North to pi/6 South, that is, pi/3 in total angle.

In this simulation, all outflow has open angle of pi/6. The equatorial outflow last 11.2 years, the orbital period for three simulations are 4 years, 6.2 years and 11.6 years respectively, so the outflow can cover at least a full orbital period. At all other time, the AGB star is in spherical wind state. Its spherical wind speed is above the escaping velocity (40 km/s).

The initial specific angular momentum of the outflow is negligible, let's say it is approximately 0. This is in line with the property of AGB and post-AGB stars.

The temperature of the simulation is fixed at 100 K.

The separation are 3 AU, 4 AU and 6 AU (See (5,3,8) data set in ​3AU4AU6AU) respectively in the following simulations, each has topview and sideview.

  1. Simulation

3AUsideview​

3AUtopview​

​4AUsideview

4AUtopview​

6AUsideview​

​6AUtopview

The equatorial plane has higher density than the polar direction. Although the AGB star has spherical outflow after the equatorial outflow, the "heavy" equatorial gas is barely supported by the successive wind while the gas in polar direction has been pushed out. However, a fraction of the polar outflow is fallen back to the equatorial plane at large radii then start to orbit the binary stars (see Muhammad Akashi and Noam Soker 2008 ​http://www.sciencedirect.com/science/article/pii/S1384107607000863). This simulation validate their assumption in 3D. We can clearly see the formation of disk in equatorial plane.

We can also see the bipolar shape of PPNe. However, the simulation zone is not big enough to see the full bipolar outflow picture.

  1. Conclusion

The conclusion is that the closer the binary stars is, the more likely the fall back disk forms or the smaller the circumbianry disk will be.

Please see my further progress of this topic in this post.

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