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TeamCOMPAS / COMPAS

COMPAS rapid binary population synthesis code
http://compas.science
MIT License
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Unphysical hard binaries are created #306

Closed ilyamandel closed 4 years ago

ilyamandel commented 4 years ago

COMPAS finally succeeded in breaking through the speed of light barrier! It can create binaries with orbital velocities of >10^{10} km/s, and separations of <1 m. Of course, such binaries are unphysical. An example is seed 606249 from the Mandel+ stochastic BPS solar metallicity run.

Preliminary testing indicates that such binaries arise when we enforce the stability of case BB mass transfer. In this case, we can have extreme hardening if the donor is much more massive than the accretor (by a factor of ~40 in this example, which has mass transfer from a HeHG star onto a 0.1 solar mass MS accretor). The extreme mass ratio in the context of essentially fully non-conservative mass transfer leads to extreme hardening. The proposed solution is to include a test in the mass transfer function that enforced stable MT resulting in an RLOF configuration should be declared a merger.

ilyamandel commented 4 years ago

I've implemented a test for stars touching (a proxy for L2 overflow and hence merger) after stable mass transfer, which resolves this issue, in PR #343 . Closing.