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| Paper: |
Asynchonous Binaries, Energy Dissipation and Turbulent Viscosity |
| Volume: |
496, Living Together: Planets, Host Stars and Binaries |
| Page: |
264 |
| Authors: |
Koenigsberger, G.; Brott, I.; Moreno, E. |
| Abstract: |
Stars in binary systems are generally modeled under the assumption that
they are in an equilibrium configuration and, in particular, that the stellar
rotation angular velocity equals the orbital angular velocity. However,
asynchronous rotation is more common than generally recognized.
All eccentric systems undergo asynchronous rotation and the angular velocity of
rotation of many stars in circular orbits differs from that of the orbital
angular velocity.
Combined with the external gravitational potential, this asynchronous rotation
causes shearing motions in the stellar layers and, given that the stellar material
is not inviscid, kinetic energy is dissipated into heat. In 1968, Zdeněk Kopal
addressed the question of whether the tidal shear energy dissipation rates, Ė,
in asynchronous binaries can lead to an internal stellar structure that differs from
that in an analogous single star. His calculation, based on the assumption that
the viscosity is purely molecular, led him to conclude that Ė is insignificant
and therefore has no effect on the internal stellar structure. However, Kopal also pointed
out the important caveat that if turbulent viscosity prevailed, then larger
values of Ė would obtain. We have revisited the question of the
magnitude of Ė using the TIDES code (Moreno 2011) and examined its
dependence on viscosity for several layers of a ZAMS 30 M☉ star with a 20 M☉
companion in a 6-day eccentric orbit. We find that conditions for turbulent viscosity
are favored when the star expands after leaving the main sequence. For example, when
the 30 M☉ star is 5 Myr old and rotating near its corrotation speed,
turbulent viscosity might be expected to appear in all layers at distances greater than
60% of the maximum stellar radius. As a consequence, tidal shear energy dissipation
may constitute a non-negligible effect in a large number of close binary systems, with
possibly interesting consequences for their internal structure and evolution. |
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