On the Lack of Circumbinary Planets Orbiting Isolated Binary Stars
Abstract
We outline a mechanism that explains the observed lack of circumbinary planets (CBPs) via coupled stellartidal evolution of isolated binary stars. Tidal forces between lowmass, shortperiod binary stars on the premain sequence slow the stellar rotations transferring rotational angular momentum to the orbit as the stars approach the tidally locked state. This transfer increases the binary orbital period, expanding the region of dynamical instability around the binary, and destabilizing CBPs that tend to preferentially orbit just beyond the initial dynamical stability limit. After the stars tidally lock, we find that angular momentum loss due to magnetic braking can significantly shrink the binary orbit, and hence the region of dynamical stability, over time, impacting where surviving CBPs are observed relative to the boundary. We perform simulations over a wide range of parameter space and find that the expansion of the instability region occurs for most plausible initial conditions and that, in some cases, the stability semimajor axis doubles from its initial value. We examine the dynamical and observable consequences of a CBP falling within the dynamical instability limit by running Nbody simulations of circumbinary planetary systems and find that, typically, at least one planet is ejected from the system. We apply our theory to the shortestperiod Kepler binary that possesses a CBP, Kepler47, and find that its existence is consistent with our model. Under conservative assumptions, we find that coupled stellartidal evolution of premain sequence binary stars removes at least one closein CBP in 87% of multiplanet circumbinary systems.
 Publication:

The Astrophysical Journal
 Pub Date:
 May 2018
 DOI:
 10.3847/15384357/aabd38
 arXiv:
 arXiv:1804.03676
 Bibcode:
 2018ApJ...858...86F
 Keywords:

 binaries: close;
 planet–star interactions;
 planets and satellites: dynamical evolution and stability;
 stars: premain sequence;
 Astrophysics  Solar and Stellar Astrophysics;
 Astrophysics  Earth and Planetary Astrophysics
 EPrint:
 26 pages, 16 figures, accepted, ApJ