Ice Lines in Circumbinary Protoplanetary Disks

Abstract

I examine the position of the ice line in circumbinary disks heated by steady mass accretion and stellar irradiation and compare with the critical semi-major axis, interior to which planetary orbits are unstable. There is a critical binary separation, dependent on the binary parameters and disk properties, for which the ice line lies within the critical semi-major axis for a given binary system. For an equal mass binary comprised of 1 M$_{\odot}$ components, this critical separation is $\approx 1.04 $AU, and scales weakly with mass accretion rate and Rosseland mean opacity ($\propto [\dot{M}\kappa_R]^{2/9}$). Assuming a steady mass accretion rate of $\dot{M} \sim 10^{-8} {\rm M_{\odot} yr^{-1}}$ and a Rosseland mean opacity of $\kappa_R\sim 1 {\rm cm^2 g^{-1}}$, I show that $\gtrsim 80%$ of all binary systems with total masses $M_{\rm tot} \lesssim 4.0 M_{\odot}$ have ice lines that lie interior to the critical semi-major axis. This suggests that rocky planets should not form in these systems, a prediction which can be tested by looking for planets around binaries with separations larger than the critical separation with \emph{Kepler} (difficult) and with microlensing.