  Recent discoveries of circumbinary planets by Kepler mission provide
motivation for understanding their birthplaces - protoplanetary disks around
stellar binaries with separations <1 AU. We explore properties and evolution of
such circumbinary disks focusing on modification of their structure caused by
tidal coupling to the binary. We develop a set of analytical scaling relations
describing viscous evolution of the disk properties, which are verified and
calibrated using 1D numerical calculations with realistic inputs. Injection of
angular momentum by the central binary suppresses mass accretion onto the
binary and causes radial distribution of the viscous angular momentum flux F_J
to be different from that in a standard accretion disk around a single star
with no torque at the center. Disks with no mass accretion at the center
develop F_J profile which is flat in radius. Radial profiles of temperature and
surface density are also quite different from those in disks around single
stars. Damping of the density waves driven by the binary and viscous
dissipation dominate heating of the inner disk (within 1-2 AU), pushing the
iceline beyond 3-5 AU, depending on disk mass and age. Irradiation by the
binary governs disk thermodynamics beyond ~10 AU. However, self-shadowing by
the hot inner disk may render central illumination irrelevant out to ~20 AU.
Spectral energy distribution of a circumbinary disk exhibits a distinctive bump
around 10 micron, which may facilitate identification of such disks around
unresolved binaries. Efficient tidal coupling to the disk drives orbital
inspiral of the binary and may cause low-mass and compact binaries to merge
into a single star within the disk lifetime. We generally find that
circumbinary disks present favorable sites for planet formation (despite wider
zone of volatile depletion), in agreement with the statistics of Kepler
circumbinary planets.
