Compact object mergers driven by gas fallback
  Recently several gravitational wave detections have shown evidence for
compact object mergers. However, the astrophysical origin of merging binaries
is not well understood. Stellar binaries are typically at much larger
separations than what is needed for the binaries to merge due to gravitational
wave emission, which leads to the so-called final AU problem. In this letter we
propose a new channel for mergers of compact object binaries which solves the
final AU problem. We examine the binary evolution following gas expansion due
to a weak failed supernova explosion, neutrino mass loss, core disturbance, or
envelope instability. In such situations the binary is possibly hardened by
ambient gas. We investigate the evolution of the binary system after a shock
has propagated by performing smoothed particle hydrodynamics simulations. We
find that significant binary hardening occurs when the gas mass bound to the
binary exceeds that of the compact objects. This mechanism represents a new
possibility for the pathway to mergers for gravitational wave events.
