Production of gravitational waves during preheating in the Starobinsky
  inflationary model
  The production of GWs during preheating in the Starobinsky model with a
nonminimally coupled auxiliary scalar field is studied through the lattice
simulation in this paper. We find that the GW spectrum $\Omega_{\rm gw}$ grows
fast with the increase of the absolute value of coupling parameter $\xi$. This
is because the resonant bands become broad with the increase of $|\xi|$. When
$\xi<0$, $\Omega_{\rm gw}$ begins to grow once the inflation ends and grows
faster than the case of $\xi>0$. $\Omega_{\rm gw}$ reaches the maximum at
$\xi=-20$ ($\xi=42$ for the case $\xi>0$) and then decreases with slight
oscillation. Furthermore we find that the GWs produced in the era of preheating
satisfy the limits from the Planck and next-generation CMB experiments.
