  The Starobinsky model of modified gravity suggested to explain dark energy
may be also considered in the astrophysical context. Recently it has been
pointed out that in contracting regions curvature oscillations around the GR
value may lead to the production of high energy particles which contribute to
the cosmic ray flux. We revisit these calculations in the Einstein frame and
show that the continuous approximation for the matter density used in the
original calculations is not valid. We show that this problem is generic in
$F(R)$-gravity models introduced to describe the dark energy. We go beyond the
approximation and find the rate of particle production to be negligible.
