  We use the gravitational instability formation scenario of cometesimals to
derive the aggregate size that can be released by the gas pressure from the
nucleus of comet 67P/Churyumov-Gerasimenko for different heliocentric distances
and different volatile ices. To derive the ejected aggregate sizes, we
developed a model based on the assumption that the entire heat absorbed by the
surface is consumed by the sublimation process of one volatile species. The
calculations were performed for the three most prominent volatile materials in
comets, namely, H_20 ice, CO_2 ice, and CO ice. We find that the size range of
the dust aggregates able to escape from the nucleus into space widens when the
comet approaches the Sun and narrows with increasing heliocentric distance,
because the tensile strength of the aggregates decreases with increasing
aggregate size. The activity of CO ice in comparison to H_20 ice is capable to
detach aggregates smaller by approximately one order of magnitude from the
surface. As a result of the higher sublimation rate of CO ice, larger
aggregates are additionally able to escape from the gravity field of the
nucleus. Our model can explain the large grains (ranging from 2 cm to 1 m in
radius) in the inner coma of comet 67P/Churyumov-Gerasimenko that have been
observed by the OSIRIS camera at heliocentric distances between 3.4 AU and 3.7
AU. Furthermore, the model predicts the release of decimeter-sized aggregates
(trail particles) close to the heliocentric distance at which the gas-driven
dust activity vanishes. However, the gas-driven dust activity cannot explain
the presence of particles smaller than ~1 mm in the coma because the high
tensile strength required to detach these particles from the surface cannot be
provided by evaporation of volatile ices. These smaller particles can be
produced for instance by spin-up and centrifugal mass loss of ejected larger
aggregates.
