  We find that crystalline states of repulsive active Brownian particles at
high activity melt into a hexatic state but this transition is not driven by an
unbinding of bound dislocation pairs as suggested by the
Kosterlitz-Thouless-Halperin-Nelson-Young (KTHNY) theory. Upon reducing the
density, the crystalline state melts into a high-density hexatic state devoid
of any defects. Decreasing the density further, the dislocations proliferate
and introduce plasticity in the system, nevertheless maintaining the hexatic
state, but eventually melting into a fluid state. Remarkably, the elastic
constants of active solids are equal to those of their passive counterparts, as
the swim contribution to the stress tensor is negligible in the solid state.
The sole effect of activity is that the stable solid regime shifts to higher
densities. Furthermore, discontinuities in the elastic constants as a function
of density correspond to changes in the defect concentrations rather than to
the solid-hexatic transition.
