Production of dark matter axions from collapse of string-wall systems
  We analyze the spectrum of axions radiated from collapse of domain walls,
which have received less attention in the literature. The evolution of
topological defects related to the axion models is investigated by performing
field-theoretic lattice simulations. We simulate the whole process of evolution
of the defects, including the formation of global strings, the formation of
domain walls and the annihilation of the defects due to the tension of walls.
The spectrum of radiated axions has a peak at the low frequency, which implies
that axions produced by the collapse of domain walls are not highly
relativistic. We revisit the relic abundance of cold dark matter axions and
find that the contribution from the decay of defects can be comparable with the
contribution from strings. This result leads to a more severe upper bound on
the axion decay constant.
