Curvature Perturbations from a Massive Vector Curvaton
  We study a ghost-free model of massive vector curvaton proposed in the
literature, where the quick decrease of the vector background expectation value
is avoided by a suitable choice of kinetic and mass functions. The curvaton
perturbations of this model have been so far computed assuming that these
functions are external classical quantities, and it was found that some special
time evolution of these functions leads to scale invariant and statistically
isotropic perturbations of the vector curvaton. However, external functions
should be understood as originating from the expectation value of some
additional field. Since these functions need to present a non-trivial evolution
during inflation, the field cannot be trivially integrated out, and, in
particular, its perturbations need to be included in the computation. We do so
in a minimal implementation of the mechanism, where the additional field is
identified with the inflaton. We show that, except for a narrow window of model
parameters, the interaction with this field generally causes the curvature
perturbations to violate statistical isotropy beyond the observational limit.
