On the consistency of warm inflation

Conditions are obtained for the existence of a warm inflationary attractor in the system of equations describing an inflaton coupled to radiation. These conditions restrict the temperature dependence of the dissipative terms and the size of thermal corrections to the inflaton potential, as well as the gradient of the inflaton potential. When these conditions are met, the evolution approaches a slow-roll limit and only curvature fluctuations survive on super-horizon scales. Formulae are given for the spectral indices of the density perturbations and the tensor/scalar density perturbation amplitude ratio in warm inflation.