  We investigate the interaction between scalar fields and radiation in the
framework of warm inflationary models by using the irreversible thermodynamics
of open systems with matter creation/annihilation. We consider the scalar
fields and radiation as an interacting two component cosmological fluid in a
homogeneous, spatially flat and isotropic Friedmann-Robertson-Walker (FRW)
Universe. The thermodynamics of open systems as applied together with the
gravitational field equations to the two component cosmological fluid leads to
a generalization of the elementary scalar field-radiation interaction model,
which is the theoretical basis of warm inflationary models, with the decay
(creation) pressures explicitly considered as parts of the cosmological fluid
energy-momentum tensor. Specific models describing coherently oscillating
scalar waves, scalar fields with a constant potential, and scalar fields with a
Higgs type potential are considered in detail. For each case exact and
numerical solutions of the gravitational field equations with scalar
field-radiation interaction are obtained, and they show the transition from an
accelerating inflationary phase to a decelerating one. The theoretical
predictions of the warm inflationary scenario with irreversible matter creation
are also compared in detail with the Planck 2018 observational data, and
constraints on the free parameters of the model are obtained.
