  GRB 101225A is a prototype of the class of blackbody-dominated (BBD)
gamma-ray bursts (GRBs). It has been suggested that BBD-GRBs result from the
merger of a binary system formed by a neutron star and the helium core of an
evolved star. We have modelled the propagation of ultrarelativistic jets
through the environment left behind the merger by means of relativistic
hydrodynamic simulations. In this paper, the output of our numerical models is
post-processed to obtain the (thermal) radiative signature of the resulting
outflow. We outline the most relevant dynamical details of the jet propagation
and connect them to the generation of thermal radiation in GRB events akin to
that of GRB 101225A. A comprehensive parameter study of the jet/environment
interaction has been performed and synthetic light curves are confronted with
the observational data. The thermal emission in our models originates from the
interaction between the jet and the hydrogen envelope ejected during the
neutron star/He core merger. We find that the lack of a classical afterglow and
the accompanying thermal emission in BBD-GRBs can be explained by the
interaction of an ultrarelativistic jet with a toroidally shaped ejecta whose
axis coincides with the binary rotation axis. The spectral inversion and
reddening happening at about 2 d in GRB 101225A can be related to the time at
which the massive shell ejected in an early phase of the common envelope
evolution of the progenitor system is completely ablated by the
ultrarelativistic jet.
