  After the detection of GRB 170817A, the first unambiguous off-axis gamma-ray
burst (GRB), several studies tried to understand the structure of GRB jets. The
initial jet structure (directly produced by the central engine) can be
partially preserved, or can be completely modified by the interaction with the
environment. In this study, we perform three-dimensional, special relativistic
hydrodynamics simulations of long GRB jets evolving through a massive
progenitor star. Different jet scenarios were considered: Top-hat, Gaussian
jets dominated by pressure or by kinetic energy, as well as a model of a
supernova (SN) plus a jet both propagating through the progenitor. We found
that, while propagating inside the progenitor star, jets with different initial
structures are nearly indistinguishable. Kinetic dominated jets are faster and
more collimated than pressure dominated jets. The dynamics of jets inside the
progenitor star strongly depends on the presence of an associated SN, which can
substantially decelerate the jet propagation. We show that the initial
structure of GRB jets is preserved, or not, mainly depending on the jet
collimation. The initial structure is preserved in uncollimated jets, i.e. jets
which move through low density environments. Meanwhile, jets which move through
dense environments are shaped by the interaction with the medium and remain
collimated.
