  In this paper we present a theoretical investigation for the ground state of
an impurity immersed in a Fermi sea. The molecular regime is considered where a
two-body bound state between the impurity and one of the fermions is formed.
Both interaction and exchange of the bound fermion take place between the dimer
and the Fermi sea. We develop a formalism based on a two channel model allowing
us to expand systematically the ground state energy of this immersed dimer with
the scattering length $a$. Working up to order $a^3$, associated to the
creation of two particle-hole pairs, reveals the first signature of the
composite nature of the bosonic dimer. Finally, a complementary variational
study provides an accurate estimate of the dimer energy even at large
scattering length.
