${\mathfrak{s}\mathfrak{o}}_n(\mathbb{C})$-modules which are free over an abelian nilradical

In this paper, we classify the category of ${\mathfrak{s}\mathfrak{o}}_n(\mathbb{C})$-modules whose restrictions to the universal enveloping algebra of the abelian nilradical of a maximal parabolic subalgebra are free of rank $1$. We show that they generically are simple, and give an explicit description of submodules in the non-simple case. This category embodies a certain connection between weight modules and non-weight modules.