Conormal Varieties on the Cominuscule Grassmannian

Let $G$ be a simply connected, almost simple group over an algebraically closed field $\mathbf{k}$, and $P$ a maximal parabolic subgroup corresponding to omitting a cominuscule root. We construct a compactification $\varphi :T^{\ast }G\mathrm{/}P\to X(u)$, where $X(u)$ is a Schubert variety corresponding to the loop group $LG$. Let $N^{\ast }X(w)\subset T^{\ast }G\mathrm{/}P$ be the conormal variety of some Schubert variety $X(w)$ in $G\mathrm{/}P$; hence we obtain that the closure of $\varphi (N^{\ast }X(w))$ in $X(u)$ is a $B$-stable compactification of $N^{\ast }X(w)$. We further show that this compactification is a Schubert subvariety of $X(u)$ if and only if $X(w_{0}w)\subset G\mathrm{/}P$ is smooth, where $w_0$ is the longest element in the Weyl group of $G$. This result is applied to compute the conormal fibre at the zero matrix in any determinantal variety.