Closed Orbits and Descents for Enhanced Standard Representations of Classical Groups

Let $G={\mathrm{G}\mathrm{L}}_n(\mathbb{F})$, ${\mathrm{O}}_n(\mathbb{F})$, or ${\mathrm{S}\mathrm{p}}_{2n}(\mathbb{F})$ be one of the classical groups over an algebraically closed field $\mathbb{F}$ of characteristic $0$, let $\breve{G}$ be the MVW-extension of $G$, and let $\mathfrak{g}$ be the Lie algebra of $G$. In this paper, we classify the closed orbits in the enhanced standard representation $\mathfrak{g}\times E$ of $G$, where $E$ is the natural representation if $G={\mathrm{O}}_n(\mathbb{F})$ or ${\mathrm{S}\mathrm{p}}_{2n}(\mathbb{F})$, and is the direct sum of the natural representation and its dual if $G={\mathrm{G}\mathrm{L}}_n(\mathbb{F})$. Additionally, for every closed $G$-orbit in $\mathfrak{g}\times E$, we prove that it is $\breve{G}$-stable, and determine explicitly the corresponding stabilizer group as well as the action on the normal space.