Given a locally finite leafless tree $ T $, various algebraic groups over local fields might appear as closed subgroups of $ \text{ Aut} (T)$. We show that the set of closed cocompact subgroups of $ \text{ Aut} (T)$ that are isomorphic to a quasi-split simple algebraic group is a closed subset of the Chabauty space of $ \text{ Aut} (T)$. This is done via a study of the integral Bruhat$-$Tits model of $ \text{ SL}_2 $ and $ \text{ SU}_3^{ L/K} $, that we carry on over arbitrary local fields, without any restriction on the (residue) characteristic. In particular, we show that in residue characteristic $ 2 $, the Tits index of simple algebraic subgroups of $ \text{ Aut} (T)$ is not always preserved under Chabauty limits.
Categories: math.GR