Conformal symmetries and MOTS stability

Let $\{\Sigma_t\}$ be a spacelike foliation of a spacetime $\mathcal{M}$, and suppose each $\Sigma_t$ is foliated by 2-surfaces $\mathcal{S}$ with spacelike unit normal in $\Sigma_t$. We show that under mild energy conditions, a MOTS $\mathcal{S}$ that intersects integral curves of past-pointing conformal Killing vector field lying in the normal space of $\mathcal{S}$ is strictly stable and evolves smoothly to a spacelike horizon. We also show that if the restriction of the divergence of the vector field to $\mathcal{S}$ is non-negative, $\mathcal{S}$ is unstable, and if negative and $\mathcal{S}$ is a 2-sphere, $\mathcal{S}$ must be strictly stable.