A Turing Kernelization Dichotomy for Structural Parameterizations of $\mathcal{F}$-Minor-Free Deletion

For a fixed finite family of graphs $\mathcal{F}$, the $\mathcal{F}$-Minor-Free Deletion problem takes as input a graph $G$ and an integer $\ell$ and asks whether there exists a set $X \subseteq V(G)$ of size at most $\ell$ such that $G-X$ is $\mathcal{F}$-minor-free. For $\mathcal{F}={K2}$ and $\mathcal{F}={K3}$ this encodes Vertex Cover and Feedback Vertex Set respectively. When parameterized by the feedback vertex number of $G$ these two problems are known to admit a polynomial kernelization. Such a polynomial kernelization also exists for any $\mathcal{F}$ containing a planar graph but no forests. In this paper we show that $\mathcal{F}$-Minor-Free Deletion parameterized by the feedback vertex number is MK[2]-hard for $\mathcal{F} = {P3}$. This rules out the existence of a polynomial kernel assuming $NP \subseteq coNP/poly$, and also gives evidence that the problem does not admit a polynomial Turing kernel. Our hardness result generalizes to any $\mathcal{F}$ not containing a $P3$-subgraph-free graph, using as parameter the vertex-deletion distance to treewidth $mintw(\mathcal{F})$, where $mintw(\mathcal{F})$ denotes the minimum treewidth of the graphs in $\mathcal{F}$. For the other case, where $\mathcal{F}$ contains a $P_3$-subgraph-free graph, we present a polynomial Turing kernelization. Our results extend to $\mathcal{F}$-Subgraph-Free Deletion.