Star 5-edge-colorings of subcubic multigraphs

The star chromatic index of a multigraph $G$, denoted ${\chi }_s^{\mathrm{\prime }}(G)$, is the minimum number of colors needed to properly color the edges of $G$ such that no path or cycle of length four is bi-colored. A multigraph $G$ is star $k$-edge-colorable if ${\chi }_s^{\mathrm{\prime }}(G)\le k$. Dvo\v{r}'ak, Mohar and \v{S}'amal [Star chromatic index, J Graph Theory 72 (2013), 313–326] proved that every subcubic multigraph is star $7$-edge-colorable, and conjectured that every subcubic multigraph should be star $6$-edge-colorable. Kerdjoudj, Kostochka and Raspaud considered the list version of this problem for simple graphs and proved that every subcubic graph with maximum average degree less than $7\mathrm{/}3$ is star list-$5$-edge-colorable. It is known that a graph with maximum average degree $14\mathrm{/}5$ is not necessarily star $5$-edge-colorable. In this paper, we prove that every subcubic multigraph with maximum average degree less than $12\mathrm{/}5$ is star $5$-edge-colorable.