Now, seeing it in perspective, it looks like an average run of the mill asterina. Once again, imo, the snails were victim to something else and the star was just cleanin up shop.
Have you considdered that an "oops" may have occured while you were on vacation? Snails cannot deal with rapid changes/swings to their environment. Perhaps the tank evaporated and it wasn't "topped off" properly. A quick spike in salinity could cause a snails demise.
Maybe one died and the others followed suit out of sympathy.
Maybe they all drank the koolaide... did you check em for Nike's?
Maybe they were siblings and all expired at the same time.
Maybe it was genocide... have you checked for gastro prints?
I'm out of bad ideas.