Feelings of what you 'should' be responsible for are immaterial. Your lease likely has the phrase "jointly and severely" in it. That means YOU are legally on the hook for their portion of the rent next month. You need to tell your landlord what's going on ASAP so they can help find replacements. Of course you should go after the roommies in court if you end up being out of pocket for their share but I would personally do everything I can to just wash my hands of them and get somebody else in their rooms.