Hi Mark,
Is the utility bill in their name? If so they would be responsible for the balance and that is taken care of between the utility company and the old tenants. The new tenants moving in would put the utilities in their name and have a zero balance.
If the utilities was in the previous owners name you are now responsible for that bill. Pay it and add it to your due diligence checklist for next time.
I wouldn't wait to figure it out with the former tenants before letting new tenants move-in. In their minds it's likely not their problem anymore since they're out already so don't be surprised if they ghost you. You may never get money from them so don't lose more money by having the unit vacant for any longer. Make sure you screen whoever moves in, don't just like anybody's third cousin twice removed from their mom's side move-in. Make sure they are qualified and if they are not use your discretion if you want to use a co-signer. And most importantly, make sure utilities are transferred to their name. Ask for account numbers prior to giving out keys.
If they paid utilities directly to the owner, still try to recoup the money, but don't hold your breath. Let them know if they don't pay by x date, you'll send their account to collections. Send it to collections and move on.