Community automatically bumps some old questions that have no accepted answers to the top of the Questions/Active page. Unfortunately, for some of them, the OP has no longer exist as in this question.
It is clear that this question will be seen again and again. I can see that some of them are well answered but the OP has left without accepting the solution.
Is there a mechanism to turn this questions into accepted position? Or we just leave them to dangle around?
Is there a mechanism to turn this questions into accepted position?
No. The system will not make any accept decisions on behalf of the asker, whether the asker has an account on this site or not.
Unfortunately, for some of them, the OP has no longer exist as in this question.
Actually for this specific question, the user doesn't exist yet, because the question was migrated from stackoverflow here. So chances are once the user sees the migration they will create an account here to interact with the answerers.
You can distinguish the cases also by looking at the username: If it is greyed-out and has the form userXYZ with XYZ being a big number, then the user was most likely removed, whereas if it is greyed out with a textual name, chances are that the question was migrated here from another site.
we just leave them to dangle around?
Yes. However note that the condition for questions getting bumped to the homepage is actually that the question has the status "unanswered" which is defined by having no accepted answer and neither having an answer with score at least +1. So if you think an answer to a bumped question is good, upvote it and if nobody else downvotes it, it won't be bumped anymore.
