Contacting potential PhD advisors while not knowing research topic?
You definitely don't need super specific research topics to reach out to potential advisors to gauge their interest in working with you. When I first applied to PhD programs, I also got in touch with potential advisors beforehand to introduce myself and tell them about my research interests just to see if they would be responsive/be interested in working with me should I be admitted. At the time, I had a couple of ideas about what I wanted to do for my dissertation, but they weren't fully formed at all. Still, I did get some positive responses from several professors which encouraged me to apply to their programs.

At the end of the day, they know that most students don't join PhD programs with well-developed research agendas, that research interests change, and even that the person you wanted to work with originally might not be the person you end up working with in the end (the last scenario happened to me in some very ugly turn of events). Point being, I wouldn't worry too much about presenting developed research questions in the initial emails to faculty. If anything it's just a good way to get an idea of professors' availability and responsiveness, and to narrow down your list of schools to which you're applying. 

And actually, given the above reference to my own negative experiences post-admission, I would add that it doesn't hurt to look for programs that have at least a couple of potential advisors to choose from.