Au Pairs? What are the limitations?

I am a single father and have recently been offered a well paying job in the Northern Virgina area. With the high pay comes plenty of travel. I dont know the exact amount of travel, but sometimes for two weeks or so.

Anyway, I was researching nannies and au pairs online. My son is 12 years old, and doesnt have any special needs, I just cant leave him alone while I travel. I read on a website that au-pairs can only work so many hours per week and have other limitations. I really just need her to cook for him while I am gone, and make sure he does his homework, and goes to school. If I am gone for 24 hours per day, is that considered that she is working 24 hours per day?

Am I better off getting a nanny? I looked at a website, but looks like a nanny will cost me about $2000 a month. Its not out of the question, but I guess if I get a nanny her life will be pretty easy. I can cook and clean for myself and my son when I am there. But I would need someone full time because my travel is with short notice and I wouldnt want to have to search for someone new every time I had to go out of town.

Thanks