A：

如果你跟牛老师做过科研，可以加上xxx's student；针对老师的领域可以写 interested in xxx（两三个词）显得你focus，然后加上Prospective Ph.D. student Fall 2017

e.g.

1. Emmanuel Candes' student interested in statistical machine learning, prospective Ph.D. student fall 2017

2. Inquiry for possible Ph.D. position in your group.

3. Prospective PhD student interested in AEMS - Reply Appreciated