As an undergraduate, I did some group research a little over a year ago which is now being compiled into a paper. My advisor at the time is contacting us to request feedback and help writing it. I am especially important to the effort, because I am the only one who knows some of the results (and proofs) well enough to be able to explain them and structure the paper around them.
However, I am just beginning a PhD, in a completely different field than my undergraduate research. This comes with a new set of challenges to focus on, including that I have to find an advisor here within my first year. I would like to be primarily focused on making the most of my PhD program and thinking about current research, rather than having to worry too much about this old research I did as an undergraduate, which I do not feel will impact my career. Even if our research gets published (which it might not), because it's not in my current field it does not seem that it should be my top priority.
In summary, I do not think that assisting with the writing of the paper is in my personal best interest. However, I also understand that I have some obligation to the research group I was in and the research we did, and that it could be rude and irresponsible, even unethical, to drop out of the picture entirely. What exactly are my obligations to writing this paper? What is the most ethical thing to do in this situation, which also preserves my interests as much as possible?
Finally, if I should avoid working too much on this paper, how do I communicate this to my undergraduate advisor and to my fellow researchers? It would be rude to cut back on the work without any explanation.