Can one use 亲爱 "dear" in a letter to a teacher one knows well?
In the specific case that triggered this question, the student, who is a male student in his late 20s, is being addressed 亲爱的[name]兄弟 by a female teacher who is about 10 years older. They have known each other for several years, and cooperate on a research project.
The student (as far as I know) always addresses her as [Surname]+老师, but since they get along very well, he is wondering if one could find a "warmer" way of addressing her.
Two questions: 1. Is it possible for him to address her in emails as 亲爱的[Surname]老师 without invoking any undesired connotations (notably without suggesting anything beyond respectful friendship). 2. Is there any alternative to 老师 in conversation? My guess is that there really isn't (except for more formal options, such as 先生, 教授 etc.), but perhaps I am wrong? Could one use, say, [Surname]+姐?