Q. Should the indefinite article “a” be used when introducing a professor emeritus? For example, “He is [a] professor emeritus of chemistry at the university.” On the one hand, “a” usually indicates that the person is not the only person with that title at the university. On the other hand, Google Ngram shows a higher preference for no article.
A. We agree that adding “a” could make sense if there’s more than one such professor at the university in question. But whether there’s one or eleven, omitting the article makes “professor emeritus” sound more like a professional title than a job description. That is, it sounds fancier.
This may explain why it would be relatively rare to refer to someone as “teacher of” chemistry or another subject (without “a”), whereas calling someone “professor of”—even without “emeritus”—is fairly common (“She is professor of chemistry at . . .”). The word professor enjoys a status that teacher does not.
In other words, there’s no definitive answer to your question. Though adding an “a” or an “an” can work well before a title held by more than one person, omitting the article before “professor” and variations of that term even when there’s more than one can be an equally good choice, one that follows an unstated convention in academia.