Now that would be Argumentum ad verecundiam or an appeal to authority. Which is a false arguement unless the person is an expert in the field. When you make an appeal to an authority the opposite side has the right to question that authority. Ad hominem attacks are only irrelevant attacks on a person, questioning an experts credentials isn't irrelevant, its central to the debate.
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A gentleman should always be able to break his fast in the manner of a gentleman where so ever he may find himself.--Good Omens