The rule that I usually use in such cases is that *an* precedes a vowel sound, while *a* is used before a consonant sound. I understand sound as different from letter - conventionally u would be called a vowel, but in unitary it actually sounds as [yu] ([ju], if we use IPA).
The question is:
- do we deal here with a simple misuse (confusing the latter and the sound that it corresponds to, but possibly also me using the incorrect rule)?
OR
- is there a dialectal difference involved here, so that some people would pronounce u of unitary as [u], in which case using an would actually make sense? If this is the case, is this difference characteristic of a geographic region, social origin or something else?
Update
I think the proposed duplicates do not really answer the question: some of them assume that [ju] is the "correct" pronunciation, whereas the other discuss the phonetic value of [j] as semivowel, that is conceivably perceptible as vowel - hence either a or an can be used. For background: my first language is Russian, which does have a separate letter й corresponding to consonant/semivowel [j], and a double set of vowel symbols: а/я ~ [a]/[ja], о/ё ~ [o]/[jo], etc. So for me [u] and [ju] are two distinct possibilities. Thus the question remains: are these indistinguishable to an English speaker OR does using one or the other is a matter of one's dialect/sociolect?
Update 2
The question has been now answered in the Linguistics community (and migrated back to English language and usage) with the reference to article A Corpus-Based Sociolinguistic Study of Indefinite Article Forms in London English. I cite below the abstract:
This article reports on work carried out as part of the project Analysis of Spoken London English Using Corpus Tools, namely, an analysis of the use of indefinite article forms in spoken London English in a corpus of transcribed interviews, combining methodologies from sociolinguistics and corpus linguistics. The authors find a relatively high frequency of a before words beginning with a vowel, where Standard English will have an. Social factors, in particular speakers’ age, ethnicity, and place of residence, are more important than linguistic factors affecting the use of a before vowels. The authors argue that the indefinite article a before vowels forms part of Multicultural London English, along with other phonological and grammatical features that have previously been documented. The indefinite article a before vowels seems to have undergone a process of reallocation in which its status has been realigned, possibly because of an increase in social acceptance of nonstandard forms.
(emphasis is mine)