A question to Americans - vowels

Kess, I think it is a great exaggeration to state that Mid-Atlantic speakers merge the vowels of cot and caught. The thesis to which you have linked says that only some speakers in that region have the merger. And other sources indicate that the Mid-Atlantic region (especially New York City) is the area where there is probably the biggest difference between these vowels for most speakers.

[ɑ] and [ɔ] were clearly distinct for the Northern talkers.

//It's because /ɑ/ is [a] (or [æ]) and [ɔ] is [ɑ]

Compare

in Denver: doll call [dɑl kɑl], hot dog [hɑt dɑg]

in Detroit: doll call [dæl kɑl], hot dog [hæt dɑg]