In RP - the standard, textbook or 'BBC' pronunciation of the south of England - the lips are relaxed for the /a:/ sound. It is a very 'open' sound, with no tension in the lips. /a:/ is an open, back, unrounded vowel. 'Open' tells you that the mouth is open and the tongue is low, 'back' tells you that the tongue is a long way back in your mouth, and 'unrounded' tells you that the lips are relaxed. The rounded version is /ɒ/ . If you round your lips, it will sound more like the /ɔ:/ sound in 'core' rather than 'car'. [British English speaker/ RP]

Here is some more information. /a/ becomes unrounded [ɑ] or rounded [ɒ] depending on the variety of English. Here is a video by a British voice coach demonstrating how a British actor can sound American (to a British audience) by always pronouncing /r/ and by replacing [ɒ] and [ɔ] with [ɑ]. https://www.youtube.com/watch?v=p5T3ubSdhSM Here is more information about cardinal vowels (definitions of [ɑ], [ɒ], [ɔ], and others). https://en.wikipedia.org/wiki/Cardinal_vowels [ɔ] - open-mid back rounded vowel (example "core" [kɔr] Am[kɔ:] Br - Collins online dictionary). [ɑ] - open back unrounded vowel (example "car" [kɑr] Am, [kɑ:] Br - Collins online dictionary). [ɒ] - open back rounded vowel

I just checked :) My lips are rounded for that sound. UK English (Australian accent)