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The 2.7 K is the 'radiation temperature' in deep space. The 'atomic temperature' - the temperature (speed) of the atoms, molecules and dust, would be from 2.7 K up to a few million K.
As for the question - when does the coma form - well, that's another can of wormholes...
The coma is basically the atmosphere of the comet and there will always be something there - just most of the time it's too thin and dim to see. Some gas will always be given off, according to the Stefan-Boltzman distribution which states that, basically, the 'temperature' that we measure is really only the average and an object at ... 100 K, for example, will actually be composed of molecules at 0 K (or slightly more) up to a thousand or so - you can actually work out the number ussing the SB law. Because of this, there will always be some outgassin - just like water at 20 C will slowly gassify as the higher energy molecules (ie faster ones) break free of the surface tension and turn to vapour.
And then size does matter too - a 10 m comet will probably be fried in the sun with no one noticing it. A 500 km comet (pluto?) might be visible at 100 AUs...
Well then, maybe you could define it when you can see it - well that won't work either. If you can't see it with your 8" SCT, then maybe you can see it with the Keck or the Hubble or with the proposed 100 m 'Suzyscope' on Ceres.
Sorry to be a bore!
Probably what you need to do is define the question first - eg for a 10 km comet, at what distance from the sun will enough gas and dust be released so that it's visible in a ... 10 " f10 SCT...
"There goes that rotten Halley's Comet! It makes me sick! I want to vomet!"
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