Re: 4 Msol limit?
"Did I miss something?"
The important thing is the difference between what mass it's possible for a black hole to have, and figuring out a way for a black hole of any particular size to actually form. In fact, it's possible for a black hole to have any mass - compress enough mass into a small enough volume and you get a black hole. The question then becomes what processes can do that. As it turns out, there only seem to be such process to form three broad classes of black hole - supermassive ones, stellar mass ones, and microscopic ones.
The latter form through high energy particle collisions, which means they don't get above a certain size due to the difficulty accelerating particles to such high energies - even extreme processes in supernovae and quasars can't produce energies high enough to form black holes as big as something like a grain of sand. Meanwhile supermassive black holes aren't understood as well, but appear to be the result of things happening at the galactic scale, and not simply from the merger of lots of smaller black holes (one of the big open questions is whether the black hole or the galaxy forms first). Then you have the stellar mass ones discussed in this article, which oddly enough have masses similar to stars and mostly form as the result of stellar processes.
What you might notice here is that there are pretty big gaps. There are no black holes with masses falling between atoms and stars, and none falling between stars and galaxies. That's not because such black holes can't exist, but simply because there isn't anything happening at those scales that actually forms them in practice. Finally getting to the actual question, the same is true around the edges of stellar-mass black holes. Yes, a black hole could have 3 solar masses, but we don't actually have a theory of how it could form. Models of things like supernovae predict 4 and up, and we've observed them to actually exist, but smaller ones just don't seem to happen. Without going into too much detail, supernovae are pretty chaotic things that involve throwing a lot of material off into space, and if you're that close to the lower limit of forming a black hole, the models predict that too much material will be lost to actually form one in the end.
In addition, simply taking something that has too little mass to form a black hole and letting it accrete more material may not actually work. A white dwarf that gains too much mass doesn't turn into a neutron star, it explodes in type Ia supernova. Depending on how much mass is thrown off, it may actually end up as a neutron star afterwards, but ironically one with lower mass than the white dwarf it formed from. Exactly what happens to a neutron star when something similar happens isn't really clear, but it's entirely possible that if you simply take a heavy neutron star and add more material, whatever happens to it could end up still not having enough mass to actually form a black hole.
tl;dr, It's not that black hole such as described in the article is impossible and its discovery will overturn decades of physics. It's simply that models and observations up to this point suggest such black holes don't actually form in practice, so finding one will need to lead to some revisions to the model, or entirely new theories, to explain where this one came from.
Biting the hand that feeds IT
