Really nice find!
Stars consisting essentially of only H and He because formed from early clouds that had no heavier elements.
the paper helps us imagine early times and the very earliest star formation
Computer simulations show how small concentrations of DM and gas (massing only a few million solar) could breed within them the so called pop III (metal-free) stars estimated at a few tens of solar mass.
Interestingly, LITTLE stars (one solar mass or less) of the sort common today CANNOT FORM without heavier elements to help radiate away excess heat generated by the collapse of the cloud.
Hydrogen and Helium are not efficient radiators. You need heavier atoms like carbon with more electron states and hopefully more complicated molecules---able to absorb energy from collision and turn it into heat radiation which leaves the cloud.
So the only way stars can form is from a very big massive cloud that forms a BIG star, so the collapsing object has enough gravity to hold together while the cloud gets hot enough to force the H and He to radiate off the excess energy of condensation.
I'm trying to understand this intuitively. The first stars were all biggies. It's curious.
And now these people are looking for protogalaxies comprised of just these first stars. Fuzzy blobs whose light contains no metal lines in its spectrum, only H and He lines.
Great find, Chronos. thanks!