I have isolated these words just because they are different to my hypotheses. What I will say please realize that I may not be right either......
The protoplanetary disk already contains many elements, and is the very reason why the disk is formed to begin with. As the the protostar collapses it gains in gravity, pulling things in, but also increases its spin and increasing the speed of the surrounding matter. The centrifugal rings can easily be identified by the composition of objects having the proper mass and speed to remain in orbit.....
1. "As the the protostar collapses it gains in gravity" I can see what you mean but gravity is dependent on mass, so the amount of gravity must remain the same. But I accept that as the density goes up the gravitation force on each fallen particle will rise also, but this would be balance by the centrifugal forces so to get continuing collapse there has to be loss of heavy particles carry this energy, and lost of heat radiation (so the velocity of the particles slows they lose centrifugal force, gravity moves the masses inwardly then gravity increases due to the increased density, heat energy is removed and so the collapse continues.
2. "but also increases its spin and increasing the speed of the surrounding matter." The compressing matter has a lower mechanical energy than the matter already flung off, so the transfer of energy by increasing the speed is debated. I will grant that in my hypotheses the radiant energy drives the flung off material further away due to radiant energy so this may slow the collapse of the flung material.
3. I agree "The centrifugal rings can easily be identified by the composition of objects having the proper mass and speed to remain in orbit....." (Except I wonder if the word is "centrifugal" when speaking of the rings. (annular is that a better word??)
Thanks again and sorry I didn't get time to research yesterday's unanswered question about the temperature of the inner Solar System.