Is fusion in an accretion disk of a black hole possible?

[Irfan Nafi]

I know that the accretion disk of a black hole gets hot enough for powerful emission of x-rays, but does that disk get hot enough for certain elements to fuse?

 

[e.bar.goum]

Yes. Though, many other processes e.g. the r-process (rapid neutron capture forming nuclei heavier than iron), p-process (proton capture), are relevant.

see:
http://arxiv.org/abs/0803.1785
http://www.lsw.uni-heidelberg.de/nic2010/talks/McLaughlin.pdf
http://online.kitp.ucsb.edu/online/grb06/surman/pdf/Surman_KITP.pdf

 

[Ken G]

Yet notice that what is meant by an "accretion disk" can be two rather different things-- we can have a disk forming as a rotating star collapses or two stars merge, which are going to be very high-density disks, and we have more standard X-ray binary disks, where you have Roche lobe overflow or a wind streaming in, and those kinds of accretion disks are going to be much lower density. So although nucleosynthesis would occur in both cases, it seems less likely to be observable or important in the low-density versions, and that might be what the OP is asking about. To what degree it occurs or is important in each situation will depend on the density and timescale, given that the temperature is indeed high enough. So certainly the answer is yes, nucleosynthesis occurs, but we should still distinguish supernova-type situations from the more standard meaning of a black hole accretion disk.

 

[Irfan Nafi]

So it is possible but the accretion disk has to be dense enough. When a rotating star collapses to form a black hole the black hole should produce a GRB so any material in that GRB (that is close enough) has a probability to fuse only if the region is dense enough for atoms or nucleons to fuse. But how does the collision of 2 high mass stars result in a black hole?

 

[Ken G]

But how does the collision of 2 high mass stars result in a black hole?

When the masses of the degenerate electron core inside the two stars come together and exceed a limit, their gravity cannot be supported by degenerate electrons (because the electron energies go relativistic), and the cores collapse into either a neutron star or black hole (either one produces a dense accretion disk if there is enough angular momentum).