- Of course, Hot Bottom Burning does other things too. For a start,
it will be burning H via the CN cycles. This will t urn C-12 into C-13
But we are adding C-12 at each pulse. This is what makes a Carbon star.
But if HBB is active, then the added C-12 can be burned into N-14 and the
not become a carbon star after all!
Here we see the C/O ratio decreasing once the HBB starts!
The C13 content increases with the C12 abundance, due to the processing of
C12 into C13. In extreme cases, when the entire envelope can be processed
many time between the pulses, then HBB can produce the equilibrium ratio
of C12/C13 about 3.5.
And a consequence of this burning is the copious production of primary
- Here are some plots for M=4, 5 and 6 Msun each with Z=0.02
(solar), Z=0.008 (Large Magellanic Cloud) and Z=0.004 (Small Magellanic
- C/O plot showing that HBB prevents
occurrence of C stars. In fact, however, once the mass-loss gets
and the envelope mass is decreased substantially, then the HBB
ceases and the dredge-up continues. So sometimes a C-star can still form,
albeit briefly and at the end of the star's life (when it is not visible
because of the lost mass obscuring it from view). The diagram has the
region corresponding to C stars (C/O > 1) shown in magenta.
- C-12/C-13 plot showing equilibrium
values when HBB dominates.
- N-14 plot showing lots of N-14 made
when HBB exists.