Re: Heavy Protons & Life Without Hydrogen

"Luke Campbell" <lwcamp@xxxxxxxxx> wrote in message
news:1185406725.896368.231700@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
On Jul 25, 1:52 pm, "Logan Kearsley" <chrono.sur...@xxxxxxxxxxx>
wrote:
"Luke Campbell" <lwc...@xxxxxxxxx> wrote in message
[...]
What you should worry about is the deuteron binding energy. This
tells you how much energy will be released upon neutron fusion. If
this is very low, you not only get less heat, you get equilibrium
between the reactions
n+n -> D+e+nu_bar
D+e -> n+n+nu
The second reaction not only absorbs energy, but the combination of
the two will cool the cloud by neutrino radiation. This could keep
the clouds quite cold.

Which *would* allow them to collapse, yes?

Yup.

Resulting in neutron-fusion stars
that initially release a lot of neutrino radiation, which slowly starts
emitting electromagnetic radiation and expanding as deuterons/tritons
(maybe
some heavier nuclei, since a neutron rich environment would be conducive
to
adding on more and more neutrons?) build up, then expands and increases
in
electromagnetic luminosity when deutrium burning starts? And they would
be
much dimmer per unit mass than stars are in our universe, due to the
combination of less energy from fusion, and lots more energy being lost
to
neutrinos.

There is the complication that as the temperature (measured in kinetic
energy per degree of freedom) increases past the binding energy you
tend to get the back reaction occurring about as often as the forward
reaction, which absorbs heat. As a consequence, you will never raise
the temperature much above the binding energy. You would end up with
cold, degenerate spheres of heavy hydrogen and possibly helium after
the neutron cloud radiated away all of its energy via neutrinos. At
these degenerate densities, you could get a runaway fusion reaction,
resulting in a supernova as the entire degenerate star fuses
suddenly. As a comparison, in real life the temperature required for
fusion is about two orders of magnitude lower than the binding energy
of a deuteron, so we don't get this phenomenon.

Neat! A supernova involves expanding and increasing in luminosity, so I was
qualitatively right, at least.
So, after that happens you end up with a new nebula that's highly depleted
in hydrogen isotopes, and enriched in heavy elements.
Which ought to result in smaller, hotter second generation stars, right? (Or
maybe not, since fusing carbon and oxygen wouldn't be as energetic as it is
in our universe either....)

How would one go about figuring out the reaction rates, given binding
energy
/ mass ratios / whatever else is relevant? (And doesn't binding energy
have
something to do with the mass ratios?)

This gets complicated. You need to know not only the binding energy
but also the reaction barrier. Probably, what you need to do is run a
lattice QCD calculation on your configuration to get the answer. This
means you need access to a supercomputer.

Crap. I don't suppose there are any general trends one could use for
estimation?

-l.
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