Deuterons used up before reaching the A=5 and 8 bottlenecks?

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Discussion Overview

The discussion revolves around the concept of bottlenecks in nucleosynthesis, specifically focusing on mass numbers A=5 and A=8. Participants explore the implications of these bottlenecks in the context of stellar processes and the fusion of deuterons and lithium.

Discussion Character

  • Exploratory, Technical explanation, Debate/contested

Main Points Raised

  • Some participants note that A=5 and A=8 are described as bottlenecks due to the absence of stable nuclei with these mass numbers.
  • One participant suggests that fusing 7Li and 2H could bypass the A=8 bottleneck, while questioning whether this reaction occurs due to the depletion of 2H before significant amounts of 7Li are produced.
  • Another participant argues that neither deuterium nor lithium accumulates significantly as they react with protons or helium shortly after their formation.
  • It is mentioned that all deuterium is consumed early in a star's life, which may affect the availability of deuterons for later reactions.
  • One participant states that deuterium fuels stars in the brown dwarf mass range, implying it is largely consumed before hydrogen fusion begins.
  • A participant recalls that the significance of the A=5 and A=8 bottlenecks may differ between big bang nucleosynthesis and later stellar burning processes.
  • Another participant highlights the importance of the A=8 bottleneck for progressing past lithium, mentioning reactions involving helium in stellar interiors.

Areas of Agreement / Disagreement

Participants express a general agreement that deuterium and lithium do not coexist in significant amounts during nucleosynthesis, but there is no consensus on the specifics of how the A=5 and A=8 bottlenecks are navigated or their relative importance in different nucleosynthesis contexts.

Contextual Notes

Some limitations include the dependence on specific stellar conditions and the unresolved nature of the reactions involving deuterons and lithium in various stages of nucleosynthesis.

Who May Find This Useful

This discussion may be of interest to those studying nucleosynthesis, stellar evolution, and the processes involved in the formation of elements in astrophysical contexts.

bcrowell
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I've seen A=5 and 8 described as bottlenecks in nucleosynthesis, because there are no stable nuclei with these mass numbers. One of my students suggested that you could, e.g., fuse 7Li+2H to produce 9Be, thereby bypassing the A=8 bottleneck. I've read that A=8 is actually surmounted by doing 4He+4He->8Be, then 8Be+4He (during the extremely short time that the 8Be holds together). Am I correct in thinking that 7Li+2H doesn't happen because by the time a star has built up an appreciable amount of 7Li, it has already used up all its 2H? What about the A=5 bottleneck?
 
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I'm no expert, but I believe that neither deuterium nor lithium are significant reaction products at the same time as the other, and neither build up to significant amounts because they react with protons or helium soon after production.

As for the A=5 bottleneck, well it too seems to be surpassed by the triple alpha process as you described in your post.
 
Yes, all the D is burned early in the star's life.
 
Deuterium is believed to fuel stars in the brown dwarf sized mass range, so it appears highly probable most of it is consumed by the time a protostar approaches the mass required to initiate hydrogen fusion.
 
I vaguely remember that those two bottlenecks are more important during big bang nucleosynthesis than during later stellar burning.

Jim Graber
 
The A = 8 bottleneck is important for getting past Li. The 3 He4 reaction (2 + 1 or all 3) takes place in the interior of stars.
 
Thanks, all, that's very helpful.
 

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