Chemical Elements produced inside the Sun

[alantheastronomer]

The R-process seems to be an exclusive result of a kilonova, the merger of two neutron stars.

Yes it's true that the R-process elements observed in this event closely matched the predicted ratios - except for the higher mass end. Since we don't yet have a working model of a supernova explosion, it's impossible to estimate the R-process yields of such an event. It's worth noting that the frequency of supernova explosions is approximately one to two per century per galaxy while, as you have noted, the timescale for the merger of binary neutron stars is at least hundreds of thousands of years.

 

[eachus]

Yes it's true that the R-process elements observed in this event closely matched the predicted ratios - except for the higher mass end. Since we don't yet have a working model of a supernova explosion, it's impossible to estimate the R-process yields of such an event. It's worth noting that the frequency of supernova explosions is approximately one to two per century per galaxy while, as you have noted, the timescale for the merger of binary neutron stars is at least hundreds of thousands of years.

Good point. If some supernovas form degenerate cores even seconds before the explosion, and don't form a white dwarf, neutron star or black hole, they will probably form, and release into the interstellar medium some R-process elements. I thought I left enough wiggle room for that. However, I suspect that most of these elements if formed will get locked away inside whatever stellar remnant is formed. Looking at the spectra of white dwarfs would be the best way to look for this. (But white dwarf production is probably the signature of the mildest supernovas.) The data for white dwarf spectra that I have looked at pretty much has iron and nickel as the heaviest elements.

Hmm. That seems to say that at some point, in heavy stars core burning migrates outward. Think of Hydrogen or Helium burning when the core is nearly depleted. The burning would migrate to where the fuel was, which can happen much faster than atoms can move in the core. When burning produces iron the burning is still moving outward. If the core then cools, by neutrino emission if no other way, it will collapse and boom! The signature of iron production outside the core, would be iron in the outer layers of recently produced white dwarfs. I'm retired, and modelling that looks like a big job with a supercomputer needed for the calculations. So if anyone wants to take that idea and run with it? Feel free.

 

[alantheastronomer]

The signature of iron production outside the core, would be iron in the outer layers of recently produced white dwarfs.

You seem to be confusing white dwarfs with neutron stars. Neutron stars are the end products of supernova explosions, while white dwarfs are the cores of lower mass stars as they turn into red giant stars and ultimately planetary nebulae. To find a more thorough description look up "Stellar Evolution" and "Supernovae" in Wikipedia.

I suspect that most of these elements if formed will get locked away inside whatever stellar remnant is formed.

There is still nucleosynthesis going on in the layers of material outside the core, and r-process elements could conceivably be produced there during the explosion, if we only knew what that mechanism was.

 

[rootone]

I find it curious that the emissions of supernovae and similar contain so much heavy elements.
Obviously they do, otherwise rocky planets like Earth could never form.
Intuitively one would think that most of the heavy elements produced end up within the remnant of a core.
Typically that is a white dwarf.or a neutron star.

 

[eachus]

You seem to be confusing white dwarfs with neutron stars. Neutron stars are the end products of supernova explosions, while white dwarfs are the cores of lower mass stars as they turn into red giant stars and ultimately planetary nebulae.

I'm not confused. I'm trying to show that even at their hottest, white dwarfs do not produce significant amounts of iron. In the hottest white dwarfs helium burns to carbon and oxygen. Forming higher weight atoms by adding additional alpha particles doesn't occur, due to the higher and higher temperatures required. However, it is possible that higher weight atoms (up to Ni56 (which decays to Fe56.) could be produced by the s-process but the lack of heavier metals, especially iron, says that there are not enough neutrons floating around to produce them. (Some white dwarfs do have traces of elements up to silicon, but it is no clear if they are collected from nearby supernova or interstellar gas.

 

[alantheastronomer]

I'm not confused. I'm trying to show that even at their hottest, white dwarfs do not produce significant amounts of iron.

Oh, I see what you're saying! Sorry, never mind.

I suspect that most of these elements if formed will get locked away inside whatever stellar remnant is formed.

Intuitively one would think that most of the heavy elements produced end up within the remnant of a core.

In high mass stars that have formed iron cores that are ready to collapse, there is still shell burning of elements leading up to iron outside of the core. It is the mixing of these layers, and the rapid expulsion of neutrons from the surface of the newly formed neutron star by whatever mechanism that causes the supernova explosion, that presumably produces the rest of the heavier elements.