APOD: The Origin of Elements (2020 Aug 09)

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neufer
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Re: APOD: The Origin of Elements (2020 Aug 09)

Post by neufer » Fri Oct 09, 2020 4:10 pm

Nvarius wrote:
Fri Oct 09, 2020 12:20 am

It appears that colliding neutron stars are not the source of the unexplained abundance of gold in our observable universe. Due to the complex and difficult molecular process required to produce gold, I am thinking that the main source of gold might be coming from black holes. Is this even a possibility?
No.
https://en.wikipedia.org/wiki/Gold wrote:
<<Traditionally, gold in the universe is thought to have formed by the r-process (rapid neutron capture) in supernova nucleosynthesis,[51] but more recently it has been suggested that gold and other elements heavier than iron may also be produced in quantity by the r-process in the collision of neutron stars. In both cases, satellite spectrometers at first only indirectly detected the resulting gold. However, in August 2017, the spectroscopic signatures of heavy elements, including gold, were observed by electromagnetic observatories in the GW170817 neutron star merger event, after gravitational wave detectors confirmed the event as a neutron star merger. Current astrophysical models suggest that this single neutron star merger event generated between 3 and 13 Earth masses of gold. This amount, along with estimations of the rate of occurrence of these neutron star merger events, suggests that such mergers may produce enough gold to account for most of the abundance of this element in the universe.

The r-process entails a succession of rapid neutron captures (hence the name) by one or more heavy seed nuclei, typically beginning with nuclei in the abundance peak centered on 56Fe. The captures must be rapid in the sense that the nuclei must not have time to undergo radioactive decay (typically via β− decay) before another neutron arrives to be captured. This sequence can continue up to the limit of stability of the increasingly neutron-rich nuclei (the neutron drip line) to physically retain neutrons as governed by the short range nuclear force. The r-process therefore must occur in locations where there exist a high density of free neutrons. Early studies theorized that 1024 free neutrons per cm3 would be required, for temperatures about 1GK, in order to match the waiting points, at which no more neutrons can be captured, with the atomic numbers of the abundance peaks for r-process nuclei. This amounts to almost a gram of free neutrons in every cubic centimeter, an astonishing number requiring extreme locations. Traditionally this suggested the material ejected from the reexpanded core of a core-collapse supernova, as part of supernova nucleosynthesis, or decompression of neutron-star matter thrown off by a binary neutron star merger. The relative contributions of these sources to the astrophysical abundance of r-process elements is a matter of ongoing research.

A limited r-process-like series of neutron captures occurs to a minor extent in thermonuclear weapon explosions. These led to the discovery of the elements einsteinium (element 99) and fermium (element 100) in nuclear weapon fallout.

The r-process contrasts with the s-process, the other predominant mechanism for the production of heavy elements, which is nucleosynthesis by means of slow captures of neutrons. The s-process primarily occurs within ordinary stars, particularly AGB stars, where the neutron flux is sufficient to cause neutron captures to recur every 10–100 years, much too slow for the r-process, which requires 100 captures per second. The s-process is secondary, meaning that it requires pre-existing heavy isotopes as seed nuclei to be converted into other heavy nuclei by a slow sequence of captures of free neutrons. The r-process scenarios create their own seed nuclei, so they might proceed in massive stars that contain no heavy seed nuclei. Taken together, the r- and s-processes account for almost the entire abundance of chemical elements heavier than iron. The historical challenge has been to locate physical settings appropriate for their time scales.>>
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Re: APOD: The Origin of Elements (2020 Aug 09)

Post by BDanielMayfield » Sat Oct 10, 2020 8:20 am

Nvarius wrote:
Fri Oct 09, 2020 12:20 am
It appears that colliding neutron stars are not the source of the unexplained abundance of gold in our observable universe.
I’ve been seeing news reports to that effect too. The suggestion is that neutron star mergers are too rare to have produced all the gold at its observed universal abundance.
Due to the complex and difficult molecular process required to produce gold, I am thinking that the main source of gold might be coming from black holes. Is this even a possibility?
Coming directly from black holes would be impossible (since nothing can escape from them), but fusion could be possible (supposition) inside a bh’s accretion disk. But for this to be main site for Au’s production it would then have to be ejected out the bh’s jets, which seems highly unlikely.

The team making the claim that NS mergers aren’t the main source for Au have also proposed that rapidly spinning very massive star core collapse SN could be the universe’s main Au producer.
Just as zero is not equal to infinity, everything coming from nothing is illogical.