Galactic 'gold mine' explains the origin of nature's heaviest elements

May 19, 2016, The Kavli Foundation
An artist's impression of two neutron stars colliding. Credit: Dana Berry / Skyworks Digital, Inc./The Kavli Foundation

The origin of many of the most precious elements on the periodic table, such as gold, silver and platinum, has perplexed scientists for more than six decades. Now a recent study has an answer, evocatively conveyed in the faint starlight from a distant dwarf galaxy.

In a roundtable discussion, published today, The Kavli Foundation spoke to two of the researchers behind the discovery about why the source of these , collectively called "r-process" , has been so hard to crack.

"Understanding how heavy, r-process elements are formed is one of hardest problems in nuclear physics," said Anna Frebel, assistant professor in the Department of Physics at the Massachusetts Institute of Technology (MIT) and also a member of the MIT Kavli Institute for Astrophysics and Space Research (MKI).

"The production of these really heavy elements takes so much energy that it's nearly impossible to make them experimentally," Frebel continued. "The process for making them just doesn't work on Earth. So we have had to use the stars and the objects in the cosmos as our lab."

The findings also demonstrate how determining the contents of stars can shed light on the history of the galaxy hosting them. Nicknamed "stellar archaeology," this approach is increasingly allowing astrophysicists to learn more about conditions in the early universe.

"I really think these findings have opened a new door for studying galaxy formation with individual stars and to some extent individual elements," said Frebel. "We are seriously connecting the really small scales of stars with the really big scales of galaxies."

In the late 1950s, nuclear physicists had worked out that extreme conditions somewhere in the cosmos, full of subatomic particles called neutrons, must serve as the forges for r-process elements, which also include familiar substances such as uranium and lead. The explosions of giant stars and the rare mergings of the densest stars in the universe, called , were the most plausible sources. But observational evidence was sorely lacking.

Researchers at the MKI have now filled this observational gap. An analysis of the starlight from several of the brightest stars in a tiny galaxy called Reticulum II, located some 100,000 light years from Earth, suggests these stars contain whopping amounts of r-process elements.

Since the stars could not have made the heavy elements on their own, some event in Reticulum II's past must have "seeded" and enriched the matter that grew into these stars. The abundances of elements in the stars squarely implicates the collision of two neutron stars.

Frebel's graduate student Alexander Ji discovered the enriched in Reticulum II while using the Magellan telescopes at the Las Campanas Observatory in Chile. He is first author on a paper about the findings, published March 31 in the journal Nature.

"When we read off the r-process content of that first star in our telescope, it just looked wrong, like it could not have come out of this galaxy!" said Ji, in the roundtable. "I spent a long time making sure the telescope was pointed at the right star."

Ji further commented on how the discovery helps to finally tell the tale of how r-process elements come to exist. "Definitely one of the things that I think attracts people to astronomy is understanding the origin of everything around us."

Enrico Ramirez-Ruiz, a professor of Astronomy and Astrophysics at the University of California, Santa Cruz, joined Ji and Frebel for the roundtable.

"I've been working on neutron star mergers for a while, so I was extremely excited to see Alex and Anna's results," said Ramirez-Ruiz, who was not involved in the research. "Their study is indeed a smoking gun that exotic neutron star mergers were occurring very early in the history of this particular dwarf galaxy, and for that matter likely in many other small galaxies. Neutron star mergers are therefore probably responsible for the bulk of the precious substances we call r-process elements throughout the universe."

Explore further: Tiny, ancient galaxy preserves record of catastrophic event

More information: Alexander P. Ji et al, R-process enrichment from a single event in an ancient dwarf galaxy, Nature (2016). DOI: 10.1038/nature17425

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16 comments

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kevin_s
4.6 / 5 (10) May 19, 2016
The article states "...in a tiny galaxy called Reticulum II, located some 100 light years from Earth...". 100 light years??? Very close to be a galaxy. I didn't realize a dwarf galaxy was that close especially given our galaxy is estimated to be 100,000 light years in diameter or .1% of the diameter of the Milky Way!
tekram
5 / 5 (12) May 19, 2016
The article states "...in a tiny galaxy called Reticulum II, located some 100 light years from Earth...". 100 light years??? Very close to be a galaxy. I didn't realize a dwarf galaxy was that close especially given our galaxy is estimated to be 100,000 light years in diameter or .1% of the diameter of the Milky Way!

"Reticulum II, located some 100 light years from Earth,"

Actual distance is 30 kpc or about 100,000 light years.
Da Schneib
5 / 5 (8) May 19, 2016
Looks like they fixed the article. Good catch, you two!
Mark Thomas
3.7 / 5 (3) May 19, 2016
"The abundances of elements in the stars squarely implicates the collision of two neutron stars."

It is not clear from the article why this had to be due to neutron star mergers and not supernovas. Furthermore, were there enough neutron star mergers to account for all the r-process elements we see?
ursiny33
1 / 5 (3) May 19, 2016
When neutron stars have a high velocity collision , its the same as neutrons having a high velocity collisions in a particle accelerator, it turns the neutrons in to their quantum particle foundations releasing electrical energy charges in the process of reverting back into the micro charges of quantum parts, in a neutron star collisions, gravity holds the quantum particles from escaping the area to build a quantum particle mass, not a black hole like you assume in your math equations,because your math doesn't take into account the structural limits of neutrons
Whydening Gyre
5 / 5 (7) May 19, 2016
"Reticulum II, located some 100 light years from Earth,"

Reticulum?!?
Hell, damn near killed 'em...
torbjorn_b_g_larsson
5 / 5 (8) May 19, 2016
@Mark: The sentence is taken squarely from the press release. [ https://www.scien...0931.htm ]

From the arxiv paper, it seems it is a two factor argument:

"The enhancement in this "r-process galaxy" is 2-3 orders of magnitude higher than what is seen in any other UFD11,14,15. This implies that a single rare event produced the r-process material in Reticulum II, whether or not gas accretion was significant in UFDs. The r-process yield is incompatible with ordinary core-collapse supernova yields16 but consistent with r-process production in neutron star mergers17."

[tbctd]
torbjorn_b_g_larsson
5 / 5 (8) May 19, 2016
[ctd]

Factor 1: "Furthermore, all nine other UFDs with neutron-capture abundances contain at least 100 times less neutron-capture material per star11,14,15. It is thus extremely likely that the neutron-capture material in Ret II was produced by just one event. If each UFD was equally likely to host an r-process event, then the probability that N r-process events occurred in Ret II but zero r-process events occurred in the other nine UFDs is (1/10)N . There is then only a ~1% chance that 2 or more events are responsible for the rprocess material in Ret II."

[tbctd]
torbjorn_b_g_larsson
5 / 5 (8) May 19, 2016
[ctd]

Factor 2: "A single CCSN is unable to explain the majority of r-process material in Ret II. Using Eu as the representative r-process element, we find most of the r-process stars in Ret II have [Eu/H] of -1 to -1.3, implying these stars formed in an environment where the Eu mass ratio MEu/MH was 10-10.3 to 10-10.6. Typical CCSN Eu yields are ~10-7.5 solar masses16, and in a UFD this material mixes into ~106 solar masses of hydrogen due to turbulent mixing during the assembly of the galaxy10. This results in MEu/MH ~ 10-13.5, three orders of magnitude below what is required to match the Eu abundances in Ret II (although consistent with every other UFD and our two non-rprocess stars)."

[ https://arxiv.org...1558.pdf ]

Enjoy (or not)!
torbjorn_b_g_larsson
5 / 5 (6) May 19, 2016
@ursiny: Who assumes black holes when it is neutrino star collisions that er considered?

Read the arxiv paper I linked to in a previous comment if you don't believe me!
Whydening Gyre
5 / 5 (4) May 19, 2016
Factor 1: "Furthermore, all nine other UFDs with neutron-capture abundances contain at least 100 times less neutron-capture material per star11,14,15. It is thus extremely likely that the neutron-capture material in Ret II was produced by just one event. If each UFD was equally likely to host an r-process event, then the probability that N r-process events occurred in Ret II but zero r-process events occurred in the other nine UFDs is (1/10)N . There is then only a ~1% chance that 2 or more events are responsible for the rprocess material in Ret II."

Since it is a small galxy, 1 event would have a proportionally larger representation. A Neutron merge might even be sufficient trigger for other contributing events...?
wduckss
1 / 5 (9) May 20, 2016
Children from kindergarten see that is dis article inaccurate. This is an elemental ignorance or lack of knowledge of basic evidence.
In the case of arrival of heavy elements of the universe on planets and satellites, the composition must be identical or is not. They're looking for evidence of 100,000 ly away and not see in front the nose. The only Earth has a diversity of (quantity) of elements (see Mars, nothing).
"Supernovae are not our creators"
http://www.svemir...per-Nova
Enthusiastic Fool
5 / 5 (5) May 20, 2016
Having read torb's comments: Could different starting conditions for the dwarf contribute? I thinking like a galaxy that forms later gets some late pop III stars so its hypernovas are more recent past. I guess Ill fumble my way through the paper to see how the metallicities/ages compare. Thanks Torb.
Enthusiastic Fool
5 / 5 (4) May 20, 2016
supernovae experiencing the magnetorotational instability have many desirable properties, synthesizing as much as ~10-5 solar masses of Eu on a timescale similar to CCSNe and at a rate more frequent than NSMs28. These properties are similar enough to NSMs that Ret II cannot yet distinguish between the two possibilities


I guess I need to look up neutron star mergers because it seems exceedingly rare in my mind. They go on to say despite having 2 possibilities local conditions point to NSM being more likely.
Mark Thomas
4.2 / 5 (5) May 20, 2016
torbjorn_b_g_larsson, thanks for your help.
ursiny33
4.5 / 5 (2) May 21, 2016
They might try measuring the heavy element contents of mini perimeter galaxies orbiting the outer edge of our closest neighbor galaxy, to so if the heavy element content differs in the dozens that orbit within our neighbor

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