Actinides in the Solar System Probably Were Formed by a Neutron Star Collision.

[NNadir]

The paper I'll discuss in this brief post is this one: A nearby neutron-star merger explains the actinide abundances in the early Solar System (Imre Bartos & Szabolcs Marka, Nature 569, 85–88 (2019) )

Although only two actinide elements occur naturally on Earth in significant quantities, thorium and uranium, it is well known from the examination of meteorites that the early solar system contained both plutonium and curium isotopes and their decay daughters. (cf. Origin of uranium isotope variations in early solar nebula condensates. (Cf Origin of uranium isotope variations in early solar nebula condensates (François L. H. Tissot*, Nicolas Dauphas and Lawrence Grossman, Science Advances 04 Mar 2016: Vol. 2, no. 3, e1501400) (Open sourced.)

A few residual atoms of Pu-244 have been discovered at the Mountain Pass Lanthanide Ore Mine in California. Detection of Plutonium-244 in Nature (Darlene Hoffman et al Nature 234, 132–134 (1971) ) Except for these few atoms, all of the Pu-244 at the time of formation of the Earth has decayed into Thorium-232. It is believed that a fair fraction of the U-235, the isotope that was the key to unlocking nuclear energy for use, represents decayed Curium-247.

These heavy elements are created in the "r-process" which involves the rapid capture of neutrons in a massive neutron flux in a stellar event, such as a supernovae.

According to the paper linked at the opening, these elements in our solar system did not originate in supernovae but can rather be traced to a single event, collision of two neutron stars.

From the abstract:

A growing body of evidence indicates that binary neutron-star mergers are the primary origin of heavy elements produced exclusively through rapid neutron capture1,2,3,4 (the ‘r-process’). As neutron-star mergers occur infrequently, their deposition of radioactive isotopes into the pre-solar nebula could have been dominated by a few nearby events. Although short-lived r-process isotopes—with half-lives shorter than 100 million years—are no longer present in the Solar System, their abundances in the early Solar System are known because their daughter products were preserved in high-temperature condensates found in meteorites5. Here we report that abundances of short-lived r-process isotopes in the early Solar System point to their origin in neutron-star mergers, and indicate substantial deposition by a single nearby merger event. By comparing numerical simulations with the early Solar System abundance ratios of actinides produced exclusively through the r-process, we constrain the rate of occurrence of their Galactic production sites to within about 1?100 per million years. This is consistent with observational estimates of neutron-star merger rates6,7,8, but rules out supernovae and stellar sources.

From the introduction:

As neutron-star mergers are rare—they only occur a few times per million years (Myr) in the Milky Way9—their production of heavy r-process elements is infrequent. This results in the strong temporal variation of the abundance of r-process elements in the interstellar medium. Other plausible r-process sources, mainly supernovae and massive stars, are orders of magnitude more frequent in the Milky Way, such that r-process production by them can be approximated as being uniform in time5,10 (the ‘uniform production’ model)...

...We used short-lived r-process isotopes to constrain the Galactic rate of occurrence (hereafter ‘rate’) of r-process production sites. These elements encode the short-term history of their production, making them a sensitive indicator of the rate of their source. Several such elements have measured abundances in the early Solar System5. These include two actinides, 247Cm (with half-life t1/2 = 15.6 Myr) and 244Pu (t1/2 = 80.8 Myr), and 129I (t1/2 = 15.7 Myr). For comparison we additionally examined 235U (t1/2 = 703.8 Myr), a radioactive actinide with longer half-life and known abundance in the early Solar System.

The authors used a computer simulation to show that the likely formation of these elements derived from the collision of two neutron stars in our local area.

We used our simulations to derive the probability distribution of (N247Cm/N244Pu)ESS for different Rmerger values. We computed the density of actinides in the interstellar medium at the location of the pre-solar nebula, about 8.3 kpc from the Galactic Centre, for a range of Galactic ages from 8,500 to 9,500 Myr. Figure 2 shows the simulated interstellar-medium abundance ratio (N247Cm/N244Pu)ISM,sim as a function of time for one Monte Carlo realization, for two different merger rates. For the higher rate shown, Rmerger = 500 Myr?1, we see that (N247Cm/N244Pu)ISM,sim is distributed closer to the abundance ratio predicted by the uniform production model than is the curve we calculate for the lower merger rate, Rmerger = 20 Myr?1 (see below). This is expected, because the uniform production model is approximately the same as an infinite source rate. This higher rate of Rmerger = 500 Myr?1, while still much lower than the Galactic core-collapse supernova rate15 of about 3 × 104 Myr?1, is inconsistent with the early Solar System abundance ratio.

Some pictures from the text:



The caption:

When neutron stars merge, they create an accreting black hole (the accretion disk is shown red). Tidal (dynamical) forces and winds from the accretion disk eject neutron-rich matter. This ejected matter (ejecta, shown grey) undergoes rapid neutron capture, producing heavy r-process elements, including actinides. The ejecta reach the pre-solar nebula and inject the heavy elements that will remain in the Solar System.

The caption:

Values are shown as functions of time measured from the formation of the Milky Way. a, The abundance ratio (N247Cm/N244Pu)ISM,sim in the interstellar medium near the pre-solar nebula for Galactic merger rates of 20 Myr?1 (blue line) and 500 Myr?1 (grey dashed line), for the early Solar System abundance ratio (N247Cm/N244Pu)ESS (ESS; red line) and for the abundance ratio predicted by the uniform production model (UP; green line), the last two both shown with 1? error regions. Also shown is the 90% confidence interval of (N247Cm/N244Pu)ISM,sim for Rmerger = 20 Myr?1 (blue shading). b, Distance of the merger with the greatest contribution of curium to the early Solar System (rdom), assuming Rmerger = 20 Myr?1. c, Fraction of curium in the early Solar System from the single dominant source, assuming Rmerger = 20 Myr?1.

The caption:

a–d, As for Fig. 2a but for abundance ratios (N247Cm/N232Th)ISM,sim (a), (N129I/N127I)ISM,sim (b), (N244Pu/N232Th)ISM,sim (c) and (N235U/N238U)ISM,sim (d).

Some conclusions:

Our computations show that the dominant source contributed a substantial part of (N247Cm)ESS. The single merger with the highest contribution deposited fdom,Cm = 70% ± 20% of the 247Cm. Because all other isotopes of curium have a much shorter half-life than 247Cm, this means that 70% ± 20% of all curium in the early Solar System was produced by a single neutron-star merger.

We carried out the same calculation for 244Pu. Owing to its longer half-life, the early Solar System abundance (N244Pu)ESS is dominated to a smaller extent by a single event than (N247Cm)ESS. We found that a single source deposited fdom,Pu = 40% ± 15% of the 244Pu. Because all other isotopes of plutonium have a much shorter half-life than 244Pu, this means that 40% ± 15% of all plutonium in the early Solar System was produced by a single neutron-star merger. Interestingly, the dominant source of plutonium is not always the same merger as the dominant curium source.

The authors claim that the total amount of r-process matter in the solar system is roughly 10^21 kg.

The neutron star collision is believed to have taken place about 80 million years before the solar system formed, roughly the half life of Pu-244.

Interesting.

I have personally always assumed that the origin of the actinides in our solar system arose from supernova, but this paper puts a new spin on things.

Happy Friday.

[Ponietz]

1. Interesting

I’m no scientist and can’t sleep, thus the post.
Can you help me with ‘local area’ to imagine the distances involved?
Also, would the black hole have dissipated by now or could it constitute the center of a neighboring galaxy? Any particular culprits suspected or are there just too many to count? Thanks.

[rickford66]

2. Well, that clears that up.

Thanks

[Ponietz]

3. NNadir does not deign to respond to the likes of me

What’s the point of posting it without discussion with us mere mortals?

[NNadir]

4. Trust me, I'm merely mortal. However, like you, I was suffering a bout of mortal insomnia...

...last night which is how the post came about. Happily for me, I went to sleep shortly after clicking "post." I would say that 90% of my posts generate no comment, which is fine, but sometimes I sit around to see if there's a comment, although last night wasn't it.

I found it interesting, since it kind of flew in the face of my expectations about the origins of actinides, an area of chemistry in which I have considerable interest.

I apologize for not getting back to you immediately, but I have a life outside of DU, believe it or not, a job, and those sorts of things, so I can't always respond immediately.

To answer your excellent questions in your post:

The paper reports that the authors believe that the neutron star collision was about 300 parsecs from the pre-solar nebula. A parsec is about 3.6 light years. It was therefore about 1000 light years away, a considerable distance. However such ejecta might be expected to be traveling at relativistic speeds.

However, as the event took place over 9 billion years ago, and the galaxy is rotating, it is unlikely that the residue of the neutron star collision, if it actually happened, is anywhere near us. It is unlikely that it is in another galaxy; the authors describe the event in terms of intragalactic events, and in fact, rely on the estimated frequency of neutron star collisions, which they contend is lower than supernova type events.

If it, the residue, is a black hole, it's fate is unknown. Only recently, using sophisticated processing did we actually get a "picture" of a black hole, although it was, by definition, not a picture per se, but a picture of its effects.

I'm not sure anyone knows how many black holes there are; perhaps someone does, but I'm unaware of that result.

Not all black holes are at the centers of galaxies, although it is believed that most or all galaxies feature one at their cores.

I hope this answers your questions.

Have a nice weekend.

[Ponietz]

5. Thanks, sincerely.

Recognizing my teachers is a passion with me. And please consider my comments to be just the banter of a student waiting for the teacher to arrive.

[NNadir]

9. You're quite welcome. Thank you for asking the questions. n/t

[Ponietz]

10. Okay...

A neutron star collision sounds like a very rare event. I was wondering if the Acinide material is just one more gargantuan fluke that contributed to the vast improbability that is earth. Sounds like we could have done nicely without the stuff.

[NNadir]

11. Without actinides, the Earth would not have land masses.

The internal heat of this planet is largely generated by the decay of uranium and, to a lesser extent, the decay of thorium and radioactive potassium.

Lord Kelvin calculated in 1892, a few years before Bequerel's discovery of radioactivity, that the Earth could not be any more than 100 million years old since it exhibited internal heat and that had to have come from the energy of accretion.

Without internal heat from radioactive decay there would be no plate tectonics, since there would be no energy to drive them. Most likely, given its water content, the Earth might have evolved as a planet covered by shallow water seas, or simply a rock with lots of icy hydrates buried in its rocks.

It's not clear that life would have evolved, since the outgassing of gases, including water, is believed to have resulted from volcanic eruptions. It might have been possible to have seas if comets collided with the earth and melted, but without some carbon dioxide, the planet would be much colder than it is.

It is now known that because of higher concentrations of thermally fissionable U-235 (which has a shorter half life than U-238) in Earth's past that natural nuclear reactors operated for some time, notably at Oklo in Gabon, but suspected in other places. Events like these probably caused significant out gassing.

As for probability:

One of the fun things that one learns if one takes a course involving statistical mechanics is exactly how improbable the arrangement, including location and velocity, of air molecules in any room at any time at any particular moment is. The probability is less than 1 in the number of seconds in all of time.

Nevertheless, the molecules in the room have this arrangement.

If we include the dimensions of space and time, and assume without any justification that time itself is cyclical and periodic, everything that is becomes inevitable, because time is infinite, and improbability no longer means anything. These ideas, with varying levels of sophistication, are at the core of multiple universe theories, which are, of course, not accessible to experiment and thus must remain speculative.

There are also quasi-scientific philosophy efforts involving the role of the observer in the universe's existence, the so called "Anthropic Principle."

I read a fun book on the topic many years ago, by Frank Tippler and John Barrow, with a forward by none other than John Wheeler.

The Anthropic Cosmological Principle

I don't necessarily buy into any of it, but it was a fun idea, and I rather liked the description of the "Weak Anthropic Principle" which states that we see the universe that we see because it is a kind of universe that could support our existence. A universe filled with only colliding neutron stars would not support our existence, since chemical bonds break in high energy radiation fields.

In any case, unfortunately, I cannot post the full paper on line, but the issue of the frequency of neutron star collisions plays a huge role in the author's calculations.

If one is truly interested, and can manage to find an academic library with open access - I have two in my area - one can read the full paper. I am very lucky to have access to these libraries, but I know that the majority of my fellow citizens in this area do not use these fabulous resources or even care that they're there.

[Ponietz]

12. My jaw drops.

Looking forward to further consideration of these ideas.
Thank you

[Igel]

6. Yeah, pardon my paranoia.

First thought through my petty little mind: "So, a black hole was formed as the material that participated in the molecular cloud formed, later producing our protoplanetary disk--and that probably not billions, but more like 10s or 100s of millions of years later. Okay. Granted that we have sibling stars that aren't exactly close, still ... Where papa Black Hole and how far away have we moved from it? Or is it going to be like Khronos, devouring its children? And might not the non-proximity of our siblings have to do with, oh, perhaps a large-ish mass that we can't quite pin down?"

Although two neutron stars would put the maximum mass at 6 solar masses, and with matter-energy conversion rates and the material expelled by the merger, it's likely to be well under 6 solar masses. Making it both not a huge threat for size and a niggling little sinister threat because of size. Not going to make a mess of things because it's on the small side, likely to be insidious because its small size makes it hard to find. Gravitational lensing around it wouldn't be great if it's within a reason number number of parsecs and it's not going to be easily seen by occultation unless we're looking for it.

Then I think of moderately unexplained perturbation effects on some KB objects and I start getting anxious. Makes me want to go to arxiv.org and rummage in a kind of black-hole induced paranoiac delirium.

[John ONeill]

7. Black holes

There's a hell of lot more stars than black holes, and probably more large wandering objects than stars. Since a collision with any of them could seriously ruin your day, it's probably better to worry about the more likely galactic visitor, and in proportion to that likelihood. In other words, not at all.

[NNadir]

8. The radial velocity of our sun, with respect to the galactic center is...

...reportedly on the order of 13 meters/second.

RADIAL VELOCITIES FOR 889 LATE-TYPE STARS (David L. Nidever,2 Geoffrey W. Marcy,2,3 R. Paul Butler,4 Debra A. Fischer,3 and Steven S. Vogt5, The Astrophysical Journal Supplement Series, 141:503–522, 2002 August).

This works out to roughly 410,250 km/year. This means that over a period of 4.5 billion years, assuming constant it has traveled 1.8 quadrillion km.

The black hole in question was not, if this paper is accurate, near the solar system; it was roughly 1000 light years away. But consider the case where they started out close, but their trajectories were 1 degree different.

In a linear case, using the sine function, their distance apart would be 32 trillion km by now.

You and I will become dead people at some unspecified future point. I am willing to guarantee that an astronomical event will not kill either of us.

As a practical matter, the greatest threat to our planet is climate change. Although the engineering of addressing that vastly higher risk is a prodigious task of a magnitude that I don't think people really appreciate, I personally it is on the edge of feasible to address that risk.

If we are killed by a black hole, a gamma burst, a supernova, or any such thing, by way of contrast, there is nothing we can do about it, just as there is nothing we can do to address our mortality.

Speaking only for myself, I am glad I'm mortal. It makes life more precious.

Have a nice weekend.