AAS 236: Laboratory Astrophysics Prize Lecture Sheds Light On Stellar Explosive Processes and Nucleosynthesis Of Chemical Elements

Jim Truran of University of Chicago, winner of  2020 Laboratory Astrophysics prize for his body of cutting edge work over a lifetime

When Janice asked me what was the best presentation - after the solar ones described last week - I had to ruminate for some time over the 3 talks just described (after the solar ones) and then chirped up: "The  leading (11:00 a.m.) Tuesday session talk on Laboratory Astrophysics! Specifically, the application of nuclear physics lab results to Astrophysics!"  The talk was actually entitled:

  "Jim Truran's Contributions to Nuclear and Laboratory Astrophysics"
 
  I was excited because for years, hell decades, I'd been hammering on connections between laboratory experimental results (i.e. in spectroscopy, plasma physics, nuclear physics) which had direct application and connections to astrophysical processes.   For example, Jim Lawler's yeoman work (at Univ. of Wisconsin)  in laboratory atomic physics which has made major contributions to stellar spectroscopy, i.e. by measuring transition probabilities which have also enabled more precise determinations of stellar elemental abundances..

 Such hard core experimental connections gave the lie to books like two recent ones of author David Lindley (e.g. 'The End Of Physics' (1993), and now 'The Dream Universe') which have argued that modern physics has lost its way amidst a morass of obscure "mathematical elegance" which bears little relevance to the real world.

Anyway, this particular AAS  lecture refers to the 2020 Laboratory Astrophysics Division prize talk given by Jim Cowan in honor of LAD prize winner, Jim Truran.   This was for his work at the Univ. of Chicago using nuclear decay chronometers to determine the ages of stellar and terrestrial material.  Prof. Truran's primary work was in discovering neutron and  explosive  processes in stars, e.g.. connected to supernovae. This entailed examining the s-process in stars and the r-process in low metallicity stars. 

It is useful here to distinguish the two, noting first that because we are referring  to neutron collisions and capture, there is no Coulomb potential barrier involved.  This is because neutrons have no charge unlike protons, which would partake in the p-p fusion reaction,  and hence have to surmount the Coulomb barrier because of their positive charge. (I.e. manifest as a repulsive effect between two like charges trying to fuse.)

In essence, we are looking at reactions occurring at relatively lower temperatures (assuming free neutrons already present in the gas), wherein the reactions result in atomic nuclei that are either stable or unstable against the beta decay reaction, i.e.

   _Z X ^A+1  +  e ^- ®   _Z+1  X ^{A + 1}   +   u   +   g

If then the beta decay half life  (b T½ )  is short compared to the time scale for neutron capture, then the neutron capture reaction is said to be a "slow" or an  s-process.   Such reactions tend to yield stable nuclei - either directly or secondarily, i.e. via beta decay.   Conversely, if    (b T½ )  is long  compared to the time scale for neutron capture  the reaction is termed a "rapid"  or r-process reaction, yielding heavy, neutron rich nuclei.  (But also more unstable - which we will get to.)

Prof. Cowan began his presentation by laying out the scope of the problem, i.e.

 

How did a few simple elements at the beginning (hydrogen, helium, lithium)  ultimately end up delivering a periodic table's worth?   Well, the answer inhered in the processes that acted in tandem for stellar nucleosynthesis.  To this end, as Prof. Cowan explained, a prime goal became understanding the solar chemical composition, e.g.


Thus, even as far back as 1959, Cameron was able to estimate solar system abundances, which - as we inspect the graph above - disclose he wasn't too far off the results obtained by Lodder (using improved techniques) some 46 years later.  What both found is that the elements are mostly made in combination but some are forged in one single process or the other, that is, the s- or r-process.   

All of this is relevant as we note the role of the s-, and r-processes in the slide and their respective  manifestations as so-called abundance peaks, a number of which can be easily isolated.

Jim Truran and collaborators showed that the way to solve the problem of solar abundances of the elements was by way of isotopic deconvolution referenced to the s- and r-process, e.g.


 Here it was clear that the key elemental peaks grouped in series and were separated by process (red or blue graph line). For example, in the s-process (blue graph line) note how  the peaks for strontium (Sr), Barium (Ba) and lead (Pb), dominate. Meanwhile, the r-process discloses peaks for selenium (Se), Xenon (Xe) Tellurium (Te) as well as for the rare earth elements (REE) and a third peak for gold (Au), Platinum (Pt) and Osmium.  Jim Cowan pointed out that these abundance models first appeared ca. 1957 and were done without computers - since none existed for such work at the time.

Going back over my own notes from a stellar evolution course in 1970, I see reference to the s-, r-processes in a 1957 paper (famously called the "B2FH" paper after the authors surname first initials: Burbidge, Burbidge, Fowler and Hoyle) in which we learn the r-process is applicable to the production of isotopes in the  mass number range:  70 <  A  <  209.   A quick glance at the slide above shows this to be spot on.   However, the s-process of neutron capture is said to be applicable to the range:  23 <  A  <  46 only.  Even more relevant here, Truran made his own discovery of explosive stellar processes -leading to nucleosynthesis - independently of  B2FH.


Cowan also stressed that the connections between nuclear physics and astrophysics underscored the need for good facilities to pursue productive research one of  the fruits of which turned out to be the historic slide shown below:


 Here  there are several components to note:  first, the tiny black boxes to the left edge of the multi-colored  'stream' expanding to the right; second, a black line wending its way to the right of the minuscule boxes, and third  a red (magenta) line to the right of that.  The black mini-boxes define what we call the "valley of  b  stability".   To the right, the black solid line defines the s-process and elements with greater radioactivity.  The red line - further right - defines the r-process in which neutron capture occurs so quickly  that we have radioactive nuclei with half life less than  1 second.  

The B2FH paper refers to neutron capture time scales from 0.01 sec - 10 sec.   We now know, thanks to Jim Truran's contributions, the neutron capture times are much much less than the   b decay times,   or from 0.01 - 0.1 sec.    This is also why the site for the r-process has been very hard to identify for decades, while we have the s-process well identified in AGB (Red Giant) stars.

Basically, by 1957 - to make a long story short - we learned how the elements were made by both explosive and neutron capture processes.  What Jim Truran's work showed, especially tied to the Facility for Rare Isotropic Beams (FRIB) is that:


Challenges to our understanding both of the nature of stellar explosions and the synthesis of heavy elements are inextricably tied to uncertainties in the underlying nuclear physics.

The gradual success in surmounting these challenges -  and the uncertainties in the nuclear physics-   initially led to supernovae as the initial suspects for the r-process. For example, the material in the Crab  Nebula was originally considered as early as 1957. A number of variations were later considered, depicted in the slide below:



The problem was that most of these models couldn't explain what the laboratory results disclosed in terms of the element abundances.  Thus, they were unable to reproduce r-process material at the heaviest peaks, like platinum and gold.   The model that came closest was the last requiring neutron star and black hole mergers.  Jim Truran also pursued other models,  such as:  helium-rich regions regions of exploding supernova and  prompt explosions of low mass, Type II supernovae.  Truran worked on these for many decades, trying to see if supernovae in the He-rich regions could explain it.   

Ultimately, the best result was obtained via a neutron star collision model from Al  Cameron, who had been Jim Truran's Ph.D. thesis adviser.

This was also reinforced after the first gravitational wave detection from  a binary star collision 2 years ago.  In the case of Cameron's model we find a multi-stage process. First, at v ~ 0.25c  an EM spectra 'kilonova' -type event (a blue spectrum after 1 day)  indicating light r-process nuclei, elements like xenon, silver.  Then a week or so later - look to the right -  we get another (red) spectrum consistent with heavy r-process nuclei - things like gold and uranium.  So far this model presents the best for determining the site of the r-process.

As an ancillary benefit, Jim Truran's work on the periodic table,  starting a decade ago with the Lanthanides (lanthanum up through technetium) has resulted in a measurement of all their atomic properties,  which then enabled more  precise abundance determinations that can be applied to stellar astrophysics. I.e. the data could then be used to make more precise abundance determinations for these elements in the stars.

More recently, Jim has worked through the iron peak elements (from Scandium through zinc) As can be seen from the slide, the chief current atomic data enhancements have been made in: the neutral species (blue coded elements), the neutral and ion species (green coded elements), and the ionized species (orange-coded elements) while the yellow (e.g. calcium) is under study and from September its atomic properties have been measured.

Jim Truran has also made contributions to galactic chemical evolution as well as carried out important studies of the s-process in carbon-detonation models of Type Ia supernovae.  More recently, he was instrumental in the development of the FLASH simulation code and its application to thermonuclear supernovae.  It will be of intense interest in the coming years to see what further major contributions Jim makes to explosive stellar events as well as the atomic and nuclear physics driving them.

See Also:

Heavy Elements Problem May Finally Be Solved By Ne...

Detection Of Gravitational Waves From Colliding Bl...

Heavy Elements Problem May Finally Be Solved By New Gravitational Waves Detection From Colliding Neutron Stars

Diagram of laser interferometer such as employed by LIGO (from Wikipedia)

The news that a collision of two neutron stars has precipitated another detection of gravitational waves by LIGO (Laser Interferometer Gravitational Wave Observatory), has the world of astrophysics abuzz and with good reason. It confirms the import of the laser interferometer principle used in gravitational wave detection (see diagram 2) and biw in the setting of another type of collision- between two neutron stars. Recall the original report occurred in February, last year, based on detecting gravitational waves from two colliding black holes in late 2015.

LIGO’s  original discovery, accepted for publication in Physical Review Letters,  

Based on the data cited in the above link, the two black holes were each roughly 30 times the mass of the Sun. They evidently merged some 1.3 billion light years from Earth. The  gravitational waves themselves were generated in the final moments before the black holes merged. The signal was brief but definitive and we on Earth have now received it.

LIGO features 2 L-shaped detectors (one in Hanford, WA, the other in Livingston, LA)  made up of two perpendicular arms totaling 2.5 miles long. Then a laser beam is split and travels along both arms, bouncing off respective mirrors to return to the L-intersection. Normally, the beams are aligned so they balance each other out and hence there's nothing to detect. But if a gravitational wave is intercepted it creates a tiny mismatch which is what LIGO detects. (One of the authors of the paper has referred to it as a "chirp".  The effect of this chirp or ripple changes the arms' lengths by a tiny amount, and that change can be detected by lasers.)

To determine the specific location of the source multiple detectors are used, both to distinguish signals from other "noise" by confirming the signal is not of earthly origin, and also to determine direction by means of triangulation.  In such triangulations a third LIGO site is used, namely one near Pisa, Italy, with a 3 km interferometer. The technique uses the fact that the gravitational waves travel at the speed of light and will reach different detectors at different times depending on their source direction. Although the differences in arrival time may be just a few milliseconds, this is sufficient to identify the direction of the origin of the wave with considerable precision.

In 1915 Albert Einstein, in a remarkable achievement of theoretical physics, used an abstruse form of math known as tensor calculus to predict the existence of gravitational waves. This was by way of showing how gravitational distortions arise when mass or energy warp space-time.  The ground breaking field equation that relates these parameters was summarized:

G _mn   =  - ½ g _mn  G=  - 8 p T _mn   

Where the  T _mn    denotes the associated  “stress-energy” tensor which incorporates internal stresses, the density of matter and its component velocities (u, v, w or in some texts: u1, u2 and u3).  

Now, the collision of a pair of neutron stars has again brought gravitational waves front and center given it may finally  resolve how the heaviest elements originated. The collision itself occurred some 130 million years ago when dinosaurs still walked the Earth, but the signal was only detected on August 17, 2016 given light propagates at a finite speed: 300,000 km/s.

Recall that the current theory of stellar evolution postulates that elements are built up in the cores of heavy stars (> 10 solar masses)by nucleosynthesis a la successive fusion reactions.  A key transition point occurs after carbon is formed in the core, and reaches a critical density and temperature to detonate. The resulting deflagration, which includes the core separating from the exploding outer layers, turns the star into an instant nuclear factory. Nickel and iron are formed as well as lighter elements in the imploding shells including of: magnesium, sulphur, silicon, manganese, chromium and a host of lesser atomic weight elements- are evolved.

The problem is that no heavier elements than iron can be envisaged although some theoreticians have postulated they can arise in the context of supernova explositions.  In the latter case a stage is reached whereby differing  neutron fluences  arise. These fluences, if conditions are right,  can then produce the elements known as actinides such as uranium and thorium but no longer containing  the A=130 abundance peak.  This is believed to occur over seconds time scales via the r-(rapid  neutron capture) process.  But the only modern nearby supernova, 1987A, has not revealed r-process enhancements. Modern thinking is that the r-process yield may be ejected from some supernovae but swallowed up in others as part of the residual neutron star.

Note that the neutron star is an ultradense stellar remnant of a  core-collapsed by supernova. The neutron star is the smallest, densest  known to exist with diameter scales typically about 12 miles wide.  One  teaspoon of neutron star material would have a mass of about a billion tons. The core is a soup of pure neutrons, while the crust is smooth, solid and 10 billion times stronger than steel. Basically to achieve neutron star status protons and electrons are forced so closely together that they fuse, merge. e- + p +  ->   n.

So the current excitement is that the gravitational wave data from these colliding neutron stars may finally solve the heavier- than- iron elements origin problem.  Dave Reitze, executive director of LIGO, said in an interview with the Guardian: “What is amazing about this discovery is it is the first time we’ve got a full picture of one of the most violent, cataclysmic events in the universe. This is the most intense observational campaign there has ever been.”

The 100-second hum picked up by LIGO has told the story of how the two neutron stars to be, each slightly heavier than the Sun, approached their demise. Initially separated by 200 miles, they circled each other 30 times a second. As they whirled inwards, accelerating to 2,000 orbits each second, the signal rose in pitch like a train whistle or ambulance alarm approaching. Two seconds later, NASA’s Fermi space telescope picked up an intense burst of gamma rays, emitted as shockwaves propagating through jets of matter funneled out of the poles during the energetic impact of the collision.

  The chief problem here is the estimate of the source volume.  This is facilitated in the neutron star case given we have that the components are separated by less than 300 km and accelerating to 2,000 orbits each second. This necessitates a tiny volume and implies that a lot of mass-energy will be distorting the associated region of space-time, and in this case leading ultimately to a black hole from the collision.  This distortion itself is bound up with the stress-energy tensor T _mn.

Note that the most common parameter describing the amplitude  for a gravitational wave is a dimensionless "strain" h = 2 ∫∫ g' dt ². ... Thus h is twice the fractional change in displacement between two nearby masses due to the gravitational wave. 

According to Prof Andreas Freise, a LIGO project scientist at the University of Birmingham: 

“Neutron stars are at this sweet spot between a star and a black hole.  When two of them collide, we expect them to immediately collapse into a black hole, leaving behind a bit of dust and stuff.”

David Shoemaker, spokesman for the LIGO Scientific Collaboration, said: “It’s [probably] the first observation of a black hole being created where there was none before, which is pretty darn cool.”

The recent observations also herald a new era of rapid-response astronomy, in which transient and unexpected cosmic events can be observed in detail for the first time. When LIGO's software picked up a signal at 13:41  GMT on 17 August, Shoemaker was one of a small team at LIGO to be alerted by a ringtone on his phone reserved for when black holes or neutron stars collide.

Within an hour, the detection had been confirmed by Virgo, a European gravitational wave detector near Pisa, the source of the signal had been triangulated to a small patch of sky and a global alert was triggered.

Prof Stephen Smartt, of Queen’s University Belfast, had been leading a five-day observation run of supernovae on the New Technology Telescope at La Silla, Chile, when the news came in. Smartt’s team, and those on other telescopes, observed the faint new blob and measured its spectrum to assess the chemical composition. The blob was a fireball of radioactive heavy chemical elements, known as a kilonova, that had been blown out from the collision at one fifth of the speed of light shortly after the gamma ray burst.

As I pointed out earlier, previously astrophysicists had speculated that the sheer rapidity of neutron capture (via r-process) would be enough to force extra neutrons into the nuclei of atoms, forging heavy elements like uranium. .  Later the conjecture altered to considering the sheer mega-force of neutron collisions in violent events to forge heavy elements like gold and platinum  But until now this idea was purely theoretical.  After all, what kind of violent cosmic event would be needed to make it work?  In the words of Prof. Freise:

“People have been looking for that forever,”

Adding:

“This is the first real confirmation that heavy elements such as gold, platinum and uranium are either solely or predominantly produced in binary neutron star collisions. The wedding band on your finger or the gold watch you’re wearing was most likely produced a billion years ago by two neutron stars colliding. That’s pretty cool.”

Let's also reference that earlier this month, three U.S. astrophysicists (Rainer, Weiss,  Barry C. Barish and Kip S. Thorne)  who played a crucial role in the development of LIGO were awarded the Nobel prize in physics for the first detection of gravitational waves. Shoemaker pointed out that two of the new laureates – and others – had been working on the project long before it captured the world’s attention.  As he observed:

“This kind of thing doesn’t happen because there are suddenly neat instruments. It’s decades of work and people working together in a collaborative way. It’s quite phenomenal.”

Indeed it is. For those interested the findings were published on Monday in a series of papers in journals including Science, Nature and Physics Review Letters.  For an abstract of the work appearing in the latter, see:

https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.119.161101