Heavy metal stars
Posted: August 30, 2001

Very high abundances of the heavy element Lead have been discovered in three distant stars in the Milky Way Galaxy. This finding strongly supports the long-held view that roughly half of the stable elements heavier than Iron are produced in common stars during a phase towards the end of their life when they burn their Helium - the other half results from supernova explosions. All the Lead contained in each of the three stars weighs about as much as our Moon.

The observations show that these "Lead stars" - all members of binary stellar systems - have been more enriched with Lead than with any other chemical element heavier than Iron. This new result is in excellent agreement with predictions by current stellar models about the build-up of heavy elements in stellar interiors.

The new observations are reported by a team of Belgian and French astronomers who used the Coude Echelle Spectrometer on the ESO 3.6-m telescope at the La Silla Observatory (Chile).

The build-up of heavy elements
Astronomers and physicists denote the build-up of heavier elements from lighter ones as "nucleosynthesis".

Only the very lightest elements (Hydrogen, Helium and Lithium) were created at the time of the Big Bang and therefore present in the early universe.

All the other heavier elements we now see around us were produced at a later time by nucleosynthesis inside stars. In those "element factories", nuclei of the lighter elements are smashed together whereby they become the nuclei of heavier ones - this process is known as nuclear fusion. In our Sun and similar stars, Hydrogen is being fused into Helium. At some stage, Helium is fused into Carbon, then Oxygen, etc.

The fusion process requires positively charged nuclei to move very close to each other before they can unite. But with increasing atomic mass and hence, increasing positive charge of the nuclei, the electric repulsion between the nuclei becomes stronger and stronger.

In fact, the fusion process only works up to a certain mass limit, corresponding to the element Iron. All elements that are heavier than Iron cannot be produced via this path.

But then, how were those heavy elements we now find on the Earth produced in the first place? From where comes the Zirconium in artificial diamonds, the Barium that colours fireworks, the Tungsten in the filaments in electric bulbs? Which process made the Lead in your car battery?

Beyond iron
The production of elements heavier than Iron takes place by adding neutrons to the atomic nuclei. These neutral particles do not feel any electrical repulsion from the charged nuclei. They can therefore easily approach them and thereby create heavier nuclei. This is indeed the way the heaviest chemical elements are built up.

There are actually two different stellar environments where this process of "neutron capture" can happen.

One place where this process occurs is inside very massive stars when they explode as supernovae. In such a dramatic event, the build-up proceeds very rapidly, via the so-called "r-process" ("r" for rapid).

The AGB stars
But not all heavy elements are created in such an explosive way.

A second possibility follows a more "peaceful" road. It takes place in rather normal stars, when they burn their Helium towards the end of their lives. In the so-called "s-process" ("s" for slow), heavier elements are then produced by a rather gentle addition of neutral neutrons to atomic nuclei.

In fact, roughly half of all the elements heavier than Iron are believed to be synthesized by this process during the late evolutionary phases of stars.

This process takes place during a specific stage of stellar evolution, known as the "AGB" phase. It occurs just before an old star expels its gaseous envelope into the surrounding interstellar space and sometime thereafter dies as a burnt-out, dim "white dwarf".

Stars with masses between 0.8 and 8 times that of the Sun are believed to evolve to AGB-stars and to end their lives in this particular way. At the same time, they produce beautiful nebulae like the "Dumbbell Nebula". Our Sun will also end its active life this way, probably some 7 billion years from now.

Low-metallicity stars
The detailed understanding of the "s-process" and, in particular, where it takes place inside an AGB-star, has been an area of active research for many years. Current state-of-the-art computer-based stellar models predict that the s-process should be particularly efficient in stars with a comparatively low content of metals ("metal-poor" or "low-metallicity" stars).

In such stars - which were born at an early epoch in our Galaxy and are therefore quite old - the "s-process" is expected to effectively produce atomic nuclei all the way up to the most heavy, stable ones, like Lead (atomic number 82) and Bismuth (atomic number 83) - since more neutrons are available per Iron-seed nucleus when there are fewer such nuclei (as compared to the solar composition). Once these elements have been produced, the addition of more s-process neutrons to those nuclei will only produce unstable elements that decay back to Lead. Hence, when the s-process is sufficiently efficient, atomic nuclei with atomic numbers around 82, that is, the Lead region, just continue to pile up.

As a result, when compared to stars with "normal" abundances of the metals (like our Sun), those low-metallicity stars should thus exhibit a significant "over-abundance" of those very heavy elements with respect to Iron, in particular of Lead.

Looking for Lead
Direct observational support for this theoretical prediction would be the discovery of some low-metallicity stars with a high abundance of Lead. At the same time, the measured amounts of all the heavy elements and their relative abundances would provide very valuable information and strongly reinforce our current understanding of heavy element nucleosynthesis.

But detecting the element Lead is not easy - the expected spectral lines of Lead in stellar spectra are relatively weak, and they are blended with many nearby absorption lines of other elements.

Moreover, bona-fide, low-metallicity AGB stars appear to be extremely rare in the solar neighborhood.

But if the necessary observations are so difficult, how is it then possible to probe nucleosynthesis in low-metallicity AGB stars?

CH-stars in binary systems
In a determined effort in this direction, a team of Belgian and French astronomers decided to try to detect the presence of Lead in some "CH-stars" that are located about 1600 light-years away, high above the main plane of our Milky Way Galaxy.

Over-abundance of some heavy elements has been observed in some "CH-stars". But CH-stars are not very luminous and have not yet evolved to the AGB phase. Hence they are totally unable to produce heavy elements. So how can there be heavy elements in the CH-stars?

This mystery was solved when it was realized that the CH-stars all belong to binary systems and that they therefore have a companion star. That companion is now a white dwarf star and was therefore at some earlier moment an AGB star!

During its AGB-phase, the companion star expelled much of its material, eventually producing the "planetary nebula" phenomenon, referred to above. In this process, a lot of its material, enriched with heavy elements produced by the "s-process" during the AGB phase, was deposited in the atmosphere of the CH-star that is now observed. The former AGB-star, now a slowly cooling, dim white-dwarf star, still orbits the CH-star.

For this reason, the atmospheric composition of a CH-star actually carries the signature of the nucleosynthesis that took place deep inside the companion AGB star at an earlier epoch. Spectroscopic observations of CH-stars thus provide the opportunity to probe the predicted s-process in low-metallicity stars.

Three stars with Lead
A necessary condition for these observations to succeed is a very high spectral resolution in order to detect the spectral line of Lead (Pb), in particular to "resolve" it among the many absorption lines from other elements, present in the stellar spectrum in this wavelength region. Moreover, a fairly large telescope is needed as the stars to be observed are relatively rare, hence distant and faint for this kind of demanding observations.

The Belgian and French astronomers decided to use the Coude Echelle Spectrometer (CES) at the ESO 3.6-m telescope on La Silla, a telescope/instrument combination offering some hope of success for these difficult observations. Spectra of three southern stars, HD 187861, HD 196944 and HD 224959, were obtained during two nights in September 2000 and found to be of excellent quality.

HD 196944
One of the three Lead stars, HD 196944 that was analyzed in the present research programme (at the center of the field). This star lies about 1600 light years away in the constellation Aquarius. At magnitude 9, it is not visible to the unaided eye, but easily seen through a small amateur telescope. Photo: UK SERC/PPARC (Particle Physics and Astronomy Research Council, formerly Science and Engineering Research Council), the Anglo-Australian Telescope Board and the Association of Universities for Research in Astronomy (AURA)
The scientists were very pleased to find that the Lead absorption line was clearly present and very strong in the spectra of all three stars. A subsequent, detailed analysis demonstrated that the three stars all have a substantial overabundance of Lead. Moreover, from the measured abundances of other elements in these spectra, it is also clear that this Lead has been formed in the s-process. The astronomers were able to prove that the Lead cannot originate from the competing "r-process" that occurs in other environments like supernova explosions.

"This is the first detection of a Lead-star", explains Sophie Van Eck from the Institut d'Astronomie et d'Astrophysique of the Université Libre de Bruxelles (Belgium). "These stars are almost exclusively enriched with Lead. Moreover, the abundances in all three stars show a remarkable similarity."

How does the s-process operate?
The high abundance of Lead in these otherwise low-metallicity stars also provides detailed clues on how the s-process operates inside the AGB stars. When a Carbon-13 nucleus (i.e. a nucleus with 6 protons and 7 neutrons) is hit by a Helium-4 nucleus (2 protons and 2 neutrons), they fuse to form Oxygen-16 (8 protons and 8 neutrons). In this process - as can be seen by adding the numbers - one neutron is released. It is exactly these surplus neutrons that become the building-blocks for making heavier elements via the s-process.

Hence the true source of the required neutrons is the Carbon-13 isotope, which is in turn produced by fusion of normal carbon (Carbon-12) and protons, i.e. hydrogen nuclei. However, an additional problem is that it seems that nowhere inside the star would there be sufficient Carbon and Hydrogen in the same place to allow this process to take off. Indeed most hydrogen nuclei have already been "used up" and have fused to heavier nuclei, including Carbon.

But the observations now prove that the s-process does happen - how is this then possible?

Mixing the star
Current models of stellar interiors suggest that a moderate, "partial" mixing occurs that occasionally drags Hydrogen down to the Carbon-rich inner regions (and some Carbon moves up into the Hydrogen-rich region). It is still not clearly understood exactly how this process operates, but the Belgian astronomers independently predicted that if such a "partial mixing process" does take place in a low-metallicity star, then Lead-stars should exist and it should also be possible to observe them.

"Our discovery of these Lead stars is without any doubt the clearest signature of that model prediction we have today", states Sophie Van Eck. "The excellent agreement between predicted and observed abundances reinforces our current understanding of the detailed operation of the s-process in the deep interiors of the stars, and thus constitutes an important piece of information on how the heaviest stable elements in the universe are formed."

Three moons and your car battery
The astronomers altogether found a mass of Lead in each of the three stars that is about the same as the mass of our Moon.

Stars like these were once the most efficient Lead factories in the Universe. It is likely that the Lead in your car battery was once produced in such a low-metallicity star. From that star, it was later dispersed into the interstellar medium and was present in the cloud of dust and gas from which the Solar System and hence our Earth was formed.