Text with all links and the photo is available at
http://www.eso.org/outreach/press-rel/pr-2001/pr-19-01.html

Summary

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 [1] who used the Coude Echelle Spectrometer on the ESO
3.6-m telescope at the La Silla Observatory (Chile).

PR Photo 26a/01: A photo of HD 196944, one of the “Lead stars”.
PR Photo 26b/01: A CES spectrum of HD 196944.

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 [2]) 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 [2]. 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 [3]. 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 [2]) 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

ESO PR Photo 26a/01

Caption: 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. Still, the detailed spectroscopic study reported in
this Press release that revealed a high abundance of Lead in this star
required a 4-m class telescope. This DSS-image are copyright by the 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 spikes seen in this photo are an optical effect in the
telescope.

In a determined effort in this direction, a team of Belgian and French
astronomers [1] decided to try to detect the presence of Lead in some
“CH-stars” [4] 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 [5]. 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

ESO PR Photo 26b/01

Caption: A high-resolution spectrum of the CH-star HD 196944, obtained
with the CES instrument on the ESO 3.6-m telescope in September 2000.
The observed spectrum (dots) shows many absorption lines from elements
that are usually seen in stars. The red line shows a model in which
elements (in particular those produced by the s-process) are present in
normal quantities, compared to Iron. The blue line instead shows a model
where s-processing has occured. It is obvious that the red line does not
fit, only the blue line reproduces the observed absorption line at
wavelength 405.781 nm caused by Lead (Pb) atoms in the atmosphere of
this star. A subsequent, detailed analysis demonstrated that HD 196944
is a true “Lead star”. Technical information about this photo is
available below.

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.

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 Universite 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 [2])
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 (7.4 x 10^22 kg).

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.

More information

The research described in this Press Release is reported in a scientific
article (“Discovery of three Lead stars” by S. Van Eck, S. Goriely, A.
Jorissen and B. Plez) that appears in the August 23, 2001 issue of the
science journal “Nature”.

Notes

[1]: The team consists of Sophie Van Eck, Stephane Goriely, Alain Jorissen
(all Institut d’Astronomie et d’Astrophysique de l’Universite Libre de
Bruxelles, Belgium) and Bertrand Plez (Groupe de Recherche en Astronomie et
Astrophysique en Languedoc, Universite de Montpellier II – GRAAL), France).
Sophie Van Eck was an ESO fellow (1999-2000).

[2] The “atomic mass” of a chemical element is the total mass of the
positively charged protons and neutral neutrons in the atomic nucleus. The
“atomic number” of a chemical element is equal to the number of protons in
the nucleus. Different isotopes of a chemical element all have the same
number of protons in the nuclei, but a different number of neutrons. For the
principal (most abundant) isotopes of the elements mentioned in this text,
the “atomic mass” (expressed in “atomic mass units” (amu)) is approximately:

Hydrogen: 1 atomic mass unit (with 1 proton in the nucleus);

Helium: 4 atomic mass units (2 protons + 2 neutrons);

Lithium: 7 atomic mass units (3 protons + 4 neutrons);

Carbon: 12 atomic mass units (6 protons + 6 neutrons);

Oxygen: 16 atomic mass units (8 protons + 8 neutrons);

Iron: 56 atomic mass units (26 protons + 30 neutrons);

Zirconium: 90 atomic mass units (40 protons + 50 neutrons);

Barium: 138 atomic mass units (56 protons + 82 neutrons);

Tungsten: 184 atomic mass units (74 protons + 110 neutrons);

Lead: 208 atomic mass units (82 protons + 126 neutrons);

Bismuth: 209 atomic mass units (83 protons + 126 neutrons)

[3] “AGB” stands for “Asymptotic Giant Branch”; a location in the
HR-diagramme (a plot of stellar colours and luminosities) of evolved stars
in which hydrogen and helium burning occurs in two concentric shells and
elements heavier than iron are produced via the s-process.

[4] The “CH-stars” owe their name to the prominent bands of the CH-molecule
observed in their spectrum.

[5] The fact that CH-stars are all double stars was discovered by the
Canadian astronomer Robert McClure in 1984.

Technical information about the photos

PR Photo 26b/01 shows a small section of the reduced spectrum of the CH-star
HD 196944, near wavelength 4050 Angstrom. It was obtained during a 90-min
exposure with the Coude Echelle Spectrometer at the ESO 3.6-m telescope on
La Silla in 16 September 2000. The spectral resolution is 135 000.

pt.

Contacts

Sophie Van Eck
Institut d’Astronomie et d’Astrophysique de l’Universite Libre de Bruxelles
Brussels, Belgium
Tel.: +32-2-650-28-63
E-Mail: svaneck@astro.ulb.ac.be

Stéphane Goriely
Institut d’Astronomie et d’Astrophysique de l’Universite Libre de Bruxelles
Brussels, Belgium
Tel.: +32-2-650-28-43
E-Mail: sgoriely@astro.ulb.ac.be

Alain Jorissen
Institut d’Astronomie et d’Astrophysique de l’Universite Libre de Bruxelles
Brussels, Belgium
Tel.: +32-2-650-28-34
E-Mail: ajorisse@astro.ulb.ac.be

Bertrand Plez
Groupe de Recherche en Astronomie et Astrophysique en Languedoc
Universite de Montpellier II
France
Tel.: +33-467-14-48-91, +33-608-16-71-57
E-Mail: plez@graal.univ-montp2.fr