Astronomy and Astrophysics – Astronomy
Scientific paper
Aug 2001
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001eso..pres...19.&link_type=abstract
ESO Press Release, 08/2001
Astronomy and Astrophysics
Astronomy
Scientific paper
La Silla Telescope Detects Lots of Lead in Three Distant Binaries
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
ESO PR Photo 26a/01 [Preview - JPEG: 350 x 400 pix - 232k] [Normal - JPEG: 700 x 800 pix - 616k]
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
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