Radioactive iron, a window to the stars (Forwarded)
- From: Andrew Yee <ayee@xxxxxxxxxxxxxxxxxxxxxx>
- Date: Mon, 25 Jun 2007 21:57:16 GMT
ESA News
http://www.esa.int
25 June 2007
Radioactive iron, a window to the stars
ESA's orbiting gamma-ray observatory, Integral, has made a pioneering
unequivocal discovery of radioactive iron-60 in our galaxy that provides
powerful insight into the workings of massive stars that pervade and shape
it.
Found drifting in space, the radioactive isotope has been sought for long.
All past reported sightings of iron-60 have been subject to controversy.
Now Integral has provided unequivocal evidence.
Since late 2002, Integral has been collecting data from across the galaxy.
It shows an enhancement in gamma rays at two characteristic energies, 1173
and 1333 kilo electron Volts. These are produced by radioactive decay of
iron-60 into cobalt-60.
Roland Diehl of the Max-Planck-Institut für extraterrestrische Physik,
headed the work and believes it is a major step forward. "These gamma-ray
lines have been detected before with some dispute. Integral, the only
instrument capable of doing this, shows that iron-60 does exist in
interstellar space in our Galaxy," he says.
More than a curiosity, its presence opens a door into the very heart of
the most massive stars in the cosmos. The majority of chemical elements
are built inside stars from raw ingredients present during star formation
from an interstellar gas cloud. In addition to hydrogen and helium
produced during the Big Bang, the gas contained enrichments, known to
astronomers as 'metals', from previous generations of stars and their
nuclear reactions.
Until this detection, astronomers had only one radioactive isotope to
probe into the current build-up of chemical elements in stars and their
distribution with respect to future star formation. That was the
radioactive isotope aluminium-26, first discovered in 1978. "The study of
aluminium-26 has developed into its own branch of astronomy," says Diehl.
Iron-60 gives astronomers valuable new insight -- although produced in the
same stars as aluminium-26, its production differs markedly. Iron-60 is
synthesised both later in a star's life and deeper inside.
As massive stars age, they develop a layered structure in which different
chemical elements are fused together. While aluminium-26 is one rung on
the ladder of nuclear reactions, iron-60 is produced from pre-existing
stable iron isotopes by a process called 'neutron capture' in the
respective layers where helium and carbon atoms are undergoing fusion.
"Iron-60 provides the entry into studying neutron capture in stars through
contemporaneous radioactivity," says Diehl. It has also prompted a number
of particle accelerators to begin more detailed studies of how easily iron
captures neutrons.
Unlike aluminium-26, iron-60 is only expelled into space when the star
explodes at the end of its life. It then decays with a half-life of 1.5
million years, producing the gamma rays that Integral detected.
The new data pins down the ratio of iron-60 to aluminium-26, which has a
half-life of 740 000 years. Previous predictions have fallen anywhere
between 10 and 100 percent. Integral shows it to be 15 percent, which
agrees well with current theoretical estimates. But theoreticians and
nuclear physicists have been stimulated by Integral's results to strive
for more precise predictions.
Although Integral clearly sees the telltale gamma rays, they are too faint
for it to map out enhancements and paucities across the Galaxy. "Mapping
the distribution of iron-60 is a job for the next generation of gamma-ray
instruments," says Diehl.
Nevertheless, the team will continue observing with Integral for as long
as they can, in the hope of gaining some coarse ideas about the isotope's
spread across the Galaxy.
Notes for editors:
The findings will appear in the article titled, "SPI observations of the
diffuse 60Fe emission in the Galaxy", by W. Wang , M. Harris, R. Diehl, H.
Halloin, B. Cordier, A.W. Strong, K. Kretschmer, J. Knödlseder, P. Jean,
G.G. Lichti, J.P. Roques, S. Schanne, A. von Kienlin, G. Weidenspointner,
C. Wunderer, accepted for publication on 26 April 2007 in the Astronomy
and Astrophysics journal.
For more information:
Roland Diehl
Max-Planck-Institut für extraterrestrische Physik, Garching, Germany
Email: Rod @ mpe.mpg.de
Christoph Winkler, ESA Integral Project Scientist
Email: Christoph.Winkler @ esa.int
[NOTE: Images and weblinks supporting this release are available at
http://www.esa.int/esaSC/SEM6FS8OY2F_index_1.html ]
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