INTEGRAL helps unravel the tumultuous recent history of the solar neighbourhood
Analysing new observations in gamma rays with ESA's INTEGRAL observatory, astronomers from the Max
Planck Institute for Extraterrestrial Physics and other institutions found evidence that only a few
million years ago massive stars enriched our cosmic neighbourhood with heavy elements. The scientists
exploited the radioactive decay of an isotope of aluminium, produced in the late stages of a massive
star's lifetime, to estimate the age of stars in the nearby Scorpius-Centaurus association, the closest
group of young and massive stars to the Sun.
A common technique used in archaeology to establish the age of fossils and other organic samples from the
past consists of measuring how much of a particular isotope of carbon, namely carbon-14, they contain. As
this radioactive isotope on a time scale of a few thousand years, the amount remaining in these ancient
fossils is a strong indicator of the epoch from which they date. The astronomers from MPE have now used
an analogous method, based on the radioactive decay of an unstable isotope of aluminium, to probe and
assess the age of stars in the Scorpius-Centaurus association, which is located about 300-500 light-years
from the Sun, and find a value of 5 million years, consistent with estimates by other methods.
COMPTEL all-sky image of 26Al gamma rays.
Image: Plüschke et al. 2001.
This dating procedure is possible because aluminium is one of the elements synthesised by massive stars
during their late evolutionary stages, and its abundance in a star cluster varies strongly with time. One
isotope of this element, namely aluminium-26, is radioactive and decays with an exponential lifetime of
about one million years.
"Conveniently for astronomers, the decay of aluminium-26 involves a similar time scale to that spanned
by the life time of massive stars, which is on the order of a few million years," explains Roland Diehl
from the Max Planck Institute for Extraterrestrial Physics in Germany, who led the recent study targeting
the gamma-ray emission from this isotope in the Scorpius-Centaurus association. "As its decay time is
'just right', measuring the abundance of aluminium-26 is an excellent tool to trace the presence of young
and massive stars, and it allows us to directly estimate their age," he adds.
The decay process of aluminium-26 results in a stable isotope of the element magnesium (26Mg) and a
number of by-products, including an extremely energetic photon observable in gamma rays at an energy
of about 1.8 MeV. Earlier observations, conducted in the 1990s with the COMPTEL instrument on NASA's
Compton Gamma-Ray Observatory, revealed for the first time the emission of aluminium-26 across the
entire sky. Subsequent data collected by ESA's INTEGRAL mission confirmed these results, probing the
global properties of this isotope throughout the plane of the Milky Way thanks to INTEGRAL's improved
The evolution of the abundance of 26Al in a stellar group.
Image: R. Voss.
The data analysed by Diehl's team focussed on the Scorpius-Centaurus association and revealed robust
evidence for recent massive star formation therein. Via stellar winds and supernova explosions, the
stars in the Scorpius-Centaurus association are currently enriching the surrounding interstellar medium
with heavy elements, including aluminium. Studying the details of these processes gives the astronomers
new insights into the influence of recent star formation on our immediate cosmic neighbourhood.
The new INTEGRAL data also allowed the astronomers to refine the estimate of the total content of
aluminium-26 in the Milky Way, which is lower by about 20 per cent than previous estimates. This is a
critical step that is required to validate our understanding of the star formation and nucleosynthesis
processes in our Galaxy. Massive stars and supernovae are the main producers of new atomic nuclei in the
universe, but in our own Galaxy is it is difficult to observe them, as they are embedded in and hidden
behind interstellar clouds. Penetrating gamma-rays now allow us to compare the predicted rate of supernova
explosions to actual measurements.
The ESA Science Program Committee decided in their November meeting to extend the INTEGRAL mission,
operated since 2002 and planned initially for 5 years, by two more years until the end of 2014.