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"We have analysed in great detail the chemical
composition of stars in three star-clusters and shown that each
cluster presents a high level of homogeneity and a very distinctive
chemical signature," says De Silva, who started this research while
working at the Mount Stromlo Observatory, Australia. "This paves the
way to chemically tagging stars in our Galaxy to common formation
sites and thus unravelling the history of the Milky Way," she adds.
"Galactic star clusters are witnesses of the
formation history of the Galactic disc," says Kenneth Freeman, also
from Mount Stromlo and another member of the team. "The analysis of
their composition is like studying ancient fossils. We are chasing
pieces of galactic DNA!"
Open star clusters are among the most important
tools for the study of stellar and galactic evolution. They are
composed of a few tens up to a few thousands of stars that are
gravitationally bound, and they span a wide range of ages. The
youngest date from a few million years ago, while the oldest (and more
rare) can have ages up to ten billion years. The well-known Pleiades,
also called the Seven Sisters, is a young bright open cluster.
Conversely, Collinder 261, which was the target of the present team of
astronomers, is among the oldest. It can therefore provide useful
information on the early days in the existence of our Galaxy.
The astronomers used UVES to observe a dozen red
giants in the open cluster Collinder 261, located about 25,000 light
years from the Galactic Centre. Giants are more luminous, hence they
are well suited for high-precision measurements. From these
observations, the abundances of a large set of chemical elements could
be determined for each star, demonstrating convincingly that all stars
in the cluster share the same chemical signature.
"This high level of homogeneity indicates that the
chemical information survived through several billion years," explains
De Silva. "Thus all the stars in the cluster can be associated to the
same prehistoric cloud. This corroborates what we had found for two
other groups of stars."
But this is not all. A comparison with the open
cluster called the Hyades, and the group of stars moving with the
bright star HR 1614, shows that each of them contains the same
elements in different proportions. This indicates that each star
cluster formed in a different primordial region, from a different
cloud with a different chemical composition.
"The consequences of these observations are
thrilling," says Freeman. "The ages of open clusters cover the entire
life of the Galaxy and each of them is expected to originate from a
different patch of 'dough'. Seeing how much sodium, magnesium, calcium,
iron and many other elements are present in each star cluster, we are
like accurate cooks who can tell the amount of salt, sugar, eggs and
flour used in different cookies. Each of them has a unique chemical
signature."
The astronomers will now aim to measure the
chemical abundances in a larger sample of open clusters. Once the
"DNA" of each star cluster is inferred, it will be possible to trace
the genealogic tree of the Milky Way. This chemical mapping through
time and space will be a way to test theoretical models.
"The path to an extensive use of chemical tagging
is still long," cautions De Silva, "but our study shows that it is
possible. When the technique is tested and proven we will be able to
get a detailed picture of the way our Galactic cradle formed." |
