Fingerprinting the Milky Way (Forwarded)

ESO Education and Public Relations Dept.

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Contact:

Gayandhi De Silva
ESO, Chile
Phone: +56 2 463 3066

Kenneth Freeman
Mount Stromlo Observatory, Australia
Phone: +61 2 6125 0264

For Immediate Release: 22 March 2007

ESO Science Release 15/07

Fingerprinting the Milky Way

Chemical Composition of Stars in Clusters Can Tell History of our Galaxy

Using ESO's Very Large Telescope, an international team of astronomers has
shown how to use the chemical composition of stars in clusters to shed
light on the formation of our Milky Way. This discovery is a fundamental
test for the development of a new chemical tagging technique uncovering
the birth and growth of our Galactic cradle.

The formation and evolution of galaxies, and in particular of the Milky
Way -- the 'island universe' in which we live, is one of the major puzzles
of astrophysics: indeed, a detailed physical scenario is still missing and
its understanding requires the joint effort of observations, theories and
complex numerical simulations. ESO astronomer Gayandhi De Silva and her
colleagues used the Ultraviolet and Visual Echelle Spectrograph (UVES) on
ESO's VLT to find new ways to address this fundamental riddle.

"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."

More Information

The research presented here is discussed in a paper in the Astronomical
Journal, volume 133, pages 1161-1175 ("Chemical homogeneity in Collinder
261 and implications for chemical tagging", by G.M. De Silva et al.).

The team is composed of Gayandhi De Silva (ESO), Kenneth Freeman, Martin
Asplund and Michael Bessell (Mount Stromlo Observatory, Australia), Joss
Bland-Hawthorn (Anglo-Australian Observatory, Australia), Remo Collet
(Uppsala University, Sweden).

National contacts for the media:

Belgium: Dr. Rodrigo Alvarez, +32-2-474 70 50
Finland: Ms. Tiina Raivo, +358 9 7748 8369
Denmark: Dr. Michael Linden-Vørnle, +45-33-18 19 97
France: Dr. Daniel Kunth, +3:3-1-44 32 80 85
Germany: Dr. Jakob Staude, +49-6221-528229
Italy: Dr. Leopoldo Benacchio, +39-347-230 26 51
The Netherlands: Ms. Marieke Baan, +31-20-525 74 80
Portugal: Prof. Teresa Lago, +351-22-089 833
Sweden: Dr. Jesper Sollerman, +46-8-55 37 85 54
Switzerland: Dr. Martin Steinacher, +41-31-324 23 82
United Kingdom: Mr. Peter Barratt, +44-1793-44 20 25

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