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Astronomers at the "Service d’Aéronomie"
(CNRS, Université Pierre et Marie Curie and the Université
de Versailles, Saint Quentin, France), the Observatoire de Paris(1)
and the University of California in Berkeley have completed a 3-D map
of the area surrounding our solar system. The solar system is inside a
"bubble" measuring 1,000 light years in diameter and filled
with very dense and hot gas at a temperature of one million degrees. The
team revealed, for the first time, a network of "walls" and
"tunnels" around this bubble, linking this cavity to other hot
gas bubbles surrounding neighboring stars. The purpose of these satellite
observations is to enable us to better predict the future evolution of
the local bubble. These results are to be announced at the meeting of
the American Astronomical Society on May 29, 2003 in Nashville,
Tennessee (USA).
An international team of astronomers(2)
, led by Rosine Lallement, research director at the CNRS, "Service
d’Aéronomie," has created a 3-D reconstruction of the
outline of the local interstellar bubble. This is a gigantic cavity in
our galaxy, measuring approximately 1,000 light-years in diameter, from
which dense gases and dust are absent. Our sun is currently crossing this
nearly empty space on its path around the center of the Milky Way. This
cavity, one million times bigger than our solar system, was probably created
by a series of supernova explosions (or a single explosion of the gamma-ray
burst type) over the last several million years. It is also possible that
it was blown through "tunnels" by massive stellar winds from
the region neighboring Scorpius-Centaurus. The extremely dense gas with
which it is filled has a temperature of one million degrees, testimony
to its tumultuous past.
The aim of observations obtained with the help of several telescopes,
mainly those of the ESO in La Silla, Chile, and also at the Observatoire
de Haute-Provence (CNRS), in Australia and in the United States, was to
map the region surrounding this local bubble. These observations are based
on the search for interstellar sodium atoms in the direction of neighboring
stars. When no atom is detected on the path to a star, this means that
only a very hot vacuum exists. "When we analyze light coming from
a more distant star and we detect a large quantity of sodium instead from
areas near the star, this means that we have gone beyond the limit of
our local vacuum and that the star is located in the dense galactic environment
or in another bubble separated from ours by a dense wall," explains
Rosine Lallement who began the project several years ago.
The existence of a network of tunnels and very hot gas bubbles has long
been by models: successive supernova explosions produce gigantic expanding
bubbles that push gas in front of them, much like a snow plough. The pushed
walls run into each other, creating compressed gas shells surrounding
the bubbles, broken up in spots. The whole structure looks like foam.
Mapping of the regions surrounding our local bubble is based on observations
of the sun, on recent results from the European satellite, Hipparcos,
concerning the distances of stars, as well as on new tomographic methods
developed by Jean-Luc Vergely, one of the members of the team. Three-D
maps show this maze of walls and tunnels linking our cavity to other neighboring
hot gas bubbles, like the Lupus-Norma, Scorpius-Centaurus and Auriga-Perseus
associations. Computer-generated images will be published soon in the
European journal Astronomy & Astrophysics.
"It is not just a question of creating a "roadmap" of
our galactic suburbs, but of understanding the physics behind the recycling
phenomenon of the interstellar environment as well, and finding an explanation
for a number of anomalies," continues Rosine Lallement. The maps
show that the local cavity goes through the galactic disk from one end
to the other and is extended by two wide tunnels that link it to the galactic
corona on the north and south sides. "Using satellite observations
with FUSE and Hubble, and a computer-generated analysis of wall movements,
we want to test the properties of hot gas in the galactic plane and in
these "chimneys" going toward intergalactic space. Are we going
to be limited to the inside of a bubble that is getting smaller and smaller,
"squeezed" by its neighbors? Or, on the contrary, is there enough
pressure in the cavity to overcome these forces and push back the walls?"
Whereas supernova shells are commonly observed, most often at their first
stages, just after the explosion, the local bubble is a much calmer and
more sprawling "ancestor" and, most importantly, we can observe
its interior close up to obtain another type of information. Thanks to
this proximity, it is possible, for example, to make a very detailed analysis
of the distribution of hot gas emission zones in this bubble as well as
their spectral characteristics, which reveal an enormous over-abundance
of metals, a property that we find in the "galactic winds" of
some galaxies with very high star formation rates that are still not understood.
The local bubble may be able to put us on the path to understanding mechanisms
that have not been taken into account up until now.
(1)
GEPI Laboratory, "Galaxies, Etoiles, Physique et Instrumentation
", CNRS Joint Research Unit, Observatoire de Paris, Université
Denis Diderot.
(2) This team includes:
- Rosine Lallement, Service d’Aéronomie, CNRS;
- Jean-Luc Vergely, Société ACRI (Sofia-Antipolis);
- Francoise Crifo, GEPI, Observatoire de Paris, Meudon;
- Barry Welsh, University of California, Berkeley.
Researcher
contact:
Rosine Lallement
Service d’Aéronomie.
Tel: +33 1 64 47 42 35
E-mail: Rosine.Lallement@aerov.jussieu.fr
Press
contact :
Martine Hasler
Tel: +33 1 44 96 46 35
E-mail: martine.hasler@cnrs-dir.fr
INSU CNRS contact:
Philippe Chauvin
Tel: +33 1 44 96 43 36 -
E-mail: philippe.chauvin@cnrs-dir.fr
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