Andrew Yee
October 21st 04, 04:01 PM
European Space Agency
Science News Release SNR 23-2004
Paris, France
20 October 2004
ESA's Hipparcos finds rebels with a cause
A team of European astronomers has discovered that many stars in the vicinity of
the Sun have unusual motions caused by the spiral arms of our galaxy, the Milky
Way. According to this research, based on data from ESA's Hipparcos observatory,
our stellar neighbourhood is the crossroads of streams of stars coming from
several directions. Some of the stars hosting planetary systems could be
immigrants from more central regions of the Milky Way.
The Sun and most stars near it follow an orderly, almost circular orbit around
the centre of our galaxy, the Milky Way. Using data from ESA's Hipparcos
satellite, a team of European astronomers has now discovered several groups of
'rebel' stars that move in peculiar directions, mostly towards the galactic
centre or away from it, running like the spokes of a wheel. These rebels account
for about 20% of the stars within 1000 light-years of the Sun, itself located
about 25 000 light-years away from the centre of the Milky Way.
The data show that rebels in the same group have little to do with each other.
They have different ages so, according to scientists, they cannot have formed at
the same time nor in the same place. Instead, they must have been forced
together. "They resemble casual travel companions more than family members,"
said Dr Benoit Famaey, Université Libre de Bruxelles, Belgium.
Famaey and his colleagues believe that the cause forcing the rebel stars
together on their unusual trajectory is a 'kick' received from one of the Milky
Way's spiral arms. The spiral arms are not solid structures but rather regions
of higher density of gas and stars, called 'density waves' and similar to
traffic hot-spots along the motorway. An approaching density wave compresses the
gas it encounters and favours the birth of new stars, but it can also affect
pre-existing stars by deflecting their motion. After the wave has passed, many
stars will thus travel together in a stream, all in the same direction, even
though they were originally on different trajectories or not even born.
This research has shown that the neighbourhood of the Sun is a crossroads of
many streams, made up of stars with different origins and chemical composition.
These streams could also account for many of the stars with planetary systems
recently discovered near the Sun.
Astronomers know that stars with planetary systems preferentially form in dense
gas clouds with a high metal content, such as those located in the more central
regions of the Milky Way. The streams discovered by Hipparcos could be the
mechanism that brought them closer to the Sun. As Famaey explains, "If these
stars are kicked by a spiral arm, they can be displaced thousands of light-years
away from their birthplace." These stars, together with their planets, can thus
have migrated closer to the Sun.
To learn more about the structure of our Milky Way, an aggregate of thousands of
millions of stars, astronomers look at the way in which stars stay together in a
coherent way or move with respect to the Sun and relative to one another. During
its four-year mission, ESA's Hipparcos satellite has measured the distance and
motion of more than a hundred thousand stars within a 1000 light-years of the
Sun. However, while Hipparcos's data show in which directions stars are moving
on the sky, they cannot tell whether stars are coming towards us or going away
from us.
By combining the Hipparcos data with ground-based measurements of their 'Doppler
shift', obtained with a Swiss telescope at the Observatoire de Haute-Provence,
France, Famaey and his colleagues could add the missing third dimension, namely
the speed with which stars approach us or recede from us. Because of the Doppler
shift, the colour of a star appears to change when it travels towards us or away
from us, becoming respectively bluer or redder and giving astronomers
information about its motion. "By combining all these first-class data, we now
have a comprehensive, three-dimensional view of how nearby stars move about us,"
said Famaey.
Scientists now wonder how widespread are the streams discovered by Famaey's team
and what role they could play in the evolution of our galaxy. "This result opens
up exciting new prospects for our understanding of the dynamics of the Milky
Way," said Dr Michael Perryman, ESA Hipparcos and Gaia project scientist. ESA's
forthcoming mission Gaia, scheduled for launch in 2011, will make it possible to
extend this investigation over a much wider region of our galaxy. Gaia will
observe more than a thousand million stars and will measure their motion in all
three dimensions simultaneously, thanks to the on-board spectrograph providing
information on their Doppler shift. "This will give us the clearest view ever of
the structure and evolution of the Milky Way," Perryman said.
Notes for editors
These results are the subject of a paper published this month in the European
journal Astronomy & Astrophysics. Besides Benoit Famaey, the list of authors
includes Alain Jorissen, Université Libre de Bruxelles, Belgium; Xavier Luri,
University of Barcelona, Spain; Michel Mayor and Stéphane Udry, University of
Geneva, Switzerland; Herwig Dejonghe, Ghent University, Belgium; Catherine
Turon, Observatory of Paris-Meudon, France. The paper, entitled "Local
kinematics of K and M giants from CORAVEL/Hipparcos/Tycho-2 data. Revisiting the
concept of superclusters" can be found at this address:
http://www.edpsciences.org/articles/aa/pdf/press-releases/PRAA200409.pdf
More about Hipparcos
ESA's Hipparcos pinpointed the positions of more than one hundred thousand
stars, two hundred times more accurately than ever before. It was launched in
1989 on an Ariane-4 rocket and completed its mission four years later in 1993.
Hipparcos data helped predict the impact of Comet Shoemaker-Levy 9 with Jupiter
in 1994 and showed that the Universe was bigger and younger than expected. The
work of Hipparcos will be refined and extended by ESA's Gaia mission.
For more information about Hipparcos please see:
* Scientific and Technical Hipparcos information
http://hipparcos.esa.int/science-e/www/area/index.cfm?fareaid=20
More about Gaia
ESA's Gaia mission will map one thousand million stars in our galaxy to
unprecedented levels of precision. It will monitor each of its target stars
about 100 times over a five-year period, precisely charting their motions and
changes in brightness. Gaia will be launched on a Russian Soyuz-Fregat in mid
2011. It is expected to discover hundreds of thousands of new celestial objects,
such as extrasolar planets and failed stars called brown dwarfs. Within our own
Solar System, Gaia should identify up to a million asteroids and comets.
For more information about Gaia please see:
* Gaia Press Kit
http://www.esa.int/esaSC/SEMBF8XLDMD_1_spk.html
* Scientific and Technical Gaia information
http://gaia.esa.int/science-e/www/area/index.cfm?fareaid=26
For more information please contact:
Benoit Famaey
Institut d'Astronomie et d'Astrophysique
Université Libre de Bruxelles
Brussels, Belgium
Tel: +44 1865 273986 (presently at the University of Oxford, UK)
E-mail: bfamaey @ astro.ulb.ac.be
Alain Jorissen
Institut d'Astronomie et d'Astrophysique
Université Libre de Bruxelles
Brussels, Belgium
Tel: +32 2 650 2834
E-mail: ajorisse @ astro.ulb.ac.be
Michael Perryman
ESA Hipparcos and Gaia Project Scientist
European Space Agency
Noordwijk, The Netherlands
Tel: +31 71 565 3833
Irina Bruckner
ESA Science Communications Office
European Space Agency
Noordwijk, The Netherlands
Tel: +31 71 565 3273
E-mail: ibruckne @ esa.int
ESA Media Relations Division
European Space Agency
Paris, France
Tel: +33 1 5369 7155
Fax: +33 1 5369 7690
IMAGE CAPTION:
[Low-resolution version (JPG)
http://esamultimedia.esa.int/images/spcs/hipparcos/hipparcos20041020a.jpg (19KB)
High-resolution version (TIFF)
http://esamultimedia.esa.int/images/spcs/hipparcos/hipparcos20041020a.tiff (1MB)]
Artist's impression of our galaxy, the Milky Way, an aggregate of thousands of
millions of stars. The spiral arms are clearly visible. They are regions of
enhanced density of stars and gas. The Sun is located near the edge of one arm,
about half-way from the galactic centre. Spiral arms can impart a kick on stars
orbiting close to them. These stars are then forced unto streams running inwards
or outwards, whereas the bulk of stars in the Milky Way move in circular orbits
around the galactic centre. Using data from ESA's Hipparcos satellite,
astronomers have now identified three such streams, reaching into the solar
neighbourhood.
Credits: S. Kerroudj, B. Famaey & A. Jorissen (Université Libre de Bruxelles)
Science News Release SNR 23-2004
Paris, France
20 October 2004
ESA's Hipparcos finds rebels with a cause
A team of European astronomers has discovered that many stars in the vicinity of
the Sun have unusual motions caused by the spiral arms of our galaxy, the Milky
Way. According to this research, based on data from ESA's Hipparcos observatory,
our stellar neighbourhood is the crossroads of streams of stars coming from
several directions. Some of the stars hosting planetary systems could be
immigrants from more central regions of the Milky Way.
The Sun and most stars near it follow an orderly, almost circular orbit around
the centre of our galaxy, the Milky Way. Using data from ESA's Hipparcos
satellite, a team of European astronomers has now discovered several groups of
'rebel' stars that move in peculiar directions, mostly towards the galactic
centre or away from it, running like the spokes of a wheel. These rebels account
for about 20% of the stars within 1000 light-years of the Sun, itself located
about 25 000 light-years away from the centre of the Milky Way.
The data show that rebels in the same group have little to do with each other.
They have different ages so, according to scientists, they cannot have formed at
the same time nor in the same place. Instead, they must have been forced
together. "They resemble casual travel companions more than family members,"
said Dr Benoit Famaey, Université Libre de Bruxelles, Belgium.
Famaey and his colleagues believe that the cause forcing the rebel stars
together on their unusual trajectory is a 'kick' received from one of the Milky
Way's spiral arms. The spiral arms are not solid structures but rather regions
of higher density of gas and stars, called 'density waves' and similar to
traffic hot-spots along the motorway. An approaching density wave compresses the
gas it encounters and favours the birth of new stars, but it can also affect
pre-existing stars by deflecting their motion. After the wave has passed, many
stars will thus travel together in a stream, all in the same direction, even
though they were originally on different trajectories or not even born.
This research has shown that the neighbourhood of the Sun is a crossroads of
many streams, made up of stars with different origins and chemical composition.
These streams could also account for many of the stars with planetary systems
recently discovered near the Sun.
Astronomers know that stars with planetary systems preferentially form in dense
gas clouds with a high metal content, such as those located in the more central
regions of the Milky Way. The streams discovered by Hipparcos could be the
mechanism that brought them closer to the Sun. As Famaey explains, "If these
stars are kicked by a spiral arm, they can be displaced thousands of light-years
away from their birthplace." These stars, together with their planets, can thus
have migrated closer to the Sun.
To learn more about the structure of our Milky Way, an aggregate of thousands of
millions of stars, astronomers look at the way in which stars stay together in a
coherent way or move with respect to the Sun and relative to one another. During
its four-year mission, ESA's Hipparcos satellite has measured the distance and
motion of more than a hundred thousand stars within a 1000 light-years of the
Sun. However, while Hipparcos's data show in which directions stars are moving
on the sky, they cannot tell whether stars are coming towards us or going away
from us.
By combining the Hipparcos data with ground-based measurements of their 'Doppler
shift', obtained with a Swiss telescope at the Observatoire de Haute-Provence,
France, Famaey and his colleagues could add the missing third dimension, namely
the speed with which stars approach us or recede from us. Because of the Doppler
shift, the colour of a star appears to change when it travels towards us or away
from us, becoming respectively bluer or redder and giving astronomers
information about its motion. "By combining all these first-class data, we now
have a comprehensive, three-dimensional view of how nearby stars move about us,"
said Famaey.
Scientists now wonder how widespread are the streams discovered by Famaey's team
and what role they could play in the evolution of our galaxy. "This result opens
up exciting new prospects for our understanding of the dynamics of the Milky
Way," said Dr Michael Perryman, ESA Hipparcos and Gaia project scientist. ESA's
forthcoming mission Gaia, scheduled for launch in 2011, will make it possible to
extend this investigation over a much wider region of our galaxy. Gaia will
observe more than a thousand million stars and will measure their motion in all
three dimensions simultaneously, thanks to the on-board spectrograph providing
information on their Doppler shift. "This will give us the clearest view ever of
the structure and evolution of the Milky Way," Perryman said.
Notes for editors
These results are the subject of a paper published this month in the European
journal Astronomy & Astrophysics. Besides Benoit Famaey, the list of authors
includes Alain Jorissen, Université Libre de Bruxelles, Belgium; Xavier Luri,
University of Barcelona, Spain; Michel Mayor and Stéphane Udry, University of
Geneva, Switzerland; Herwig Dejonghe, Ghent University, Belgium; Catherine
Turon, Observatory of Paris-Meudon, France. The paper, entitled "Local
kinematics of K and M giants from CORAVEL/Hipparcos/Tycho-2 data. Revisiting the
concept of superclusters" can be found at this address:
http://www.edpsciences.org/articles/aa/pdf/press-releases/PRAA200409.pdf
More about Hipparcos
ESA's Hipparcos pinpointed the positions of more than one hundred thousand
stars, two hundred times more accurately than ever before. It was launched in
1989 on an Ariane-4 rocket and completed its mission four years later in 1993.
Hipparcos data helped predict the impact of Comet Shoemaker-Levy 9 with Jupiter
in 1994 and showed that the Universe was bigger and younger than expected. The
work of Hipparcos will be refined and extended by ESA's Gaia mission.
For more information about Hipparcos please see:
* Scientific and Technical Hipparcos information
http://hipparcos.esa.int/science-e/www/area/index.cfm?fareaid=20
More about Gaia
ESA's Gaia mission will map one thousand million stars in our galaxy to
unprecedented levels of precision. It will monitor each of its target stars
about 100 times over a five-year period, precisely charting their motions and
changes in brightness. Gaia will be launched on a Russian Soyuz-Fregat in mid
2011. It is expected to discover hundreds of thousands of new celestial objects,
such as extrasolar planets and failed stars called brown dwarfs. Within our own
Solar System, Gaia should identify up to a million asteroids and comets.
For more information about Gaia please see:
* Gaia Press Kit
http://www.esa.int/esaSC/SEMBF8XLDMD_1_spk.html
* Scientific and Technical Gaia information
http://gaia.esa.int/science-e/www/area/index.cfm?fareaid=26
For more information please contact:
Benoit Famaey
Institut d'Astronomie et d'Astrophysique
Université Libre de Bruxelles
Brussels, Belgium
Tel: +44 1865 273986 (presently at the University of Oxford, UK)
E-mail: bfamaey @ astro.ulb.ac.be
Alain Jorissen
Institut d'Astronomie et d'Astrophysique
Université Libre de Bruxelles
Brussels, Belgium
Tel: +32 2 650 2834
E-mail: ajorisse @ astro.ulb.ac.be
Michael Perryman
ESA Hipparcos and Gaia Project Scientist
European Space Agency
Noordwijk, The Netherlands
Tel: +31 71 565 3833
Irina Bruckner
ESA Science Communications Office
European Space Agency
Noordwijk, The Netherlands
Tel: +31 71 565 3273
E-mail: ibruckne @ esa.int
ESA Media Relations Division
European Space Agency
Paris, France
Tel: +33 1 5369 7155
Fax: +33 1 5369 7690
IMAGE CAPTION:
[Low-resolution version (JPG)
http://esamultimedia.esa.int/images/spcs/hipparcos/hipparcos20041020a.jpg (19KB)
High-resolution version (TIFF)
http://esamultimedia.esa.int/images/spcs/hipparcos/hipparcos20041020a.tiff (1MB)]
Artist's impression of our galaxy, the Milky Way, an aggregate of thousands of
millions of stars. The spiral arms are clearly visible. They are regions of
enhanced density of stars and gas. The Sun is located near the edge of one arm,
about half-way from the galactic centre. Spiral arms can impart a kick on stars
orbiting close to them. These stars are then forced unto streams running inwards
or outwards, whereas the bulk of stars in the Milky Way move in circular orbits
around the galactic centre. Using data from ESA's Hipparcos satellite,
astronomers have now identified three such streams, reaching into the solar
neighbourhood.
Credits: S. Kerroudj, B. Famaey & A. Jorissen (Université Libre de Bruxelles)