A Galactic Fossil

ESO 23/07 - Science Release

10 May 2007
For Immediate Release

Star is Found to be 13.2 Billion Years Old

How old are the oldest stars? Using ESO's VLT, astronomers recently measured the age of a star located in our Galaxy. The star, a real fossil, is found to be 13.2 billion years old, not very far from the 13.7 billion years age of the Universe. The star, HE 1523-0901, was clearly born at the dawn of time.

"Surprisingly, it is very hard to pin down the age of a star", the lead author of the paper reporting the results, Anna Frebel, explains. "This requires measuring very precisely the abundance of the radioactive elements thorium or uranium, a feat only the largest telescopes such as ESO's VLT can achieve."

ESO PR Photo 23a/07
The 'Cosmic Clock'

This technique is analogous to the carbon-14 dating method that has been so successful in archaeology over time spans of up to a few tens of thousands of years. In astronomy, however, this technique must obviously be applied to vastly longer timescales.

For the method to work well, the right choice of radioactive isotope is critical. Unlike other, stable elements that formed at the same time, the abundance of a radioactive (unstable) isotope decreases all the time. The faster the decay, the less there will be left of the radioactive isotope after a certain time, so the greater will be the abundance difference when compared to a stable isotope, and the more accurate is the resulting age.

Yet, for the clock to remain useful, the radioactive element must not decay too fast - there must still be enough left of it to allow an accurate measurement, even after several billion years.

"Actual age measurements are restricted to the very rare objects that display huge amounts of the radioactive elements thorium or uranium," says Norbert Christlieb, co-author of the report.

ESO PR Photo 23b/07
Uranium Line in the Spectrum
of an Old Star

Large amounts of these elements have been found in the star HE 1523-0901, an old, relatively bright star that was discovered within the Hamburg/ESO survey [1]. The star was then observed with UVES on the Very Large Telescope (VLT) for a total of 7.5 hours.

A high quality spectrum was obtained that could never have been achieved without the combination of the large collecting power Kueyen, one of the individual 8.2-m Unit Telescopes of the VLT, and the extremely good sensitivity of UVES in the ultraviolet spectral region, where the lines from the elements are observed.

For the first time, the age dating involved both radioactive elements in combination with the three other neutron-capture elements europium, osmium, and iridium.

"Until now, it has not been possible to measure more than a single cosmic clock for a star. Now, however, we have managed to make six measurements in this one star", says Frebel.

Ever since the star was born, these "clocks" have ticked away over the eons, unaffected by the turbulent history of the Milky Way. They now read 13.2 billion years.

The Universe being 13.7 billion years old, this star clearly formed very early in the life of our own Galaxy, which must also formed very soon after the Big Bang.

More Information

This research is reported in a paper published in the 10 May issue of the Astrophysical Journal ("Discovery of HE 1523-0901, a Strongly r-Process Enhanced Metal-Poor Star with Detected Uranium", by A. Frebel et al.).
The team includes Anna Frebel (McDonald Observatory, Texas) and John E. Norris (The Australian National University), Norbert Christlieb (Uppsala University, Sweden, and Hamburg Observatory, Germany), Christopher Thom (University of Chicago, USA, and Swinburne University of Technlogy, Australia), Timothy C. Beers (Michigan State University, USA), Jaehyon Rhee (Center for Space Astrophysics, Yonsei University, Korea, and Caltech, USA).

Note

[1]: The Hamburg/ESO sky survey is a collaborative project of the Hamburger Sternwarte and ESO to provide spectral information for half of the southern sky using photographic plates taken with the now retired ESO-Schmidt telescope. These plates were digitized at Hamburger Sternwarte.

Contact

Anna Frebel
McDonald Observatory, Texas
Phone: +1 512-461-7907
Email: anna (at) astro.as.utexas.edu

Norbert Christlieb
Department of Astronomy and Space Physics, Uppsala University, Sweden
Phone: +46-18-471-5982
Mobile: +49-176-67 67 14 08
E-mail: norbert (at) astro.uu.se

National contacts for the media:
Belgium - Dr. Rodrigo Alvarez +32-2-474 70 50 rodrigo.alvarez@oma.be
Czech Republic - Pavel Suchan +420 267 103 040 suchan@astro.cz
Finland - Ms. Tiina Raivo +358 9 7748 8369 tiina.raivo@aka.fi
Denmark - Dr. Michael Linden-Vørnle +45-33-18 19 97 mykal@tycho.dk
France - Dr. Daniel Kunth +33-1-44 32 80 85 kunth@iap.fr
Germany - Dr. Jakob Staude +49-6221-528229 staude@mpia.de
Italy - Dr. Leopoldo Benacchio +39-347-230 26 51 benacchio@inaf.it
The Netherlands - Ms. Marieke Baan +31-20-525 74 80 mbaan@science.uva.nl
Portugal - Prof. Teresa Lago +351-22-089 833 mtlago@astro.up.pt
Spain - Dr. Miguel Mas-Hesse +34918131196 mm@laeff.inta.es
Sweden - Dr. Jesper Sollerman +46-8-55 37 85 54 jesper@astro.su.se
Switzerland - Dr. Martin Steinacher +41-31-324 23 82 martin.steinacher@sbf.admin.ch
United Kingdom - Mr. Peter Barratt +44-1793-44 20 25 peter.barratt@stfc.ac.uk

 

Astronomers Find First Earth-like Planet in Habitable Zone

ESO 22/07 - Science Release

25 April 2007
For Immediate Release

The Dwarf Carried Other Worlds Too!

Astronomers have discovered the most Earth-like planet outside our Solar System to date, an exoplanet with a radius only 50% larger than the Earth and capable of having liquid water. Using the ESO 3.6-m telescope, a team of Swiss, French and Portuguese scientists discovered a super-Earth about 5 times the mass of the Earth that orbits a red dwarf, already known to harbour a Neptune-mass planet. The astronomers have also strong evidence for the presence of a third planet with a mass about 8 Earth masses.

ESO PR Photo 22a/07 The Planetary System Around Gliese 581

This exoplanet - as astronomers call planets around a star other than the Sun - is the smallest ever found up to now [1] and it completes a full orbit in 13 days. It is 14 times closer to its star than the Earth is from the Sun. However, given that its host star, the red dwarf Gliese 581 [2], is smaller and colder than the Sun - and thus less luminous - the planet nevertheless lies in the habitable zone, the region around a star where water could be liquid! The planet's name is Gliese 581 c.

"We have estimated that the mean temperature of this super-Earth lies between 0 and 40 degrees Celsius, and water would thus be liquid," explains Stéphane Udry, from the Geneva Observatory (Switzerland) and lead-author of the paper reporting the result. "Moreover, its radius should be only 1.5 times the Earth's radius, and models predict that the planet should be either rocky - like our Earth - or fully covered with oceans," he adds.

ESO PR Photo 22c/07 The star Gliese 581

"Liquid water is critical to life as we know it," avows Xavier Delfosse, a member of the team from Grenoble University (France). "Because of its temperature and relative proximity, this planet will most probably be a very important target of the future space missions dedicated to the search for extra-terrestrial life. On the treasure map of the Universe, one would be tempted to mark this planet with an X."

The host star, Gliese 581, is among the 100 closest stars to us, located only 20.5 light-years away in the constellation Libra ("the Scales"). It has a mass of only one third the mass of the Sun. Such red dwarfs are intrinsically at least 50 times fainter than the Sun and are the most common stars in our Galaxy: among the 100 closest stars to the Sun, 80 belong to this class.

"Red dwarfs are ideal targets for the search for low-mass planets where water could be liquid. Because such dwarfs emit less light, the habitable zone is much closer to them than it is around the Sun," emphasizes Xavier Bonfils, a co-worker from Lisbon University. Planets lying in this zone are then more easily detected with the radial-velocity method [3], the most successful in detecting exoplanets.

ESO PR Photo 22d/07 Velocity Variations of Gl 581

Two years ago, the same team of astronomers already found a planet around Gliese 581 (see ESO 30/05). With a mass of 15 Earth-masses, i.e. similar to that of Neptune, it orbits its host star in 5.4 days. At the time, the astronomers had already seen hints of another planet. They therefore obtained a new set of measurements and found the new super-Earth, but also clear indications for another one, an 8 Earth-mass planet completing an orbit in 84 days. The planetary system surrounding Gliese 581 contains thus no fewer than 3 planets of 15 Earth masses or less, and as such is a quite remarkable system.

The discovery was made thanks to HARPS (High Accuracy Radial Velocity for Planetary Searcher), perhaps the most precise spectrograph in the world. Located on the ESO 3.6-m telescope at La Silla, Chile, HARPS is able to measure velocities with a precision better than one metre per second (or 3.6 km/h)! HARPS is one of the most successful instruments for detecting exoplanets and holds already several recent records, including the discovery of another 'Trio of Neptunes' (ESO 18/06, see also ESO 22/04).

ESO PR Video 22/07 Watch the video!

The detected velocity variations are between 2 and 3 metres per second, corresponding to about 9 km/h! That's the speed of a person walking briskly. Such tiny signals could not have been distinguished from 'simple noise' by most of today's available spectrographs.

"HARPS is a unique planet hunting machine," says Michel Mayor, from Geneva Observatory, and HARPS Principal Investigator. "Given the incredible precision of HARPS, we have focused our effort on low-mass planets. And we can say without doubt that HARPS has been very successful: out of the 13 known planets with a mass below 20 Earth masses, 11 were discovered with HARPS!"

HARPS is also very efficient in finding planetary systems, where tiny signals have to be uncovered. The two systems known to have three low mass planets - HD 69830 and Gl 581 - were discovered by HARPS.

"And we are confident that, given the results obtained so far, finding a planet with the mass of the Earth around a red dwarf is within reach," affirms Mayor.

More Information

This research is reported in a paper submitted as a Letter to the Editor of Astronomy and Astrophysics ("The HARPS search for southern extra-solar planets : XI. An habitable super-Earth (5 MEarth) in a 3-planet system", by S. Udry et al.) The paper is available from http://obswww.unige.ch/~udry/udry_preprint.pdf.
The team is composed of Stéphane Udry, Michel Mayor, Christophe Lovis, Francesco Pepe, and Didier Queloz (Geneva Observatory, Switzerland), Xavier Bonfils (Lisbonne Observatory, Portugal), Xavier Delfosse, Thierry Forveille, and C.Perrier (LAOG, Grenoble, France), François Bouchy (Institut d'Astrophysique de Paris, France), and Jean-Luc Bertaux (Service d'Aéronomie du CNRS, France)

Notes

[1]: Using the radial velocity method, astronomers can only obtain a minimum mass (as it is multiplied by the sine of the inclination of the orbital plane to the line of sight, which is unknown). From a statistical point of view, this is however often close to the real mass of the system. Two other systems have a mass close to this. The icy planet around OGLE-2005-BLG-390L, discovered by microlensing with a network of telescopes including one at La Silla (ESO 03/06), has a (real) mass of 5.5 Earth masses. It, however, orbits much farther from its small host star than the present one and is hence much colder. The other is one of the planets surrounding the star Gliese 876. It has a minimum mass of 5.89 Earth masses (and a probable real mass of 7.53 Earth masses) and completes an orbit in less than 2 days, making it too hot for liquid water to be present.

[2]: Gl 581, or Gliese 581, is the 581th entry in the Gliese Catalogue, which lists all known stars within 25 parsecs (81.5 light years) of the Sun. It was originally compiled by Gliese and published in 1969, and later updated by Gliese and Jahreiss in 1991.

[3]: This fundamental observational method is based on the detection of variations in the velocity of the central star, due to the changing direction of the gravitational pull from an (unseen) exoplanet as it orbits the star. The evaluation of the measured velocity variations allows deducing the planet's orbit, in particular the period and the distance from the star, as well as a minimum mass.

Contact

Stéphane Udry, Michel Mayor
Observatory of Geneva University, Switzerland
Phone: +41 22 379 22 00
Email: Stephane.Udry (at) obs.unige.ch, Michel.Mayor (at) obs.unige.ch

Xavier Delfosse, Thierry Forveille
LAOG, France
Phone: +33 476 51 42 06
Email: Xavier.Delfosse (at) obs.ujf-grenoble.fr, Thierry.Forveille (at) obs.ujf-grenoble.fr

Xavier Bonfils
Lisbonne Observatory, Portugal
Phone: +351 21 361 67 43
Email: xavier.bonfils (at) oal.ul.pt

National contacts for the media:
Belgium - Dr. Rodrigo Alvarez +32-2-474 70 50 rodrigo.alvarez@oma.be
Finland - Ms. Tiina Raivo +358 9 7748 8369 tiina.raivo@aka.fi
Denmark - Dr. Michael Linden-Vørnle +45-33-18 19 97 mykal@tycho.dk
France - Dr. Daniel Kunth +33-1-44 32 80 85 kunth@iap.fr
Germany - Dr. Jakob Staude +49-6221-528229 staude@mpia.de
Italy - Dr. Leopoldo Benacchio +39-347-230 26 51 benacchio@inaf.it
The Netherlands - Ms. Marieke Baan +31-20-525 74 80 mbaan@science.uva.nl
Portugal - Prof. Teresa Lago +351-22-089 833 mtlago@astro.up.pt
Spain - Dr. Miguel Mas-Hesse +34918131196 mm@laeff.inta.es
Sweden - Dr. Jesper Sollerman +46-8-55 37 85 54 jesper@astro.su.se
Switzerland - Dr. Martin Steinacher +41-31-324 23 82 martin.steinacher@sbf.admin.ch
United Kingdom - Mr. Peter Barratt +44-1793-44 20 25 Peter.Barratt@pparc.ac.uk

 

The Impossible Siblings

ESO 18/07 - Science Release

29 March 2007
For Immediate Release

Unique Data Collected on Double Asteroid Antiope

Combining precise observations obtained by ESO's Very Large Telescope with those gathered by a network of smaller telescopes, astronomers have described in unprecedented detail the double asteroid Antiope, which is shown to be a pair of rubble-pile chunks of material, of about the same size, whirling around one another in a perpetual pas de deux. The two components are egg-shaped despite their very small sizes.

The asteroid (90) Antiope was discovered in 1866 by Robert Luther from Dusseldorf, Germany. The 90th asteroid ever discovered, its name comes from Greek mythology. In 2000, William Merline and his collaborators found that the asteroid was composed of two similarly-sized components, making it a truly 'double' asteroid, one of the very first of this kind in the main belt of asteroids that lies between the orbits of Mars and Jupiter.

ESO PR Photo 18a/07 The Antiope Doublet

"The way double asteroids have formed in the main belt is still unclear," says Pascal Descamps, from the Paris Observatory and lead-author of the paper presenting the new results. "The Antiope system provides us with a unique opportunity to know more about this class of objects and we decided to study it in detail," he adds.

Descamps, with colleague Franck Marchis from the University of California at Berkeley, USA, therefore initiated a large campaign of observations for more than two and a half years starting in January 2003. They used the NACO instrument on ESO's Very Large Telescope at Cerro Paranal for the larger part, while using one of the Keck telescopes for some additional observations in 2005.

NACO allows the astronomers to perform adaptive optics observations, providing images that are mostly free from the blurring effect of the atmosphere. With these, it was always possible to separate clearly the two components of the Antiope system, thereby obtaining a large set of very precise measurements of their positions.

"With this unique set of data, we could determine with utmost precision the course of the two pieces of cosmic rock as they turn around each other," says Marchis. "We found that the two objects are separated by 171 km, and that they perform their celestial dance in 16.5 hours. In fact, we now know this orbital period with a precision of better than half a second."

With the orbit determined, the astronomers could derive the total mass of the system: 828 millions million tons, and found the two objects were rotating around their own axes at the same speed as they orbit each other. Thus, in the same way than the Moon does to the Earth, they always present to each other the same side (something astronomers call 'tidal locking'). Moreover, the two asteroids rotate in the same plane as they orbit each other.

ESO PR Photo 18b/07 Double Asteroid (NACO/VLT)

The adaptive optics observations could, however, never resolve the shape of the individual components as they are too small. "But with the new orbit, we could precisely predict that from the end of May to the end of November 2005 the system would present eclipses and occultations," says Marchis. "Such 'mutual events' are unique opportunities to learn a great deal about this double asteroid."

The astronomers invited observers around the world to turn their eyes on the asteroid pair to measure the drops in brightness resulting from the predicted events. Over the six-month period, amateurs and professionals from as far afield as Brazil, Chile, France, Réunion Island, South Africa, and the USA, observed repeated occultations as well as shadows passing over one of the pair.

With this new data, Descamps, Marchis and their team, found enough evidence that the two mountain-like chunks of material forming the Antiope system have the shape of ellipsoids, that is, slightly deformed spheres, almost similar in size: 93.0 x 87.0 x 83.6 km and 89.4 x 82.8 x 79.6 km, respectively. Each asteroid in the pair is thus roughly the size of a large city.

Perhaps the most astonishing result is the fact that the two components have a shape close to the one predicted by the French scientist Edouard Roche in 1849 for self-gravitating, rotating fluid objects orbiting each other and tidally locked.

Of course, the asteroids are not gaseous nor liquids, they are solids, but their internal structure must be so loose that their bodies can readjust themselves due to the gravitational influence of the companion.

The scientists were also able to derive the density of the objects, only a quarter higher than the density of water. This means the asteroids are very porous, having 30 percent empty space, and thereby suggesting a rubble-pile structure. This structure could explain why it was easier for the asteroids to reach equilibrium shapes, while being so small.

"Despite this intensive study, the origin of this unique doublet still remains a mystery," says Descamps. "The formation of such a large double system is an improbable event and represents a formidable challenge to theory. One possibility is that a parent body was spun up so much that it took the shape of an apple core, then split into two similar-sized pieces."

More Information

This work is reported in a paper published in the journal Icarus ("Figure of the double Asteroid 90 Antiope from adaptive optics and lightcurve observations", by P. Descamps et al.).

The team is composed of P. Descamps, F. Marchis, F. Vachier, F. Colas, J. Berthier, D. Hestroffer, R. Viera-Martins, and M. Birlan (Observatoire de Paris, France), T. Michalowski and M. Polinska (Adam Mickiewicz University, Poznan, Poland), M. Assafin (Observatorio do Valongo/UFRJ, Brazil), P.B. Dunckel (Rattlesnake Creek Observatory, USA), W. Pych (Nicolaus Copernicus Astronomical Center, Warsaw, Poland), J.-P. Teng-Chuen-Yu, A. Peyrot, B. Payet, J. Dorseuil, Y. Léonie, and T. Dijoux (Makes Observatory, Réunion Island, France). F. Marchis is also at the University of California at Berkeley, USA.

Contacts

Franck Marchis
University of California, Berkeley, USA
Phone: +1 (510) 642 3958 or +1 (510) 599 0604
Email: fmarchis (at) berkeley.edu

Pascal Descamps, Daniel Hestroffer, Jerome Berthier
IMCCE, Observatoire de Paris, France
Phone: +33 1 4051 2268 or +33 1 4051 2260
Email: descamps (at) imcce.fr, hestroffer (at) imcce.fr, berthier (at) imcce.fr

National contacts for the media:
Belgium - Dr. Rodrigo Alvarez +32-2-474 70 50 rodrigo.alvarez@oma.be
Finland - Ms. Tiina Raivo +358 9 7748 8369 tiina.raivo@aka.fi
Denmark - Dr. Michael Linden-Vørnle +45-33-18 19 97 mykal@tycho.dk
France - Dr. Daniel Kunth +33-1-44 32 80 85 kunth@iap.fr
Germany - Dr. Jakob Staude +49-6221-528229 staude@mpia.de
Italy - Dr. Leopoldo Benacchio +39-347-230 26 51 benacchio@inaf.it
The Netherlands - Ms. Marieke Baan +31-20-525 74 80 mbaan@science.uva.nl
Portugal - Prof. Teresa Lago +351-22-089 833 mtlago@astro.up.pt
Sweden - Dr. Jesper Sollerman +46-8-55 37 85 54 jesper@astro.su.se
Switzerland - Dr. Martin Steinacher +41-31-324 23 82 martin.steinacher@sbf.admin.ch
United Kingdom - Mr. Peter Barratt +44-1793-44 20 25 Peter.Barratt@pparc.ac.uk

 

Fingerprinting the Milky Way

ESO 15/07 - Science Release

22 March 2007
For Immediate Release

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.

ESO PR Photo 15/07 The Cluster Collinder 261

"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).

Contact

Gayandhi De Silva
ESO, Chile
Phone: +56 2 463 3066
Email: gdesilva (at) eso.org

Kenneth Freeman
Mount Stromlo Observatory, Australia
Phone: +61 2 6125 0264
Email: kcf (at) mso.anu.edu.au

National contacts for the media:
Belgium - Dr. Rodrigo Alvarez +32-2-474 70 50 rodrigo.alvarez@oma.be
Finland - Ms. Tiina Raivo +358 9 7748 8369 tiina.raivo@aka.fi
Denmark - Dr. Michael Linden-Vørnle +45-33-18 19 97 mykal@tycho.dk
France - Dr. Daniel Kunth +33-1-44 32 80 85 kunth@iap.fr
Germany - Dr. Jakob Staude +49-6221-528229 staude@mpia.de
Italy - Dr. Leopoldo Benacchio +39-347-230 26 51 benacchio@inaf.it
The Netherlands - Ms. Marieke Baan +31-20-525 74 80 mbaan@science.uva.nl
Portugal - Prof. Teresa Lago +351-22-089 833 mtlago@astro.up.pt
Sweden - Dr. Jesper Sollerman +46-8-55 37 85 54 jesper@astro.su.se
Switzerland - Dr. Martin Steinacher +41-31-324 23 82 martin.steinacher@sbf.admin.ch
United Kingdom - Mr. Peter Barratt +44-1793-44 20 25 Peter.Barratt@pparc.ac.uk

 

Star Family Seen Through Dusty Fog

ESO 12/07 - Science Release

13 March 2007
For Immediate Release

New Globular Cluster Found in Milky Way

Images made with ESO's New Technology Telescope at La Silla by a team of German astronomers reveal a rich circular cluster of stars in the inner parts of our Galaxy. Located 30,000 light-years away, this previously unknown closely-packed group of about 100,000 stars is most likely a new globular cluster.

Star clusters provide us with unique laboratory conditions to investigate various aspects of astrophysics. They represent groups of stars with similar ages, chemical element abundances and distances. Globular clusters, in particular, are fossils in the Milky Way that provide useful information. With ages of about 10 billion years, they are among the oldest objects in our Galaxy - almost as old as the Universe itself. These massive, spherical shaped star clusters are therefore witnesses of the early, mysterious ages of the Universe.

ESO PR Photo 12/07 The Newly Identified Cluster

"Moreover, the properties of globular clusters are deeply connected with the history of their host galaxy," says Dirk Froebrich from the University of Kent, and lead-author of the paper presenting the results. "We believe today that galaxy collisions, galaxy cannibalism, as well as galaxy mergers leave their imprint in the globular cluster population of any given galaxy. Thus, when investigating globular clusters we hope to be able to use them as an acid test for our understanding of the formation and evolution of galaxies," he adds.

In our own Galaxy about 150 globular clusters are known, each containing many hundreds of thousands of stars. In contrast to their smaller and less regularly shaped siblings - open clusters - globular clusters are not concentrated in the galactic disc; rather they are spherically distributed in the galactic halo, with increasing concentration towards the centre of the Galaxy. Until the mid 1990s, globular clusters were identified mostly by eye - from visual inspection of photographic plates. However, these early searches are likely to have missed a significant number of globular clusters, particularly close to the disc of the Galaxy, where dense clouds of dust and gas obscure the view. In the early times of extragalactic astronomy this area was called the 'Zone of Avoidance' because extragalactic stellar systems appeared to be very rare in this part of the sky.

Searching for the missing globular clusters in our Galaxy requires observations in the infrared, because infrared radiation is able to penetrate the thick 'galactic fog'. Using modern, sensitive infrared detectors, this is now possible.

Completing the census is not only a challenge for its own sake, as finding new globular clusters is useful for several additional reasons. For example, analysing their orbits allows astronomers to draw conclusions about the distribution of mass in the Galaxy. Star clusters can therefore be used as probes for the large-scale structure of the Milky Way.

"It has been estimated that the region close to the Galactic Centre might contain about 10 so far unknown globular clusters and we have started a large campaign to unveil and characterise them," explains Helmut Meusinger, from the Thüringer Landessternwarte Tautenburg, Germany, and part of the team.

The astronomers carried out a systematic and automated large-scale (14,400 square degrees) search for globular cluster candidates in the entire Galactic Plane, based on the near-infrared Two Micron All Sky Survey (2MASS). Eventually, only about a dozen candidate objects remained.

The astronomers observed these candidates with the SofI instrument attached to ESO's New Technology Telescope (NTT) at La Silla (Chile), taking images through three different near-infrared filters. The new images are ten times deeper and have a much better angular resolution than the original 2MASS images, thereby allowing the astronomers to resolve at least partly the dense accumulation of stars in the globular cluster candidates.

One of these candidates had the number 1735 in the list of Froebrich, Scholz, and Raftery, and is therefore denoted as FSR 1735.

"The unique images we have obtained reveal that the nebulous appearance of the cluster in previous images is in fact due to a large number of faint stars," says Froebrich. "The images show a beautiful, rich, and circular accumulation of stars."

From a detailed analysis of the properties of the cluster, the astronomers arrive at the conclusion that the cluster is about 30,000 light-years away from us and only 10,000 light-years away from the Galactic Centre, close to the Galactic Plane.

"All the evidence supports the interpretation that FSR 1735 is a new globular cluster in the inner Milky Way," says Aleks Scholz, from the University of St Andrews, UK, and another member of the team. "However, to be sure, we now need to measure the age of the cluster accurately, and this requires still deeper observations."

The cluster is about 7 light-years wide (slightly less than twice the distance between the Sun and its nearest star, Proxima Centauri) but contains about 100,000 stars for a total estimated mass of 65,000 times the mass of the Sun. The stars contain between 5 and 8 times less heavy elements than the Sun.

"On its way to our Solar System, the light coming from the stars in the FSR 1735 cluster has to penetrate a thick cloud of dust and gas," says Meusinger. "This is one of the reasons why this cluster was hard to find in previous surveys."

"Is this now the last missing globular cluster in our galaxy?," asks Scholz. "We really can't be sure. The opaque interiors of the Milky Way may well have more surprises in store."

More Information

The team is composed of Dirk Froebrich (University of Kent, UK), Helmut Meusinger (Thüringer Landessternwarte Tautenburg, Germany), and Aleks Scholz (University of St Andrews, Scotland, UK).
This research is presented in an article in press in the Monthly Notices of the Royal Astronomical Society ("FSR 1735 - A new globular cluster candidate in the inner Galaxy", by Froebrich et al.).

Contact

Dirk Froebrich
University of Kent, Canterbury, UK
Phone: +44-1227-827346
Email: df (at) star.kent.ac.uk

Helmut Meusinger
Thüringer Landessternwarte Tautenburg, Germany
Phone: +49-36427-86362
Email: meus (at) tls-tautenburg.de

Aleks Scholz
University of St. Andrews, Scotland, UK
Phone: +44-1334-461666
Email: as110 (at) st-andrews.ac.uk

National contacts for the media:
Belgium - Dr. Rodrigo Alvarez +32-2-474 70 50 rodrigo.alvarez@oma.be
Finland - Ms. Tiina Raivo +358 9 7748 8369 tiina.raivo@aka.fi
Denmark - Dr. Michael Linden-Vørnle +45-33-18 19 97 mykal@tycho.dk
France - Dr. Daniel Kunth +33-1-44 32 80 85 kunth@iap.fr
Germany - Dr. Jakob Staude +49-6221-528229 staude@mpia.de
Italy - Dr. Leopoldo Benacchio +39-347-230 26 51 benacchio@inaf.it
The Netherlands - Ms. Marieke Baan +31-20-525 74 80 mbaan@science.uva.nl
Portugal - Prof. Teresa Lago +351-22-089 833 mtlago@astro.up.pt
Sweden - Dr. Jesper Sollerman +46-8-55 37 85 54 jesper@astro.su.se
Switzerland - Dr. Martin Steinacher +41-31-324 23 82 martin.steinacher@sbf.admin.ch
United Kingdom - Mr. Peter Barratt +44-1793-44 20 25 Peter.Barratt@pparc.ac.uk

 

Solar Power at Play

ESO 11/07 - Science Release

7 March 2007
For Immediate Release

Observing the Spin-Up of an Asteroid

For the very first time, astronomers have witnessed the speeding up of an asteroid's rotation, and have shown that it is due to a theoretical effect predicted but never seen before. The international team of scientists used an armada of telescopes to discover that the asteroid's rotation period currently decreases by 1 millisecond every year, as a consequence of the heating of the asteroid's surface by the Sun. Eventually it may spin faster than any known asteroid in the solar system and even break apart.

ESO PR Photo 11a/07 Asteroid 2000 PH5

"The Yarkovsky-O'Keefe-Radzievskii-Paddack (YORP) effect is believed to alter the way small bodies in the Solar System rotate," said Stephen Lowry (Queens University Belfast, UK), lead-author of one of the two companion papers in which this work is reported [1, 2].

"The warming caused by sunlight hitting the surfaces of asteroids and meteoroids leads to a gentle recoil effect as the heat is released," he added. "By analogy, if one were to shine light on a propeller over a long enough period, it would start spinning."

Although this is an almost immeasurably weak force, its effect over millions of years is far from negligible. Astronomers believe the YORP effect may be responsible for spinning some asteroids up so fast that they break apart, perhaps leading to the formation of double asteroids. Others may be slowed down so that they take many days to complete a full turn. The YORP effect also plays an important role in changing the orbits of asteroids between Mars and Jupiter, including their delivery to planet-crossing orbits, such as those of near-Earth asteroids. Despite its importance, the effect has never been seen acting on a solar system body, until now.

Using extensive optical and radar imaging from powerful Earth-based observatories, astronomers have directly observed the YORP effect in action on a small near-Earth asteroid, known as (54509) 2000 PH5.

Shortly after its discovery in 2000, it was realised that asteroid 2000 PH5 would be the ideal candidate for such a YORP detection. With a diameter of just 114 metres, it is relatively small and so more susceptible to the effect. Also, it rotates very fast, with one 'day' on the asteroid lasting just over 12 Earth minutes, implying that the YORP effect may have been acting on it for some time. With this in mind, the team of astronomers undertook a long term monitoring campaign of the asteroid with the aim of detecting any tiny changes in its rotation speed.

Over a 4-year time span, Stephen Lowry, Alan Fitzsimmons and colleagues took images of the asteroid at a range of telescope sites including ESO's 8.2-m Very Large Telescope array and 3.5-m New Technology Telescope in Chile, the 3.5-m telescope at Calar Alto, Spain, along with a suite of other telescopes from the Czech Republic, the Canary Islands, Hawaii, Spain and Chile. With these facilities the astronomers measured the slight brightness variations as the asteroid rotated.

ESO PR Photo 11b/07 Radar Images of 2000 PH5

Over the same time period, the radar team led by Patrick Taylor and Jean-Luc Margot of Cornell University employed the unique capabilities of the Arecibo Observatory in Puerto Rico and the Goldstone radar facility in California to observe the asteroid by 'bouncing' a radar pulse off the asteroid and analysing its echo.

"With this technique we can reconstruct a 3-D model of the asteroid's shape, with the necessary detail to allow a comparison between the observations and theory," said Taylor.

After careful analysis of the optical data, the asteroid's spin rate was seen to steadily increase with time, at a rate that can be explained by the YORP theory. Critically, the effect was observed year after year, for more than 4 years. Furthermore, this number was elegantly supported via analysis of the combined radar and optical data, as it was required that the asteroid is increasing its spin rate at exactly this rate in order for a satisfactory 3-D shape model to be determined.

ESO PR Video 11c/07 Watch the Asteroid Move!

To predict what will happen to the asteroid in the future, Lowry and his colleagues performed detailed computer simulations using the measured strength of the YORP effect and the detailed shape model. They found that the orbit of the asteroid about the Sun could remain stable for up to the next 35 million years, allowing the rotation period to be reduced by a factor of 36, to just 20 seconds, faster than any asteroid whose rotation has been measured until now.

"This exceptionally fast spin-rate could force the asteroid to reshape itself or even split apart, leading to the birth of a new double system," said Lowry.

Notes

[1] Stephen C. Lowry, Alan Fitzsimmons, Petr Pravec, David Vokrouhlicky, Hermann Boehnhardt, Patrick A. Taylor, Jean-Luc Margot, Adrian Galad, Mike Irwin, Jonathan Irwin, and Peter Kusnirak (2007). Direct Detection of the Asteroidal YORP Effect, Published online in Science Express.

[2] Patrick A. Taylor, Jean-Luc Margot, David Vokrouhlicky, Daniel J. Scheeres, Petr Pravec, Stephen C. Lowry, Alan Fitzsimmons, Michael C. Nolan, Steven J. Ostro, Lance A. M. Benner, Jon D. Giorgini, Christopher Magri (2007). Spin Rate of Asteroid (54509) 2000 PH5 Increasing due to the YORP Effect, Published online in Science Express.

Contacts

Stephen Lowry, Alan Fitzsimmons
Astrophysics Research Centre
Queen's University Belfast, UK
Phone: +44 28 9097-3692, +44 7834-318834
Email: s.c.lowry (at) qub.ac.uk, a.fitzsimmons (at) qub.ac.uk

Patrick Taylor, Jean-Luc Margot
Department of Astronomy
Cornell University, USA
Phone: +1 607-255-2727, +1 607-255-1810
Email: ptaylor (at) astro.cornell.edu, jlm (at) astro.cornell.edu

Hermann Boehnhardt
Max-Planck Institute for Solar System Research
Katlenburg-Lindau, Germany
Phone: +49 5556-979-545
Email: boehnhardt (at) mps.mpg.de

Petr Pravec
Astronomical Institute AS CR
Ondrejov, Czech Republic
Phone: +420 323-620352, +420 737-322815
Email: ppravec (at) asu.cas.cz, suchan (at) astro.cz

National contacts for the media:
Belgium - Dr. Rodrigo Alvarez +32-2-474 70 50 rodrigo.alvarez@oma.be
Finland - Ms. Tiina Raivo +358 9 7748 8369 tiina.raivo@aka.fi
Denmark - Dr. Michael Linden-Vørnle +45-33-18 19 97 mykal@tycho.dk
France - Dr. Daniel Kunth +33-1-44 32 80 85 kunth@iap.fr
Germany - Dr. Jakob Staude +49-6221-528229 staude@mpia.de
Italy - Dr. Leopoldo Benacchio +39-347-230 26 51 benacchio@inaf.it
The Netherlands - Ms. Marieke Baan +31-20-525 74 80 mbaan@science.uva.nl
Portugal - Prof. Teresa Lago +351-22-089 833 mtlago@astro.up.pt
Sweden - Dr. Jesper Sollerman +46-8-55 37 85 54 jesper@astro.su.se
Switzerland - Dr. Martin Steinacher +41-31-324 23 82 martin.steinacher@sbf.admin.ch
United Kingdom - Mr. Peter Barratt +44-1793-44 20 25 Peter.Barratt@pparc.ac.uk

 

It Is No Mirage!

ESO 02/07 - Science Release

8 January 2007
For Immediate Release

Large Telescopes Team Up to Help Astronomers Discover a Trio of Quasars

Using ESO's Very Large Telescope and the W.M. Keck Observatory, astronomers at the Ecole Polytechnique Fédérale de Lausanne in Switzerland and the California Institute of Technology, USA, have discovered what appears to be the first known triplet of quasars. This close trio of supermassive black holes lies about 10.5 billion light-years away towards the Virgo (The Virgin) constellation.

"Quasars are extremely rare objects," says George Djorgovski, from Caltech and leader of the team that made the discovery. "To find two of them so close together is very unlikely if they were randomly distributed in space. To find three is unprecedented."

The findings are being reported at the winter 2007 meeting of the American Astronomical Society in Seattle, USA.

ESO PR Photo 02/07 The Trio of Quasars

Quasars are extraordinary luminous objects in the distant universe, thought to be powered by supermassive black holes at the heart of galaxies. A single quasar could be a thousand times brighter than an entire galaxy of a hundred billion stars, and yet this remarkable amount of energy originates from a volume smaller than our solar system. About a hundred thousand quasars have been found to date, and among them several tens of close pairs, but this is the first known case of a close triple quasar system.

Quasars (QUAsi StellAR Sources) were first discovered in 1963 by the Dutch-American astronomer Maarten Schmidt at the Palomar Observatory (California, USA) and the name refers to their 'star-like' appearance on the images obtained at that time. Distinguishing them from stars is thus no easy task and discovering a close trio of such objects is even less obvious.

The feat could only be accomplished by combining images from two of the largest ground-based telescopes, ESO's 8.2-m Very Large Telescope at Cerro Paranal, in Chile, and the W. M. Keck Observatory's 10-m telescope atop Mauna Kea, Hawaii, as well as using a very sophisticated and efficient image sharpening method.

The distant quasar LBQS 1429-008 was first discovered in 1989 by an international team of astronomers led by Paul Hewett of the Institute of Astronomy in Cambridge, England. Hewett and his collaborators found a fainter companion to their quasar, and proposed that it was a case of gravitational lensing. According to Einstein's general theory of relativity, if a large mass (such as a big galaxy or a cluster of galaxies) is placed along the line of sight to a distant quasar, the light rays are bent, and an observer on Earth will see two or more close images of the quasar â" a cosmic mirage. The first such gravitational lens was discovered in 1979, and hundreds of cases are now known. However, several groups over the past several years cast doubts that this system is a gravitational lens, and proposed instead that it is a close physical pair of quasars.

What the Caltech-Swiss team has found is that there is a third, even fainter quasar associated with the previously known two. The three quasars have the same redshift, hence, are at the same distance from us.

The astronomers performed an extensive theoretical modeling, trying to explain the observed geometry of the three images as a consequence of gravitational lensing. "We just could not reproduce the data," says Frédéric Courbin of Lausanne. "It is essentially impossible to account for what we see using reasonable gravitational lensing models."

Moreover, there is no trace of a possible lensing galaxy, which would be needed if the system were a gravitational lens. The team has also documented small, but significant differences in the properties of the three quasars. These are much easier to understand if the three quasars are physically distinct objects, rather than gravitational lensing mirages. Combining all these pieces of evidence effectively eliminated lensing as a possible explanation.

"We were left with an even more exciting possibility that this is an actual triple quasar," says Georges Meylan, also from Lausanne. The three quasars are separated by only about 100,000 to 150,000 light-years, which is about the size of our own Milky Way.

Gravitational lensing can be used to probe the distribution of dark and visible mass in the universe, but quasar pairs -and now a triplet- provide astronomers with a different kind of insight.

"Quasars are believed to be powered by gas falling into supermassive black holes," says Djorgovski. "This process happens very effectively when galaxies collide or merge, and we are observing this system at the time in the cosmic history when such galaxy interactions were at a peak."

If galaxy interactions were responsible for the quasar activity, having two quasars close together would be much more likely than if they were randomly distributed in space. This may explain the unusual abundance of binary quasars, which have been reported by several groups. "In this case, we are lucky to catch a rare situation where quasars are ignited in three interacting galaxies," says Ashish Mahabal, one of the Caltech scientists involved in the study.

Discoveries of more such systems in the future may help astronomers understand better the fundamental relationship between the formation and evolution of galaxies, and the supermassive black holes in their cores, now believed to be common in most large galaxies, our own Milky Way included.

This work is also described in a paper submitted to the Astrophysical Journal Letters. The team is composed of S. George Djorgovski, Ashish Mahabal, and Eilat Glikman of Caltech (USA), Frédéric Courbin, Georges Meylan and Dominique Sluse of the Ecole Polytechnique Fédérale de Lausanne (Switzerland), and David Thompson of the University of Arizona's Large Binocular Telescope Observatory (USA).

 

Contacts

Georges Meylan
Ecole Polytechnique Fédérale de Lausanne
Phone (Sauverny): +41 22 379 24 25
Phone (Lausanne): +41 21 693 06 44
Email: Georges.Meylan@epfl.ch

S. George Djorgovski
(attending the AAS meeting in Seattle on Jan. 7-8)
Caltech Astronomy, USA
Phone: +1 (626) 395-4415
Email: george@astro.caltech.edu

National contacts for the media:
Belgium - Dr. Rodrigo Alvarez +32-2-474 70 50 rodrigo.alvarez@oma.be
Finland - Ms. Riitta Tirronen +358 9 7748 8369 riitta.tirronen@aka.fi
Denmark - Dr. Michael Linden-Vørnle +45-33-18 19 97 mykal@tycho.dk
France - Dr. Daniel Kunth +33-1-44 32 80 85 kunth@iap.fr
Germany - Dr. Jakob Staude +49-6221-528229 staude@mpia.de
Italy - Dr. Leopoldo Benacchio +39-347-230 26 51 benacchio@inaf.it
The Netherlands - Ms. Marieke Baan +31-20-525 74 80 mbaan@science.uva.nl
Portugal - Prof. Teresa Lago +351-22-089 833 mtlago@astro.up.pt
Sweden - Dr. Jesper Sollerman +46-8-55 37 85 54 jesper@astro.su.se
Switzerland - Dr. Martin Steinacher +41-31-324 23 82 martin.steinacher@sbf.admin.ch
United Kingdom - Mr. Peter Barratt +44-1793-44 20 25 Peter.Barratt@pparc.ac.uk