Mathematics – Logic
Scientific paper
Feb 1999
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999eso..pres....3.&link_type=abstract
ESO Press Release, 02/1999
Mathematics
Logic
Scientific paper
New Spectrograph Explores the Skies from La Silla
While a major effort is now spent on the Very Large Telescope and its advanced instruments at Paranal, ESO is also continuing to operate and upgrade the extensive research facilities at La Silla, its other observatory site.
ESO PR Photo 03a/99
ESO PR Photo 03a/99
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Caption to PR Photo 03a/99 : This photo shows the ESO 1.52-m telescope, installed since almost 30 years in its dome at the La Silla observatory in the southern Atacama desert. The new FEROS spectrograph is placed in an adjacent, thermally and humidity controlled room in the telescope building (where a classical coudé spectrograph was formerly located). The light is guided from the telescope to the spectrograph by 14-m long optical fibres.
Within this programme, a new and powerful spectrograph, known as the Fibre-fed Extended Range Optical Spectrograph (FEROS) , has recently been built by a consortium of European institutes. It was commissioned in late 1998 at the ESO 1.52-m telescope by a small team of astronomers and engineers and has already produced the first, interesting scientific results.
FEROS is able to record spectra of comparatively faint stars. For instance, it may be used to measure the chemical composition of stars similar to our Sun at distances of up to about 2,500 light-years, or to study motions in the atmospheres of supergiant stars in the Magellanic Clouds. These satellite galaxies to the Milky Way are more than 150,000 light-years away and can only be observed with telescopes located in the southern hemisphere. First FEROS observations uncover an unusual star
ESO PR Photo 03b/99
ESO PR Photo 03b/99
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Caption to PR Photo 03b/99 : This diagramme shows the spectrum of the Lithium rich giant star S50 in the open stellar cluster Be21 , compared to that of a normal giant star ( S156 ) in the same cluster. The comparatively strong absorption line at the centre, at wavelength 6708 Å (671 nm), is caused by Lithium atoms (Li I) in the upper layers of the star's atmosphere. Lines from Iron (Fe I) and Calcium (Ca I) atoms are also present in this spectral region. While they are of about equal strength in the two stars, the Lithium line is not seen in the comparison spectrum of S156 . Stellar evolution theories do not predict the presence of Lithium in a giant star like S50 .
Technical information: FEROS obtained two spectra (each of 90 min exposure) of S50 , both showing this strong Lithium line and thus proving that it cannot have been caused by an instrumental effect. These spectra also illustrate the great amount of information that may be obtained in each exposure with FEROS - the shown spectral interval is just 1/280 of the total range recorded. The (visual) magnitude of S50 is 15.6, i.e., about 7,000 times fainter than what can be seen with the unaided eye.
During the first tests of FEROS at the 1.52-m telescope, spectra were obtained of many different stars. Some of these observational data could be used for scientific purposes and, in one case, led to the discovery of unusual properties of a giant star in a stellar cluster. Its spectrum shows an unexplained large amount of the cosmologically important, light element Lithium, cf. PR Photo 03b/99 . The star is thus an obvious object for further, even more detailed studies with ESO's Very Large Telescope (VLT).
This giant star, designated as S50 , is a member of the open-type stellar cluster Be21 (less dense than globular clusters). This cluster is of special interest, since its stars contain few elements heavier than hydrogen and helium. It is located in the direction opposite to the Galactic Center and the distance has been measured as approximately 16,000 light-years. All of its stars were formed at the same time, about 2,000 - 2,500 million years ago; this corresponds to half of the age of the Solar System. The study of stars in this cluster provides important information about the chemical evolution of the Milky Way galaxy. The significance of Lithium
Lithium is not a very common element in daily life (except in batteries and certain medical drugs), but it is of great interest in astronomy. It is the heaviest element that is supposed to have been created in measurable quantities in the early Universe, soon after the Big Bang.
All stars destroy most of their Lithium soon after their formation, although some manage to produce this element again at a later stage of their evolution [1]. There may be a substantial loss of Lithium from evolved stars into the interstellar medium (ISM). This element is indeed observed in the ISM. Calculations have shown that the primordial (original) abundance of Lithium was about ten times less than what is now measured in the ISM.
The present abundance of Lithium in the Sun is over 100 times less than in the ISM. Large quantities of this element would certainly not be expected in a star as old as S50, especially since violent motions in the atmospheres of such giant stars very efficiently mix the material in the upper layers with that from the star's inner regions where the ongoing nuclear processes quickly destroy any Lithium. Still, the FEROS spectra show the presence in S50 of Lithium in quantities similar to that in the ISM - or in the proto-solar nebula from which the Sun and the planets formed, about 4,600 million years ago!
The spectra of many hundreds of giant stars in the solar neighbourhood have been recorded, but only a few have shown such an unusual presence of Lithium. This is the first time that a Lithium rich giant star has been found in a stellar cluster and for which a comparatively accurate age can be determined. In fact, S50 appears to contain more of this fragile element than any other giant star observed so far. What is the origin of the Lithium in S50?
How can this unexpected observation be explained? The astronomers do not know, but suggest two possible causes.
One might be the recent infall of a large planet or a brown dwarf star (an object too small to become a star and hence without nuclear processes, cf. ESO PR 07/97 ) into the atmosphere of S50 . Another is that the star experiences a very short evolutionary period very rarely observed [2] and during which Lithium is produced and brought to the upper atmosphere.
According to our current knowledge of stellar evolution, S50 is due to lose much of its mass through a strong stellar wind during the next few million years. Its Lithium will then be returned to the ISM and thereby contribute to the above mentioned enrichment of this medium. Future observations
There is little doubt that this star and many other giant stars in stellar clusters will be high on the list of objects that will soon be observed with the next large instrument to be installed at the VLT on Paranal. Some months after the First Light event of the second VLT 8.2-m Unit Telescope (UT2) in March 1999, the UVES high-dispersion spectrograph will be mounted on this large telescope. This powerful telescope/instrument combination will also be able to extend this type of astronomical studies to fainter and more distant stars, in the Milky Way as well as in the Magellanic Clouds.
Still, the VLT UT2 will also have many other tasks to perform. It is therefore important that FEROS is available as an effective and dedicated spectroscopic facility that is bound to uncover many other unusual objects in the southern sky. FEROS - a high-dispersion spectrograph fed by optical fibres
FEROS is a state-of-the-art high-resolution spectrograph, based on an advanced concept. The light from celestial objects is collected by the 1.52-m telescope and transferred to the new instrument through optical fibres.
It was built in a collaboration between the Heidelberg State Observatory , the Copenhagen University Astronomical Observatory and ESO . The Heidelberg State Observatory was responsible for the overall design and construction, as well as the data reduction software; this institution
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