Physics – Space Physics
Scientific paper
Oct 2002
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002eso..pres...19.&link_type=abstract
ESO Press Release, 10/2002
Physics
Space Physics
Scientific paper
VLT UVES Observes Most Metal-Deficient Star Known [1]
Summary
A faint star in the southern Milky Way, designated HE 0107-5240 , has been found to consist virtually only of hydrogen and helium . It has the lowest abundance of heavier elements ever observed , only 1/200,000 of that of the Sun - 20 times less than the previous record-holding star.
This is the result of a major ongoing research project by an international team of astronomers [2]. It is based on a decade-long survey of the southern sky, with detailed follow-up observations by means of the powerful UV-Visual Echelle Spectrograph (UVES) on the 8.2-m VLT KUEYEN telescope at the ESO Paranal Observatory in Chile.
This significant discovery now opens a new window towards the early times when the Milky Way galaxy was young, possibly still in the stage of formation. It proves that, contrary to most current theories, comparatively light stars like HE 0107-5240 (with 80% of the mass of the Sun) may form in environments (nearly) devoid of heavier elements.
Since some years, astronomers have been desperately searching for stars of the very first stellar generation in the Milky Way, consisting only of hydrogen and helium from the Big Bang. None have been detected so far and doubts have arisen that they exist at all.
The present discovery provides new hope that it will ultimately be possible to find such stellar relics from the young Universe and thereby to study "unpolluted" Big Bang material.
PR Photo 25a/02 : The sky region around the very metal-deficient star HE 0107-5240 . PR Photo 25b/02 : Comparison of UVES spectra of stars with different metal abundances. Stellar generations in the Milky Way galaxy
The Milky Way galaxy in which we live formed from a gigantic cloud of gas, when the Universe was still young, soon after the initial Big Bang. At the beginning, this gas was presumably composed almost exclusively of hydrogen and helium atoms produced during the Big Bang.
However, once the first stars formed by contraction in that gas, many heavier elements were built up by nuclear processes in their interiors. As time passed, many of the stars of this and following stellar generations returned the processed matter to their surroundings at the ends of their lives, either during violent supernova explosions or via strong "stellar winds". In this way, the interstellar gas in the Milky Way system has ever since been continuously enriched with heavier elements. Stars of later generations like our Sun now contain those elements produced by their ancestors and we are indeed ourselves made up of them.
Consequently, the early (and hence, old) stars in the Milky Way mainly differ from younger stars by containing very small amounts of such elements . Hunting the earliest stars
Have some of those earliest stars survived to our days? In theory, at least, it would be possible that some of the lighter ones - having the longest lifetimes - are still around. But if so, where are they?
During the past three decades, astronomers have desperately tried to find bona-fide representatives of the very first stellar generation(s) in the Milky Way, i.e. stars with no or, at most, extremely low abundance of elements other than hydrogen and helium. The researchers usually refer to such objects as Population III stars , the other two populations being stars with heavy-element abundances like the Sun (Population I) or somewhat less (Population II) [3]. The Hamburg/ESO survey
Now, a group of astronomers from Germany, Sweden, Australia, Brazil and the USA [2] has found a giant star that has a concentration of heavy elements 200,000 times lower than the Sun, or about 20 times less than the previous "record" for this kind of star. It thus provides the researchers with a unique window towards the early stages of the formation of the Milky Way and a fine opportunity to study stellar gas with a composition close to that produced during the Big Bang.
This is one important outcome of a systematic search for the most metal-deficient stars that is currently being carried out at Hamburger Sternwarte [4]. Over a period of more than 10 years, a large collection of photographic pictures of the southern sky were obtained with the ESO 1-m Schmidt Telescope, a wide-angle telescope at the La Silla observatory in Chile that has now been decommissioned. Thanks to a large glass prism in the front of the telescope, every object in the observed sky field - stars as well as galaxies - was imaged as a small spectrum, providing a first rough idea about its type and composition.
The main aim of this "Hamburg/ESO survey" (with Dieter Reimers , Associate Director of the Hamburger Sternwarte, as Principal Investigator and Lutz Wisotzki , now at Astrophysikalisches Institut Potsdam, Germany, as Project Scientist) was to find quasars (particularly active centres of galaxies), a task that was accomplished most successfully, cf. e.g., ESO PR 10/97 and ESO PR 08/00 (Report F).
A very welcome by-product of this survey has been a rich harvest of very metal-poor stars . This part of the project is led by Norbert Christlieb , also from the Hamburg Observatory, and now on sabbatical leave at the Research School of Astronomy and Astrophysics of the Australian National University (Canberra, Australia).
Using fast computers and advanced pattern-recognition software to analyze the photographic exposures and thus to sift through millions of registered stellar spectra, about 8000 candidates for very metal-poor stars were found. These stars are now being scrutinized spectroscopically one-by-one with many medium-sized telescopes all over the world. Confirmed candidates are then observed with the largest telescopes in the world in order to obtain very detailed spectra (of high spectral resolution), which allow the astronomers to determine their chemical composition accurately. The very metal-deficient star HE 0107-5240
ESO PR Photo 25a/02
ESO PR Photo 25a/02
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ESO PR Photo 25b/02
ESO PR Photo 25b/02
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Caption : PR Photo 25a/02 shows a small sky field with the very metal-deficient star HE 0107-5240 at the centre (reproduced from the Digital Sky Survey [STScI Digitized Sky Survey, (C) 1993, 1994, AURA, Inc. all rights reserved - cf. http://archive.eso.org/dss/dss]). PR Photo 25b/02 displays a comparison of a region of the spectrum of the Sun (top) with that of CD -38 245 , the previously most iron-deficient star known (2nd from top), the new record-holder HE 0107-5240 (3rd from top), and a (hypothetical) Population III star [4], consisting only of elements produced in the Big Bang, i.e. hydrogen and helium, and traces of lithium. As can be seen, the spectral absorption lines become progressively weaker with decreasing content of heavier elements. While there is 1 iron atom for every 31,000 hydrogen atoms in the atmosphere of the Sun, in HE 0107-5240 this ratio is about 200,000 times smaller, or only 1 iron atom for every 6.8 billion hydrogen atoms! The two spectra in the middle show that HE 0107-5240 is indeed much more metal-poor than the previous record-holder CD -38 245 - the iron (Fe) lines in the spectrum of HE 0107-5240 are weaker (or absent) and the Nickel (Ni) line is not visible at all.
One of these stars has been designated HE 0107-5240 ("HE" stands for Hamburg/ESO Survey, and the number denotes the approximate position of the star on the sky). It is about ten thousand times fainter than the faintest stars that can be seen with the unaided eye. It is located in the direction of the southern constellation Phoenix, at a distance of about 36,000 light-years.
This star was observed in December 2001 with the UV-Visual Echelle Spectrograph (UVES) on the 8.2-m VLT KUEYEN telescope at the ESO Paranal Observatory (Chile). From these spectra, Norbert Christlieb and his colleagues at the Dept. of Astronomy and Space Physics, University of Uppsala (Sweden) and at the Munich University Observatory (Germany) were able to determine the chemical compositi
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