Astronomy and Astrophysics – Astrophysics
Scientific paper
Aug 2002
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002eso..pres...15.&link_type=abstract
ESO Press Release, 08/2002
Astronomy and Astrophysics
Astrophysics
Scientific paper
VLT Observes Wolf-Rayet Stars in Virgo Cluster Galaxies [1]
Summary
Do very massive stars form in metal-rich regions of the Universe and in the nuclei of galaxies ? Or does "heavy element poisoning" stop stellar growth at an early stage, before young stars reach the "heavyweight class"?
What may at the first glance appear as a question for specialists actually has profound implications for our understanding of the evolution of galaxies, those systems of billions of stars - the main building blocks of the Universe.
With an enormous output of electromagnetic radiation and energetic elementary particles, massive stars exert a decisive influence on the surrounding (interstellar) gas and dust clouds . They also eject large amounts of processed elements, thereby participating in the gradual build-up of the many elements we see today. Thus the presence or absence of such stars at the centres of galaxies can significantly change the overall development of those regions and hence, presumably, that of the entire galaxy.
A team of European astronomers [2] has now directly observed the presence of so-called Wolf-Rayet stars (born with masses of 60 - 90 times that of the Sun or more) within metal-rich regions in some galaxies in the Virgo cluster, some 50 million light-years away. This is the first unambiguous detection of such massive stellar objects in metal-rich regions .
PR Photo 20a/02 : H II regions in the Virgo cluster galaxy NGC 4254 . PR Photo 20b/02 : Multi-object-slit observation of galaxy NGC 4303 . PR Photo 20c/02 : Spectrum of H II region in NGC 4254 with Wolf-Rayet signatures. Production of heavy elements in the Universe
Most scientists agree that the Universe in which we live underwent a dramatic event, known as the Big Bang , approximately 15,000 million years ago. During the early moments, elementary particles were formed which after some time united into more complex nuclei and in turn resulted in the production of hydrogen and helium atoms and their isotopes, with a sprinkling of the light element lithium.
At our epoch, the visible ("baryonic") matter in the Universe still mostly consists of hydrogen and helium. However, progressively heavier elements have been built up via fusion processes in the interior of stars ever since the Big Bang. Some of the heaviest elements are also produced when massive stars die in gigantic stellar explosions, observed as "supernovae".
This gradual process, referred to as "chemical evolution" , occurs with different speeds in different regions of the Universe, being fastest in those regions where star formation is most intense.
In the relatively "quiet" region of the Milky Way galaxy where our Solar System was born some 4,600 million years ago, it took nearly 10,000 million years to produce all the heavy elements now found in our neighbourhood . Contrarily, in the innermost regions ( the "nuclei" ) of normal galaxies and especially in so-called "active galaxies", the same or even higher heavy-element "enrichment" levels were reached in much shorter time, less than about 1,000 to 2,000 million years. This is the result of observations of particularly active galaxy nuclei ("quasars") in the distant (i.e., early) Universe. Star formation in highly enriched environments
Little is presently known about such highly enriched environments. Since astronomers refer to elements heavier than hydrogen and helium as "metals" , they talk about "metal-rich" regions . This is readily observable from the presence of strong lines from heavier elements in the spectra of the interstellar gas in such regions.
A central, still unresolved question is whether under such special conditions, stars can still form with the same diversity of masses, as this happens in other, less extreme areas of the Universe . Indeed, some current theories of star formation and certain indirect observations appear to indicate that very heavy stars - with masses more than 20 - 30 times that of our Sun - could not possibly form in metal-rich regions.
This would be because the very strong radiation from nascent stars in such environments would be most efficiently "stopped" by the surrounding material. That leads to a repulsive effect, which would rapidly disperse the remains of the natal cloud and thereby halt any further growth beyond a certain limit. Deprived of "food", those young stellar objects would be unable to grow beyond a certain, limited mass.
Stars with masses up to 100 - 200 times that of the Sun are known to exist in more "normal" regions. However, if the above ideas were true, there would be no such "heavy-weight" stars in "metal-rich" regions. Whether this is really so or not has important implications for a correct understanding of the nuclei of galaxies, the properties of massive galaxies and, in general, for all evolved regions of the Universe. VLT observes star-forming nebulae in distant galaxies
ESO PR Photo 20a/02
ESO PR Photo 20a/02
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ESO PR Photo 20b/02
ESO PR Photo 20b/02
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Caption : PR Photo 20a/02 shows an image of the Virgo cluster spiral galaxy NGC 4254, with the "metal-rich" H II regions indicated that were observed with the VLT. In PR Photo 20b/02 , the very efficient multi-object-slit observing technique with the multi-mode instrument FORS1 is demonstrated on the Virgo cluster galaxy NGC 4303 . Nineteen moveable slits at the instrument focal plane are positioned so that the faint light from several H II regions in this galaxy can pass into the spectrograph, while the much stronger "background" light (from the nearby areas in the galaxy and, to a large extent, from the Earth's upper atmosphere) is blocked by the mask. This technique is explained in more detail in ESO PR Photos 38c-d/98.
Using the ESO Very Large Telescope (VLT) at the Paranal Observatory , a team of French, Swiss, and Spanish astronomers [2] were able for the first time to detect signs of a large number of extremely massive stars inside "metal-rich" star-forming regions . This observation-based result thus contradicts the above mentioned theory.
The observations aimed at obtaining optical spectra of numerous such star-forming regions, located in a number of galaxies in the Virgo galaxy cluster , that is seen in the constellation of that name at a distance of about 50 million light-years, cf. PR Photo 20a-b/02 . It is at the centre of a supercluster of galaxies in the outskirts of which the "Local Group" - with the Milky Way galaxy where we live - is located.
These nebulae - also known as "H II regions" because of their content of ionized hydrogen - are very dim and therefore difficult to observe. However, the astronomers were able to obtain detailed spectra of excellent quality, thanks to the large light-collecting power of the 8.2-m VLT ANTU telescope, together with the FORS1 instrument, here used in the very efficient multi-spectra mode. Massive stars in NGC 4254
ESO PR Photo 20c/02
ESO PR Photo 20c/02
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Caption : PR Photo 20c/02 shows the observational evidence of the presence of hot and massive "Wolf-Rayet" stars [3] in a metal-rich H II region (designated "-014+081") in the spiral galaxy NGC 4254, a member of the Virgo cluster of galaxies at a distance of about 50 million light-years. Comparison spectra of two types of Wolf-Rayet stars (WC and WN) in the Milky Way galaxy are shown. The characteristic spectral features of ionized helium (He II) and double and triple ionized carbon (C III, C IV) are identical.
Spectra of about ninety "metal-rich" HII regions were secured in the course of only one observing night. Almost thirty of them clearly show unambiguous "spectral fingerprints" of so-called Wolf-Rayet stars [3], a type of stars also known in the Milky Way galaxy, cf. PR Photo 20c/02 . They are the descendants of the most massive stars known, and the quality of the V
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