Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:

 

Heavy Stars Thrive among Heavy Elements

23.08.2002


VLT Observes Wolf-Rayet Stars in Virgo Cluster Galaxies [1]

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.

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

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

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 VLT spectra is such that the presence of as few as two Wolf-Rayet stars in one H II region could be detected, even at this large distance!

A detailed analysis of the comprehensive observational data has shown that stars with masses of at least 60 - 90 times that of the Sun are definitely formed in the "metal-rich" regions in those Virgo galaxies. Furthermore, the ratio of these heavy stars to less massive ones is found to be identical to that observed in "normal" environments.

Important implications

These new results provide important information for our understanding of star formation, one of the central issues of modern astrophysics. They show beyond doubt that the formation of very massive stars is not suppressed in an environment with strong chemical enrichment.

Most galactic nuclei, massive and interacting galaxies and related objects are metal-rich and this new finding therefore implies that they must also harbour massive stars. The VLT observations provide the first clear and direct evidence for this.

Massive stars play a leading role in shaping the complex interactions between the many components of a galaxy - stars, interstellar gas and cold molecular clouds. With their enormous output of electromagnetic radiation and strong winds of elementary particles and, not least, by means of gigantic supernova explosions at the end of their short lives, they thoroughly stir up the interstellar gas and dust in their surroundings. Moreover, they are responsible for the production of the bulk of the heavy elements now observed in the Universe. No picture of the evolution of galaxies can therefore be complete without taking into account the presence (or absence) of massive stars.

In more immediate terms, the fact that massive stars exist in metal-rich environments will also have a direct implication for the interpretation of spectra of remote galaxies.

Future observations In the wake of this successful result, supplementary observations are now being planned with various ESO facilities in order to obtain a better understanding of the complex phenomenon of massive star formation in all kinds of galaxies, including those in the nearby Universe and also primordial galaxies.

This will involve, among others, infrared observations of young galaxies in which intensive star-forming processes are now going on ("starburst galaxies") with the Thermal Infrared Multimode Instrument (TIMMI2) on the ESO 3.6-m telescope at the La Silla Observatory (Chile), and later with the VLT Mid Infrared Spectrometer/Imager (VISIR), a future, extremely powerful mid-infrared sensitive instrument. The infrared technique allows to study the earliest phases of massive star formation, deep inside the natal clouds. In addition, highly promising searches for very remote galaxies, in the process of forming their first stars, are now underway with the Infrared Spectrometer And Array Camera (ISAAC) at the VLT.

More information

The information presented in this Press Release is based on a research article in the European research journal "Astronomy & Astrophysics" ("VLT observations of metal-rich extragalactic HII regions. I. Massive star populations and the upper end of the IMF" by Maximilien Pindao, Daniel Schaerer, Rosa M. Gonzalez Delgado and Grazyna Stasinska. It is available on the web at http://arXiv.org/abs/astro-ph/0208226.

Richard West | alfa
Further information:
http://arXiv.org/abs/astro-ph/0208226
http://www.eso.org/outreach/press-rel/pr-2002/pr-15-02.html

More articles from Physics and Astronomy:

nachricht New material for splitting water
19.06.2018 | American Institute of Physics

nachricht Carbon nanotube optics provide optical-based quantum cryptography and quantum computing
19.06.2018 | DOE/Los Alamos National Laboratory

All articles from Physics and Astronomy >>>

The most recent press releases about innovation >>>

Die letzten 5 Focus-News des innovations-reports im Überblick:

Im Focus: Overdosing on Calcium

Nano crystals impact stem cell fate during bone formation

Scientists from the University of Freiburg and the University of Basel identified a master regulator for bone regeneration. Prasad Shastri, Professor of...

Im Focus: AchemAsia 2019 will take place in Shanghai

Moving into its fourth decade, AchemAsia is setting out for new horizons: The International Expo and Innovation Forum for Sustainable Chemical Production will take place from 21-23 May 2019 in Shanghai, China. With an updated event profile, the eleventh edition focusses on topics that are especially relevant for the Chinese process industry, putting a strong emphasis on sustainability and innovation.

Founded in 1989 as a spin-off of ACHEMA to cater to the needs of China’s then developing industry, AchemAsia has since grown into a platform where the latest...

Im Focus: First real-time test of Li-Fi utilization for the industrial Internet of Things

The BMBF-funded OWICELLS project was successfully completed with a final presentation at the BMW plant in Munich. The presentation demonstrated a Li-Fi communication with a mobile robot, while the robot carried out usual production processes (welding, moving and testing parts) in a 5x5m² production cell. The robust, optical wireless transmission is based on spatial diversity; in other words, data is sent and received simultaneously by several LEDs and several photodiodes. The system can transmit data at more than 100 Mbit/s and five milliseconds latency.

Modern production technologies in the automobile industry must become more flexible in order to fulfil individual customer requirements.

Im Focus: Sharp images with flexible fibers

An international team of scientists has discovered a new way to transfer image information through multimodal fibers with almost no distortion - even if the fiber is bent. The results of the study, to which scientist from the Leibniz-Institute of Photonic Technology Jena (Leibniz IPHT) contributed, were published on 6thJune in the highly-cited journal Physical Review Letters.

Endoscopes allow doctors to see into a patient’s body like through a keyhole. Typically, the images are transmitted via a bundle of several hundreds of optical...

Im Focus: Photoexcited graphene puzzle solved

A boost for graphene-based light detectors

Light detection and control lies at the heart of many modern device applications, such as smartphone cameras. Using graphene as a light-sensitive material for...

All Focus news of the innovation-report >>>

Anzeige

Anzeige

VideoLinks
Industry & Economy
Event News

Munich conference on asteroid detection, tracking and defense

13.06.2018 | Event News

2nd International Baltic Earth Conference in Denmark: “The Baltic Sea region in Transition”

08.06.2018 | Event News

ISEKI_Food 2018: Conference with Holistic View of Food Production

05.06.2018 | Event News

 
Latest News

Carbon nanotube optics provide optical-based quantum cryptography and quantum computing

19.06.2018 | Physics and Astronomy

How to track and trace a protein: Nanosensors monitor intracellular deliveries

19.06.2018 | Life Sciences

New material for splitting water

19.06.2018 | Physics and Astronomy

VideoLinks
Science & Research
Overview of more VideoLinks >>>