Heavy Stars Thrive among Heavy Elements

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.