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.

Images online at http://www.eso.org/outreach/press-rel/pr-2002/pr-15-02.html

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   [ESO PR Photo 20b/02]  ESO PR Photo
                         20a/02                                20b/02

 [Preview - JPEG: 400 x 507 pix - 59k  [Preview - JPEG: 400 x 440 pix -
 [Normal - JPEG: 1236 x 800 pix -      96k]
 728k]                                 [Normal - JPEG: 800 x 879 pix -
 [Full-Res - JPEG: 1791 x 2271 pix -   1.2M]
 2.1M]                                 [Full-Res - JPEG: 2076 x 2282 pix -
                                       3.7M]

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  Caption: PR Photo 20c/02 shows the
                         20c/02        observational evidence of the
                                       presence of hot and massive
                                       "Wolf-Rayet" stars [3] in a
 [Preview - JPEG: 603 x 400 pix - 68k  metal-rich H II region (designated
 [Normal - JPEG: 1206 x 800 pix -      "-014+081") in the spiral galaxy NGC
 168k]                                 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 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.

Notes

[1]: This ESO press release is issued in coordination between ESO and the
Observatoire Midi-Pyrenees.

[2]: The team consists of Daniel Schaerer (Principal Investigator;
Observatoire Midi-Pyrenees, Toulouse, France), Maximilien Pindao
(Observatoire de Geneve, Switzerland), Rosa M. Gonzalez Delgado (Instituto
de Astrofisica de Andalucia, Granada, Spain) and Grazyna Stasinska
(Observatoire de Meudon, France).

[3]: Wolf-Rayet stars are named after two 19th-century French astronomers,
Charles Wolf and Georges Rayet.

Contact

Daniel Schaerer
Laboratoire d’Astrophysique
CNRS/UMR 5572
Observatoire Midi-Pyrenees
Toulouse, France
Phone: +33 5 61 33 2929/2898
email: schaerer@ast.obs-mip.fr