The Orion Nebula: The Jewel in the Sword – Major VLT Research Project Opens with Beautiful Infrared View

Orion the Hunter is perhaps the best known constellation in
the sky, well placed in the evening at this time of the year for
observers in both the northern and southern hemispheres, and instantly
recognisable. And for astronomers, Orion is surely one of the most
important constellations, as it contains one of the nearest and most
active stellar nurseries in the Milky Way, the galaxy in which we
live.

Here tens of thousands of new stars have formed within the past
ten million years or so – a very short span of time in
astronomical terms. For comparison: our own Sun is now 4,600 million
years old and has not yet reached half-age. Reduced to a human
time-scale, star formation in Orion would have been going on for just
one month as compared to the Sun’s 40 years.

Just below Orion’s belt, the hilt of his sword holds a great jewel
in the sky, the beautiful Orion Nebula. Bright enough to be
seen with the naked eye, a small telescope or even binoculars show the
nebula to be a few tens of light-years’ wide complex of gas and dust,
illuminated by several massive and hot stars at its core, the famous
Trapezium stars.

However, the heart of this nebula also conceals a secret from the
casual observer. There are in fact about one thousand very young
stars about one million years old within the so-called
Trapezium Cluster, crowded into a space less than the distance
between the Sun and its nearest neighbour stars. The cluster is very
hard to observe in visible light, but is clearly seen in the above
spectacular image of this area (ESO PR 03a/01), obtained in
December 1999 by Mark McCaughrean (Astrophysical Institute
Potsdam, Germany) and his collaborators [1] with
the infrared multi-mode ISAAC
instrument on the ESO Very Large Telescope (VLT) at Paranal (Chile).

Many details are seen in the new ISAAC image

Indeed, at visible wavelengths, the dense cluster of stars at the
centre is drowned out by the light from the nebula and obscured by
remnants of the dust in the gas from which they were formed. However,
at longer wavelengths, these obscuring effects are reduced, and the
cluster is revealed. In the past couple of years, several of the
world’s premier ground- and space-based telescopes have made new
detailed infrared studies of the Orion Nebula and the
Trapezium Cluster, but the VLT image shown here is the
“deepest” wide-field image obtained so far.

The large collecting area of the VLT and the excellent seeing of
the Paranal site combined to yield this beautiful image, packed full
of striking details. Powerful explosions and winds from the most
massive stars in the region are evident, as well as the contours of
gas sculpted by these stars, and more finely focused jets of gas
flowing from the smaller stars.

Sharper images from the VLT

It is even possible to see disks of dust and gas surrounding a few
of the young stars, as silhouettes in projection against the bright
background of the nebula. Many of these disks are very small and
usually only seen on images obtained with the Hubble Space Telescope
(HST) [2] . However, under the best seeing
conditions on Paranal, the sharpness of VLT images at infrared
wavelengths approaches that of the HST in this spectral band,
revealing some of these disks, as shown in PR Photo 03d/01.

Indeed, the theoretical image sharpness of the 8.2-m VLT is more
than three times better than that of the 2.4-m HST. Thus, the VLT will
soon yield images of small regions with even higher resolution by
means of the High-Resolution
Near-Infrared Camera (CONICA) and the Nasmyth Adaptive Optics
System (NAOS) that will compensate the smearing effect
introduced by the turbulence in the atmosphere. Later on, extremely
sharp images will be obtained when all four VLT telescopes are
combined to form the Very Large Telescope
Interferometer (VLTI). With these new facilities, astronomers
will be able to make very detailed studies – among others, they will
be looking for evidence that the dust and gas in these disks might be
agglomerating to form planets.

Free-floating planets in Orion?

Recently, research teams working at other telescopes have claimed
to have already seen planets in the Orion Nebula, as very dim objects,
apparently floating freely between the brighter stars in the
cluster. They calculated that if those objects are of the same age as
the other stars, if they are located in the cluster, and if present
theoretical predictions of the brightness of young stars and planets
are correct, then they should have masses somewhere between 5 and 15
times that of planet Jupiter.

Astronomer Mark McCaughrean is rather sceptical about this:
“Calling these objects “planets” of course sounds exciting, but
that interpretation is based on a number of assumptions. To me it
seems equally probable that they are somewhat older, higher-mass
objects of the “brown dwarf” type from a previous generation of star
formation in Orion, which just happen to lie near the younger
Trapezium Cluster today. Even if these objects were confirmed to have
very low masses, many astronomers would disagree with them being
called planets, since the common idea of a planet is that it should be
in orbit around a star“.

He explains: “While planets form in circumstellar disks, current
thinking is that these Orion Nebula objects probably formed in the
same way as do stars and brown dwarfs, and so perhaps we’d be better
off talking about them just as low-mass brown dwarfs” and also
notes that “similar claims of “free-floating planets” found in
another cluster associated with the star Sigma Orionis have also been
met with some scepticism“.

Here, as in other branches of science, claim, counter-claim,
scepticism and amicable controversy are typical elements of the
scientific search for the truth. Thus the goal must now be to look at
these objects in much more detail, and to try to determine their real
properties and formation history.

Comprehensive VLT study of Orion well underway

This is indeed one of the main aims of the present major VLT study,
of which the image shown here is decidedly a good start and a great
“appetizer”! In fact, even the present photo – that is based on quite
short exposures with a total of only 13.5 min at each image point (4.5
min in each of the three bands) – is already of sufficient quality to
raise questions about some of the “very low-mass
objects”. McCaughrean acknowledges that “some of these very
faint objects were right at the limit of earlier studies and hence the
determination of their brightnesses was less precise. The new, more
accurate VLT data show several of them to be intrinsically brighter
than previously thought and thus more massive; also some other objects
seem not to be there at all“.

Clearly, the answer is to look even deeper in order to get more
accurate data and to discover more of these objects. More infrared
images were obtained for the present programme in December 2000 by the
VLT team. They will now be combined with the earlier data shown here
to create a very deep survey of the central area of the Orion
Nebula.

One of the great strengths of the VLT is its comprehensive
instrumentation programme, and the team intends to carry out a
detailed spectral analysis of the very faintest objects in the
cluster, using the VLT VIMOS and NIRMOS
multiobject spectrometers, as these become available.

Only then, by analysing all these data, will it become possible to
determine the masses, ages, and motions of the very faintest members
of the Trapezium Cluster, and to provide a solid answer to the
tantalising question of their origin.

The beautiful infrared image shown here may just be a first
“finding chart” made at the beginning of a long-term research project,
but it already carries plenty of new astrophysical information. For
the astronomers, images like these and the follow-up studies will help
to solve some of the fascinating and perplexing questions about the
birth and early lives of stars and their planetary systems.

Note

[1] The new VLT data covering the Orion Nebula
and Trapezium Cluster were obtained as part of a long-term project by
Mark McCaughrean (Principal Investigator, Astrophysical
Institute Potsdam [AIP], Germany), João Alves (ESO,
Garching, Germany), Hans Zinnecker (AIP) and Francesco
Palla (Arcetri Observatory, Florence, Italy). The data also form
part of the collaborative research being undertaken by the European
Commission-sponsored Research Training Network on “The Formation and
Evolution of Young Star Clusters” (RTN1-1999-00436), led by the
Astrophysical Institute Potsdam, and including the Arcetri Observatory
in Florence (Italy), the University of Cambridge (UK), the University
of Cardiff (UK), the University of Grenoble (France), the University
of Lisbon (Portugal) and the CEA Saclay (France).

[2] To compare the present VLT infrared image
with the more familiar view of the Orion Nebula in optical light, the
ST-ECF has prepared an
image covering a similar field from data taken with the NASA/ESA
Hubble Space Telescope WFPC2 camera and extracted and processed by
Jeremy Walsh from the ESO/ST-ECF archive. This
4-colour composite emphasises the light from the gaseous nebula rather
than from the stars, and there is dramatic difference from the
infrared view which sees much deeper into the region. The HST image is
available at http://www.stecf.org/epo/support/orion/.

Technical information about the photos

Contact

Mark McCaughrean
Astrophysikalisches Institut
Potsdam
Germany
Tel.: +49 331 749 9525
URL: http://www.aip.de/~mjm

ESO PR Photos 03a-d/01 may be reproduced, if credit is given
to Mark McCaughrean and the European Southern Observatory (ESO).