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The Orion Nebula: The Jewel in the Sword

Major VLT Research Project Opens with Beautiful Infrared View

17 januari 2001

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 Press Release eso0104), 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 ESO Press Photo eso0104d.

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.

Noter

[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 online.

Mer information

Technical information about the photos

ESO Press Photo eso0104a of the Orion Nebula and the Trapezium Cluster was made using the near-infrared camera ISAAC on the ESO 8.2-m VLT ANTU telescope on December 20 - 21, 1999. The full field measures approx. 7 x 7 arcmin, covering roughly 3 x 3 light-years (0.9 x 0.9 pc) at the distance of the nebula (about 1500 light-years, or 450 pc). This required a 9-position mosaic (3 x 3 grid) of ISAAC pointings; at each pointing, a series of images were taken in each of the near-infrared J s - (centred at 1.24 µm wavelength), H- (1.65 µm), and K s - (2.16 µm) bands. North is up and East left.

The total integration time for each pixel in the mosaic was 4.5 min in each band. The seeing FWHM (full width at half maximum) was excellent, between 0.35 and 0.50 arcsec throughout. Point sources are detected at the 3-sigma level (central pixel above background noise) of 20.5, 19.2, and 18.8 magnitude in the J s -, H-, and K s -bands, respectively, mainly limited by the bright background emission of the nebula.

After removal of instrumental signatures and the bright infrared sky background, all frames in a given band were carefully aligned and adjusted to form a seamless mosaic. The three monochromatic mosaics were then unsharp-masked and scaled logarithmically to reduce the enormous dynamic range and enhance the faint features of the outer nebula. The mosaics were then combined to create this colour-coded image, with the J s -band being rendered as blue, the H-band as green, and the K s -band as red. A total of 81 individual ISAAC images were merged to form this mosaic.

ESO Press Photos eso0104b-c show smaller sections of the large image; the areas are 2.6 x 3.2 and 4.2 x 3.8 arcmin (1.1 x 1.4 and 1.8 x 1.6 light-years), respectively.

ESO Press Photo eso0104d is based on J s band data only, to ensure good visibility (maximum contrast) of the Orion 114-426 silhouette disk against the background nebula. The three highest spatial resolution images covering this region were accurately aligned to form a mosaic with a resolution of 0.4 arcsec FWHM (180 Astronomical Units [AU]) in the vicinity of the disk. A 29 x 29 arcsec (0.2 x 0.2 light-year) section of this smaller mosaic was cut out and the square root of the intensity taken to enhance the disk. The disk is roughly 2 arcsec or 900 AU in diameter. North is up, East left.

Kontakter

Mark McCaughrean
Astrophysikalisches Institut Potsdam
Potsdam, Germany
Tel: +49 331 749 9525

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