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Zooming into the skin of the Orion hunter

Combining the information from the ALMA interferometer and the IRAM 30-meter single-dish, an international team of astronomers led by Javier Goicoechea (CSIC) obtained the most detailed image of the Orion Bar, the frontier between the atomic and molecular gas in the closest massive star forming region from the solar system. This image is of considerable interest for the study of the morphology and the activity of this fascinating region of the sky

The Orion Nebula is at the top of the sword of the Orion constellation. This famous nebula is one of the most popular celestial attractions for amateur astronomers. The obtained colorful images reflect the intense UV illumination of the interstellar gas by a generation of young massive stars nicknamed The Trapezium. The presence of massive stars is also the reason why this region is one of the most studied by professional astronomers.

Located at 1350 light years from Earth, this is indeed the region of formation of massive stars closest to Earth. Astrophysicists thus observe this region in the prospect of revealing the secrets of formation of these monster stars that deliver about 200 000 times the solar luminosity!

Javier Goicoechea explains: "Up to now, we had an extremely static view of what happens to the molecular gas in this transition. This has changed drastically with ALMA. We obtained an image of exquisite sensitivity with a precision of 1 arcsecond, i.e., the angular size under which the solar system would be seen at the distance of Orion."

However, as any interferometer, ALMA alone would have only been sensitive to the small scale structures at the edge of the Orion Bar. This would have given a biased view of this edge. To correct for this, the team used single-dish observations obtained with the IRAM 30-meter telescope and combined both data sets to get the right answer. Jerome Pety adds: "Basically, we took a picture of the skin of the hunter constellation, while ALMA alone would have only been sensitive to the skin pores!"


Le bord du nuage moléculaire d'Orion illuminé par les étoiles de l'amas du Trapèze : En partant de la gauche, la première image est obtenue avec le télescope de l'IRAM de 30m seul, la deuxième avec ALMA uniquement et la troisième est issue de la combinaison des deux jeux de données. Il y a un nombre environ 100 fois plus élevé de pixels dans l'image ALMA comparée à l'image 30m. Par comparaison une télévision HD offre un nombre 4 fois plus grand de pixels qu'une télévision standard. Crédits : IRAM/ALMA/Pety.

Javier Goicoechea continues: "This combination reveals that the edge of the molecular cloud is composed of small filaments and globules more or less organized in periodic patterns at the edge of an extended molecular cloud. The explanation for such patterns is counter-intuitive. Indeed, in a static view, the intense UV radiation from the Trapezium stars should only dissociate molecules and ionize atoms. However, in a dynamic view, the UV field brings so much energy to the edge of the molecular cloud that it shocks it and compresses its outside layers to form the thin filaments and small globules."


Ces images présentent la surface du vaste nuage moléculaire situé derrière la grande nébuleuse d'Orion à 1400 années lumière de la Terre. L'image à gauche présente une vision générale de la région, obtenue avec l'instrument HAWK-I installé sur le Very Large Telescope. La zone plus petite observée avec l'interféromètre Atacama Large Millimeter/submillimeter Array (ALMA) est identifiée par un rectangle blanc. Le panneau de droite présente l'image ALMA et l'impressionnante structure révélée pour la première fois. Crédits : ESO.

Jerome Pety enthousiastically finishes: "This compressed layer could help to protect the rest of the cloud from the harsh UV radiation. Behind the compression zone the cloud temperature decreases fast and the cloud becomes quiet enough to continue forming new generation of stars."

Further Resources

  • Nature Publication : Javier R. Goicoechea, Jérôme Pety, Sara Cuadrado, José Cernicharo, Edwige Chapillon, Asunción Fuente, Maryvonne Gerin, Christine Joblin, Nuria Marcelino & Paolo Pilleri. Compression and ablation of the photo-irradiated molecular cloud the Orion Bar. Nature, 537, 207-209, 8 septembre 2016.
  • CNRS Press Release :
  • IRAM Press Release :

IRAP Contact

  • Chiristine Joblin,
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