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!”

The edge of the Orion molecular cloud illuminated by the stars of the Trapezium Cluster: Starting from the left, the first image is obtained with the IRAM 30m telescope alone, the second with ALMA alone and the third is from the combination of the two datasets. There are about 100 times more pixels in the ALMA image compared to the 30m image. In comparison, an HD television has 4 times more pixels than a standard television. Credits: 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.”

These images show the surface of the vast molecular cloud behind the large Orion Nebula 1400 light years from Earth. The image on the left shows a general view of the region, obtained with the HAWK-I instrument installed on the Very Large Telescope. The smaller area observed with the Atacama Large Millimeter/submillimeter Array (ALMA) interferometer is identified by a white rectangle. The right panel shows the ALMA image and the impressive structure revealed for the first time. Credits: 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 : www.insu.cnrs.fr/node/5988
  • IRAM Press Release : http://www.iram-institute.org/EN/news/2016/134.html

IRAP Contact

  • Chiristine Joblin, christine.joblin@irap.omp.eu

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