Deepest Ever Look into Orion
VLT infrared images reveal unexpected horde of low-mass objects
12 July 2016
ESO’s HAWK-I infrared instrument on the Very Large Telescope (VLT) in Chile has been used to peer deeper into the heart of Orion Nebula than ever before. The spectacular picture reveals about ten times as many brown dwarfs and isolated planetary-mass objects than were previously known. This discovery poses challenges for the widely accepted scenario for Orion’s star formation history.
An international team has made use of the power of the HAWK-I infrared instrument on ESO’s Very Large Telescope (VLT) to produce the deepest and most comprehensive view of the Orion Nebula  to date. Not only has this led to an image of spectacular beauty, but it has revealed a great abundance of faint brown dwarfs and isolated planetary-mass objects. The very presence of these low-mass bodies provides an exciting insight into the history of star formation within the nebula itself.
The famous Orion Nebula spans about 24 light-years within the constellation of Orion, and is visible from Earth with the naked eye, as a fuzzy patch in Orion’s sword. Some nebulae, like Orion, are strongly illuminated by ultraviolet radiation from the many hot stars born within them, such that the gas is ionised and glows brightly.
The relative proximity of the Orion Nebula  makes it an ideal testbed to better understand the process and history of star formation, and to determine how many stars of different masses form.
Amelia Bayo (Universidad de Valparaíso, Valparaíso, Chile; Max-Planck Institut für Astronomie, Königstuhl, Germany), a co-author of the new paper and member of the research team, explains why this is important: "Understanding how many low-mass objects are found in the Orion Nebula is very important to constrain current theories of star formation. We now realise that the way these very low-mass objects form depends on their environment."
This new image has caused excitement because it reveals a unexpected wealth of very-low-mass objects, which in turn suggests that the Orion Nebula may be forming proportionally far more low-mass objects than closer and less active star formation regions.
Astronomers count up how many objects of different masses form in regions like the Orion Nebula to try to understand the star-formation process . Before this research the greatest number of objects were found with masses of about one quarter that of our Sun. The discovery of a plethora of new objects with masses far lower than this in the Orion Nebula has now created a second maximum at a much lower mass in the distribution of star counts.
These observations also hint tantalisingly that the number of planet-sized objects might be far greater than previously thought. Whilst the technology to readily observe these objects does not exist yet, ESO’s future European Extremely Large Telescope (E-ELT), scheduled to begin operations in 2024, is designed to pursue this as one of its goals.
Lead scientist Holger Drass (Astronomisches Institut, Ruhr-Universität Bochum, Bochum, Germany; Pontificia Universidad Católica de Chile, Santiago, Chile) enthuses: “Our result feels to me like a glimpse into a new era of planet and star formation science. The huge number of free-floating planets at our current observational limit is giving me hope that we will discover a wealth of smaller Earth-sized planets with the E-ELT.”
 Nebulae such as the famous one in Orion are also known as H II regions to indicate that they contain ionised hydrogen. These immense clouds of interstellar gas are sites of star formation throughout the Universe.
 This information is used to create something called the Initial Mass Function (IMF) — a way of describing how many stars of different masses make up a stellar population at its birth. This provides an insight into the stellar population’s origins. In other words, determining an accurate IMF, and having a solid theory to explain the origin of the IMF is of fundamental importance in the study of star formation.
This research was presented in a paper entitled “The bimodal initial mass function in the Orion Nebula Cloud”, by H. Drass et al., published in Monthly Notices of the Royal Astronomical Society.
The team is composed of H. Drass (Astronomisches Institut, Ruhr-Universität Bochum, Bochum, Germany; Pontificia Universidad Católica de Chile, Santiago, Chile), M. Haas (Astronomisches Institut, Ruhr-Universität Bochum, Bochum, Germany), R. Chini (Astronomisches Institut, Ruhr-Universität Bochum, Bochum, Germany; Universidad Católica del Norte, Antofagasta, Chile), A. Bayo (Universidad de Valparaíso, Valparaíso, Chile; Max-Planck Institut für Astronomie, Königstuhl, Germany) , M. Hackstein (Astronomisches Institut, Ruhr-Universität Bochum, Bochum, Germany), V. Hoffmeister (Astronomisches Institut, Ruhr-Universität Bochum, Bochum, Germany), N. Godoy (Universidad de Valparaíso, Valparaíso, Chile) and N. Vogt (Universidad de Valparaíso, Valparaíso, Chile).
ESO is the foremost intergovernmental astronomy organisation in Europe and the world’s most productive ground-based astronomical observatory by far. It is supported by 16 countries: Austria, Belgium, Brazil, Czechia, Denmark, France, Finland, Germany, Italy, the Netherlands, Poland, Portugal, Spain, Sweden, Switzerland and the United Kingdom, along with the host state of Chile. ESO carries out an ambitious programme focused on the design, construction and operation of powerful ground-based observing facilities enabling astronomers to make important scientific discoveries. ESO also plays a leading role in promoting and organising cooperation in astronomical research. ESO operates three unique world-class observing sites in Chile: La Silla, Paranal and Chajnantor. At Paranal, ESO operates the Very Large Telescope, the world’s most advanced visible-light astronomical observatory and two survey telescopes. VISTA works in the infrared and is the world’s largest survey telescope and the VLT Survey Telescope is the largest telescope designed to exclusively survey the skies in visible light. ESO is a major partner in ALMA, the largest astronomical project in existence. And on Cerro Armazones, close to Paranal, ESO is building the 39-metre European Extremely Large Telescope, the E-ELT, which will become “the world’s biggest eye on the sky”.
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