The secret life of the Orion Nebula: Dancing filaments and a possible new way to form large star clusters
Whole clusters of stars, including some of the most massive specimens, can form in comparatively short time. Based on an examination of a filament of gas and dust that includes the well-known Orion nebula, Amelia Stutz and Andrew Gould of the Max Planck Institute for Astronomy propose a new model for this quick mode of star formation. They provide evidence that the filament in question is a flexible structure, held together by gravity and stabilized by magnetic fields, and undulating back and forth. This and the locations and properties of nearby star clusters suggest instabilities similar to those known in plasma physics could be responsible for the quick formation of star clusters.
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The Orion Nebula, known from numerous telescopic images showing it bright red and full of intricate structure, is one of the most famous astronomical objects. MPIA's Amelia Stutz and Andrew Gould believe it could be much more than that: the key to understanding how some big molecular clouds manage to form a whole star cluster worth of stars within a few million years, astronomically speaking a very short time.
The proposal by Stutz and Gould is based on maps of the distribution of mass in a gas-and-dust structure called the integral-shaped filament (cf. figure 1), of which the Orion Nebula Cluster is a part, and on earlier studies of magnetic fields in that filament. (The filament's name reflect that it is shaped like a drawn-out S, similar to the integral sign used in mathematics.) The two astronomers present a scenario in which the balance between magnetic and gravitational influences makes the filament into a flexible, undulating entity completely unlike the usual picture of collapsing gas clouds.
Key evidence for this novel picture is provided by the distribution of protostars and young stars in and around the filament. Protostars are still in the process of formation, contracting until sufficiently high density and temperature in their core regions allows for large-scale nuclear fusion to ignite. Protostars are light enough get wafted along when the filament undulates back and forth, while the much more compact young stars get left behind, or shot out as by a slingshot. This would explain why, indeed, protostars are exclusively found along the dense spine of the filament, while young stars are mostly outside.
Potentially the biggest consequence of the new scenario could be that it suggests a new mechanism for how star clusters can form in astronomically short time-scales. The locations of star clusters in and around the integral-shaped filament suggest that initially, the filament stretched much further to the North. Over millions of years, the filament formed one star cluster after another, with each fully-formed cluster dispersing the gas and dust around it. That is why, at present, one can see three clusters in and around the filament – the oldest at the greatest distance from the current Northern tip of the filament, the second-oldest with some wisps of filament around it, and the youngest, still forming stars, the Orion Nebula Cluster smack in the middle of the filament.
The interplay of magnetic fields and gravity suggests certain kinds of instabilities that could be responsible for this serial star-cluster formation. But at this point, Stutz and Gould’s proposal is just that – a proposal based on the properties of the integral-shaped filament, but not (yet) a fully-fledged model for cluster-type star formation. Over the next years, appropriate simulations by theoreticians as well as additional observations, both of the integral-shaped filament and of promising candidate objects that might be the result of similar processes, should decide whether the Orion A molecular cloud is a rather special case, or whether star clusters, born in a pinch within dancing, magnetically confined filaments, are the natural way for our universe to produce lots of stars in a hurry.
The results described here have been published as Amelia M. Stutz and Andrew Gould, "Slingshot Mechanism in Orion: Kinematic Evidence For Ejection of Protostars by Filaments" in Astronomy & Astrophysics.