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.
In-depth information: "The secret life of the Orion Nebula: Dancing filaments and a possible new way to form large star clusters"
The big picture of star formation is rather simple: Take a very cold cloud made of hydrogen gas and dust; whenever regions of this cloud are cold enough, they will collapse under their own gravity and form stars.
The devil is in the details. For a start, there appear to be two distinct modes of star formation. In ordinary, smaller molecular clouds, one or a few stars will form until, after about 3 million years, the gas in the cloud has dispersed. Larger clouds live much longer until about 30 million years. They can exhibit a much more spectacular kind of star formation, resulting in a stellar cluster's worth of stars, including very massive stars.
There are, at this moment, some proposals, but no well-tested model for how this cluster mode of star formation can work, and why such clouds are able to produce such a rich yield on comparably short astronomical time scales of millions of years. Most tentative explanations include some kind of chain reaction, with the formation of the first stars in the cloud triggering the next wave of stellar births. The triggers could be the supernova explosions of the most massive newly-formed (and hence most short-lived) stars.
A filament of gas and dust
Now, Amelia M. Stutz and Andrew Gould of the Max Planck Institute for Astronomy have proposed an alternative mechanism for clustered star formation. The mechanism involves magnetic fields in and around the molecular cloud and a dynamical instability known as the pinch effect, and it is based on observations of the most famous star formation region in the night sky: the Orion nebula, visible even with the naked eye as an indistinct white smudge, and gloriously red and finely structured in optical images of advanced amateur astronomers, or taken with the Hubble Space Telescope. If you switch on infrared vision, the Orion nebula has another, hidden side that is intimately connected with the new mechanism proposed by Stutz and Gould.
A key piece of evidence for the model is a map of dust in the Orion A star-forming region published by Stutz and MPIA's Jouni Kainulainen in May 2015, based on observations of the region with the Herschel Space Telescope, at far-infrared wavelengths between 160 and 500 μm. At such wavelengths, astronomers can detect the thermal radiation emitted by the cold dust in molecular clouds. Stutz then used the data to create a density map of cold dust within the region (more precisely: a column density map, projecting the dust density onto the two dimensional sphere of the night sky).
With a distance of around 1300 light-years, Orion A is the closest massive star-forming region in our cosmic neighborhood. The most prominent feature in the region is called the integral-shaped filament (ISF), a distribution of gas and dust (in a proportion of about 110 to 1) shaped somewhat like an elongated letter S, nearly a hundred light-years long. The famous Orion Nebula, visible with the naked eye in the constellation Orion, is located within in the middle section of that filament.
In essence, the integral-shaped filament is a somewhat deformed cylinder, with a diameter that varies slightly along its length. Diameter variations can be estimated from the two-dimensional astronomical image. The slender filament, a few light-years across, is enveloped by a much larger mass distribution, a few dozens of light-years across. Kinematic data, namely spectral lines associated with carbon monoxide-13 [in particular, the line 13CO(2-1)] and other tracers of high-density matter, can be used to reconstruct how matter moves within the filament, confirming the general structure.
Using a simplified model of the filament and the Herschel data, Stutz and Gould were able to reconstruct the distribution of mass within the filament as well as the gravitational potential inside and in the surrounding regions of space. As expected, the filament has a particularly dense "spine" running along the middle. Both inside and around the filament, up to a distance of around 25 light-years from the central spine, the density varies with a simple power law, proportional to r –1.6.
Cosmic slinky: magnetic fields around the filament
So far, so good. But when Stutz began to examine the positions of protostars and young stars relative to the filament, the result was something of a surprise. The protostars – stars-to-be that are still contracting, and have not yet become sufficiently dense for hydrogen fusion to set in within their central regions – were behaving exactly as expected: they were situated along the filament's central ridge, that is, in the region of greatest density, exactly where one would expect star-forming collapse to occur. But the young stars (pre-main-sequence Class II stars, to give the technical term) were mostly located outside the filament. Something must have accelerated either the filament or the stars, causing the separation. How did the stars get kicked out of the filament, or how did the filament veer away from the stars?
An important ingredient of the explanation came from another special property of the filament. In 1997, an analysis by Carl Heiles of the University of California at Berkeley, based on a spectral analysis of the 21 cm line of atomic hydrogen, had suggested that the integral-shaped filament is surrounded by a magnetic field. The spectral line gets split in two in a magnetic field, due to the so-called Zeeman effect. Additional information about the filament's magnetic field was provided by later analyses of the polarization of light from this region (Matthews & Wilson 2000).
Even though neither Heiles nor his successors published this conclusion: These observational results are consistent with a helical magnetic field, whose field lines envelop the filament like a gigantic cosmic slinky. The magnetic field lines are anchored to the filament via a small fraction of ionized, and hence electrically charged, particles in the hydrogen gas.
A state that is prone to disturbances
By comparing the results of the magnetic field measurements with their reconstruction of the gravitational potential, Stutz and Gould found that magnetic and gravitational influences on the filament appear to be of comparable strength. Close to the filament, the energy stored in the magnetic field is about the same as the gravitational binding energy. Shifting a portion of the filament by a few 3 light-years from the center of the enveloping mass distribution takes about the same energy as the energy stored in that portion's magnetic fields.
Under these circumstances, the filament becomes much more than a collection of dust and gas, held together by its own gravity. It becomes a gigantic flexible structure, almost 100 light-years long, which can move and oscillate as a whole – and it appears that some of its regions can also become unstable! These properties hold the key to explaining not only the curiously lonely young stars but, as it turns out, could also explain the quick formation of whole star clusters found near the filament.
Gravity versus magnetism
Generally, magnetic fields within a molecular cloud have a stabilizing effect. Any compression of the cloud, including its magnetic field lines, would bring the field lines closer together; this would correspond to a stronger field, and thus to a higher energy contained in the magnetic field. The result is magnetic pressure, opposing any compression of the cloud's volume. The gravitational field, of course, has the opposite effect, exerting its influence to pull the cloud's particles closer together, effecting compression or even the collapse of a cloud.
For a longish object like the integral-shaped filament, the magnetic field energy is more concentrated toward the spine than the gravitational field energy. Within the first few light-years from the central spine, magnetic and gravitational influences are almost equal. Move away further from the spine, and gravity will dominate.
In a situation where magnetic and gravitational energy are equal, or almost equal, these two influences cancel out almost completely. Under such conditions, small changes in the cloud's volume will hardly change the overall energy at all. This means there is hardly any pressure, or tension, opposing such small changes. These are ideal conditions for small-scale instability: minute disturbances and fluctuations, which are always present in real-world situations, are not suppressed, but will be able to travel along the filament, building up to larger oscillations which can set the whole filament in motion.
One particular kind of disturbance that is bound to occur in such a situation is related to transversal deformations, in other words: with parts of the filament swinging out sideways. Such sideways motion is checked by the gravity of the surrounding, larger envelope of matter, though: Whenever the filament moves too far off center within its massive envelope, gravity will begin to dominate and pull the stray portion of the filament back towards the central line. Transversal deformation and the gravitational influence pulling out-of-line bits back combine into an undulating motion of the filament as a whole.
Slingshot effect: Young stars and the filament
This, then, is the explanation proposed by Stutz and Gould for the curiously displaced stars: Protostars form along the spine of the filament, where the gas and dust have the greatest density. But the filament as a whole is in undulating motion, oscillating left and right on a time scales of about 600 000 years, with its gas, dust and magnetic field shifting in unison due to the coupling of the magnetic field to the ions sprinkled throughout the gas, the collisions between the ions and the gas particles, and between gas particles and dust. As long as the protostar has not completely collapsed, it is light enough to be wafted along with the filamentary motion, remaining in position someplace in the filament’s spine. Not so once the collapse is complete, and the protostar has transformed into a compact young star of considerable density: Such a considerably more compact object is not as easily nudged along by collisions with the surrounding gas particles. It will get left behind as the filament undulates on or, more likely, shot out as by a slingshot as it continues with the speed it had gathered when it still moved with the filament.
In consequence, protostars can be found along the spine of the filament, while completely formed young stars can mostly be found outside – the protostars are wafted along, while the young stars are left behind, or shot out, as the filament moves on. This provides a natural explanation both for the appearance of the integral-shaped filament and for the positioning and the velocities of protostars and young stars in and around it.
A conspicuous series of stellar clusters
But there is more to this picture of oscillating filaments, bound systems made of gas, and dust, held together by gravity and stabilized by magnetic field lines. Indications of this come from the arrangement of star clusters next to and within the filament, which has a clear North-South structure. The Northern end of the integral-shaped filament is separated from the next large molecular cloud, called Orion B and located still further in the North, by a marked gap. Below Orion B, there are three star clusters, about equally spaced from North to South: NGC 1981, then NGC 1977, both located in the gap, and then the Orion Nebula Cluster (ONC) smack in the middle of the integral-shaped filament. Even further South is the transition from the integral-shaped filament to the markedly wider and more irregular rest of the Orion A cloud, known as L1641.
In addition to the spatial sequence, there appears to be a temporal sequence. NGC 1981 is around 5 million years old, and its massive, bright stars have had sufficient time to disperse any left-over gas from the cluster's birth. At two million years of age, NGC 1977 is markedly younger, and still contains left-over gas and dust from the epoch of its formation. Hydrogen gas excited by the radiation of the cluster's brightest stars, and dust reflecting stellar radiation, give NGC 1977 an appealingly colorful structure. The Orion Nebula Cluster is younger still, and in this cluster, star formation is still in full swing.
For Stutz and Gould, this suggested progressive star formation episodes: Initially, all three regions would have been located within the integral-shaped filament, but over millions of years, the formation of clusters would have begun at the North end of the filament, with each cluster dispersing the filament's gas and dust in its wider neighborhood. For the oldest visible cluster NGC 1981, dispersion has been so complete that there is no connection with the existing filament at all. (Had there been older clusters still, their stars would likely have dispersed over the intervening time, making them impossible to identify from current observations.)
To the west of NGC 1977, there is a thin whisp of gas and dust that Stutz and Gould have dubbed the "ghost filament," and which appears to be left over from the time when NGC 1977 formed, presumably also within the integral-shaped filament. The Orion Nebula Cluster, on the other hand, is still firmly embedded, although it is doing its best to destroy the filament from the inside by forming numerous bright and massive stars.
A new mechanism of star formation
The filament appears to have shrunk from the North, each section being frayed and dispersed as each new star cluster formed. Stutz and Gould propose that this systematic process could be a direct consequence of the interplay of magnetic fields and gravity causing parts of the filaments to collapse. The precise physical mechanism behind this is unclear, although there are intriguing analogies with so-called pinching instabilities known from plasma physics.
Magnetic fields complicate matters, and for that reason their influence on astronomical scales is frequently left out of the simplest kinds of model. The work of Stutz and Gould shows that, when it comes to massive star formation, that might well be a big mistake. At this point, however, the formation scenario they propose is an intriguing suggestion, but far from an established mechanism. How to move forward, then?
The first question is for the theoreticians, who should be able to simulate what is going on in a filament such as the one studied by Stutz and Gould. A quantitative description of this kind of situation, which includes an understanding of just what kind of instability could cause the serial star cluster production observed in Orion A, would be a key step in establishing the newly proposed mechanism more firmly.
What to observe next?
Next, future observations promise more stringent tests of the mechanism, and the first of these concern a more detailed study of the Orion Nebular Cluster as the current location for the new mode of star formation. The model of Stutz and Gould predicts that protostars in the ONC should be aligned rather tightly along the spine of the filament. A competing model, known as “cold collapse,” posits that this kind of rapid star formation is the result of numerous filamentary substructures of the region in question, each sufficiently dense for new stars to form. Consequently, the cold collapse model predicts that protostars should be clustered not along the spine of the ISF, but on numerous sub-filaments, which are in turn falling down towards the filament’s spine.
Unfortunately, in the available data taken with the Herschel and Spitzer space telescopes, the region of the Orion Nebular Cluster is so bright that it has overwhelmed the telescopes’ detectors – in the images, this region is saturated, and there is no way to distinguish gas, young stars and protostars. Observations with the ALMA observatory, which is sensitive to millimeter and submillimeter wavelengths, could map this region in detail, and find out which prediction best matches the observational data.
Then, there is the question of identifying the same processes in other astronomical objects – after all, the importance of the newly proposed mechanism will depend on how wide-spread it is. Could this be the standard way of forming lots of stars quickly, including heavyweights, or is it a rare mechanism, depending on very special conditions to be found in Orion A but hardly anywhere else, and hence of minor importance only for the big picture of how galaxies form stars?
One suitable observational candidate is the star-forming region NGC 1333 in the constellation Perseus, roughly 1000 light-years from Earth, which appears to share key properties with Orion A. The molecular cloud known as Cepheus OB3, at 2300 light-years, could be an even more promising candidate. Its morphology shows interesting filaments and, more importantly, a number of star clusters located in the filaments’ immediate vicinity.
Testing the mechanism proposed by Stutz and Gould would require detailed studies of the dense gas and its motion, and of protostars and young stars in and around these filaments. Some of the data is already available in the archive of data left behind by the now defunct Herschel Space Observatory. Other data, such as precise localizations of protostars and large area maps of the motion of gas within the Cepheus OB3 clouds will need to be acquired, using the ALMA observatory. Once this data is in, Stutz and Gould should be able to 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.