Astrophysics

Waves on Orion's shores

Waves have been discovered in the molecular cloud surrounding the Orion nebula, generated by shearing flows in the cloud. This finding provides clues to the way filamentary substructures form in the interstellar medium.

Massive stars have a profound effect on evolution in the cosmos. Their brilliant light can outshine all other stars in star-forming galaxies, and their cores are the source of most elements other than hydrogen and helium. Their intense radiation, stellar winds and supernova explosions sculpt surrounding interstellar gas clouds and determine the clouds' state. The Orion nebula1,2, located about 400 parsecs from the Sun, is the nearest site of ongoing massive-star formation. It is one of the best-known environments in which interactions between massive stars and the surrounding interstellar medium (ISM) can be studied in detail. Orion continues to provide new insights into the physics of the ISM, particularly into the interactions between massive stars and the surrounding clouds that gave birth to them. Reporting in this issue (page 947), Berné et al.3 provide evidence for a new class of such interactions — waves at the interface between the nebula and the background molecular cloud.

The dense, molecular phase of the ISM is the raw material for star and planet formation. Massive stars drive a 'galactic ecology', recycling matter from interstellar clouds into stars and back again into the ISM. Our Milky Way galaxy contains between 10 billion and 100 billion stars, and there is sufficient material in its ISM to make billions more. Over the past 13 billion years, most of the Milky Way's hydrogen and helium that originated in the Big Bang has been converted into long-lived, low-mass stars. But just a few per cent of the mass of these gases formed short-lived massive stars. The most massive were born with more than 100 times the mass of our Sun, shine millions of times more brightly and explode as supernovae within a few million years of their birth. The least massive stars that eventually explode are those born with about 8 times the Sun's mass, and that live for about 40 million years. Contrast this with the life expectancy of our Sun: roughly 10 billion years.

Since the formation of our Galaxy and the birth of our Solar System 4.5 billion years ago, many generations of massive stars have come and gone. Each generation converted much of its hydrogen and helium into heavier elements and, on exploding, recycled this heavier material to enrich the ISM. Today, between 1 and 2% of the mass of the ISM consists of elements forged by the thermonuclear fires in the cores of massive stars.

During their brilliant lives, massive stars ionize their parent clouds, driving stellar winds that create expanding bubbles of plasma. Over the past 12 million years, the Orion region's giant molecular clouds (Fig. 1) have given birth to tens of thousands of stars, including dozens of massive ones. The Orion nebula, which has spawned more than 1,000 low-mass stars and at least half a dozen massive ones, is the youngest region of massive-star birth in the Orion constellation.

Figure 1: The constellation of Orion.
figure1

ESA

This image was taken by the Planck satellite using high-frequency radio emissions, and covers a region of 13°×13°. The turquoise structures consist of interstellar dust and are mostly associated with molecular clouds in which stars are forming. The two largest clouds, Orion A and Orion B, have a combined mass of about 100,000 times that of the Sun. The red regions trace hydrogen gas that has been ionized by dozens of massive stars over the past 13 million years. The supernova explosions of some of these stars have created a bubble that is squeezing and compressing the clouds. The large red arc, known as Barnard's loop, marks the outer edge of this bubble. The Orion nebula, located in front of the compressed northern end of the Orion A cloud, and in the interior of the hot, supernova-heated bubble, marks the most recent site of massive-star birth in Orion. Berné et al.3 have discovered waves in the molecular cloud of Orion.

Within the past million years, the nebula's luminous, hot, massive stars have started to dissociate the background molecular cloud, creating a 'blister' of plasma that has a temperature of 10,000 kelvin. In this process, ultraviolet radiation cleaves the bonds of molecules, heats and ionizes atoms, and accelerates the resulting plasma to form a wind blowing towards Earth and over parts of the adjacent dense cloud. Doppler shifts of the plasma's spectrum have shown that the plasma flows with speeds of tens of kilometres per second, forming a shearing layer over parts of the background cloud4. This situation might be familiar to anyone who has spent time on a beach, watching waves breaking on the shore. Distant winds blowing over the water form a shearing layer and excite the formation of waves. Berné et al.3 now report that shearing flows between the plasma in the nebula and the background molecular cloud form a hydrodynamical effect known as a Kelvin–Helmholtz instability (KHI), which generates waves on the surface of the cloud — the 'shores' of the Orion nebula.

But why should we care about waves in Orion? There are several reasons. The waves discovered by Berné and colleagues are an example of how energy and momentum are injected into the ISM by massive stars, and provide clues about the self-regulation of star formation. For example, instabilities such as KHIs can develop in a complex manner, growing in amplitude until the wave energy degrades into turbulence — analogous to the chaos that follows the breaking of ocean waves on a shore. Berné and colleagues' results also indicate new directions for the numerical modelling of interstellar plasmas.

Unlike ocean waves, however, the shear flows in Orion are mediated by ultraviolet radiation. This radiation penetrates the shearing layer of the KHI, creating an insulating sheet of plasma that separates the dense cloud from the nebula's out-flowing wind, which should prevent the KHI from developing. How, then, did the waves develop in the presence of ultraviolet radiation? Berné and colleagues' answers3 provide clues about the history of the Orion nebula. They propose that the waves formed before the birth of the nebula's most massive star. At this time, there was less ultraviolet radiation flooding the environment, and so the layer separating the shearing plasma flow from the cloud would have been thinner than today.

Radiation-mediated shear flows may be a universal feature of interactions in the ISM. The origin of star-forming molecular clouds, and of their filamentary substructure5, remains a hotly debated topic among astronomers. One popular theory6,7 posits that dense clouds form behind shock waves, where lower-density ISM flows converge head-on as a result of expanding bubbles, stellar winds, supernova explosions or spiral density waves (regions of higher density associated with the spiral arms of galaxies). Head-on collisions of clouds, however, are much less likely than collisions with shear flows. The study and modelling of ultraviolet-mediated shear flows, such as those in Orion, may thus provide new insights into the formation of filamentary molecular clouds. On Earth's shores and in its atmosphere, the convergence of flows with shear sometimes gives rise to long cylinders of vortical motion (so-called von Kármán vortex streets8). In the ISM, perhaps ultraviolet-mediated converging and shearing flows, combined with entrained magnetic fields and self-gravity, provide an explanation for the origins of filamentary clouds.

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Bally, J. Waves on Orion's shores. Nature 466, 928–929 (2010). https://doi.org/10.1038/466928a

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