Interactions between young stars and their parent molecular clouds are poorly understood. High-resolution observations of the Orion nebula now reveal these interactions, which have implications for star formation. See Letter p.207
Stars are not static objects — they form, evolve and are then destroyed. In galaxies such as the Milky Way, the gas and dust between stars, which comprise the interstellar medium, accumulate in giant molecular clouds. The densest parts of these clouds eventually collapse under their own weight to create stars. On page 207, Goicoechea et al.1 report their latest observations of one of the closest stellar nurseries, the Orion molecular cloud. They find evidence of strong interactions between young massive stars and the cloud, shedding light on some unknown aspects of star formation.
The general process of star formation is fairly well understood, but many details remain a mystery. In particular, there is a lack of information on the formation of the most massive stars (those about 8–150 times more massive than the Sun). Such massive stars are rare, but they are the primary sources of light in the Milky Way — some are a few hundred thousand times more luminous than the Sun2.
When massive stars form, they start to emit energetic radiation, largely in the ultraviolet region of the electromagnetic spectrum. This UV radiation destroys the molecules in the surrounding cloud, creating a layer of atomic gas around young massive stars. In this layer, in the region closest to the star, the radiation is energetic enough to ionize the atoms, forming a bubble of ionized gas. At the edge of this bubble, the most energetic UV photons have already been absorbed, and the atomic gas can survive. The transition zone between the edge of the bubble and the molecular gas is called the photodissociation region (PDR)3. Just like human skin, the PDR protects molecules in the cloud from harmful UV radiation.
Goicoechea and colleagues show that it is possible to observe a PDR with sufficient resolution to directly study how the molecular cloud is pushed away and dispersed by the stellar radiation and winds of young massive stars. The authors use the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile to study a PDR of the Orion molecular cloud: the Orion Bar (Fig. 1). The data set presented by Goicoechea et al. and the level of detail revealed by the ALMA observations are unparalleled, allowing unknown aspects of star formation to be explored.
The Orion Bar is an archetypal PDR that makes an ideal study candidate for two reasons: it is close to Earth, and it is oriented edge-on, which allows astronomers a good look at how radiation is absorbed as it enters the molecular cloud. The local gas density in the PDR controls how quickly this absorption occurs. In low-density gas (a few hundred to a few thousand gas particles per cubic centimetre), the medium gradually becomes opaque to radiation, whereas denser gas (a few million particles per cubic centimetre) becomes opaque much more suddenly. In the Orion Bar, this gradual absorption happens on a scale of 15 arcseconds4 (equivalent to less than 1% of the full Moon's angular diameter) before the UV radiation is sufficiently absorbed so that the molecules can survive.
The measurement of 15 arcseconds surprised astronomers because standard models of PDRs5 can explain this value only if the gas in the Orion Bar has a low density. However, radiation observed from the Orion Bar requires high-density gas (a few million particles per cubic centimetre) to explain its emission6,7. In theoretical models, such a high density would require a smaller distance than 15 arcseconds between the atomic and molecular gas layers. In other words, UV radiation is observed to penetrate deeply into the cloud, whereas it should be absorbed by high-density gas.
Earlier studies6,7,8 tried to reconcile this discrepancy by suggesting that the Orion Bar consists of clumps of dense gas embedded in a thinner gas. Such a structure would allow for both high-density molecular emission and deeper penetration of UV radiation into the cloud. Goicoechea et al. are the first to directly observe such clumps of dense gas in the Orion Bar. Their results have strong implications for models of PDRs, because they demonstrate that even such an archetypal PDR does not have the stratified transition between atomic and molecular gas layers that was previously assumed9.
The authors' results also provide some explanation for the evolution of the Orion Bar. They find evidence of a high-pressure wave expanding into the molecular cloud, which is consistent with the picture of an expanding bubble of ionized gas created by the young massive star in its centre. The bubble pushes against the molecular cloud, compressing dense regions, while dispersing less-dense regions. However, because of the experimental limitations of ALMA, Goicoechea and colleagues only observe a small region of the Orion Bar, in a snapshot of time and at a limited wavelength. To rule out the possibility that the authors observed an atypical region with respect to PDRs in general, it will be necessary to consider a larger sample size, including PDRs with various local physical conditions.
An expanding bubble of ionized gas is one of the prime candidates proposed to explain how the interaction of young stars with their parent interstellar clouds controls the efficiency of star formation10. Without these interactions, star formation would be about 10 to 100 times more efficient than what is observed. The detailed nature and relative importance of these interactions with respect to other factors that influence star formation remain largely unknown. Therefore, any direct observation of these processes, as presented by Goicoechea and colleagues, provides a step towards a better understanding of star formation and, consequently, of how the Sun and the Solar System formed.Footnote 1
Goicoechea, J. R. et al. Nature 537, 207–209 (2016).
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