The outburst of a Sun-like star offers a rare opportunity to witness the making of silicate crystals in the star's planet-forming disk, providing key information about the formation of comets and the Solar System.
We live in a dusty Universe. Dust is a ubiquitous feature of the cosmos, and impinges directly or indirectly on most fields of modern astronomy. The most common cosmic-dust species — the silicates — occurs in a wide variety of astrophysical environments, ranging from comets and protoplanetary disks (planet-forming dust disks around young stars) to the most distant galaxies known, which formed when the Universe was just a few hundred million years old. The way in which atoms in silicate grains are arranged — that is, whether they are arranged in a random manner or in an ordered lattice structure, as in their amorphous and crystalline forms, respectively — provides information about their origin, in particular about their parent regions.
The origin of crystalline silicates in comets has been a matter of debate since their first detection 20 years ago1. Crystalline silicates are unexpected if comets are, as is widely believed, remnants of primordial material from the cold, outer parts of the protoplanetary dust disk from which the Solar System has formed, the solar nebula2. Although it is recognized that comets do evolve during their storage in the far reaches of the Solar System3, they are undoubtedly the most pristine bodies in the Solar System.
On page 224 of this issue, Ábrahám et al.4 present the first convincing evidence for the formation of crystalline silicates through thermal annealing of amorphous silicates in the hot, inner disk around an eruptive star. In a complementary study, Vinković5 (page 227) proposes a viable, novel mechanism to transport the newly formed crystalline silicates from the hot, inner regions of protoplanetary disks to their cold, outer, comet-forming regions. Together, these results4,5 offer a solution to the long-standing puzzle of the origin of crystalline silicates in comets and protoplanetary disks, and provide insight into the formation of comets and planetary systems.
As far as comet-formation theory goes, comets formed in the cold, outer regions of the solar nebula, at distances of at least 5 astronomical units (AU) from the Sun (1 AU is the distance from Earth to the Sun). They have been stored in reservoirs as far as 30–10,000 AU from the Sun, and, having formed early in the life of the Solar System, which is about 4.5 billion years old, have remained cold ever since. In observed samples of comets, the presence of highly volatile, frozen molecules, such as carbon monoxide, and molecular nitrogen (which in comets is a rare gas species), indicates that comets formed at very low temperatures, as low as about 30 kelvin. Moreover, the remarkable similarity between such volatile ices — in particular those of water, carbon monoxide, ammonia and methane — in interstellar material and in comets strengthens the link between comets and the pristine interstellar materials of the solar nebula3.
Recently, crystalline silicates were identified in the dust samples collected from comet 81P/Wild 2 by the Stardust spacecraft6. Their presence in other comets has also been revealed by infrared (IR) spectral signatures7: the IR spectra display sharp emission features at several specific wavelengths that are characteristic of crystalline silicates. These distinct emission features are also seen in protoplanetary disks around young stars8, suggesting a similar origin for crystalline silicates in comets and in these dust disks.
So where did the observed silicate crystals in comets come from? Apparently, they were not inherited from the interstellar medium, simply because interstellar silicates are predominantly amorphous9. They clearly did not form in cometary nuclei, which are believed to be assembled at temperatures below 30 K (ref. 2), or in the cold, outer regions of the solar nebula, where comets were accreted about 4.5 billion years ago and where materials have never experienced temperatures higher than 100 K. The crystallization of the original, amorphous silicates through thermal annealing requires temperatures of at least ∼1,000 K (ref. 10), in contradiction with the scenario in which comets were formed and stored in cold environments2.
The standard speculation has been that the volatile ices and crystalline silicates found in comets are of different origins. Whereas volatile ices may be pristine interstellar material surviving from the time the Solar System formed, crystalline silicates can originate from amorphous silicates that were transformed to crystalline form by thermal annealing in the hot, inner solar nebula, and were then transported outwards and incorporated into comets (Fig. 1).
However, it is Ábrahám et al.4 who provide the first concrete observational evidence for thermal annealing of amorphous silicates. They present mid-IR spectra, in the 5.2–37-micrometre wavelength range, of the star EX Lupi, obtained at two epochs separated by an interval of about 3 years. EX Lupi is a prototypical young Sun-like eruptive star that undergoes large, repetitive outbursts. The first-epoch spectrum, obtained when EX Lupi was in a quiescent phase, displays a broad, smooth 9.7-μm emission band, a telltale signature of amorphous silicates. By contrast, the second-epoch spectrum, acquired when the star was in the middle of an outburst, exhibits several sharp peaks characteristic of crystalline silicates superimposed on the broad, 9.7-μm band of amorphous silicates. These features are similar to those observed in comet spectra.
Ábrahám et al.4 interpret the observations as ongoing crystal formation: crystalline silicates are produced by thermal annealing in the surface layer of the star's inner disk (about 0.5 AU from the star) by heat from the outburst, which increases the visual brightness of the star by a factor of about 100. Alternative explanations for the observed spectral peaks, such as illumination of existing crystals residing in outer disk areas or the stirring up of crystals from the disk's mid-plane, are ruled out by modelling.
So the question that naturally arises is how these newly produced silicate crystals are carried outwards from the inner, crystal-formation zone to the cold, comet-forming zone to be incorporated into comets? Several mechanisms have been suggested, including turbulent mixing11 of dust grains in the mid-plane of the solar nebula and the 'X-wind' model, in which dust grains are ballistically launched above the disk's mid-plane and transported outwards12. But these models seem to have difficulty in explaining the observed levels of transport13. Vinković5 proposes a transport model based on the non-radial component of the radiation-pressure force. He shows that the radiation pressure from the star, combined with that from the disk's near-IR light, could push grains outwards along the disk's surface irrespective of its curvature.
But Vinković's theory is valid for micrometre-sized dust grains, and crystalline-silicate grains that big cannot emit much light at their characteristic mid-IR wavelengths. If only micrometre-sized silicate crystals were transported to the outer disk regions, neither protoplanetary disks nor comets would exhibit the observed sharp emission features of crystalline silicates. It would be interesting to see whether other mechanisms such as turbulent mixing and the 'X-wind' model would effectively carry submicrometre grains, which are efficient mid-IR emitters, outwards and incorporate them into comets. It is also possible that some — but not all — crystalline silicates are made in situ in cometary comae14.
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