FEATURE ARTICLE
How Were the Comets Made?
Explaining the composition of these 4.5 billion-year-old relics may require scientists to revise their models of the primitive solar nebula
Joseph A. Nuth III
Clues from Other Stars
Fortunately, we needn't rely solely on theoretical models to understand the processes that took place in the solar nebula. At this very moment there are countless numbers of protostars in various stages of birth within our Galaxy. Although protostellar systems comparable in mass to the ancient solar nebula are too dim to observe, the behavior of more massive protostars—Herbig Ae and Be stars—can be studied to a certain extent. Despite being more than twice the mass of our Sun, these protostars are generally believed to behave and evolve in a fashion similar to the protosun.
Carol Grady of NASA's Goddard Space Flight Center and her colleagues have shown that at least one Herbig system (HD 163296) exhibits a collimated bipolar wind that is consistent with the X-wind model, as well as an uncollimated outflow that may lie parallel to the disk plane. This uncollimated outflow may be the key to understanding how the dust grains annealed near the protostar make their way to the outer reaches of the nebula.
Unfortunately, because the closest of these stars is about 300 light-years away, detailed observations of these objects are extremely difficult to make, even using the state-of-the-art instruments aboard the Hubble Space Telescope (Figure 8). It is not yet possible to examine these objects with sufficient resolution to see the inner portions of the disk or even its geometry (the angle of the plane of the disk with respect to the Earth). Without the geometry of the system it is impossible to gauge the strength and extent of the winds that might be emanating from the inner regions of these systems. Nevertheless, Grady's observations are tantalizing. If some form of uncollimated wind recycles even a fraction of the material in the accretion disk, it would have enormous consequences for the chemistry of the nebula.
Herbig stellar systems are interesting for another reason as well. Infrared spectra of older Herbig Ae and Be stars indicate that much of the dust surrounding the stars comes from infalling comets, which shed the particles on their inward voyage. Collectively, the infrared spectra of many Herbig systems reveal a distinct trend: The dust around the youngest observable stars is amorphous, but it becomes increasingly crystalline as the age of the nebula increases. Dust grains are highly unstable over the period of the nebula's lifetime. Small grains are blown out of the system by photon pressure, whereas larger grains spiral into the protostar, where they are destroyed. In order for us to see them in the older systems, the dust grains must be released over time from cometesimals and comets; otherwise they would have been destroyed.
These observations provide further evidence that processes inside protostellar nebulae are responsible for producing the crystalline grains. They also suggest that the crystalline silicates are not created by a process such as the impact shock associated with amorphous dust falling onto the stellar accretion disk, otherwise even the youngest stars would have crystalline silicates. But it also suggests that as the nebula ages, there is a steady accumulation of the crystalline grains produced in the solar nebula that must be transported to the region where the comets are made.
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