enhanced by relevant studies of basic molecular processes.
Late stages of stellar evolution and the feedback to the interstellar medium. Fundamental tests of stellar models are provided by the study of non-radial oscillations of stars. If important information can be obtained for the brightest stars through high-resolution spectroscopy from ground, a dramatic impulse to asteroseismology is expected in the near future form the space massions like Corot, expected to be launched at the end of 2006, and, subsequently, Kepler. Space missions are also the only way to access the ultraviolet and X-ray window, on one side, and mid- and far-infrared, on the other. Both domains are fundamental for the study of both the fast and slow winds from evolved stars. Spitzer and Herschel will play a major role at long wavelengths along with millimetric observations from Alma. At shorter wavelengths after the current workhorses HST, XMM and Chandra the situations is still undefined.
4.5 How do planetary systems form and evolve?
4.5.1 Background
Circumstellar discs are ubiquitous around low-mass pre-main-sequence stars. Discs appear to ex- ist around more massive stars, but are more difficult to separate from the surrounding molecular cocoon, because of the short evolution time scale of these objects. These discs are believed to play a major role in the assembly of the central star and in the formation of a planetary system. Disc evolution is controlled on one side by the decline of the accretion rate, and on the other by changes in dust properties. High spatial resolution studies of discs at millimetre frequencies have shown that grains in the outer disc (at radii larger than 30–50 AU from the star) have grown in many cases to very large sizes (up to few cm); mid-infrared spectroscopy, on the other hand, indi- cates that there is a residual population of micron-size silicates on the disc surface, at least within a few tens of AU from the star. Much smaller grains (less than 5–10 nm) and macromolecules (PAHs) are also present on the disc surfaces, even in rather evolved objects, and have an impor- tant role in determining the gas physical and chemical conditions.
The properties of the dust population, including its size distribution, are very likely the result of sedimentation and coagulation, which, in turn, depend on the gas motions. Grains in the disc midplane may form larger aggregates that eventually lead to planetesimal formation, which, in turn, are the building blocks of the rocky core of planets.
When the planetary system is assembled, the disc is almost completely devoid of gas and the dust component is reduced to a few lunar masses. In these discs, the dust is not pristine, but represent the debris of the planetary formation process, for this reason these are called ‘debris discs’. The study of debris discs is a powerful indirect proble of the planetary system they host.
4.5.2 Key questions
The evolution of the dust-to-gas ratio in discs is a major unsolved issue. The main constituent of the gas, molecular hydrogen, is essentially unobservable, except in a small and warm region of the disc where the mid-infrared lines of H2can be produced. The study of the gas properties will
mostly remain a very indirect process which must rely on observations of much less abundant species, such as CO or molecular ions. We have only a rudimentary grasp of very basic properties such as the initial gas and dust disc mass, disc sizes, and the distribution of material in the disc. The nature of the interaction of the disc with the environment is an open issue. There is evidence that discs, accretion onto the central star, and the ejection of material in powerful jets are con- nected, but there is no self-consistent model of a disc-star-jet system. This is partly due to the lack
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Figure 4.6: Dust evolution and planets formation in circumstellar discs. Left: artistic view of the formation of pebbles in circumstellar discs as suggested from millimeter wave observations of the TW Hydrae system. Right: simulation of the formation of a gap in a disc around a young star due to the gravitational effect of a newly formed giant planet.
of detailed observations of the inner regions of these systems where most of the interaction occurs and partly due to our limited understanding of the physical processes in discs (e.g., viscosity). Understanding the physics of discs would progress significantly if an evolutionary sequence could be drawn. Given the difficulties in constraining the ages of pre-main-sequence stars, this can only be addressed through a statistical approach, and will be a major undertaking: so far only a handful of objects have been studied in terms of gas content, indirectly through CO. This evolutionary sequence may well be different in star forming regions of various environments. Tidal processes in dense stellar environments are expected to affect the disc properties. For exam- ple, current observations suggest that circumstellar discs disappear within about 10 million years, while Uranus, Neptune and Kuiper-Belt objects require about 100 million years to form (§ 5.4). While the idea that planets form in protoplanetary discs in the early phases of the stellar life is generally accepted, many key questions are still unanswered: is the formation of planetary systems a robust and common process (how frequent is the formation of a planetary system)? What are the demographics of planetary systems (is our own Solar System a common product of the planetary formation process)? How strongly is the formation of planetary systems influenced by the properties and evolution of the central star (are the conditions that led to the formation of our own Solar System ‘special’)?
The actual mechanism of planetesimal formation, the first step in planet formation, is also still debated. A tail of smaller grains is predicted, either as a leftover of the initial solid population, or as the result of collisional fragmentation of larger bodies. The sequence of events depends on a number of assumptions, both on the disc properties (e.g., the relative velocity of colliding particles) and grain properties (e.g., the sticking probability). Calculations have been performed only for the conditions of the primordial Solar nebula, and need to be expanded and improved. The observational results on the properties of the grain population, which have revealed grain sizes up to few centimeters can provide important constraints to the models. Another potentially strong constraint could come from the spatial distribution of the large grains within the disc.
4.6. WHAT ARE THE DEMOGRAPHICS OF PLANETS IN THE GALAXY? 57