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a power law:

where P lies between 0.5 and 0.65 (Beckwith et al., 1990). This value, which is less than that of molecular clouds, seems to favour a clumpy, and perhaps fractal, structure.

5.3 Planetary Disks and Debris Disks

With the protoplanetary disks just described, the mass of the disk is typically one-hundredth of the stellar mass (see Sect. 5.2.5). This mass is often too low for direct observation of the disk by imagery, but it may be deduced from measurements of its infrared flux. Hundreds of protoplanetary disks have been discovered in this way. Over the course of time, the disks will still lose a large fraction of their mass - they are objects of Class III, see Fig. 5.13 - through the effects of the violent stellar winds that accompany the T-Tauri and FU Orionis phases (or both) in the young star. The typical duration of this transition phase is shorter than ten million years. It appears to be shorter, the more massive the disk is initially.

The observation of thin disks is more difficult than for disks of Class II. We do, however, know of a few example of evolved disks, and these enable us to study their properties. The first of these is that of Beta Pictoris, first detected by IRAS in 1983 from its infrared excess, and was subsequently imaged with a coronagraph in 1984. P Pictoris is a star on the Main Sequence, and the disk surrounding it is relatively thin, when compared with the disks surrounding younger objects. Another interesting example observed recently is that of HR 4796A, an evolved, pre-Main-Sequence star, which exhibits a denser disk (less evolved) than that of P Pic. Some ten debris disks have been observed in scattered light. Some disks have also been mapped by submillimetre interferometry at 850 |m, in particular Fomalhaut, Vega, P Pic, and e Eri. These observations enable the structure of the cold dust to be mapped and to reveal density fluctuations or gaps, which are possible signatures of the presence of a planet (Meyer et al., 2006). In the case of e Eri, an exoplanet has, in fact, been detected by velocimetry, and the presence of a second exoplanet is suspected. Table 5.1 lists the properties of several debris disks observed by visible, infrared, or millimetric imagery.

Most of the debris disks have a very tenuous, or even non-existent, gaseous component. The latter may, however, be observed in certain cases. Studies undertaken with the Spitzer infrared telescope have shown that these exhibit major variations in the gas:dust ratio, and certain disks, such as that of HD 105, being devoid of gas (Meyer et al. 2006).

Table 5.1 Characteristics of some resolved debris disks (adapted from Meyer et al., 2006)

Star

Spectral type

Age (Myr)

Size of the disk (AU)

Albedo

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