Existence of Accretion Disks

Observational evidence has established recently the ubiquity of large disks of dust around very young stars. The crucial observations came first from the Infrared Astronomical Satellite (IRAS). Observed in infrared (typically from 10 to 100 |m), a large number of very young stars were much brighter than expected from their visual magnitudes (Rowan-Robinson, 1985). This infrared excess was interpreted as coming from the radiation of a large circumstellar disk of cold dust, and most of these stars were shown to be very young T Tauri stars (Bertout, 1989). Some, like FU Orionis, were still probably accreting mass (Hartmann and Kenyon, 1985). Finally, optical pictures in the visual, obtained by hiding the central star in the field of the telescope, have detected and resolved dusty disks of sizes 500-1000 Astronomical Units (AU) among the nearest candidates, like in ยก3 Pictoris (Smith and Terrile, 1984).

The existence of numerous accretion disks has therefore been substantiated recently and has become the accepted explanation on the way Nature succeeds in making single stars: namely, by shedding the angular momentum in excess to the expanding margin of the disk during the buildup of the central mass. The first consequence of this explanation is that many single stars are likely to make a planetary system by the evolution of their accretion disk. Of course, the accretion disks are only visible around very young stars because at that time, and dust is fine enough to radiate a large amount of infrared in spite of its small total mass. As soon as it coalesces and accretes into larger objects (like planetesimals or eventually planets), their total cross section becomes much smaller and they become much more difficult to detect from afar.

Accretion disks have also become popular in a different context, namely, to explain the mechanism that produces the energy released in X-ray stars and quasars. The gravitational potential well that surrounds very dense compact objects is deep enough for a particle spiraling inward to transform a reasonable fraction of its rest mass to radiation. The deeper the gravitational well, the hotter the accretion disk, but this is not the type of interest here; we have mentioned them now not so much to avoid any confusion later, as to emphasize the generality of the accretion disk mechanism to extract energy and angular momentum from a central spinning mass.

For our concern, observational evidence revealing the ubiquity of large disks of dust around very young stars has given a sudden respectability to the theory that describes Laplace's "Solar Nebula" by a viscous accretion disk, even if the cause of the viscosity has not been properly elucidated.

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