Some properties of protoplanetary disks

1 Some properties of protoplanetary disks 3.1.1 Disk masses 

Given the paucity of disks around solar-type stars that are truncated at relatively short radii, a number of theories have been offered to explain why the remaining mass in the Solar System is confined to relatively small radial distances despite the solar nebula beginning in a much more extended configuration. Among the possibilities are (1) a passing star stripped off the outer material in the Solar System either before or after the gas in the solar nebula had dissipated (Pfalzner et al. 2005), (2) ultraviolet radiation from stars in the Sun s neighborhood heated the outer gas 

In most cases, models of protoplanetary disks assume that the disk is axisymmetric and that the surface density falls off monotonically with the distance from the star. This is assumed because it is the general expectation for such disks based on collapse calculations, and, admittedly, for the sake of simplicity. However, some disks do show signs of radial or non-axisymmetric structure, indicating that such treatments are not always valid. Examples include AB Aurigae (Fukagawa et al. 2004 Pietu et al. 2005 see Fig. 3.1) or AA Tau with its rotating bump that periodically obscures the star (Bouvier et al. 2003). 

In some cases disks may have structures that are unstable to perturbations driven by the gravitational force that the disk exerts on itself. This is quantified by the Toomre Q parameter, given by  

PRC95-21 T ci QPO - C Grady INOAO al NASA Goddard Space Fiigm Cameo and nasa 

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