Structure and Instabilities of an Irradiated Viscous Protoplanetary Disk

We investigate the structure and the stabilities of a protoplanetary disk, which is heated by viscous process in itself and by its central star. The disk is set to rotate with the Keplerian velocity and has the surface density distribution of the minimum mass solar nebula. We assume the vertical hydrostatic equilibrium and the radiative equilibrium at each point, and solve the two-dimensional radiative transfer equation by means of the Short Characteristic method in the spherical coordinate in order to determine the disk structure. Our calculation shows that at the outer region of the disk with a distance from the central star of x > 1AU the radiative heating from the inner disk dominates the viscous heating even near the midplane. It is because of the high temperature distribution in the optically thin surface layer and the relatively high disk height (z∞ ∼ 0.7x at x ∼ 1AU) as a consequence of the irradiation from the inner hot region of the disk. In addition, we examine the convective and the magnetorotational instabilities of the disk. As a result, the whole disk is convectively stable since the dusty region is not heated by the viscous dissipation from the midplane but by the radial radiative heating. On the other hand, almost all the disk is magnetorotationally unstable except for the region near the equatorial plane of 2AU < x < 10AU. Finally we discuss the growth and the size distribution of dust particles in the disk, which suggests that there exist cm-sized particles in the surface layer, namely, in the exposed region of the disk. Subject headings: accretion, accretion disks — circumstellar matter — instabilities — planetary systems: protoplanetary disks — radiative transfer