Accretion disks around massive stars: hydrodynamic structure, stability, and dust sublimation

We investigate the structure of accretion disks around massive protostar applying steady state models of thin disks. The thin disk equations are solved with proper opacities for dust and gas taking into account the huge temperature variation along the disk. We explore a wide parameter range concerning stellar mass, accretion rate, and viscosity parameter alpha. The most essential finding is a very high temperature of the inner disk. For e.g., a 10 M sun protostar with an accretion rate of ~10-4 M sun yr-1, the disk midplane temperature may reach almost 105 K. The disk luminosity in this case is about 104 L sun and, thus, potentially higher than that of a massive protostar. We motivate our disk model with similar hot disks around compact stars. We calculate a dust sublimation radius by turbulent disk self-heating of more than 10 AU, a radius, which is 3 times larger than that caused by stellar irradiation. We discuss implications of this result on the flashlight effect and the consequences for the radiation pressure of the central star. In deference to disks around low-mass protostars, our models suggest rather high values for the disk turbulence parameter alpha <= 1. However, disk stability to fragmentation due to thermal effects and gravitational instability would require a lower alpha value. For alpha = 0.1, we find stable disks out to 80 AU. Essentially, our model allows us to compare the outer disk to some of the observed massive protostellar disk sources, and from that, extrapolate the disk structure close to the star which is yet impossible to observe