The Effect of Internal Dissipation and Surface Irradiation on the Structure of Disks and the Location of the Snow Line around Sun‐like Stars

Abstract

In theory of accretion disks, angular momentum and mass transfer are associated with the generation of energy through viscous dissipation. In the construction of SED models of protostellar disks, the stellar irradiation is usually assumed to be the dominant heating source. Here we construct a new set of self-consistent analytical disk models by taking into account both sources of thermal energy and the thermal structure of the disk across the midplane. We deduce a set of general formulae for the relationship between the mass accretion rate and the surface density profile. We apply it to determine the structure of protostellar disks under a state of steady accretion and derive the radial distribution of surface density and midplane temperature. The incorporation of the viscous heating in our model reduces the disk flaring angle and leads to lower photospheric temperatures than previously thought. Around T Tauri stars, the snow line can evolve from outside 10 AU during FU Orionis outbursts, to 2 AU during the quasi-steady accretion phase, to 0.7 AU when the accretion rate falls to about 10-9 M☉ yr-1, and finally reexpand beyond 2.2 AU during the protostellar-to-debris disk transition. The nonmonotonous evolution of the snow line may lead to the observed isotopic composition of water on both Venus and Earth. We also infer the presence of a marginally opaque, isothermal region with a surface density distribution similar to that of the MSN model. With a 40% higher temperature than that in the region immediately within, this transition may lead to an upturn in the SEDs in the MIR (24-70 μm) wavelength range. The optically thin, outermost regions of the disk have a shallow surface density profile of the dust that is consistent with millimeter observations of spatially resolved disks.