  We construct a new set of self-consistent analytical disk models by taking
into account both viscous and radiative sources of thermal energy. We analyze
the non-isothermal structure of the disk across the mid-plane for optically
thick disks, and use the standard two-temperature model in the case of
optically thin disks. We deduce a set of general formula for the relationship
between the mass accretion rate and the surface density profile. Our results
recover those of Chiang & Goldreich in the optically thin regions, but extend
their work for the opaque regions of the disk.
  For the purpose of illustration, we apply our theory in this paper to
determine the structure of protostellar disks around T Tauri stars under a
state of steady accretion and derive the corresponding radial distribution
function of various disk properties such as surface density and temperature
near the mid-plane. We calculate the position of the snow line around a
sun-like T Tauri star, and deduce that it can evolve from well outside 10 AU
during FU Orionis outbursts, to about 4 AU during passive accretion phase, to
the present-day orbital radius of Venus and finally re-expand to over 2.2 AU
during the protostellar- to-debris disk transition. This non-monotonous
evolution of the snow line may provide some novel and deterministic explanation
for the total water content and its isotopic composition of both Venus and the
Earth. In the optically thin, outermost regions of the disk we find that the
surface density profile of the dust varies roughly as 1/r, which is consistent
with mm observations of spatially resolved disk of Mundy et al. (2000).
