We discuss the properties of an accretion disk around a star with parameters
typical of classical T Tauri stars (CTTS), and with the average accretion rate
for these disks. The disk is assumed steady and geometrically thin. The
turbulent viscosity coefficient is expressed using the alpha prescription and
the main heating mechanisms considered are viscous dissipation and irradiation
by the central star. The energy is transported by radiation, turbulent
conduction and convection.
We find that irradiation from the central star is the main heating agent of
the disk, except in the innermost regions, R less than 2 AU. The irradiation
increases the temperature of the outer disk relative to the purely viscous
case. As a consequence, the outer disk (R larger than 5 AU) becomes less dense,
optically thin and almost vertically isothermal, with a temperature
distribution T proportional to R^{-1/2}. The decrease in surface density at the
outer disk, decreases the disk mass by a factor of 4 respect to a purely
viscous case. In addition, irradiation tends to make the outer disk regions
stable against gravitational instabilities.Comment: 41 pages, 14 postscript figures, LaTeX, accepted by Ap
ABSTRACT. Cloud environment is thought to play a critical role in determining the mechanism of formation of massive stars. In this contribution we review the physical characteristics of the environment around recently formed massive stars. Particular emphasis is given to recent high angular resolution observations which have improved our knowledge of the physical conditions and kinematics of compact regions of ionized gas and of dense and hot molecular cores associated with luminous O and B stars. We will show that this large body of data, gathered during the last decade, has allowed signiÐcant progress in the understanding of the physical processes that take place during the formation and early evolution of massive stars.
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