  We present the results of high angular resolution millimeter observations of
gas and dust toward G31.41+0.31 and G24.78+0.08, two high-mass star forming
regions where four rotating massive toroids have been previously detected by
Beltran et al. (2004). The CH3CN (12-11) emission of the toroids in G31.41+0.31
and core A1 in G24.78+0.08 has been modeled assuming that it arises from a
disk-like structure seen edge-on, with a radial velocity field. For G31.41+0.31
the model properly fits the data for a velocity v_rot~1.7 km/s at the outer
radius R_out~13400 AU and an inner radius R_inn~1340 AU, while for core A1 in
G24.78+0.08 the best fit is obtained for v_rot~2.0 km/s at R_out~7700 AU and
R_inn~2300 AU. Unlike the rotating disks detected around less luminous stars,
these toroids are not undergoing Keplerian rotation. From the modeling itself,
however, it is not possible to distinguish between constant rotation or
constant angular velocity, since both velocity fields suitably fit the data.
The best fit models have been computed adopting a temperature gradient of the
type T proportional R^{-3/4}, with a temperature at the outer radius T_out~100
K for both cores. The M_dyn needed for equilibrium derived from the models is
much smaller than the mass of the cores, suggesting that such toroids are
unstable and undergoing gravitational collapse. The collapse is also supported
by the CH3^{13}CN or CH3CN line width measured in the cores, which increases
toward the center of the toroids. The estimates of v_inf and \dot M_acc are ~2
km/s and 3x10^{-2} M_sun/yr for G31.41+0.31, and ~1.2 km/s and ~9x10^{-3}
M_sun/yr for G24.78+0.08 A1. Such large accretion rates could weaken the effect
of stellar winds and radiation pressure and allow further accretion on the
star.
