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We present the results of high angular resolution millimeter observations ofgas and dust toward G31.41+0.31 and G24.78+0.08, two high-mass star formingregions where four rotating massive toroids have been previously detected byBeltran et al. (2004). The CH3CN (12-11) emission of the toroids in G31.41+0.31and core A1 in G24.78+0.08 has been modeled assuming that it arises from adisk-like structure seen edge-on, with a radial velocity field. For G31.41+0.31the model properly fits the data for a velocity v_rot~1.7 km/s at the outerradius R_out~13400 AU and an inner radius R_inn~1340 AU, while for core A1 inG24.78+0.08 the best fit is obtained for v_rot~2.0 km/s at R_out~7700 AU andR_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 orconstant angular velocity, since both velocity fields suitably fit the data.The best fit models have been computed adopting a temperature gradient of thetype T proportional R^{-3/4}, with a temperature at the outer radius T_out~100K for both cores. The M_dyn needed for equilibrium derived from the models ismuch smaller than the mass of the cores, suggesting that such toroids areunstable and undergoing gravitational collapse. The collapse is also supportedby the CH3^{13}CN or CH3CN line width measured in the cores, which increasestoward the center of the toroids. The estimates of v_inf and \dot M_acc are ~2km/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 effectof stellar winds and radiation pressure and allow further accretion on thestar.Comment: 27 pages, including 25 figures, 12 tables;accepted for publication by  A&

A detailed study of the rotating toroids in G31.41+0.31 and G24.78+0.08