HIGH-RESOLUTION CH3CN OBSERVATIONS TOWARDS HOT CORES

The main goal of this paper is to study the spatial distribution and t he temperature, density, and velocity gradients of the hot molecular g as in regions of massive star formation, on scales ranging from simila r to 10 '' to similar to 1 ''. To this purpose, we have used the IRAM Plateau de Bure Interferometer (PdBI) to make high angular resolution (similar to 2 '') maps in several CH3CN lines of the molecular clumps associated with the two ultracompact HII regions G10.47+0.03 and G31.4 1+0.31. Part of the results for G31.41 have already been published (Ce saroni et al. 1994b): here we present the observations of G10.47 and d iscuss them in relationship to the results obtained for G31.41. We hav e produced PdBI maps of both sources in the ground state CH3CN(6-5), ( CH3CN)-C-13(6-5) and vibrationally excited (v(8)=1) CH3CN(6-5) transit ions, and in the 2.7 mm continuum emission and (CO)-C-13(1-0) line. We also obtained hybrid maps in the CH3CN(6-5) lines, by merging the int erferometer data with the IRAM 30-m telescope maps of Olmi et al. (199 6). We derive estimates of the size and mass of the hot molecular core s known from earlier ammonia and methyl cyanide observations. The conc lusions drawn by Cesaroni et al. (1994b) for G31.41 are rediscussed he re in view of new results obtained in the (CO)-C-13(1-0) line and we s how that the disk interpretation for the observed velocity gradient in CH3CN is to be favoured against the outflow hypothesis; such an outfl ow is detected in the (CO)-C-13(1-0) transition. As in the case of G31 .41, we demonstrate that most of the continuum emission at 2.7 mm seen towards G10.47 is due to dust and derive a clump mass of similar to 2 000 M.. We study the velocity field in G10.47 and detect two trends: a n east-west velocity shift is observed in CH3CN, while in (CO)-C-13 th e velocity increases steadily from south to north. We discuss the poss ibility that towards G10.47, we observe a disk (in CH3CN) and an outfl ow (in (CO)-C-13). We conclude that in both sources we are observing l ow mass halos surrounding massive cores; such cores seem to be flatten ed rotating structures, which lie at the centre of bipolar outflows.