We have used the institute for radio astronomy in the millimeter range (IRAM) Plateau de Bure Interferometer to make high angular resolution observations of the star-forming region surrounding the ultracompact H II region G31.41+0.31. We have produced maps in the ground state CH3CN(6-5), CH3(13)CN(6-5) and vibrationally excited CH3CN(6-5) transitions, and in the 3 mm continuum emission. From these, we derive estimates of the size and mass of the hot molecular core known from earlier ammonia observations. The core angular diameter as measured in both methyl cyanide and the 3 mm continuum is approximately 1 sec corresponding to approximately 0.04 pc. If the continuum emission is due to heated dust, we derive a mass of approximately 1000 solar mass, but with large uncertainties. A remarkable velocity gradient (approximately 400 km/s/pc) in the SW-NE direction is observed in CH3CN implying an equilibrium mass approximately 1000 solar mass, consistent with the value quoted above. We discuss two possible scenarios (disk or outflow) for explaining this gradient and conclude that the molecular core must be dynamically unstable.
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Cesaroni et al. (1994) studied this question.