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We report high angular resolution observations, made with the Atacama LargeMillimeter Array in band 6, of high excitation molecular lines of CH3CN and SO2and of the H29a radio recombination line towards the G345.0061+01.794 B HC H iiregion, in order to investigate the physical and kinematical characteristics ofits surroundings. Emission was detected in all observed components of theJ=14-13 rotational ladder of CH3CN and in the 30(4,26) - 30(3,27) and 32(4,28)- 32(3,29) lines of SO2. The peak of the velocity integrated molecular emissionis located ~0.4" northwest of the peak of the continuum emission. Thefirst-order moment images and channel maps show a velocity gradient, of 1.1 kms-1 arcsec-1, across the source, and a distinctive spot of blueshifted emissiontowards the peak of the zero-order moment. The rotational temperature is foundto decrease from 252+-24 Kelvin at the peak position to 166+-16 Kelvin at itsedge, indicating that our molecular observations are probing a hot molecularcore that is internally excited. The emission in the H29a line arises from aregion of 0.65" in size, whose peak is coincident with that of the dustcontinuum, has a center velocity of V_LSRK= -18.1+-0.9 km s-1 and a width(FWHM) of 33.7+-2.3 km s-1. We model the kinematical characteristics of the"central blue spot" feature as due to infalling motions, suggesting a centralmass of 126.0+-8.7 M_solar. Our observations indicate that this HC H ii regionis surrounded by a compact structure of hot molecular gas, which is rotatingand infalling toward a central mass, that is most likely confining the ionizedregion.

The environments of hyper-compact H II regions. I. G345.0061+01.794 B