Infall Motions in the Hot Core Associated with Hypercompact HII Region G345.0061+01.794 B

We report high angular resolution observations, made with the Atacama Large Millimeter Array in band 6, of high excitation molecular lines of CH_3CN and SO_2 and of the H29α radio recombination line towards the G345.0061+01.794 B HC H II region, in order to investigate the physical and kinematical characteristics of its surroundings. Emission was detected in all observed components of the J=14→13 rotational ladder of CH_3CN and in the 30_4,26-30_3,27 and 32_4,28-32_3,29 lines of SO_2. The peak of the velocity integrated molecular emission is located ∼0.^''4 northwest of the peak of the continuum emission. The first-order moment images and channel maps show a velocity gradient, of 1.1 km s^-1 arcsec^-1, across the source, and a distinctive spot of blueshifted emission towards the peak of the zero-order moment. The rotational temperature is found to decrease from 252±24 Kelvin at the peak position to 166±16 Kelvin at its edge, indicating that our molecular observations are probing a hot molecular core that is internally excited. The emission in the H29α line arises from a region of 0.^''65 in size, where its peak coincides with that of the dust continuum. We model the kinematical characteristics of the "central blue spot" feature as due to infalling motions, suggesting a central mass of 172.8±8.8 M_⊙. Our observations indicate that this HC H II region is surrounded by a compact structure of hot molecular gas, which is rotating and infalling toward a central mass, that is most likely confining the ionized region. The observed scenario is reminiscent of a "butterfly pattern" with an approximately edge-on torus and ionized gas roughly parallel to its rotation axis.