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

Abstract

We report high angular resolution observations, made with the Atacama Large Millimeter Array in band 6, of high excitation molecular lines of $\mathrm{C}{\mathrm{H}}_3\mathrm{CN}$ and $\mathrm{S}{\mathrm{O}}_2$ and of the H29$\alpha$ 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$\to$13 rotational ladder of $\mathrm{C}{\mathrm{H}}_3\mathrm{CN}$ and in the $30_{4,26}-30_{3,27}$ and $32_{4,28}-32_{3,29}$ lines of $\mathrm{S}{\mathrm{O}}_2$. The peak of the velocity integrated molecular emission is located $\sim$0$.{}^{\prime \prime }$4 northwest of the peak of the continuum emission. The first-order moment images and channel maps show a velocity gradient, of 1.1 km s${}^{-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$\pm 24$ Kelvin at the peak position to 166$\pm 16$ Kelvin at its edge, indicating that our molecular observations are probing a hot molecular core that is internally excited. The emission in the H29$\alpha$ line arises from a region of 0$.{}^{\prime \prime }$65 in size, where its peak coincides with that of the dust continuum. We model the kinematical characteristics of the "central blue spot" feature as due to infalling motions, suggesting a central mass of 172.8$\pm 8.8M_{\odot }$. 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.

Accepted by ApJ