We present a detailed characterization of the population of compact radio-continuum sources in W51 A using subarcsecond VLA and ALMA observations. We analyzed their 2-cm continuum, the recombination lines (RLs) H77$alpha$ and H30$alpha$, and the lines of $rm H_{2}CO(3_{0,3}-2_{0,2})$, $rm H_{2}CO(3_{2,1}-2_{2,0})$, and $rm SO(6_{5}-5_{4})$. We derive diameters for 10/20 sources in the range $D sim 10^{-3}$ to $sim 10^{-2}$ pc, thus placing them in the regime of hypercompact HII regions (HC HIIs). Their continuum-derived electron densities are in the range $n_{rm e} sim 10^4$ to $10^5$ cm$^{-3}$, lower than typically considered for HC HIIs. We combined the RL measurements and independently derived $n_{rm e}$, finding the same range of values but significant offsets for individual measurements between the two methods. We found that most of the sources in our sample are ionized by early B-type stars, and a comparison of $n_{rm e}$ vs $D$ shows that they follow the inverse relation previously derived for ultracompact (UC) and compact HIIs. When determined, the ionized-gas kinematics is always (7/7) indicative of outflow. Similarly, 5 and 3 out of the 8 HC HIIs still embedded in a compact core show evidence for expansion and infall motions in the molecular gas, respectively. We hypothesize that there could be two different types of $hypercompact$ ($D< 0.05$ pc) HII regions: those that essentially are smaller, expanding UC HIIs; and those that are also $hyperdense$ ($n_{rm e} > 10^6$ cm$^{-3}$), probably associated with O-type stars in a specific stage of their formation or early life.
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