SIMBAD references

We present a detailed characterization of the population of compact radio-continuum sources in W51 A using subarcsecond Very Large Array and Atacama Large Millimeter/submillimeter Array observations. We analyze their 2 cm continuum, the recombination lines (RLs) H77α and H30α, and the lines of H₂CO(3_0,3-2_0,2), H₂CO(3_2,1-2_2,0) , and SO(6₅-5₄). We derive diameters for 10/20 sources in the range D ∼ 10^–3 to ∼10^–2 pc, thus placing them in the regime of hypercompact H II regions (HC H II's). Their continuum-derived electron densities are in the range n_e ∼ 10⁴ -10⁵ cm^–3, lower than typically considered for HC H II's. We combined the RL measurements and independently derived n_e, finding the same range of values but significant offsets for individual measurements between the two methods. We find that most of the sources in our sample are ionized by early B-type stars, and a comparison of n_e versus D shows that they follow the inverse relation previously derived for ultracompact (UC) and compact H II's. When determined, the ionized-gas kinematics is always (7/7) indicative of outflow. Similarly, 5 and 3 out of the 8 HC H II's 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) H II regions: those that essentially are smaller, expanding UC H II's; and those that are also hyperdense (n_e > 10⁶ cm^–3), probably associated with O-type stars in a specific stage of their formation or early life.