Recombination Lines and Molecular Gas from Hypercompact HII regions in
  W51 A
  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 (RL's) 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 HII's). 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 HII's. 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 HII's.
When determined, the ionized-gas kinematics is always (7/7) indicative of
outflow. Similarly, 5 and 3 out of the 8 HC HII'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) HII regions: those that essentially are smaller,
expanding UC HII's; 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.
