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 ${\mathrm{H}}_2\mathrm{C}\mathrm{O}(3_{0,3}-2_{0,2})$, $\rm H_{2}CO(3_{2,1}-2_{2,0})$,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 $n_{\mathrm{e}}\sim {10}^4$ to ${10}^5$ cm${}^{-3}$, lower than typically considered for HC HII’s. We combined the RL measurements and independently derived $n_{\mathrm{e}}$, 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 $n_{\mathrm{e}}$ 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 $hyperdense$ ($n_{\mathrm{e}}>{10}^6$ cm${}^{-3}$), probably associated with O-type stars in a specific stage of their formation or early life.