Recombination Lines and Molecular Gas from Hypercompact H ii regions in W51 A

Abstract

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) H77 α and H30 α , and the lines of ,  , and . 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  ∼ 10 4 –10 5 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  > 10 6 cm −3 ), probably associated with O-type stars in a specific stage of their formation or early life.