Astrophys. J., 899, 94-94 (2020/August-3)
Recombination lines and molecular gas from hypercompact HII regions in W51 A.
RIVERA-SOTO R., GALVAN-MADRID R., GINSBURG A. and KURTZ S.
Abstract (from CDS):
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(30,3-20,2), H2CO(32,1-22,0) , 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 ne ∼ 104 -105 cm–3, lower than typically considered for HC H II's. We combined the RL measurements and independently derived ne, 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 ne 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 (ne > 106 cm–3), probably associated with O-type stars in a specific stage of their formation or early life.
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H II regions - Star formation - Radio astronomy - Giant molecular clouds
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