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Session 66 - Young Clusters, High-Mass YSOs and Their Environment.
Oral session, Tuesday, January 16
Salon del Rey Central, Hilton
HII regions in the earliest stages of their evolution are typically embedded deep in molecular clouds and unobservable at optical frequencies. At radio frequencies, however, the ionized gas distribution and kinematics may be imaged with high spatial (\sim0\farcs5) and spectral (\sim5 km s^-1) resolution. We have undertaken an extensive, multi-frequency, multi-configuration observational program of four star-forming regions at different galactocentric radii using the Very Large Array (VLA). We have observed the luminous regions Sagittarius B2 (Sgr B2, near the Galactic center) and W49A (D=11.4 kpc), and two smaller regions: K3-50 (D=8.5 kpc) and NGC 6334 (D=1.7 kpc). These radio continuum and recombination line observations have revealed important common characteristics between the regions, and have raised new questions about the early stages of massive star formation. Within each of the two largest regions (Sgr B2 and W49A), there is a striking variety of morphologies (spherical, bipolar, shell and ``cometary'') and a large number of resolved sources. In Sgr B2 we have imaged 49 HII regions within a \sim2.5 pc radius, centered on the bright source F. In W49A, 41 regions have been imaged within a \sim3.5 pc radius of the central source G. In each of the four studied regions we also image the ionized gas kinematics, crucial to the interpretation of the morphologies observed. In a small number of sources we have detected very broad recombination lines (\DeltaV_FWHM\geq 60 km s^-1) and rising spectral indices (\alpha\sim 1) at centimeter wavelengths. We have used our VLA observations of the ionized gas along with existing molecular gas observations of these sources to place the types of HII regions into an evolutionary sequence that can account for the observed morphologies, distributions and kinematics.
