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Session 106 - HII Regions and Ultracompact HII Regions.
Display session, Thursday, January 16
Metropolitan Ballroom,
We have observed the H66-alpha radio recombination line (RRL) emission at 1.3cm and the H52-alpha RRL emission at 7mm towards the ultracompact HII region M17-UC1 and the nearby arc-shaped structure. The angular resolution of the H66-alpha data, obtained using the Very Large Array in the C and DnC arrays, is 1"; the H52-alpha observations, taken in the D array, have an angular resolution of 2.3" by 1.5". H2O maser emission, at 1.3 cm, from the region of M17-UC1 was also observed in the D array with a beam of 4". We present these observations, derive the physical parameters of UC1 and the arc-shaped structure, and compare them with the values of Felli, Churchwell, and Massi(1984). We discuss the source as an example of shock induced star formation consisting of a hot, young, massive star surrounded by ionized material resulting from a stellar wind outflow. We compare M17-UC1 with six other sources known to have unusually broad RRL emission. The compact HII region (size 0.6" or 0.006 pc) is one of a growing number of HII regions with broad (>35 km/s) RRLs. M17-UC1 has an H66alpha line width (FWHM) of 47 km/s and an H52alpha line width of 41 km/s. Both lines have a central velocity of +19 km/s. The line parameters of the arc, about 1" or 0.01 pc to the east of UC1, are remarkably constant over an area extending 16.5" by 3" (0.165 pc by 0.03 pc), with a line width of 36 km/s and a central velocity of +16 km/s. The derived LTE electron temperature is 6600 K in UC1 and 8200 K in the arc region. Observations of the H20 maser emission toward UC1 reveals four masers within .5' of UC1. Of these, two are newly detected. The maser emission is near, but not coincident with, the position of the ultracompact source. Recent studies of the relation between water masers and ultracompact HII regions (Codella et al, 1996; Hofner and Churchwell, in press) suggest that such masers are among the earliest indicators of young stellar evolution, coincident with hot, dense, molecular clumps and molecular outflows in post-shock regions even before the development of an ionized region.
