Ultracompact HII Regions: Can Old Models Explain New Observations ?

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Session 37 -- Interferometry
Display presentation, Tuesday, 9:30-6:30, Pauley Room

[37.03] Ultracompact HII Regions: Can Old Models Explain New Observations ?

R.A. Gaume (NRL), W.M. Goss, M.J. Claussen (NRAO-AOC)

It has been suggested that O stars spend up to 20\% of their few $10^6$ year lifetimes in molecular clouds (Wood \& Churchwell 1989 ApJ 340, 265), with a large fraction of this period spent in the ultracompact HII region stage. However, based on standard models, an ultracompact HII region, driven by thermal and stellar wind pressures, should have a lifetime considerably shorter than this, expanding beyond the ultracompact phase in a few $10^3$ years (Wood \& Churchwell 1989 ApJ Supp 69 831).

New observations of several star forming complexes are presented which find far greater numbers of ultracompact HII regions than have been previously believed to exist. For example, in the Sgr B2 region, Gaume \& Claussen (1990 ApJ 351, 538) found 19 separate ultracompact HII components. New, higher frequency, spatial resolution, and sensitivity observations show nearly 50 individual ultracompact HII regions, many of which have diameters $<$ 2000 AU.

Viable models of HII regions must explain the large discrepancy between the observed numbers of ultracompact HII regions and their short predicted lifetimes. A review of relevant models, and their application to new observations will be made.

Tuesday program listing