Ionization Parameter: A Diagnostic of Radiation Pressure Dominated HII Regions

Details Ionization Parameter: A Diagnostic of Radiation Pressure Dominated HII Regions Ionization Parameter: A Diagnostic of Radiation Pressure Dominated HII Regions

Jan 2011

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011aas...21713207y&link_type=abstract

American Astronomical Society, AAS Meeting #217, #132.07; Bulletin of the American Astronomical Society, Vol. 43, 2011

Astronomy and Astrophysics

Astronomy

Scientific paper

When irradiation is sufficiently intense, the structure of an HII region will be dominated by radiation pressure and stellar winds, rather than ionized gas pressure. This state is of considerable interest because of its role in the formation of massive stars, the disruption of giant molecular clouds, and the evolution of starburst galaxies. We discuss the usefulness of the ionization parameter U, as often derived from observed line ratios between species which exist only in ionized gas, as a diagnostic for the radiation pressure-dominated state. In ionization-bounded directions, U cannot exceed a maximum value Umax determined by equilibrium between radiation and gas pressure forces. Lower values of U will occur, however, when the pressure of shocked stellar winds is significant, or when neutral gas is broken into clumps with sufficiently small radii of curvature. Applying these considerations to a prominent ionized shell around 30 Doradus and to the inner starburst region of M82, along with Cloudy simulations, we conclude that both are dominated by a combination of radiation pressure and shocked winds.

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