Excitation conditions in HII regions

Thesis/Dissertation · Fri Jan 01 04:00:00 EST 1982

OSTI ID:6150304

Herter, T L

The infrared fine structure lines, because of their relatively weak dependence on density and temperature, provide not only a means of determining abundances in HII regions but also a way of examining the excitation conditions of these gaseous nebulae. In particular, the ArII (6.99 ..mu..m)/ArIII (8.99 ..mu..m) and SIII (18.71 ..mu..m)/SIV (10.51 ..mu..m) ratios give an abundance independent way of comparing observation and theory. The 8 to 13 ..mu..m observations of several compact HII regions have been obtained in order to help understand the physical conditions which exist in these nebulae, while previous observations of ArII, ArIII, SIII, and SIV for 11 HII regions are used to compute observed ionic ratios of ArII/ArIII and SIII/SIV which can be compared with model calculations. Model calculations of the ionization structure of HII regions are performed assuming a constant electron temperature and the on-the-spot approximation for the treatment of the diffuse radiation field. Charge transfer reactions are included and the model atmospheres of Kurucz (1979) are used for the flux distribution of the exciting stars. These model calculations indicate that assuming a classical, uniform density, dust-free HII region results in predicted ArII/ArIII and SIII/SIV ratios which disagree with observations by one to two orders of magnitude. It does not appear that optically thin clumping or neutral condensations with shadowing can account for the observed line ratios, however the addition of selectively absorbing dust with an appreciable optical depth in the ultraviolet (of order unity) provides a simple mechanism which yields results in rough agreement with the observations.

• Research Organization: Rochester Univ., NY (USA)
• OSTI ID: 6150304
• Country of Publication: United States
• Language: English

Related Subjects

640105* -- Astrophysics & Cosmology-- Galaxies
71 CLASSICAL AND QUANTUM MECHANICS
GENERAL PHYSICS
ARGON IONS
ATMOSPHERES
CHARGE EXCHANGE
CHARGED PARTICLES
CHEMICAL COMPOSITION
COSMIC DUST
COSMIC RADIO SOURCES
DATA
DUSTS
ELECTROMAGNETIC RADIATION
ENERGY-LEVEL TRANSITIONS
EXCITATION
EXPERIMENTAL DATA
H2 REGIONS
INFORMATION
INFRARED RADIATION
IONIZATION
IONS
MATHEMATICAL MODELS
MULTICHARGED IONS
NUMERICAL DATA
RADIATIONS
STELLAR ATMOSPHERES
SULFUR IONS
THEORETICAL DATA