This thesis is concerned with testing models of the infrared Hel 2.058^m/Br7 ratio as a function of effective temperature in HII regions. This ratio is known to be sensitive to effective temperatures in the range Te //=32-38,000K. Near-infrared observational data from ultracompact (UC) HII regions, compact HII regions and giant HII regions or the nuclei of starburst galaxies is presented and analysed for this purpose.
Observations of UCHII regions show that HII region models with an electron densities of ne = 104cm-3, electron temperatures in the range Te=5,000-10,000K and a Galactic helium abundance can reproduce the observed Hel 2.058 /im/Br7 ratios in these objects, subject to one condition. As the electron temperature is increased from Te=5,000K, microturbulence is incorporated into the model which counteracts the increase in the Hel 2.058 /¿m/Br7 ratio with electron temperature. Microturbulent velocities of the order ~20kms_1 are required at Te=10,000K, in agreement with observed radio recombination line and high resolution Hel and HI measurements.
Lower Hel 2.058/xm/Br7 ratios are generally found in larger objects classed as compact HII regions, consistent with lower electron densities or lower stellar effective temperatures, or a combination of both effects in such objects. Alternative electron densities have been calculated from radio continuum measurements and provide evidence to suggest that electron densities are slightly lower in most of the compact HII regions than in the UCHII regions.
Detailed density gradient modelling of the Hel 2.058/rm/Br7 ratio has been performed, which reveals the need for realistic density distributions and explicit integration over the volume of the source when attempting to model specific HII regions. Accurate integrated electron densities or density gradients act as an alternative to microturbulence to bring theoretical Ilel 2.058/xm/Br7 ratios into agreement with observations for electron temperature models higher than Te=5,000K.
Finally, a large sample of starburst galaxies ale analysed. The Hel 2.058 /xm/Br7 ratios are much lower than found in compact or UCHII regions consistent with the proposal that the integrated Hel 2.058¿mr/Br7 emission is generally dominated by low density giant HII regions present in these galaxies. Effective temperatures derived from the Hel 2.058/rm/Bry ratio are consistent with those estimated from FIR fine structure lines. From consideration of new models of the Hel 2.058¿¿m/Bry ratio, the combination of optical HeI/H/3 data with Hel 2.058 /¿m/Bry observations extends the sensitivity of these model ratios to a wider range of effective temperatures.