We explore massive star feedback in starburst systems, employing both theoretical approaches and observational studies. First we investigate this topic using ionization parameter U as a tool to measure radiation pressure feedback in HII regions. Starburst galaxies display log10 U <= -2.3, while single HII regions never show very high U. In our theoretical study, we show that in a resolved HII region, U is limited to log10 U <= -1 when radiation pressure is important. The observed ionization parameter Uobs cannot exceed this maximum, and stellar winds and photoevaporative flows in the region can reduce Uobs. On the other hand, we show that the limit log10 U <= -2.3 of starburst galaxies is likely due to radiation pressure and internal dust extinction in their HII regions. The effect of stellar wind pressure cannot be large relative to radiation pressure and internal dust extinction, and HII regions in starburst are not adiabatic wind bubbles.
We then study the characteristic diagnostic [OIII]/Hbeta and [NII]/Halpha line ratios of different HII region models. We found that at a fixed ionizing spectrum and metallicity, there is a unique mapping between the line ratios and U and HII region density. We also confirm that under strong stellar winds with the influence of radiation pressure, an HII region cannot reach high U. The assumption of an adiabatic wind bubble with high U breaks down when reaching the high luminosity required to produce high U values.
In our observational studies, we present the first fully calibrated H2 1-0 S(1) image of 30 Doradus, together with a Brgamma image. The morphologies of H2 emission traces the CO-emitting clouds well, and H2 spatially correlates with Brgamma , implying that the observed H2 emission arises from the photodissociation regions of 30 Doradus. With the H2 to Brgamma line ratio analysis and photoionization modeling efforts, we suggest that H2 emission comes from molecular gas of density 10^2 to 10^3 cm^-.3, at a depth of AV ~ 0.34. We further suggest that radiation pressure is not important in 30 Doradus; stellar winds and photoevaporative flows are at work to reduce the Uobs of 30 Doradus.