Extragalactic nebular emission line analysis with realistic EUV spectral energy distributions of massive stars of different metallicities as a tool for measuring interstellar chemical abundances

• Applicant: Professor Dr. Adalbert Wilhelm Alois Pauldrach (†)
• Subject Area: Astrophysics and Astronomy
• Term: from 2009 to 2014
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 87508410

H װ regions play a crucial role in the measurement of the chemical composition of the interstellar medium. Like planetary nebulae, they serve as laboratories for atomic physics and provide fundamental data about heavy element abundances that constrain models of galactic chemical evolution.However, major discrepancies still exist between observed emission line strengths and those predicted by nebular models. These discrepancies cannot simply be attributed to variations in the nebular chemical abundances, and thus indicate a more fundamental deficiency in our understanding of these emission line systems. Foremost among the uncertainties in modelling H װ regions are issues concerning the spectral energy distribution (SED) of the ionizing radiation source, most often hot stars of spectral type O . Since the radiation in the ionizing spectral range cannot be directly observed because of absorption in the interstellar medium, nebular models must rely on SEDs predicted by stellar atmosphere models. Indirectly, the comparison of observed emission line strengths and those predicted by nebular models thus also offers a way to verify the SEDs predicted by stellar atmosphere models. Only when the combination of nebular and stellar models is reliably shown to reproduce the observations can nebular diagnostics be used to its full potential as a quantitative tool for measuring interstellar abundances.One of the key influences on the nebular spectra is the metallicity (the abundance of heavy elements), both nebular and stellar. The existence of radial gradients of the metallicity within galaxies is well documented, as are differences in overall metallicity from one galaxy to another. But whereas nebular modelling often involves testing of different nebular metallicities against their influence on the predicted spectra, adequate grids of stellar atmospheres and realistic SEDs for different metallicities are still missing. This is objectionable because the influence of stellar metallicity on nebular line strength ratios, via its effect on the SEDs, has been shown to be even larger than that of variations in the nebular metallicity.In this project we propose to study a large sample of H װ regions of different metallicities - from super-solar to significantly subsolar - using a consistent grid of model atmosphere SEDs covering a large range of metallicities. These SEDs will be computed by us using state-of-the-art model atmospheres that consistently take into account the attenuation of the ionizing flux by the spectral lines of heavy elements and the hydrodynamics of the radiatively driven wind and its influence on the SEDs. This grid of SEDs, together with detailed nebular models and available high-quality observations of H װ regions taken with the Spitzer Space Telescope - the observations are available to us from two completed observing runs in Cycles 1 and 2 -, will provide a useful tool to constrain models for galactic chemical evolution.

• DFG Programme: Research Grants