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
Final Report Abstract
The main objective of the supported project was the computation of a grid of synthetic spectra of hot massive stars, suitable as input for nebular models which serve as a tool for determining the properties of H II regions in different galaxies. Among these properties the chemical composition is of key importance because the distribution of metals within galaxies can be used as tracer for the galactic evolution. The analysis is performed by comparing observed emission line strengths with those from numerical simulations employing different parameters. However, there still exist notable differences between the observations and the values predicted by the nebular models. The spectral energy distributions (SEDs) of the ionizing sources are among the most important parameters for the accurate modelling of H II regions. The dominant source of ionization in classical H II regions are hot massive stars of spectral type O with effective temperatures above 30 000 K, which emit a large fraction of their radiation in the extreme ultraviolet (EUV) spectral range. Although it is not possible to observe this ionizing EUV radiation directly due to its almost complete absorption by the interstellar gas, the spectral energy distribution in the EUV range determines the ionization structure of the elements in the gas surrounding the stars and thus also the nebular emission spectra in the observable optical and infrared wavelength range. On the one hand, this means that nebular models can serve as an invaluable tool for probing the nature of the ionizing sources. (Conversely, abundance determinations from measured emission line fluxes must carefully account for the influence of the spectral energy distribution of the radiation source on the individual nebular ionization fractions and resulting emission line strengths.) On the other hand, it poses a serious problem for the verification and calibration of the nebular simulations: because the EUV radiation of the ionizing stars cannot be directly observed, no independent test of the assumptions made regarding the ionizing SEDs in the nebular models exists, and the SEDs must be provided by stellar atmosphere models with their own inherent approximations and uncertainties. Discrepancies between observed nebular line strengths and those predicted by photoionization models are likely to be resolved only through a concerted effort between nebular modelling and stellar atmospheric modelling. Accordingly, researchers in stellar atmospheres are paying increasing attention to the usefulness of nebular observations, particularly of H II regions, in validating and constraining their models. However, nebular models have often been used to analyze the influence of the metallicity of the gas on the resulting emission line spectra, but so far a corresponding parameter grid of accurate metallicity-dependent stellar SEDs have been missing. This is problematic because the influence of the stellar metallicity on the SEDs, and thereby on the emission line spectra of the surrounding gas can be as important as the chemical composition of the gas itself. To remedy this shortcoming, we created a grid of 44 stellar atmosphere models with effective temperatures between 30 000 K and 55 000 K and with metallicities ranging from one tenth to twice the solar value. Spectra of our model stars were used to describe the ionizing sources for H II region models used for the comparison with infrared spectra obtained with the Spitzer space telescope for H II regions in nearby galaxies with different metallicity. Motivated by the recent discovery of very massive stars (well above the heretofore generally accepted upper mass limit of around 150 M⊙ ) in the R 136 Cluster in the Large Magellanic Cloud, we have extended our model grid to this mass range and beyond, focusing in particular on a consistent description of the wind properties of these objects. Apart from the computed SEDs, our results for the mass loss rates are of importance regarding the evolutionary fates of such stars as possible products of stellar mergers in dense cluster environments, providing a plausible scenario for the formation of intermediate-mass black holes. As an application of these SEDs we have analyzed the possible contribution of very massive stars to the reionization history of the hydrogen and the helium in the universe. Our main conclusion is that the formation of very massive stars may significantly increase the stellar contribution to the full ionization of the intergalactic helium. Due to the large commitment of the involved persons substantial results have been obtained, which will form a solid basis for future projects.
Publications
- “Spitzer Observations of M33 and the hot star, HII region connection.” MNRAS 387, 45-62 (2008)
Rubin, R. H., Simpson, J. P., Colgan, S. W. J., Dufour, R. J., Brunner, G., McNabb, I. A.. Pauldrach, A. W. A., Erickson, E. F., Haas, M. R. & Citron, R. I.
- “Spitzer finds neon’s and sulfur’s sweet spot: part III, NGC 6822.” IAUS 265, 249-250 (2009)
Rubin, R. H., McNabb, I. A., Simpson, J. P., Dufour, R. J., Pauldrach, A. W. A., Colgan, S. W. J., Craven, T. W., Gitterman, E. D. & Lo, C. C.
- “Radiation-driven winds of hot luminous stars. XVI. Expanding atmospheres of massive and very massive stars and the evolution of dense stellar clusters.” A&A, 538, A75 (2012)
Pauldrach, A. W. A, Vanbeveren, D. & Hoffmann, T. L.
(See online at https://doi.org/10.1051/0004-6361/201117621) - “Three-dimensional modeling of ionized gas. I. Did very massive stars of different metallicities drive the second cosmic reionization?” A&A, 555, A35 (2013)
Weber, J. A., Pauldrach, A. W. A., Knogl, J. S. Hoffmann, T. L.
(See online at https://doi.org/10.1051/0004-6361/201220897)
