Self-consistent atmospheric models of very massive stars and their early evolution in dense stellar systems

• Applicant: Professor Dr. Adalbert Wilhelm Alois Pauldrach (†)
• Subject Area: Astrophysics and Astronomy
• Term: from 2013 to 2020
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 246058756

• Final Report Year: 2020

Final Report Abstract

The aim of our project has been to provide an extensive study of the spectral energy distributions (SEDs) of massive (main-sequence and supergiant) O-type stars and of very massive stars (VMS) that might have formed as a result of stellar mergers inside dense stellar clusters. The project comprised the computation and interpretation of synthetic spectra as well as both theoretical and observational studies of the impact of their SEDs on the ionization and temperature structure, and the resulting observable emission-line spectrum, of the surrounding gas. Compared to similar studies our focus is on the effects of metallicity of the emitted spectrum. Our group managed to successfully complete large parts of the initially planned project owing to the commitment and dedication of all involved persons, and in spite of the limited funding significant results could be achieved and published. As the main results of our project we could show that • while the rate of hydrogen-ionizing photons emitted by the stellar atmosphere is not strongly influenced by the stellar metallicity (for a given effective temperature and radius), the emission shows a strong metallicity dependence for photon energies above the ionization threshold of neutral helium. The stellar metallicity can thus strongly affect the ratio of diagnostically important ion pairs like N ii/N iii, O ii/O iii, Ne ii/Ne iii, and S iii/S iv, whose abundance ratios depend on the amount of ionizing stellar flux in the corresponding energy ranges. Neglecting the metallicity dependence of the stellar SED could therefore lead to considerable discrepancies in the analysis of ionized gas, such as of “classical” H ii regions. • the observed line ratios of Ne ii/Ne iii and S iii/S iv in the mid-infrared spectral range obtained with the Spitzer Space Telescope are in good agreement with the stellar SEDs predicted by our supergiant models. • the predicted higher effective temperatures of very massive stars that may have formed by stellar collisions and which considerably exceed the mass range of “normal” O-type stars result in the emission of photons above the ionization edge of He ii at 54.4 eV, and can thereby lead to a detectable signature in the emission spectrum of H ii regions. In fact, line emission from O iv (which requires photons of at least 54.9 eV to form) can actually be observed in starburst galaxies. Here, very massive stars might provide at least in part a complementary explanation for the observed emission.

Publications

• Three-dimensional modeling of ionized gas. II. Spectral energy distributions of massive and very massive stars in stationary and time-dependent modeling of the ionization of metals in H ii regions. A&A 583, A63 (2015)
  J. A. Weber, A. W. A. Pauldrach, T. L. Hoffmann
  
• Spitzer observations of extragalactic H ii regions. III. NGC 6822 and the hot star, H ii region connection. MNRAS 459, 1875 (2016)
  R. H. Rubin, J. P. Simpson, S. W. J. Colgan, R. J. Dufour, J. Kader, I. A. McNabb, A. W. A. Pauldrach, J. A. Weber
  
• Numerical models for the diffuse ionized gas in galaxies. II. Three-dimensional radiative transfer in inhomogeneous interstellar structures as a tool for analyzing the diffuse ionized gas. A&A 622, A115 (2019)
  J. A. Weber, A. W. A. Pauldrach, T. L. Hoffmann