Hot Stars: Old-Fashioned or Trendy?

2012

A&A

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Context. The commonly accepted mass-luminosity relation of central stars of planetary nebulae (CSPNs) might not be universally valid. While the atmospheric parameters T eff , log g, and the He abundances have in the past been determined using fits to photospheric H and He absorption lines from plane-parallel model atmospheres, the masses and luminosities could not be derived independently this way, and were instead taken from theoretical evolutionary models. Improvements in hydrodynamically selfconsistent modelling of the stellar atmospheric outflows now allow using fits to the wind-sensitive features in the UV spectra to consistently determine the stellar radii, masses, and luminosities without assuming a mass-luminosity relation. Recent application to a sample of CSPNs in an earlier paper of this series raised questions regarding the validity of the mass-luminosity relation of central stars of planetary nebulae. Aims. The results of the earlier UV analysis are reassessed by means of a simultaneous comparison of both the observed optical and UV spectra with corresponding synthetic spectra. The synthetic optical and UV spectra are computed simultaneously from the same atmospheric models, using the same model atmosphere code. Synthetic spectra for the two central stars NGC 6826 and NGC 2392 are computed using parameter sets from two different published analyses to check their compatibility to the observations. Methods. Using the different published stellar parameter sets, derived on the one hand by a consistent UV analysis, and on the other hand from fits to optical H and He lines, we calculate corresponding optical and UV spectra with our model atmosphere code. We have improved this model atmosphere code by implementing Stark broadening for hydrogen and helium lines, thus allowing us to obtain consistent optical H and He line profiles simultaneously with our state-of-the-art modelling of the UV-spectrum. Results. Optical line profiles computed with the consistent parameter sets from the UV analysis yield line profiles with good agreement to the observations (with small discrepancies in the emission lines corresponding to about a factor of one half in the mass loss rate). Spectra computed with the stellar parameter sets from the optical analysis in the literature and corresponding consistent wind parameters, however, show large discrepancies to the observed spectra, in the optical as well as in the UV. We conclude that the published optical analyses give good fits to the observed spectrum only because the wind parameters assumed in these analyses are inconsistent to their stellar parameters. By enforcing consistency between stellar and wind parameters, stellar parameters are obtained which disagree with the core-mass-luminosity relation for the two objects analyzed. This disagreement is also evident from a completely different approach: an investigation of the dynamical wind parameters.

Section: Methodsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Radiation-driven winds of hot luminous stars

Kaschinski

2012

A&A

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“…4 will be discussed as well. A more comprehensive description of the numerical methods can be found in Pauldrach et al (2001) and Pauldrach (2003) and references therein.…”

Section: The Radiative Transfer Modelmentioning

confidence: 99%

Non-LTE models for synthetic spectra of type Ia supernovae / hot stars with extremely extended atmospheres

Sauer

2006

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Context. Realistic atmospheric models that link the properties and the physical conditions of supernova ejecta to observable spectra are required for the quantitative interpretation of observational data of type Ia supernovae (SN Ia) and the assessment of the physical merits of theoretical supernova explosion models. The numerical treatment of the radiation transport -yielding the synthetic spectrain models of SN Ia ejecta in early phases is usually carried out in analogy to atmospheric models of "normal" hot stars. Applying this analogy indiscriminately leads to inconsistencies in SN Ia models because a diffusive lower boundary, while justified for hot stars, is invalid for hydrogen and helium-deficient supernova ejecta. In type Ia supernovae the radiation field does not thermalize even at large depths, and large optical depths are not reached at all wavelengths. Aims. We aim to derive an improved description of the lower boundary that allows a more consistent solution of the radiation transfer in SN Ia and therefore yields more realistic synthetic spectra. Methods. We analyze the conditions that lead to a breakdown of the conventional diffusion approximation as the lower boundary in SN Ia. For the radiative transfer, we use a full non-LTE code originally developed for radiatively driven winds of hot stars, with adaptations for the physical conditions in SN Ia. In addition to a well-tested treatment of the underlying microphysical processes, this code allows a direct comparison of the results for SN Ia and hot stars. Results. We develop a semi-analytical description that allows us to overcome some of the limiting assumptions in the conventional treatment of the lower boundary in SN Ia radiative transfer models. We achieve good agreement in a comparison between the synthetic spectrum of our test model and an observed spectrum.

“…In all of these applications a close connection between observations and accurate theoretical modeling of the stellar spectra is required, and this is also the case for the physical properties and observational features of H ii regions, which are closely connected with the spectra and luminosities of their ionizing sources: hot massive stars (cf. Rubin et al 1991;Sellmaier et al 1996;Giveon et al 2002;Pauldrach 2003;Sternberg et al 2003).…”

Section: Introductionmentioning

confidence: 99%

Three-dimensional modeling of ionized gas

Weber

2015

A&A

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Context. H ii regions play a crucial role in the measurement of the chemical composition of the interstellar medium and provide fundamental data about element abundances that constrain models of galactic chemical evolution. Discrepancies that still exist between observed emission line strengths and those predicted by nebular models can be partly attributed to the spectral energy distributions (SEDs) of the sources of ionizing radiation used in the models as well as to simplifying assumptions made in nebular modeling. Aims. One of the main influences on the nebular spectra is the metallicity, both nebular and stellar, which shows large variations even among nearby galaxies. Although nebular modeling 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 lacking. This is unfortunate because the influence of stellar metallicity on nebular line strength ratios, via its effect on the SEDs, is of similar importance as variations in the nebular metallicity. To overcome this deficiency we have computed a grid of model atmosphere SEDs for massive and very massive O-type stars covering a range of metallicities from significantly subsolar (0.1 Z ) to supersolar (2 Z ). Methods. The SEDs have been computed using a state-of-the-art model atmosphere code that takes into account the attenuation of the ionizing flux by the spectral lines of all important elements and the hydrodynamics of the radiatively driven winds and their influence on the SEDs. For the assessment of the SEDs in nebular simulations we have developed a (heretofore not available) 3D radiative transfer code that includes a time-dependent treatment of the metal ionization. Results. Using the SEDs in both 1D and 3D nebular models we explore the relative influence of stellar metallicity, gas metallicity, and inhomogeneity of the gas on the nebular ionization structure and emission line strengths. We find that stellar and gas metallicity are of similar importance for establishing the line strength ratios commonly used in nebular diagnostics, whereas inhomogeneity of the gas has only a subordinate influence on the global line emission. Conclusions. Nebular diagnostics as a quantitative tool for measuring the abundances in the interstellar gas can be used to its full potential only when the influence of SEDs, metallicity, and geometric structure of the nebula are taken into account. For these purposes, detailed stellar SEDs like those of our grid are an essential ingredient for the photoionization models used to predict nebular emission line spectra.