arXiv subjects

T. L. Hoffmann

Publications and source records attributed to T. L. Hoffmann.

18 recordsLinked to original sources

arXiv2018

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

Aims: We systematically explore a plausible subset of the parameter space involving effective temperatures and metallicities of the ionizing stellar sources, the effects of the hardening of their radiation by surrounding leaky HII regions with different escape fractions, as well as different scenarios for the clumpiness of the DIG, and compute the resulting line strength ratios for a number of diagnostic optical emission lines. Methods: For the ionizing fluxes we compute a grid of stellar spectral energy distributions (SEDs) from detailed, fully non-LTE model atmospheres that include the effects of stellar winds and line blocking and blanketing. To calculate the ionization and temperature structure in the HII regions and the diffuse ionized gas we use spherically symmetric photoionization models as well as state-of-the-art three-dimensional (3D) non-LTE radiative transfer simulations, considering hydrogen, helium, and the most abundant metals. Results: We provide quantitative predictions of how the line ratios from HII regions and the DIG vary as a function of metallicity, stellar effective temperature, and escape fraction from the HII region. The range of predicted line ratios reinforces the hypothesis that the DIG is ionized by (filtered) radiation from hot stars; however, comparison of observed and predicted line ratios indicates that the DIG is typically ionized with a softer SED than predicted by the chosen stellar population synthesis model. Even small changes in simulation parameters like the clumping factor can lead to considerable variation in the ionized volume. Both for a more homogeneous gas and a very inhomogeneous gas containing both dense clumps and channels with low gas density, the ionized region in the dilute gas above the galactic plane can cease to be radiation-bounded, allowing the ionizing radiation to leak into the intergalactic medium.

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arXiv2017

Carbon line formation and spectroscopy in O-type stars

The determination of chemical abundances constitutes a fundamental requirement for obtaining a complete picture of a star. Particularly in massive stars, CNO abundances are of prime interest, due to the nuclear CNO-cycle and various mixing processes which bring these elements to the surface. We aim at enabling a reliable carbon spectroscopy for our unified NLTE atmosphere code FASTWIND. We develop a new carbon model atom including CII/III/IV/V, and discuss problems related to carbon spectroscopy in O-type stars. We describe different tests to examine the reliability of our implementation, and investigate which mechanisms influence the carbon ionization balance. By comparing with high-resolution spectra from six O-type stars, we check in how far observational constraints can be reproduced by our new carbon line synthesis. Carbon lines are even more sensitive to a variation of temperature, gravity, and mass-loss rate, than hydrogen/helium lines. We are able to reproduce most of the observed lines from our stellar sample, and to estimate those specific carbon abundances which bring the lines from different ions into agreement. For hot dwarfs and supergiants earlier than O7, X-rays from wind-embedded shocks can impact the synthesized line strengths, particularly for CIV, potentially affecting the abundance determination. We have demonstrated our capability to derive realistic carbon abundances by means of FASTWIND, using our recently developed model atom. We found that complex effects can have a strong influence on the carbon ionization balance in hot stars. For a further understanding, the UV range needs to be explored as well. By means of detailed nitrogen and oxygen model atoms available to use, we will be able to perform a complete CNO abundance analysis for larger samples of massive stars, and to provide constraints on corresponding evolutionary models and aspects.

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arXiv2016

Atmospheric NLTE-Models for the Spectroscopic Analysis of Blue Stars with Winds. III. X-ray emission from wind-embedded shocks

X-rays/EUV radiation emitted from wind-embedded shocks in hot, massive stars can affect the ionization balance in their outer atmospheres, and can be the mechanism responsible for the production of highly ionized species. To allow for these processes in the context of spectral analysis, we have implemented such emission into our unified, NLTE model atmosphere/spectrum synthesis code FASTWIND. The shock structure and corresponding emission is calculated as a function of user-supplied parameters. We account for a temperature and density stratification inside the post-shock cooling zones, calculated for radiative and adiabatic cooling in the inner and outer wind, respectively. The high-energy absorption of the cool wind is considered by adding important K-shell opacities, and corresponding Auger ionization rates have been included into the NLTE network. We tested and verified our implementation carefully against corresponding results from various alternative model atmosphere codes, and studied the effects from shock emission for important ions from He, C, N, O, Si, and P. Surprisingly, dielectronic recombination turned out to play an essential role for the ionization balance of OIV/OV around Teff = 45,000 K. Finally, we investigated the behavior of the mass absorption coefficient, kappa_nu(r), important in the context of X-ray line formation in massive star winds. In almost all considered cases, direct ionization is of major influence, and Auger ionization significantly affects only NVI and OVI. The approximation of a radially constant kappa_nu is justified for r > 1.2 Rstar and lambda < 18 A, and also for many models at longer wavelengths. To estimate the actual value of this quantity, however, the HeII opacities need to be calculated from detailed NLTE modeling, at least for wavelengths longer than 18 to 20 A, and information on the individual CNO abundances has to be present.

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arXiv2015

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 HII regions

HII 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 simplifying assumptions made in nebular modeling. 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. 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 Zsun) to supersolar (2 Zsun). 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. 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.

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arXiv2014

Non-LTE models for synthetic spectra of type Ia supernovae. III. An accelerated lambda iteration procedure for the mutual interaction of strong spectral lines in SN Ia models with and without energy deposition

Context. Spectroscopic analyses to interpret the spectra of the brightest supernovae from the UV to the near-IR provide a powerful tool with great astrophysical potential for the determination of the physical state of the ejecta, their chemical composition, and the SNe distances even at significant redshifts. Methods. We report on improvements of computing synthetic spectra for SNIa with respect to i) an improved and sophisticated treatment of thousands of strong lines that interact intricately with the "pseudo-continuum" formed entirely by Doppler- shifted spectral lines, ii) an improved and expanded atomic database, and iii) the inclusion of energy deposition within the ejecta. Results. We show that an accelerated lambda iteration procedure we have developed for the mutual interaction of strong spectral lines appearing in the atmospheres of SNeIa solves the longstanding problem of transferring the radiative energy from the UV into the optical regime. In detail we discuss applications of the diagnostic technique by example of a standard SNIa, where the comparison of calculated and observed spectra revealed that in the early phases the consideration of the energy deposition within the spectrum-forming regions of the ejecta does not qualitatively alter the shape of the spectra. Conclusions. The results of our investigation lead to an improved understanding of how the shape of the spectrum changes radically as function of depth in the ejecta, and show how different emergent spectra are formed as a result of the particular physical properties of SNe Ia ejecta and the resulting peculiarities in the radiative transfer. This provides an important insight into the process of extracting information from observed SNIa spectra, since these spectra are a complex product of numerous unobservable SNIa spectral features which are thus analyzed in parallel to the observable spectral features.

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arXiv2013

Non-LTE models for synthetic spectra of type Ia supernovae. IV. A modified Feautrier scheme for opacity-sampled pseudo-continua at high expansion velocities and application to synthetic SN Ia spectra

Context. Type Ia supernovae (SN Ia) have become an invaluable cosmological tool as their exceptional brightness makes them observable even at very large distances (up to redshifts around z~1). To investigate possible systematic differences between local and distant SN Ia requires detailed models whose synthetic spectra can be compared to observations, and in which the solution of the radiative transfer is a key ingredient. One commonly employed method is the Feautrier scheme, which is generally very robust, but which can yield wrong results under certain conditions that frequently occur in the modelling of supernova ejecta or even in the radiatively driven expanding atmospheres of hot stars. Methods. We use a sophisticated model atmosphere code which takes into account the non-LTE effects and high velocity gradients that strongly affect the physics of SN Ia atmospheres at all wavelengths to simulate the formation of SN Ia spectra by the thousands of strong spectral lines which intricately interact with the "pseudo-continuum" formed entirely by these Doppler-shifted lines themselves. We focus to an investigation of the behavior of the Feautrier scheme under these conditions. Results. Synthetic spectra of SN Ia, a complex product of computer models replicating numerous physical processes that determine the conditions of matter and radiation in the ejecta, are affected by large spatial jumps of the line-dominated opacities and source functions for which the application of even well-established methods may harbor certain pitfalls. We analyze the conditions that can lead to a breakdown of conventional procedures and we derive for the Feautrier radiative transfer solver a modified description which yields more accurate results in the given circumstances.

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arXiv2013

Radiation-driven winds of hot luminous stars. XVIII. The reliability of stellar and wind parameter determinations from spectral analysis of selected central stars of planetary nebulae and the possibility of single-star SN Ia progenitors

Context. The uncertainty in the degree to which radiation-driven winds of hot stars might be affected by small inhomogeneities in the density leads to a corresponding uncertainty in the determination of the atmospheric mass loss rates from the strength of optical recombination lines. Furthermore, the optical recombination lines also react sensitively to even small changes in the density structure resulting from the (often assumed instead of computed) velocity law of the outflow. This raises the question of how reliable the parameter determinations from such lines are. Results. The discrepancy between the optical and the UV analyses is confirmed to be the result of a missing consistency between stellar and wind parameters in the optical analysis. The influence of the density (velocity field) is of the same order as that of moderate clumping factors. Moderate clumping factors leave the UV spectra mostly unaffected, indicating that the influence on the ionization balance, and thus on the radiative acceleration, is small. Instead of the erratic behavior of the clumping factors claimed from the optical analyses, our analysis based on the velocity field computed from radiative driving yields similar clumping factors for all CSPNs. The shock temperatures and the ratios of X-ray to bolometric luminosity required to reproduce the highly ionized Ovi line agree with those known from massive O stars, again confirming the similarity of O-type CSPN and massive O star atmospheres. Conclusions. The similarity of the winds of O-type CSPNs and those of massive O stars justifies using the same methods based on the dynamics of radiation-driven winds in their analysis, thus supporting the earlier result that several of the CSPNs in the sample have near-Chandrasekhar-limit masses and may thus be possible single-star progenitors of type Ia supernovae.

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arXiv2013

Three-Dimensional Modelling of Ionized Gas. I. Did very massive stars of different metallicities drive the second cosmic reionization?

The first generation of stars that formed directly from the primordial gas played a crucial role in the early phase of the reionization of the universe. Because of the short lifetimes of these stars the metals produced in their cores were quickly returned to the environment, from which early PopII stars with a different initial mass function and different SEDs were formed, already much earlier than the time at which the universe became completely reionized at a redshift of z~6. Using a state-of-the-art model atmosphere code we calculate realistic SEDs of very massive stars (VMSs) of different metallicities to serve as input for the 3-dimensional multi-frequency radiative transfer code we have developed to simulate the temporal evolution of the ionization of the inhomogeneous interstellar and intergalactic medium, using multiple stellar clusters as sources of ionizing radiation. Our tool handles distributions of numerous radiative sources characterized by high resolution synthetic SEDs, and yields occupation numbers of the required energy levels of the most important elements which are treated in NLTE consistently with the 3d radiative transfer. We further demonstrate that the increasing metallicity of the radiative sources in the transition from PopIII stars to PopII stars has a strong impact on the hardness of the emitted spectrum, and hence on the reionization history of helium. A top-heavy stellar mass distribution characterized by VMSs forming in chemically evolved clusters of high core mass density may not only provide the progenitors of intermediate-mass and supermassive black holes (SMBHs), but also play an important role for the reionization of HeII. The number of VMSs required to reionize HeII by a redshift of z~2.5 is astonishingly close to the number of VMSs required to explain galactic SMBHs if one assumes that these have been formed by mergers of smaller black holes.

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arXiv2012

Numerical Models for the Diffuse Ionized Gas in Galaxies. I. Synthetic spectra of thermally excited gas with turbulent magnetic reconnection as energy source

Aims: The aim of this work is to verify whether turbulent magnetic reconnection can provide the additional energy input required to explain the up to now only poorly understood ionization mechanism of the diffuse ionized gas (DIG) in galaxies and its observed emission line spectra. Methods: We use a detailed non-LTE radiative transfer code that does not make use of the usual restrictive gaseous nebula approximations to compute synthetic spectra for gas at low densities. Excitation of the gas is via an additional heating term in the energy balance as well as by photoionization. Numerical values for this heating term are derived from three-dimensional resistive magnetohydrodynamic two-fluid plasma--neutral-gas simulations to compute energy dissipation rates for the DIG under typical conditions. Results: Our simulations show that magnetic reconnection can liberate enough energy to by itself fully or partially ionize the gas. However, synthetic spectra from purely thermally excited gas are incompatible with the observed spectra; a photoionization source must additionally be present to establish the correct (observed) ionization balance in the gas.

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arXiv2012

Radiation-driven winds of hot luminous stars XVII. Parameters of selected central stars of PN from consistent optical and UV spectral analysis and the universality of the mass-luminosity relation

Context: The commonly accepted mass-luminosity relation of central stars of planetary nebulae (CSPNs) might not be universally valid. While earlier optical analyses could not derive masses and luminosities independently (instead taking them from theoretical evolutionary models) hydrodynamically consistent modelling of the stellar winds allows using fits to the UV spectra to consistently determine also stellar radii, masses, and luminosities without assuming a mass-luminosity relation. Recent application to a sample of CSPNs raised questions regarding the validity of the theoretical mass-luminosity relation of CSPNs. Aims: The results of the earlier UV analysis are reassessed by means of a simultaneous comparison of observed optical and UV spectra with corresponding synthetic spectra. Methods: Using published stellar parameters (a) from a consistent UV analysis and (b) from fits to optical H and He lines, we calculate simultaneous optical and UV spectra with our model atmosphere code, which has been improved by implementing Stark broadening for H and He lines. Results: Spectra computed with the parameter sets from the UV analysis yield good agreement to the observations, but spectra computed with the stellar parameters from the published optical analysis and using corresponding consistent wind parameters show large discrepancies to both the observed optical and UV spectra. The published optical analyses give good fits to the observed spectrum only because the wind parameters assumed in these analyses are inconsistent with 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 objects analyzed. This disagreement is also evident from a completely different approach: an investigation of the dynamical wind parameters.

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arXiv2011

Radiation-driven winds of hot luminous stars. XVI. Expanding atmospheres of massive and very massive stars and the evolution of dense stellar clusters

Context: Starbursts, and particularly their high-mass stars, play an essential role in the evolution of galaxies. The winds of massive stars not only significantly influence their surroundings, but the mass loss also profoundly affects the evolution of the stars themselves. In addition to the evolution of each star, the evolution of the dense cores of massive starburst clusters is affected by N-body interactions, and the formation of very massive stars via mergers may be decisive for the evolution of the cluster. Aims: To introduce an advanced diagnostic method of O-type stellar atmospheres with winds, including an assessment of the accuracy of the determinations of abundances, stellar and wind parameters. Methods: We combine consistent models of expanding atmospheres with detailed stellar evolutionary calculations of massive and very massive single stars with regard to the evolution of dense stellar clusters. Accurate predictions of the mass loss rates of very massive stars requires a highly consistent treatment of the statistical equilibrium and the hydrodynamic and radiative processes in the expanding atmospheres. Results: We present computed mass loss rates, terminal wind velocities, and spectral energy distributions of massive and very massive stars of different metallicities, calculated from atmospheric models with an improved level of consistency. Conclusions: Stellar evolutionary calculations using our computed mass loss rates show that low-metallicity very massive stars lose only a very small amount of their mass, making it unlikely that very massive population III stars cause a significant helium enrichment of the interstellar medium. Solar-metallicity stars have higher mass-loss rates, but these are not so high to exclude very massive stars formed by mergers in dense clusters from ending their life massive enough to form intermediate-mass black holes.

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arXiv2007

Central stars of planetary nebulae in the Galactic bulge

Optical high-resolution spectra of five central stars of planetary nebulae (CSPN) in the Galactic Bulge have been obtained with Keck/HIRES in order to derive their parameters. Since the distance of the objects is quite well known, such a method has the advantage that stellar luminosities and masses can in principle be determined without relying on theoretical relations between both quantities. By alternatively combining the results of our spectroscopic investigation with evolutionary tracks, we obtain so-called spectroscopic distances, which can be compared with the known (average) distance of the Bulge-CSPN. This offers the possibility to test the validity of model atmospheres and present date post-AGB evolution. We analyze optical H/He profiles of five Galactic Bulge CSPN (plus one comparison object) by means of profile fitting based on state of the art non-LTE modeling tools, to constrain their basic atmospheric parameters (Teff, log g, helium abundance and wind strength). Masses and other stellar radius dependent quantities are obtained from both the known distances and from evolutionary tracks, and the results from both approaches are compared. The major result of the present investigation is that the derived spectroscopic distances depend crucially on the applied reddening law. An "average extinction law" leads to a distance of 10.7 +- 1.2 kpc, which is considerably larger than the Galactic Center distance of 8 kpc. However, we find a remarkable internal agreement of the individual spectroscopic distances of our sample objects. Due to the uncertain reddening correction, the analysis presented here cannot yet be regarded as a consistency check for our method, and a rigorous test of the CSPN evolution theory becomes only possible if this problem has been solved.

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arXiv2006

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

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 spectra - in 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. We 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. 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. 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.

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arXiv2003

Radiation-driven winds of hot luminous stars. XV. Constraints on the mass-luminosity relation of central stars of planetary nebulae

We present a new model atmosphere analysis of nine central stars of planetary nebulae. This study is based on a new generation of realistic stellar model atmospheres for hot stars; state-of-the-art, hydrodynamically consistent, spherically symmetric model atmospheres that have been shown to correctly reproduce the observed UV spectra of massive Population I O-type stars. The information provided by the wind features (terminal velocity, mass loss rate) permits to derive the physical size of each central star, from which we can derive the stellar luminosity, mass and distance, without having to assume a relation between stellar mass and luminosity taken from the theory of stellar structure and AGB and post-AGB evolution. The results of our analysis are quite surprising: we find severe departures from the generally accepted relation between post-AGB central star mass and luminosity.

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arXiv2002

Radiation driven atmospheres of O-type stars: constraints on the mass-luminosity relation of central stars of planetary nebulae

Recent advances in the modelling of stellar winds driven by radiation pressure make it possible to fit many wind-sensitive features in the UV spectra of hot stars, opening the way for a hydrodynamically consistent determination of stellar radii, masses, and luminosities from the UV spectrum alone. It is thus no longer necessary to assume a theoretical mass-luminosity relation. As the method has been shown to work for massive O-stars, we are now able to test predictions from the post-AGB evolutionary calculations quantitatively for the first time. Here we present the first rather surprising consequences of using the new generation of model atmospheres for the analysis of a sample of central stars of planetary nebulae.

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arXiv2001

Gravitationally lensed high redshift galaxies in the field of 1E0657-56

We present images and long-slit spectra obtained with FORS1 at UT1 of the VLT centered on the gravitational arc of the galaxy cluster 1E0657-56 (z = 0.296). The cluster is one of the hottest, most massive clusters known so far and acts as a powerful gravitational telescope, amplifying the flux of background sources by up to a factor of 17. We present photometric results together with the spectra of the gravitational arc (z = 3.24) and four additional amplified high redshift objects (z = 2.34 to 3.08) that were also included in the slit by chance coincidence. A magnification map has been obtained from a lens model derived from the multiple image systems. We compare our observed spectra with models and briefly discuss the stellar contents of these galaxies. Furthermore we measured the equivalent widths of the CIV 1550 and SiIV 1400 absorption lines for the objects behind 1E0657-56 studied here, as well as for some additional starburst galaxies (nearby and at high z). For CIV we find an increasing absorption equivalent width with decreasing redshift. We discuss whether this correlation could be related to the increase of metallicity with the age of the universe.

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arXiv2000

Radiation-driven winds of hot stars. XIII. A description of NLTE line blocking and blanketing towards realistic models of expanding atmospheres

We present significant improvements of our approach in constructing detailed atmospheric models and synthetic spectra for hot luminous stars: 1. A sophisticated and consistent description of line blocking and blanketing. Our solution concept renders the line blocking influence on the ionizing fluxes (mainly the EUV and UV are affected) in identical quality as the synthetic high resolution spectra representing the observable region. Line blanketing is properly accounted for in the energy balance. 2. A considerably improved and enhanced atomic data archive providing the basis for a detailed multilevel NLTE treatment of the metal ions (from C to Zn) and an adequate representation of line blocking and the radiative line acceleration. 3. A revised inclusion of EUV and X-ray radiation produced by cooling zones originating from shock heated matter. This new tool not only provides an easy to use method for O-star diagnostics, whereby physical constraints on the properties of stellar winds, stellar parameters, and abundances can be obtained via a comparison of observed and synthetic spectra, but also allows the astrophysically important information about the ionizing fluxes of hot stars to be determined automatically. Results illustrating this are discussed by means of a basic model grid calculated for O-stars with solar metallicity. To further demonstrate the astrophysical potential of our new method we provide a first detailed spectral diagnostic determination of the stellar parameters, the wind parameters, and the abundances by an exemplary application to the O9.5Ia supergiant alpha Cam.

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arXiv2000

Spectra of High Redshift Galaxies using a Cluster as a Gravitational Telescope

Using the Focal Reducer Spectrograph (FORS) at the Very Large Telescope (VLT) during the FORS commissioning time in December 1998 we took long slit spectra of the gravitational arc visible on images of the galaxy cluster 1E 0657 (z = 0.296). This cluster is one of the hottest (massive) cluster known so far and hence perfectly acts as a gravitational telescope magnifying the flux of background sources up to a factor of 17. Here we present the spectra of the gravitational arc (z = 3.23) and 4 additional high redshift objects (z=2.35 to 3.09), that also fall on the slit by chance coincidence. We briefly discuss the stellar contents of these galaxies and show first models of the observed spectra. Furthermore we point out the effectivity of using FORS in combination with available gravitational telescopes.

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