Universitatssternwarte Munich

München, Germany

Universitatssternwarte Munich

München, Germany
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Bohringer H.,Max Planck Institute for Extraterrestrial Physics | Dolag K.,Max Planck Institute for Astrophysics | Dolag K.,Universitatssternwarte Munich | Chon G.,Max Planck Institute for Extraterrestrial Physics
Astronomy and Astrophysics | Year: 2012

Context: The largest uncertainty for cosmological studies using clusters of galaxies is introduced by our limited knowledge of the statistics of galaxy cluster structure and the related scaling relations between observables and cluster mass. A large effort has therefore been made to compile global galaxy cluster properties, in particular those obtained through X-ray observations, and to study their scaling relations. However, the scaling schemes used in the literature differ. Aims: The present paper aims to clarify this situation by providing a thorough review of the scaling laws within the standard model of large-scale structure growth and to discuss various steps in practical approximations. Methods: We derived the scaling laws for X-ray observables and cluster mass within the pure gravitational structure growth scenario. Using N-body simulations we tested the recent formation approximation used in earlier analytic approaches. It involves a redshiftdependent overdensity parameter. We find this approximation less precise than using a fiducial radius based on a fixed overdensity with respect to critical density. Results: Inspired by the comparison of the predicted scaling relations with observations, we propose a first-order modification of the scaling scheme to include the observed effects of hydrodynamics in structure formation. This modification involves a cluster-mass dependent gas-mass fraction. We also discuss the observational results of the reshift evolution of the most important scaling relations and find that a redshift dependence of the gas mass to total mass relation also has to be invoked within our modification scheme. Conclusions: We find that the current observational data are, within their uncertainties, consistent with the proposed modified scaling laws. © ESO 2012.

Rivero Gonzalez J.G.,Universitatssternwarte Munich | Puls J.,Universitatssternwarte Munich | Massey P.,Lowell Observatory | Najarro F.,CSIC - National Institute of Aerospace Technology
Astronomy and Astrophysics | Year: 2012

Context. The classification scheme proposed by Walborn et al. (2002, AJ, 123, 2754), based primarily on the relative strengths of the N IVλ4058 and N IIIλ4640 emission lines, has been used in a variety of studies to spectroscopically classify early O-type stars. Owing to the lack of a solid theoretical basis, this scheme has not yet been unIVersally accepted though. Aims. We provide first theoretical predictions for the N IVλ4058/N IIIλ4640 emission line ratio in dependence of various parameters, and confront these predictions with results from the analysis of a sample of early-type LMC/SMC O-stars. Methods. Stellar and wind parameters of our sample stars are determined by line profile fitting of hydrogen, helium and nitrogen lines, exploiting the helium and nitrogen ionization balance. Corresponding synthetic spectra are calculated by means of the NLTE atmosphere/spectrum synthesis code fastwind. Results. Though there is a monotonic relationship between the N IV/N III emission line ratio and the effectIVe temperature, all other parameters being equal, theoretical predictions indicate additional dependencies on surface gravity, mass-loss, metallicity, and, particularly, nitrogen abundance. For a gIVen line ratio (i.e., spectral type), more enriched objects should be typically hotter. These basic predictions are confirmed by results from the alternatIVe model atmosphere code cmfgen. The effectIVe temperatures for the earliest O-stars, inferred from the nitrogen ionization balance, are partly considerably hotter than indicated by previous studies. Consistent with earlier results, effectIVe temperatures increase from supergiants to dwarfs for all spectral types in the LMC. The relation between observed N IVλ4058/N IIIλ4640 emission line ratio and effectIVe temperature, for a gIVen luminosity class, turned out to be quite monotonic for our sample stars, and to be fairly consistent with our model predictions. The scatter within a spectral sub-type is mainly produced by abundance effects. Conclusions. Our findings suggest that the Walborn et al. (2002, AJ, 123, 2754) classification scheme is able to provide a meaningful relation between spectral type and effectIVe temperature, as long as it is possible to discriminate for the luminosity class. In terms of spectral morphology, this might be difficult to achieve in low-Z environments such as the SMC, owing to rather low wind-strengths. According to our predictions, the major bias of the classification scheme is due to nitrogen content, and the overall spectral type-Teff relation for low-metallicity (e.g., SMC) O-stars might be non-monotonic around O3.5/O4. © 2012 ESO.

Montesano F.,Universitatssternwarte Munich | Montesano F.,Max Planck Institute for Extraterrestrial Physics | Sanchez A.G.,Max Planck Institute for Extraterrestrial Physics | Phleps S.,Max Planck Institute for Extraterrestrial Physics
Monthly Notices of the Royal Astronomical Society | Year: 2012

We obtain cosmological constraints from a measurement of the spherically averaged power spectrum of the distribution of about 90000 luminous red galaxies (LRGs) across 7646deg2 in the Northern Galactic Cap from the seventh data release (DR7) of the Sloan Digital Sky Survey. The errors and mode correlations are estimated thanks to the 160 LasDamas mock catalogues, created in order to simulate the same galaxies and to have the same selection as the data. We apply a model that can accurately describe the full shape of the power spectrum with the use of a small number of free parameters. Using the LRG power spectrum, in combination with the latest measurement of the temperature and polarization anisotropy in the cosmic microwave background (CMB), the luminosity-distance relation from the largest available Type 1a supernovae (SNIa) data set and a precise determination of the local Hubble parameter, we obtain cosmological constraints for five different parameter spaces. When all the four experiments are combined, the flat ΛCDM model is characterized by, Ωb= 0.045 ± 0.001, ns= 0.963 ± 0.011, σ8= 0.802 ± 0.021 and H0= 71.2 ± 1.4kms-1Mpc-1. When we consider curvature as a free parameter, we do not detect deviations from flatness: Ωk= (1.6 ± 5.4) × 10-3, when only CMB and the LRG power spectrum are used; the inclusion of the other two experiments does not improve this result substantially. We also test for possible deviations from the cosmological constant paradigm. Considering the dark energy equation of state parameter wDE as time independent, we measure, if the geometry is assumed to be flat, otherwise. When describing wDE through a simple linear function of the scale factor, our results do not evidence any time evolution. In the next few years new experiments will allow us to measure the clustering of galaxies with a precision much higher than achievable today. Models like the one used here will be a valuable tool in order to achieve the full potentials of the observations and obtain unbiased constraints on the cosmological parameters. © 2012 The Authors Monthly Notices of the Royal Astronomical Society © 2012 RAS.

Dolag K.,Universitatssternwarte Munich | Dolag K.,Max Planck Institute for Astrophysics | Kachelriess M.,Norwegian University of Science and Technology | Ostapchenko S.,Norwegian University of Science and Technology | And 2 more authors.
Astrophysical Journal Letters | Year: 2011

High-energy photons from blazars can initiate electromagnetic pair cascades interacting with the extragalactic photon background. The charged component of such cascades is deflected and delayed by extragalactic magnetic fields (EGMFs), thereby reducing the observed point-like flux and potentially leading to multi-degree images in the GeV energy range. We calculate the fluence of 1ES 0229+200 as seen by Fe r m i-LAT for different EGMF profiles using a Monte Carlo simulation for the cascade development. The non-observation of 1ES 0229+200 by Fermi-LAT suggests that the EGMF fills at least 60% of space with fields stronger than O(10-16 to 10-15) G for lifetimes of TeV activity of O(102 to 104) yr. Thus, the (non-)observation of GeV extensions around TeV blazars probes the EGMF in voids and puts strong constraints on the origin of EGMFs: either EGMFs were generated in a space filling manner (e.g., primordially) or EGMFs produced locally (e.g., by galaxies) have to be efficiently transported to fill a significant volume fraction as, e.g., by galactic outflows. © 2011. The American Astronomical Society.

Sundqvist J.O.,Universitatssternwarte Munich | Puls J.,Universitatssternwarte Munich | Feldmeier A.,Institute For Physik Und Astronomie
Astronomy and Astrophysics | Year: 2010

Context. The mass-loss rate is a key parameter of hot, massive stars. Small-scale inhomogeneities (clumping) in the winds of these stars are conventionally included in spectral analyses by assuming optically thin clumps, a void inter-clump medium, and a smooth velocity field. To reconcile investigations of different diagnostics (in particular, unsaturated UV resonance lines vs. Hα radio emission) within such models, a highly clumped wind with very low mass-loss rates needs to be invoked, where the resonance lines seem to indicate rates an order of magnitude (or even more) lower than previously accepted values. If found to be realistic, this would challenge the radiative line-driven wind theory and have dramatic consequences for the evolution of massive stars. Aims. We investigate basic properties of the formation of resonance lines in small-scale inhomogeneous hot star winds with non-monotonic velocity fields. Methods. We study inhomogeneous wind structures by means of 2D stochastic and pseudo-2D radiation-hydrodynamic wind models, constructed by assembling 1D snapshots in radially independent slices. A Monte-Carlo radiative transfer code, which treats the resonance line formation in an axially symmetric spherical wind (without resorting to the Sobolev approximation), is presented and used to produce synthetic line spectra. Results. The optically thin clumping limit is only valid for very weak lines. The detailed density structure, the inter-clump medium, and the non-monotonic velocity field are all important for the line formation. We confirm previous findings that radiation-hydrodynamic wind models reproduce observed characteristics of strong lines (e.g., the black troughs) without applying the highly supersonic "microturbulence" needed in smooth models. For intermediate strong lines, the velocity spans of the clumps are of central importance. Current radiation-hydrodynamic models predict spans that are too large to reproduce observed profiles unless a very low mass-loss rate is invoked. By simulating lower spans in 2D stochastic models, the profile strengths become drastically reduced, and are consistent with higher mass-loss rates. To simultaneously meet the constraints from strong lines, the inter-clump medium must be non-void. A first comparison to the observed Phosphorus V doublet in the O6 supergiant λ Cep confirms that line profiles calculated from a stochastic 2D model reproduce observations with a mass-loss rate approximately ten × higher than that derived from the same lines but assuming optically thin clumping. Tentatively this may resolve discrepancies between theoretical predictions, evolutionary constraints, and recent derived mass-loss rates, and suggests a re-investigation of the clump structure predicted by current radiation-hydrodynamic models. © ESO, 2010.

Najarro F.,CSIC - National Institute of Aerospace Technology | Hanson M.M.,University of Cincinnati | Puls J.,Universitatssternwarte Munich
Astronomy and Astrophysics | Year: 2011

Context. Mass-loss, occurring through radiation driven supersonic winds, is a key issue throughout the evolution of massive stars. Two outstanding problems are currently challenging the theory of radiation-driven winds: wind clumping and the weak-wind problem. Aims. We seek to obtain accurate mass-loss rates of OB stars at different evolutionary stages to constrain the impact of both problems in our current understanding of massive star winds. Methods. We perform a multi-wavelength quantitative analysis of a sample of ten Galactic OB-stars by means of the atmospheric code cmfgen, with special emphasis on the L-band window. A detailed investigation is carried out on the potential of Br α and Pfγ as mass-loss and clumping diagnostics. Results. For objects with dense winds, Brα samples the intermediate wind while Pfγ maps the inner one. In combination with other indicators (UV, Hα, Br γ) these lines enable us to constrain the wind clumping structure and to obtain "true" mass-loss rates. For objects with weak winds, Brα emerges as a reliable diagnostic tool to constrain Ṁ. The emission component at the line Doppler-core superimposed on the rather shallow Stark absorption wings reacts very sensitively to mass loss already at very low Ṁ values. On the other hand, the line wings display similar sensitivity to mass loss as Hα, the classical optical mass loss diagnostics. Conclusions. Our investigation reveals the great diagnostic potential of L-band spectroscopy to derive clumping properties and mass-loss rates of hot star winds. We are confident that Brα will become the primary diagnostic tool to measure very low mass-loss rates with unprecedented accuracy. © 2011 ESO.

Rivero Gonzalez J.G.,Universitatssternwarte Munich | Puls J.,Universitatssternwarte Munich | Najarro F.,CSIC - National Institute of Aerospace Technology
Astronomy and Astrophysics | Year: 2011

Context. Evolutionary models of massive stars predict a surface enrichment of nitrogen, due to rotational mixing. Recent studies within the VLT-FLAMES survey of massive stars have challenged (part of) these predictions. Such systematic determinations of nitrogen abundances, however, have been mostly performed only for cooler (B-type) objects. For the most massive and hottest stars, corresponding results are scarce. Aims. This is the first paper in a series dealing with optical nitrogen spectroscopy of O-type stars, aiming at the analysis of nitrogen abundances for stellar samples of significant size, to place further constraints on the early evolution of massive stars. Here we concentrate on the formation of the optical N III lines at γγ 4634?4640?4642 that are fundamental for the definition of the different morphological "f"-classes. Methods.We implement a new nitrogen model atom into the NLTE atmosphere/spectrum synthesis code fastwind, and compare the resulting optical N III spectra with other predictions, mostly from the seminal work by Mihalas & Hummer (1973, ApJ, 179, 827, "MH"), and from the alternative code cmfgen. Results. Using similar model atmospheres as MH (not blanketed and wind-free), we are able to reproduce their results, in particular the optical triplet emission lines. According to MH, these should be strongly related to dielectronic recombination and the drain by certain two-electron transitions. However, using realistic, fully line-blanketed atmospheres at solar abundances, the key role of the dielectronic recombinations controlling these emission features is superseded-for O-star conditions-by the strength of the stellar wind and metallicity. Thus, in the case of wind-free (weak wind) models, the resulting lower ionizing EUV-fluxes severely suppress the emission. As the mass loss rate is increased, pumping through the N III resonance line(s) in the presence of a near-photospheric velocity field (i.e., the Swings-mechanism) results in a net optical triplet line emission. A comparison with results from cmfgen is mostly satisfactory, except for the range 30 000 K≤ Teff ≤ 35 000 K, where CMFGEN triggers the triplet emission at lower T eff than fastwind. This effect could be traced down to line overlap effects between the N III and O III resonance lines that cannot be simulated by fastwind so far, due to the lack of a detailed O III model atom. Conclusions. Since the efficiency of dielectronic recombination and "two electron drain" strongly depends on the degree of lineblanketing/-blocking, we predict the emission to become stronger in a metal-poor environment, though lower wind-strengths and nitrogen abundances might counteract this effect. Weak winded stars (if existent in the decisive parameter range) should display less triplet emission than their counterparts with "normal" winds. © 2011 ESO.

Sundqvist J.O.,Universitatssternwarte Munich | Puls J.,Universitatssternwarte Munich | Feldmeier A.,Institute For Physik Und Astronomie | Owocki S.P.,University of Delaware
Astronomy and Astrophysics | Year: 2011

Context. Mass loss is essential for massive star evolution, thus also for the variety of astrophysical applications relying on its predictions. However, mass-loss rates currently in use for hot, massive stars have recently been seriously questioned, mainly because of the effects of wind clumping. Aims. We investigate the impact of clumping on diagnostic ultraviolet resonance and optical recombination lines often used to derive empirical mass-loss rates of hot stars. Optically thick clumps, a non-void interclump medium, and a non-monotonic velocity field are all accounted for in a single model. The line formation is first theoretically studied, after which an exemplary multi-diagnostic study of an O-supergiant is performed. Methods. We used 2D and 3D stochastic and radiation-hydrodynamic wind models, constructed by assembling 1D snapshots in radially independent slices. To compute synthetic spectra, we developed and used detailed radiative transfer codes for both recombination lines (solving the "formal integral") and resonance lines (using a Monte-Carlo approach). In addition, we propose an analytic method to model these lines in clumpy winds, which does not rely on optically thin clumping. Results. The importance of the "vorosity" effect for line formation in clumpy winds is emphasized. Resonance lines are generally more affected by optically thick clumping than recombination lines. Synthetic spectra calculated directly from current radiation-hydrodynamic wind models of the line-driven instability are unable to in parallel reproduce strategic optical and ultraviolet lines for the Galactic O-supergiant λ Cep. Using our stochastic wind models, we obtain consistent fits essentially by increasing the clumping in the inner wind. A mass-loss rate is derived that is approximately two times lower than what is predicted by the line-driven wind theory, but much higher than the corresponding rate derived when assuming optically thin clumps. Our analytic formulation for line formation is used to demonstrate the potential importance of optically thick clumping in diagnostic lines in so-called weak-winded stars and to confirm recent results that resonance doublets may be used as tracers of wind structure and optically thick clumping. Conclusions. We confirm earlier results that a re-investigation of the structures in the inner wind predicted by line-driven instability simulations is needed. Our derived mass-loss rate for λ Cep suggests that only moderate reductions of current mass-loss predictions for OB-stars are necessary, but this nevertheless prompts investigations on feedback effects from optically thick clumping on the steady-state, NLTE wind models used for quantitative spectroscopy. © 2011 ESO.

Massey P.,Lowell Observatory | Neugent K.F.,Lowell Observatory | Hillier D.J.,University of Pittsburgh | Puls J.,Universitatssternwarte Munich
Astrophysical Journal | Year: 2013

The model atmosphere programs FASTWIND and CMFGEN are both elegantly designed to perform non-LTE analyses of the spectra of hot massive stars, and include sphericity and mass-loss. The two codes differ primarily in their approach toward line blanketing, with CMFGEN treating all of the lines in the co-moving frame and FASTWIND taking an approximate approach which speeds up execution times considerably. Although both have been extensively used to model the spectra of O-type stars, no studies have used the codes to independently model the same spectra of the same stars and compare the derived physical properties. We perform this task on 10 O-type stars in the Magellanic Clouds. For the late-type O supergiants, both CMFGEN and FASTWIND have trouble fitting some of the He I lines, and we discuss causes and cures. We find that there is no difference in the average effective temperatures found by the two codes for the stars in our sample, although the dispersion is large, due primarily to the various difficulties each code has with He I. The surface gravities determined using FASTWIND are systematically lower by 0.12 dex compared to CMFGEN, a result we attribute to the better treatment of electron scattering by CMFGEN. This has implications for the interpretation of the origin of the so-called mass discrepancy, as the masses derived by FASTWIND are on average lower than inferred from stellar evolutionary models, while those found by CMFGEN are in better agreement. © 2013. The American Astronomical Society. All rights reserved.

Donnert J.,National institute for astrophysics | Donnert J.,Max Planck Institute for Astrophysics | Dolag K.,Max Planck Institute for Astrophysics | Dolag K.,Universitatssternwarte Munich | And 2 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2013

We present the first high-resolution magnetohydrodynamic simulation of cosmic-ray electron re-acceleration by turbulence in clustermergers.We use an idealized model for cluster mergers, combined with a numerical model for the injection, cooling and re-acceleration of cosmic-ray electrons, to investigate the evolution of cluster-scale radio emission in these objects. In line with theoretical expectations, we for the first time show in a simulation that re-acceleration of cosmic-ray electrons has the potential to reproduce key observables of radio haloes. In particular, we show that clusters evolve being radio loud or radio quiet, depending on their evolutionary stage during the merger. We thus recover the observed transient nature of radio haloes. In the simulation, the diffuse emission traces the complex interplay between the spatial distribution of turbulence injected by the halo infall and the spatial distribution of the seed electrons to re-accelerate. During the formation and evolution of the halo, the synchrotron emission spectra show the observed variety: from power laws with spectral index of 1-1.3 to curved and ultra-steep spectra with index >1.5. © 2013 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society.

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