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Meneghetti M.,National institute for astrophysics | Meneghetti M.,National Institute of Nuclear Physics, Italy | Bartelmann M.,ZAH | Dahle H.,University of Oslo | Limousin M.,Aix - Marseille University
Space Science Reviews | Year: 2013

The existence of an arc statistics problem was at the center of a strong debate in the last fifteen years. With the aim to clarify if the optical depth for giant gravitational arcs by galaxy clusters in the so called concordance model is compatible with observations, several studies were carried out which helped to significantly improve our knowledge of strong lensing clusters, unveiling their extremely complex internal structure. In particular, the abundance and the frequency of strong lensing events like gravitational arcs turned out to be a potentially very powerful tool to trace the structure formation. However, given the limited size of observational and theoretical data-sets, the power of arc statistics as a cosmological tool has been only minimally exploited so far. On the other hand, the last years were characterized by significant advancements in the field, and several cluster surveys that are ongoing or planned for the near future seem to have the potential to make arc statistics a competitive cosmological probe. Additionally, recent observations of anomalously large Einstein radii and concentrations in galaxy clusters have reinvigorated the debate on the arc statistics problem. In this paper, we review the work done so far on arc statistics, focussing on what is the lesson we learned and what is likely to improve in the next years. © 2013 Springer Science+Business Media Dordrecht.

Freytag B.,Ecole Normale Superieure de Lyon | Freytag B.,Instituto Nazionale Of Astrofisica | Steffen M.,Leibniz Institute for Astrophysics Potsdam | Ludwig H.-G.,ZAH | And 4 more authors.
Journal of Computational Physics | Year: 2012

High-resolution images of the solar surface show a granulation pattern of hot rising and cooler downward-sinking material - the top of the deep-reaching solar convection zone. Convection plays a role for the thermal structure of the solar interior and the dynamo acting there, for the stratification of the photosphere, where most of the visible light is emitted, as well as for the energy budget of the spectacular processes in the chromosphere and corona. Convective stellar atmospheres can be modeled by numerically solving the coupled equations of (magneto)hydrodynamics and non-local radiation transport in the presence of a gravity field. The CO5BOLD code described in this article is designed for so-called " realistic" simulations that take into account the detailed microphysics under the conditions in solar or stellar surface layers (equation-of-state and optical properties of the matter). These simulations indeed deserve the label " realistic" because they reproduce the various observables very well - with only minor differences between different implementations. The agreement with observations has improved over time and the simulations are now well-established and have been performed for a number of stars. Still, severe challenges are encountered when it comes to extending these simulations to include ideally the entire star or substellar object: the strong stratification leads to completely different conditions in the interior, the photosphere, and the corona. Simulations have to cover spatial scales from the sub-granular level to the stellar diameter and time scales from photospheric wave travel times to stellar rotation or dynamo cycle periods. Various non-equilibrium processes have to be taken into account. Last but not least, realistic simulations are based on detailed microphysics and depend on the quality of the input data, which can be the actual accuracy limiter. This article provides an overview of the physical problem and the numerical solution and the capabilities of CO5BOLD, illustrated with a number of applications. © 2011 Elsevier Inc.

Wedemeyer S.,University of Oslo | Ludwig H.-G.,ZAH | Steiner O.,Kiepenheuer Institute for Solar Physics
Astronomische Nachrichten | Year: 2013

We present first results from three-dimensional radiation magnetohydrodynamic simulations of M-type dwarf stars with CO5BOLD. The local models include the top of the convection zone, the photosphere, and the chromosphere. The results are illustrated for models with an effective temperature of 3240 K and a gravitational acceleration of log g = 4.5, which represent analogues of AD Leo. The models have different initial magnetic field strengths and field topologies. This first generation of models demonstrates that the atmospheres of M dwarfs are highly dynamic and intermittent. Magnetic fields and propagating shock waves produce a complicated fine-structure, which is clearly visible in synthetic intensity maps in the core of the Ca II K spectral line and also at millimeter wavelengths. The dynamic small-scale pattern cannot be described by means of one-dimensional models, which has important implications for the construction of semi-empirical model atmospheres and thus for the interpretation of observations in general. Detailed three-dimensional numerical simulations are valuable in this respect. Furthermore, such models facilitate the analysis of small-scale processes, which cannot be observed on stars but nevertheless might be essential for understanding M-dwarf atmospheres and their activity. An example are so-called "magnetic tornadoes", which have recently been found on the Sun and are presented here in M-dwarf models for the first time. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Beeck B.,Max Planck Institute for Solar System Research | Collet R.,Max Planck Institute for Astrophysics | Steffen M.,Leibniz Institute for Astrophysics Potsdam | Asplund M.,Max Planck Institute for Astrophysics | And 6 more authors.
Astronomy and Astrophysics | Year: 2012

Context: Radiative hydrodynamic simulations of solar and stellar surface convection have become an important tool for exploring the structure and gas dynamics in the envelopes and atmospheres of late-type stars and for improving our understanding of the formation of stellar spectra. Aims: We quantitatively compare results from three-dimensional, radiative hydrodynamic simulations of convection near the solar surface generated with three numerical codes (CO 5 BOLD, MURaM, and Stagger) and different simulation setups in order to investigate the level of similarity and to cross-validate the simulations. Methods.For all three simulations, we considered the average stratifications of various quantities (temperature, pressure, flow velocity, etc.) on surfaces of constant geometrical or optical depth, as well as their temporal and spatial fluctuations. We also compared observables, such as the spatially resolved patterns of the emerging intensity and of the vertical velocity at the solar optical surface as well as the center-to-limb variation of the continuum intensity at various wavelengths. Results: The depth profiles of the thermodynamical quantities and of the convective velocities as well as their spatial fluctuations agree quite well. Slight deviations can be understood in terms of differences in box size, spatial resolution and in the treatment of non-gray radiative transfer between the simulations. Conclusions: The results give confidence in the reliability of the results from comprehensive radiative hydrodynamic simulations. © ESO 2012.

Heidt J.,ZAH | Nilsson K.,University of Turku
Astronomy and Astrophysics | Year: 2011

We present and discuss polarimetric observations of 182 targets drawn from an optically selected sample of 240 probable BL Lac candidates out of the SDSS compiled by Collinge et al. (2005, AJ, 129, 2542). In contrast to most other BL Lac candidate samples extracted from the SDSS, its radio- and/or X-ray properties have not been taken into account for its derivation. Thus, because its selection is based on optical properties alone, it may be less prone to selection effects inherent in other samples derived at different frequencies, so it offers a unique opportunity to extract the first unbiased BL Lac luminosity function that is suitably large in size. We found 124 out of 182 targets (68%) to be polarized, 95 of the polarized targets (77%) to be highly polarized (>4%). The low-frequency peaked BL Lac candidates in the sample are on average only slightly more polarized than the high-frequency peaked ones. Compared to earlier studies, we found a high duty cycle in high polarization (∼66+2-14} to be >4% polarized) in high-frequency peaked BL Lac candidates. This may come from our polarization analysis, which minimizes the contamination by host galaxy light. No evidence of radio-quiet BL Lac objects in the sample was found. Our observations show that the probable sample of BL Lac candidates of Collinge et al. (2005) indeed contains a large number of bona fide BL Lac objects. High S/N spectroscopy and deep X-ray observations are required to construct the first luminosity function of optically selected BL Lac objects and to test more stringently for any radio-quiet BL Lac objects in the sample. © 2011 ESO.

Koch A.,ZAH | Lind K.,Max Planck Institute for Astrophysics | Michael Rich R.,University of California at Los Angeles
Astrophysical Journal Letters | Year: 2011

We report on the discovery of a super-Li-rich turnoff (TO) star in the old (12Gyr), metal-poor ([Fe/H] = -2.1dex) globular cluster (GC) NGC6397, based on high-resolution MIKE/Magellan spectra. This star shows an unusually high lithium abundance of A(Li)NLTE = 4.03 0.06 0.14dex (or, 4.21, accounting for possible contamination from a binary companion) that lies above the canonical Li-plateau by a factor of 100. This is the highest Li enhancement found in a Galactic GC dwarf star to date. We discuss several enhancement mechanisms, but none can unambiguously explain such a high overabundance. The spectrum of the star shows a possible indication of binarity, but its line strengths and chemical element abundance ratios are fully compatible with other TO stars in this GC, seemingly ruling out mass transfer from an asymptotic giant branch companion as origin of the high A(Li). A possible cause is an interaction with a red giant that has undergone cool bottom processing. © 2011 The American Astronomical Society. All rights reserved.

Ludwig H.-G.,ZAH
EAS Publications Series | Year: 2011

I present ideas of Gaia's impact on the determination of the properties of stars primarily connected to the study of their atmospheres. This mainly relates to effective temperatures, gravities and high-fidelity chemical abundances obtained by combining envisioned Gaia measurements with ground-based spectroscopy ranging from single objects to well-selected stellar populations. I further discuss the impact of Gaia on the study of the kinematics of atmospheric flows. © EAS, EDP Sciences 2011.

Chini R.,Ruhr University Bochum | Chini R.,Católica del Norte University | Hoffmeister V.H.,Ruhr University Bochum | Nasseri A.,Ruhr University Bochum | And 2 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2012

The formation of stars above about 20M ⊙ and their apparently high multiplicity remain heavily debated subjects in astrophysics. We have performed a vast high-resolution radial velocity spectroscopic survey of about 250 O- and 540 B-type stars in the southern Milky Way which indicates that the majority of stars (>82percent) with masses above 16M ⊙ form close binary systems while this fraction rapidly drops to 20percent for stars of 3M ⊙. The binary fractions of O-type stars among different environment classes are: clusters (72 ± 13percent), associations (73 ± 8percent), field (43 ± 13percent) and runaways (69 ± 11percent). The high frequency of close pairs with components of similar mass argues in favour of a multiplicity originating from the formation process rather than from a tidal capture in a dense cluster. The high binary frequency of runaway O stars that we found in our survey (69percent compared to 19-26percent in previous surveys) points to the importance of ejection from young star clusters and thus supports the competitive accretion scenario. © 2012 The Authors. Monthly Notices of the Royal Astronomical Society © 2012 RAS.

Quirrenbach A.,ZAH | Reffert S.,ZAH | Bergmann C.,ZAH
AIP Conference Proceedings | Year: 2011

We present results from a radial-velocity survey of 373 giant stars at Lick Observatory, which started in 1999. The previously announced planets ι Dra b and Pollux b are confirmed by continued monitoring. The frequency of detected planetary companions appears to increase with metallicity. The star ν Oph is orbited by two brown dwarf companions with masses of 22.3 MJup and 24.5 MJup in orbits with a period ratio close to 6:1. It is likely that the two companions to ν Oph formed in a disk around the star. © 2011 American Institute of Physics.

After a ten-year design and construction period, a new universal device for astronomical observation at the world's largest single telescope, the Large Binocular Telescope in Arizona (USA) has been finalised and tested successfully. The highly complex instrument named LUCI allows astronomers to record images and spectra in the infrared with outstanding quality. It was developed by researchers of the Centre for Astronomy of Heidelberg University (ZAH) in cooperation with colleagues from the Max Planck Institute for Astronomy in Heidelberg and the Max Planck Institute for Extraterrestrial Physics in Garching. The measuring system will undergo further calibrations in the coming weeks. Once they are complete, LUCI will be available to astronomers for regular observation activities. Researchers hope that the data collected with the new instrument will give them an insight into the "nursery" of stars and even allow them to observe planets that circle remote suns.

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