Time filter

Source Type

Heidelberg, Germany

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. Source

Chini R.,Ruhr University Bochum | Chini R.,Catolica 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. Source

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. Source

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. Source

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. Source

Discover hidden collaborations