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Vladilo G.,National institute for astrophysics | Abate C.,University of Trieste | Abate C.,Astronomical Institute Utrecht | Yin J.,University of Trieste | And 3 more authors.
Astronomy and Astrophysics | Year: 2011

Silicates are an important component of interstellar dust that has been poorly investigated in high redshift galaxies. As a preliminary step to studying silicates at high redshift, we survey silicon depletions in damped Ly α (DLA) systems. Silicon depletion is mild in the Galactic interstellar medium (ISM) and is expected to be weaker in most DLA systems, so we introduce a method for improving the accuracy of DLA depletion measurements. We compare abundance ratios measured in the gas with calculations of total abundance ratios of gas and dust predicted by models of galactic chemical evolution tailored for DLA systems. To tune the model parameters, we use the dust-free observational diagram S/Zn versus Zn/H, and we also compare the look back time estimated from the absorption redshift with the evolutionary time predicted by the model. By applying our method to a large set of DLA column densities, we succeeded in measuring the depletion of silicon in 74 systems. For comparison, we also measure iron and magnesium depletions (105 and 10 systems, respectively) with the same method. The mean depletion of silicon that we derive, δSi ≃ -0.27 ± 0.16 dex, is surprisingly close to that of iron, δFe ≃ -0.42 ± 0.28 dex, despite iron being much more depleted than silicon in the Galactic ISM. Silicon depletion in DLA systems does not correlate with metallicity, at variance with iron depletion, for which we confirm a rise with [Fe/H] found in previous work. Magnesium depletion seems to behave more in accordance with silicon than with iron. The different behaviors of the silicon and iron depletions suggests a complex history of dust production at the early stages of galactic chemical evolution. © 2011 ESO.

Lommen D.J.P.,Leiden University | Van Dishoeck E.F.,Leiden University | Van Dishoeck E.F.,Max Planck Institute for Extraterrestrial Physics | Maddison S.T.,Swinburne University of Technology | And 9 more authors.
Astronomy and Astrophysics | Year: 2010

Context. Young stars are formed with dusty discs around them. The dust grains in the disc are originally of the same size as interstellar dust, i.e., of the order of 0.1 μm. Models predict that these grains will grow in size through coagulation. Observations of the silicate features around 10 and 20 μm are consistent with growth from submicron to micron sizes in selected sources whereas the slope of the spectral energy distribution (SED) at mm and cm wavelengths traces growth up to mm sizes and larger. Aims. We here look for a correlation between these two grain growth indicators. Methods. A large sample of T-Tauri and Herbig-Ae/Be stars, spread over the star-forming regions in Chamaeleon, Lupus, Serpens, Corona Australis, and the Gum nebula in Vela, was observed with the Spitzer Space Telescope at 5-13 μm, and a subsample was observed with the SMA, ATCA, CARMA, and VLA at mm wavelengths. We complement this subsample with data from the literature to maximise the overlap between μm and mm observations and search for correlations in the grain-growth signatures. Synthetic spectra are produced to determine which processes may produce the dust evolution observed in protoplanetary discs. Results. Dust disc masses in the range <1 to 7×10-4M· are obtained. The majority of the sources have a mm spectral slope consistent with grain growth. There is a tentative correlation between the strength and the shape of the 10-μm silicate feature and the slope of the SED between 1 and 3 mm. The observed sources seem to be grouped per star-forming region in the 10-μm-feature vs. mm-slope diagram. The modelling results show that, if only the maximum grain size is increased, first the 10-μm feature becomes flatter and subsequently the mm slope becomes shallower. To explain the sources with the shallowest mm slopes, a grain size distribution shallower than that of the interstellar medium is required. Furthermore, the strongest 10-μm features can only be explained with bright (L ∼6 dothot (Teff = 4000 K) central stars. Settling of larger grains towards the disc midplane results in a stronger 10-μm feature, but has a very limited effect on the mm slope. Conclusions. A tentative correlation between the strength of the 10-μm feature and the mm slope is found, which would imply that the inner and outer disc evolve simultaneously. Dust with a mass dominated by large, ~mm-sized, grains is required to explain the shallowest mm slopes. Other processes besides grain growth, such as the clearing of an inner disc by binary interaction, may also be responsible for the removal of small grains. Observations with future telescopes with larger bandwidths or collecting areas are required to provide the necessary statistics to study these processes of disc and dust evolution. © 2010 ESO.

Tautz R.C.,Astronomical Institute Utrecht | Dominici D.,TU Berlin
Physics Letters, Section A: General, Atomic and Solid State Physics | Year: 2010

Exact expressions for certain Kapteyn series of the first and second kind are obtained. Such sums are needed in analytical theories describing Cosmic Ray scattering in turbulent magnetic fields. Also, some functions defined by sums involving derivatives of Bessel functions are considered. Numerical methods and applications of the results are discussed. © 2010 Elsevier B.V. All rights reserved.

Yurkin M.A.,Novosibirsk State University | Min M.,Astronomical Institute Utrecht | Hoekstra A.G.,University of Amsterdam
Physical Review E - Statistical, Nonlinear, and Soft Matter Physics | Year: 2010

We compared three formulations of the discrete dipole approximation (DDA) for simulation of light scattering by particles with refractive indices m=10+10i, 0.1+i, and 1.6+0.01i. These formulations include the filtered coupled dipoles (FCD), the lattice dispersion relation (LDR) and the radiative reaction correction. We compared the number of iterations required for the convergence of the iterative solver (proportional to simulation time) and the accuracy of final results. We showed that the LDR performance for m=10+10i is especially bad, while the FCD is a good option for all cases studied. Moreover, we analyzed the detailed structure of DDA errors and the spectrum of the DDA interaction matrix to understand the performance of the FCD. In particular, this spectrum, obtained with the FCD for particles smaller than the wavelength, falls into the bounds, physically implied for the spectrum of the infinite-dimensional integral scattering operator, contrary to two other DDA formulations. Finally, such extreme refractive indices can now be routinely simulated using modern desktop computers using the publicly available ADDA code, which includes an efficient implementation of the FCD. © 2010 The American Physical Society.

Tautz R.C.,Astronomical Institute Utrecht
Plasma Physics and Controlled Fusion | Year: 2010

The problem of cosmic ray scattering in magnetohydrodynamic turbulence is revisited using a recently developed Monte Carlo simulation code. Diffusion coefficients are obtained using magnetostatic turbulence, slab Alfvénic turbulence with and without electric fields and isotropic magnetosonic turbulence with and without electric fields. All non-magnetostatic simulations are carried out for different values of the Alfvén velocity so that the effects of different particle to Alfvén velocity ratios can be studied. For slab turbulence, it is shown that Alfvén waves guarantee the recovery of diffusion, as predicted by analytical studies. The inclusion of electric fields, however, immediately leads to superdiffusion, if the Alfvén velocity is not too small. Furthermore, the energy gain through stochastic acceleration processes is investigated both qualitatively and quantitatively. The comparison with previous simulation reveals both similarities and interesting differences. © 2010 IOP Publishing Ltd.

De Mink S.E.,University of Bonn | De Mink S.E.,Astronomical Institute Utrecht | Cantiello M.,University of Bonn | Langer N.,University of Bonn | And 2 more authors.
AIP Conference Proceedings | Year: 2010

Rotation can have severe consequences for the evolution of massive stars. It is now considered as one of the main parameters, alongside mass and metallicity that determine the final fate of single stars. In massive, fast rotating stars mixing processes induced by rotation may be so efficient that helium produced in the center is mixed throughout the envelope. Such stars evolve almost chemically homogeneously. At low metallicity they remain blue and compact, while they gradually evolve into Wolf-Rayet stars and possibly into progenitors of long gamma-ray bursts. In binaries this type of evolution may occur because of (I) tides in very close binaries, as a result of (II) spin up by mass transfer, as result of (III) a merger of the two stars and (IV) when one of the components in the binary was born with a very high initial rotation rate. As these stars stay compact, the evolutionary channels are very different from what classical binary evolutionary models predict. In this contribution we discuss examples of nearly chemically homogeneous evolution in very close tidally-locked binaries. Even in such very close massive binaries, the stars may remain compact and avoid mass transfer, while Roche lobe overflow and a merger would be inevitable in the classical picture. This type of evolution may provide an alternative path to form tight Wolf-Rayet binaries and massive black hole binaries. © 2010 American Institute of Physics.

Lerche I.,Martin Luther University of Halle Wittenberg | Tautz R.C.,Astronomical Institute Utrecht
Journal of Physics A: Mathematical and Theoretical | Year: 2010

In discussing the signatures available from high intensity Compton scattering, a problem of considerable interest in photon scattering from active galactic nuclei and their emission jets, Harvey et al (2009 Phys. Rev. A 79 063407) showed that the properties of some other than conventional Kapteyn series play fundamental roles in determining the spectral output both with respect to frequency and with respect to emission angle. While they were able to provide bounds to the required series in terms of known Kapteyn series that could be summed analytically in closed form, only numerical analysis could take their investigation further as they demonstrated. The purpose of this paper is to show that the many Kapteyn series involved in the scattering problem can all be reduced either to analytic form or to a single Kapteyn series that cannot be evaluated in closed form but for which an integral representation is available. This reduction is of considerable benefit in controlling the correctness and accuracy of numerical investigations; the reduction also provides significant insight into some basic procedures for summing such unconventional Kapteyn series as well as allowing a better understanding of the dependence of the scattering on the physical parameters involved than would be the case directly from the series. © 2010 IOP Publishing Ltd.

Chies-Santos A.L.,Astronomical Institute Utrecht | Larsen S.S.,Astronomical Institute Utrecht | Kuntschner H.,European Southern Observatory | Anders P.,Astronomical Institute Utrecht | And 4 more authors.
Astrophysics and Space Science Proceedings | Year: 2011

Some photometric studies of extragalactic globular cluster (GC) systems using the optical and near-infrared colour combination have suggested the presence of a large fraction of intermediate-age (2-8 Gyrs) GCs. We investigate the age distributions of GC systems in 14 E/S0 galaxies. We carry out a differential comparison of the .g - z/ vs. .g - K/ two-colour diagrams for GC systems in the different galaxies in order to see whether there are indications of age differences. We also compare the different GC systems with a few simple stellar population models. No significant difference is detected in the mean ages of GCs among elliptical galaxies. S0 galaxies on the other hand, show evidence for younger GCs. Surprisingly, this appears to be driven by the more metal-poor clusters. The age distribution of GCs in NGC4365 seems to be similar to that of other large ellipticals (e.g. NGC4486, NGC4649). Padova SSPs with recently released isochrones for old ages (14Gyrs) show less of an offset with respect to the photometry than previously published models. We suggest that E type galaxies assembled most of their GCs in a shorter and earlier period than S0 type galaxies. The latter galaxy type seems to have a more extended period of GC formation/assembly. © Springer-Verlag Berlin Heidelberg 2011.

Tautz R.C.,Astronomical Institute Utrecht
Astrophysics and Space Science | Year: 2010

The particle distribution function that describes two interpenetrating plasma streams is re-investigated. It is shown how, based on the Maxwell-Boltzmann-Jüttner distribution function, which has been derived almost a century ago, a counterstreaming distribution function can be derived that uses velocity space. Such is necessary for various analytical calculations and numerical simulations that are reliant on velocity coordinates rather than momentum space. The application to the electrostatic two-stream instability illustrates the differences caused by the use of the relativistic distribution function. © 2010 The Author(s).

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