ACS observations of massive lensing clusters permit an order of magnitude increase in the numbers of multiply-lensed background galaxies identified behind a given cluster. We have developed a code to take the pixels belonging to any given image and generate counter-images with full resolution, so that multiple systems are convincingly and exhaustively identified. Over 130 images of 35 multiply lensed galaxies are found behind A1689, including many radial arcs and also tiny counter-images projected on the center of mass. The derived mass profile is found to flatten steadily towards the center, like and NFW profile, with a mean slope $d{\log{\Sigma}}/d{\log{r}}\approx-0.55\pm0.1$, over the range $r<250$ kpc/h, which is somewhat steeper than predicted for such a massive halo. We also clearly see the expected geometric increase of bend angles with redshift, however, given the low redshift of A1689, $z=0.18$, the dependence on cosmological parameters is weak, but using higher redshift clusters from our GTO program we may derive a more competitive constraint.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

Using observational data published as phase curves of the effective magnetic field strength $B_{\rm e}(P)$ and the surface field $B_{\rm s}(P)$, magnetic models of three stars with long rotational periods are calculated by the Magnetic Charge Distribution method. For two of these stars (HD 2453 and HD 12288), the structure of the magnetic field can be described well by a central dipole model. The third star (HD 200311) is better fitted by a model of a displaced dipole, being decentered by $\triangle r = 0.08 R$ along the dipole axis.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

The progenitors of magnetic white dwarfs are believed to be magnetic Ap and Bp stars because the fields in these stars are structured and are present in the stellar core. As in Ap/Bp stars the magnetic fields in white dwarfs are, in most cases, dipolar or quadrupolar with various offsets. Although the present space density of Ap/Bp progenitors would be sufficient to account for the density of magnetic white dwarfs in young populations such as found in the Palomar-Green survey, we show that it would be insufficient to generate the density of known magnetic white dwarfs in the older solar neighborhood. Assuming magnetic flux conservation during the final stages of evolution, we find that Ap/Bp stars would evolve into white dwarfs with magnetic fields exceeding $10^7$ G, assuming a minimum polar field of 200 G in Ap/Bp stars, thereby leaving many magnetic white dwarfs with lower fields without likely progenitors.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

The Vimos-VLT Deep Survey is a spectroscopic survey aiming at collecting more than 50000 spectra down to a limiting magnitude $I_{\mathrm{AB}}=24$, and 100000 down to $I_{\mathrm{AB}}=22.5$, on a total of about 16 deg$^2$ without any color or morphology preselection. We present the $N(z)$ distribution up to $z\sim 5$, obtained from a purely magnitude-limited sample down to $I_{\mathrm{AB}}=24$, which is an important input to weak-lensing studies. We discuss the evolution of the galaxy luminosity function up to $z\sim 2$, which exhibits a very strong increase in the typical galaxy luminosity $\Delta M^*\simeq -2.5$ in the U band compared to the local value. Surveys like the VVDS also allow to study the galaxy bias as a function of redshift without assumption about its linearity, an assumption that we find to be violated in some cases. A low bias is found, and the linear bias is shown to increase with redshift.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

We summarize of more than 25 years of research with the three filter, intermediate-band, $\Delta a$ photometric system. It investigates the flux depression at $\lambda 5200$ found in magnetic chemically peculiar (CP) objects. Starting with photoelectric measurements it has steadily developed introducing new and more efficient filters as well as the modern CCD technique. So far more than twenty papers were devoted to searching for new CP stars in our Milky Way up to distances of 5000 pc and even in the Large Magellanic Cloud. In the latter, the first extragalactic CP stars were detected. In addition, we have presented theoretical isochrones and synthetic colors from the latest available stellar atmospheres. The theoretical predictions agree very well with observations allowing not only to determine the reddening and age of open clusters from our photometry but also to investigate the flux depression at $\lambda 5200$ in more detail. As an outlook, we present a new approach to search for chemically peculiar horizontal branch stars in globular clusters and to detect stellar variability of various objects observed during our photometric observations.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

Present day estimates of the Hubble constant based on Cepheids and on the cosmic microwave background radiation are uncertain by roughly 10% (on the conservative assumption that the universe may not be perfectly flat). Gravitational lens time delay measurements can produce estimates that are less uncertain, but only if a variety of major difficulties are overcome. These include a paucity of constraints on the lensing potential, the degeneracies associated with mass sheets and the central concentration of the lensing galaxy, multiple lenses, microlensing by stars, and the small variability amplitude typical of most quasars. To date only one lens meets all of these challenges. Several suffer only from the central concentration degeneracy, which may be lifted if one is willing to assume that systems with time delays are either like better constrained systems with non-variable sources, or alternatively, like nearby galaxies.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

The existence of Maia variables has been in dispute since 1955. They are supposed to be located between the blue edge of the classical instability strip and the red border of the slowly pulsating B stars, hence in a domain of the HRD where no excitation mechanism for pulsation is yet known. But luminosity variations were discovered in time series of $\alpha$ Draconis, an A0III Maia candidate star, with a period of about 53 minutes and an amplitude of less than 0.002 mag. Spectroscopic time series indicate radial velocity variations with the same period and an amplitude of about $40{\rm m\,s}^{-1}$. Alpha Dra is a single-lined spectroscopic binary with an orbital period of 51.4 days and a distance between the components of about 0.46 AU. Tidal interaction may therefore be responsible for pulsation. If true, the pulsation amplitude should be modulated with the orbit as is indeed indicated by recent observations.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

High-resolution spectroscopic and spectropolarimetric data of the rapidly oscillating Ap star HD 24712 (HR 1217, DO Eri) has been analysed including modelling the vertical elemental abundance structures. We study the interaction and the relation of the vertical (stratification) and the horizontal (spots) abundance characteristics of Fe and the stellar magnetic field. By this synopsis and the relation of our results to the analysis of high resolution and high time resolved observations (Sachkov et al. 2005) we are likely to gain new insights about the atmospheric structure and the geometry, the origin, and the evolution of the magnetic fields of roAp stars.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

Cosmic shear is an essential cosmological tool, breaking degeneracies inherent to CMB data and providing an independent check of cosmological parameters. Upcoming cosmic shear surveys with photometric redshift information will enable tighter constraints to be placed on cosmological parameters, and allow us to explore how dark matter evolves. A Monte Carlo method to rapidly simulate mock surveys enables us to estimate the covariance matrix for the shear correlation functions, and hence the expected errors on cosmological parameter estimates given survey specifications. We also make brief remarks on the separation of the cosmic shear signal from any due to intrinsic galaxy alignments.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

A review of recent work on pulsating A stars is presented. The types of pulsating stars are the roAp, $\delta$ Scuti, $\lambda$ Bootis, Am stars, pre-Main Sequence pulsating A stars and $\alpha$ Cygni stars (pulsating A supergiants). Population II pulsating A stars such as SX Phe (blue stragglers) and RR Lyraes are also discussed. The emphasis is on the physics that can be derived from the study of stellar pulsations and suggestions for further progress.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

The study of orbital parameters of multiple Ap stars may lead to new suggestions about the origin and magnetic properties of the components. 53 Cam is one of the best studied binaries among Ap stars. However, the nature of its secondary star remains unclear despite the wealth of spectroscopic observations. The system was first directly resolved by speckle interferometry method in 1980 at the 4 m KPNO telescope McAlister et al. (1983). The authors supposed that the magnitude difference between the components could be close to zero. From that time, 16 speckle measurements of the system were made, including 7 observations collected with the SAO 6 m telescope. In addition to relative positions of the components, speckle observations from the 6 m telescope provide high accuracy magnitude difference estimates.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

Some theoretical problems associated with the presence of large-scale magnetic fields in A stars are reviewed. Possible implications of some recent theoretical and observational results are discussed, with particular attention to the survival of fields from the ISM and their long-term stability. A more coherent picture of the origin of the strong large-scale fields seen in the magnetic CP stars may be beginning to emerge.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

Chemically peculiar stars are ideal astrophysical laboratories for furnishing a wide range of theoretical and observational aspects of our knowledge of the physical processes in stars. Recent dramatic improvements in the quality of the observational data and refinements of the modelling techniques led to an emergence of a new branch of stellar astrophysics which is focused on the reconstruction and the understanding of the origin of the three-dimensional structures in stellar surface layers. In this contribution I present an overview of recent results of the detailed modelling of the chemical nonuniformities, the magnetic and the pulsation velocity fields in the atmospheres of A stars. New Doppler imaging analyses of the magnetic field and the chemical inhomogeneities reveal an unexpected complexity of the surface formations and suggest that nonmagnetic phenomena play an important role in shaping the geometry of chemical spots. Consideration of the line profile shapes observed at high spectral and time resolution has made it possible to probe the radial dependence of the chemical abundances and the pulsation characteristics of cool pulsating Ap stars. An extension of Doppler mapping to the reconstruction of non-radial stellar oscillation structure delivers a solution of the long-standing problem of the pulsational geometry of roAp stars and helps to elucidate the interrelation between the pulsations, the magnetic field and the stellar rotation.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

Diffraction limited 30m class telescopes will play an important role in gravitational lensing studies, coming online in approximately 2015. As imaging telescopes they will complement the $\sim$6m JWST, probing to smaller angular scales in greatly magnified objects near critical lines and for measuring shear of objects below the JWST angular scale, such as luminous super-star clusters at high redshift. The high source density will allow more detailed mass mapping in the weak lensing regime and will be useful in breaking the cosmology-lens potential degeneracy in strong lensing. As multi-object spectrographs 30m telescopes should provide spectra over the entire optical and near infrared spectrum region. The statistical distribution of redshifts needed to invert projected shear measurements and calibration of photometric redshifts for “tomography” will be available to flux levels around 5-10 nano-Jansky (approx 29.5 m$_{AB}$). However, a one nJy object is expected to require $\sim$500 hours to acquire a redshift, which is most of the dark time in an observing season. Accordingly “gravitational telescopes” will be an important tool for probing the very faint high redshift universe, magnifying a few square arc-seconds at a time by factors of 10-1000.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

The measurement of weak gravitational lensing is currently limited to a precision of $\sim$10% by instabilities in galaxy shape measurement techniques and uncertainties in their calibration. The potential of large, on-going and future cosmic shear surveys will only be realised with the development of more accurate image analysis methods. We present a description of several possible shear measurement methods using the linear “shapelets” decomposition. Shapelets provides a complete reconstruction of any galaxy image, including higher-order shape moments that can be used to generalise the KSB method to arbitrary order. Many independent shear estimators can then be formed for each object, using linear combinations of shapelet coefficients. These estimators can be treated separately, to improve their overall calibration; or combined in more sophisticated ways, to eliminate various instabilities and a calibration bias. We apply several methods to simulated astronomical images containing a known input shear, and demonstrate the dramatic improvement in shear recovery using shapelets. A complete IDL software package to perform image analysis and manipulation in shapelet space can be downloaded from www.astro.caltech.edu/~rjm/shapelets/.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

Diffusion of elements in a stellar plasma is strongly modified by the presence of magnetic fields for two primary reasons. The first is that the average motions of ions in outer atmospheres are, because of their charge, substantially constrained by the magnetic field. Both its intensity and orientation play a role. The second is the Zeeman desaturation of absorption lines that often produces amplifications of the radiative accelerations. These effects are important and must lead to the building of complex surface abundance structures. I will present how these two effects are generally modeled and what results have, up to now, been obtained. Future developments will also be considered.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

The global bipolar structure of the magnetic surface field is asymmetric for many stars, making its analytical description by an expansion of spherical harmonics problematic because of the coordinates. Landstreet (1970) proposed that the asymmetry could be understood assuming a decentered magnetic dipole in the stellar interior. Its surface field can be calculated for different arrangements inside and outside the star using the Magnetic Charge Distribution method. We demonstrate the effect for the two cases of the shift of the dipole along and perpendicular to the radial direction.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

Radiation hydrodynamics simulations have been used to produce numerical models of the convective surface layers of a number of stars, including the Sun and other stars on or above the main-sequence, white dwarfs of type DA, and red supergiants.

While granulation of main-sequence solar-type stars resembles that of the Sun, the convective velocity fields of F-type stars are much more violent and accompanied by strong pulsations. The properties of the thin convection zone(s) of A-type stars differ again (see Fig. 1). In this contribution, the pattern and dynamics of their surface granulation, the photospheric velocity fields and their effect on line profiles are investigated, based on new 3-D models of surface convection in main-sequence A-type stars with $T_{\rm eff}$=8500 K and 8000 K. Furthermore, we will look below the surface to study overshoot and the interaction of the surface convection zone and the deeper helium II convection zone.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

The existence of lensed quasars with splitting angles larger than $7''$ has been predicted on the basis of the cold dark matter model. We searched for these large-separation lensed quasars from the spectroscopically classified $\sim40,000$ quasars obtained by the Sloan Digital Sky Survey (SDSS), and succeeded in discovering the first lensed quasar by a cluster of galaxies, SDSS J1004+4112: It consists of four images, and the maximum separation is $14.62''$. Here we describe the discovery and the follow-up observations of this system, as well as the latest results of the large-separation lens search in the SDSS. We also present the new statistical model which fully incorporates non-sphericity of dark halos, and compare the theoretical predictions with the SDSS results.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

We present predictions for cosmological parameter constraints from combined measurements of second- and third-order aperture mass statistics of cosmic shear. The generalized third-order aperture mass is introduced and its relation to the convergence bispectrum is given. This quantity contains (in principle) all information about the bispectrum. Using ray-tracing simulations, we perform a Fisher matrix analysis for various cosmological parameters and show that the combination of $\langle M_{\rm ap}^2\rangle$ and $\langle M_{\rm ap}^3\rangle$ improves the parameter estimation significantly.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html