Stars in various parts of the HR diagram often have atmospheres in which the departure from the simplest kind of plane-parallel model is largely dominated by a single physical effect. For example, massive stars and giants exhibit symptoms of strong winds and lower Main Sequence stars are very strongly influenced by the presence of deep and energetic envelope convection. Main Sequence A stars, in contrast, appear to display the competing effects of several physical effects of comparable magnitude. The effects which can be detected by observation include large and relatively simple magnetic fields, strong surface convection, pulsation (often in multiple modes), diffusion of specific species under the competing influences of gravity and radiative acceleration, and (more indirectly) internal turbulent mixing, weak winds, and non-thermal heating. This situation makes these stars extremely useful as laboratories to explore and to understand the physics of these various phenomena, and how these effects interact with each other. This review will summarize some of the interconnections that are gradually being understood and emphasize some of the major remaining problems.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

We present new numerical models of line-driven stellar winds of A supergiants. Statistical equilibrium (NLTE) equations of the most abundant elements are solved, and properly obtained occupation numbers are used to calculate consistent radiative force and radiative heating terms. Wind density, velocity, and temperature are calculated as solutions of the model's hydrodynamical equations. Our models allow for the calculation of the wind mass-loss rate and terminal velocity.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

Globular clusters offer ideal laboratories to test the predictions of stellar evolution. When doing so with spectroscopic analyses during the 1990s, however, the parameters we derived for hot horizontal branch stars deviated systematically from theoretical predictions. The parameters of cooler, A-type horizontal branch stars, on the other hand, were consistent with evolutionary theories. In 1999, two groups independently suggested that diffusion effects might cause these deviations, which we verified subsequently. I will discuss these observations and analyses and their consequences for interpreting observations of hot horizontal branch stars.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

Between 2000 and 2003 more than 600 hours of photometric data of the $\delta$ Scuti star 44 Tauri (HD 26322) were collected in Strömgren $v$ and $y$ as well as in Johnson $V$. Observations were carried out at Washington Camp (Arizona), the Sierra Nevada Observatory (Spain) and the Ege University Observatory (Turkey). After analyzing the data with Period98 (Sperl 1998) more than 30 frequencies were found to be significant. We present the data and the results of our observational campaigns.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

Based on a preliminary analysis of 3 nights of high-dispersion spectral observations of HD 101065 (Przybylski's star) obtained on the ESO 3.6-m telescope + HARPS we find rapid multiperiodic radial velocity oscillations of at least three pulsation modes. The detailed analysis of the frequency space and probable splitting based on 4 nights of data will be given later.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

When a layer of heavy matter is above lighter matter in a star, the inverse $\mu $-gradient may lead to thermohaline (or double-diffusive) convection. This has been studied in the past for helium-rich atmospheres, but it may also occur for metal-rich layers. It has recently been studied for the accretion of hydrogen poor material onto the host stars of exoplanets. These stars present a metallicity excess compared to stars in which no planets have been detected. However, the reason for this excess is still a subject of debate. It may be primordial or the result of accretion, or both. In this last case, thermohaline convection may lead to “metallic fingers” which partially dilute the accreted matter inside the star. Such an effect can also be important in the chemically peculiar A stars in which metals accumulate in the atmospheric layers.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

Abundance determinations obtained from spectroscopic observations of Am stars provide information concerning the transport processes present in these stars. In this paper we have used models of Am stars which include gravitational settling, thermal diffusion, and radiative accelerations for 24 elements. We used a specific model of rotation induced mixing which has reproduced anomalies in other types of stars. For this preliminary study, models of $1.7 M_{\odot}$ and $1.9 M_{\odot}$ have been computed. A comparison of the predicted abundances to the observed ones for the Praesepe star HD 73045 sets constraints on rotational mixing.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

We discuss how strong lensing by galaxy clusters may be used to study the properties of dark matter halos and the expansion history of the universe. First, we show how the characteristics of $\Lambda$CDM clusters determine their lensing properties, and show how these properties are manifested in some of the new giant arcs discovered behind SDSS clusters. Next, we compare the statistics of strong lensing by clusters expected in the $\Lambda$CDM model to the observed statistics of giant arcs and wide-separation quasars. Finally, we discuss the cosmographic uses of clusters with multiple arcs, pointing out several sources of noise which can produce $\gtrsim100$ % errors in derived cosmological parameters.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

We present a novel method, based on the Doppler imaging inversion technique, which tries to construct a two-dimensional ‘image’ of the pulsation velocity field using time series observations of stellar spectra. This method is applied to study the geometry of nonradial oscillations in the roAp star HR 3831. The image of pulsational perturbations at the surface of this star is the first stellar pulsation map derived without assuming the spherical harmonic formalism. Our Doppler reconstruction directly demonstrates an alignment of the roAp pulsations with the stellar magnetic field axis. It also reveals a significant distortion of the dominant oblique $\ell=1$ oscillation mode by the stellar magnetic field. This first detailed characterization of the magnetic and rotation effects on pulsations opens possibilities for the direct testing of recent theories of magnetoacoustic oscillations in roAp stars.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

Research in roAp stars is being vigorously pursued, both theoretically and observationally by many groups. We report the discovery of a 21-min period, luminous roAp star, HD 116114. Longer periods for more luminous stars have been predicted theoretically and this is the first discovery of such a star. We discuss a model for the blue-to-red line profile variability observed in some roAp stars involving a shock wave high in the atmosphere of roAp stars, yet show that the H$\alpha$ line in 33 Lib has the blue-to-red-to-blue line profile variability expected for subsonic dipolar pulsation concentrated towards the pulsation pole. Further we report for 33 Lib unprecedented observations of the amplitudes and phases of its principal mode at 2.015 mHz and its first harmonic of that at 4.030 mHz.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

Observations of chemically peculiar (CP) stars have been conducted for decades and have revealed a variety of spectrum anomalies, most prominent among them are line enhancements of heavy elements. The earlier observations were limited to the optical region and the use of less sensitive detectors, yet are responsible for much of our current characterization of the CP star phenomenon. More recent observations embrace a wider expanse of the electromagnetic spectrum and employ more sensitive detectors that continue to unveil new levels of spectrum peculiarity. The traditional criteria used to distinguish normal from peculiar stars have become blurred, thus in some sense replacing the concept of peculiarity with one of continuity. This presentation will address the observations of the traditional nonmagnetic CP star groups over the past decade, paying particular attention to new avenues of research that have a bearing upon the interpretation of the atmospheres of CP stars and the origins of this phenomenon.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

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