The parameter-free model of diffusion in the atmospheres of HgMn stars (Michaud 1986, Michaud et al. 1979) predicts that helium should sink below the $\hbox{He\,{\sc ii}}$ ionization zone that the diffusion of other elements may take place, and that all HgMn stars should have deficits of helium in their photospheres, with a minimum deficit of 0.3 dex. In this study, the Smith & Dworetsky (1993) sample of HgMn stars and normal comparison stars is examined, and the helium abundances determined by spectrum synthesis using échelle spectra taken at Lick Observatory and the AAT. The prediction is confirmed. All HgMn stars are deficient in He by as much as 1.5 dex. Also, two HgMn stars, HR 7361 and HR 7664, show clear evidence of helium stratification.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

We present results from weak lensing analyses of 20 high-redshift clusters in the ESO Distant Cluster Survey (EDisCS) from both deep optical VLT and HST/ACS imaging. We find that the EDisCS sample is composed primarily of clusters that are less massive than those in current X-ray selected samples at similar redshifts, but that all of the fields are likely to contain massive clusters rather than superpositions low mass groups. We find good agreement between the weak lensing measurements in the VLT and ACS images. Finally we determine the noise level in the shear measurements for the VLT fields as a function of exposure time and seeing and demonstrate that future ground-based surveys which plan to perform deep optical imaging for use in weak lensing measurements must achieve point-spread functions smaller than a median of $0\farcs 6$ FWHM.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

The Michigan Catalogue (Vol. I – V) is an excellent database to study the distribution of CP2 stars in the galactic field in the Southern hemisphere. A cross-check with the Bidelman-Catalogue (1973) and the “Catalogue Général des Étoiles Ap et Am” (Renson 1991) shows that in the Michigan Catalogue 3.6% of all stars in the relevant spectral region are CP2 stars. Any discrepant classified stars could be verified photometrically in the $\Delta a$-system. The distribution of CP2 stars does not show any particular preferred region in the Southern hemisphere. The ages of these stars and an evolutionary dependence of the CP2 effect was another important question. We conclude that the CP2 stars occupy the full Main Sequence band.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

We present samples from a spectroscopic atlas of the sharp-lined, magnetic Chemically Peculiar star HR 8216 (A6pCr) based on spectrograms obtained with the long camera of the 1.22-m telescope of the Dominion Astrophysical Observatory using a Reticon detector. For the spectral region $\lambda\lambda 3830-4770$ the inverse dispersion is $2.4\,\AA\,mm^{-1}$ with a 2 pixel resolution of 0.072 Å. At the continuum the mean signal-to-noise ratio is 200. The wavelengths in the laboratory frame, the equivalent widths, and the identifications of the various spectral features are given. This atlas should provide useful guidance for studies of similar stars and for atomic physicists interested in improving atomic line parameters.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

The complicated and intricate time-dependent behavior of the magnetic field of $\beta$ Lyrae from 1980 to 2004 is discussed.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

Gravitational microlensing is a powerful tool to detect compact matter on mass scales ranging from roughly $10^{-6} M_\odot$ to $10^6 M_\odot$ . Of particular importance is the fact that lensing is sensitive to both luminous and dark matter. There are two practically important regimes of microlensing: cosmological and Galactic. The former deals with the effect of stellar mass objects at cosmological distances on even more distant quasars, the latter treats stellar mass objects in the Milky Way or its Halo on stars in the Magellanic Clouds, the Galactic Bulge or M31. Microlensing has been detected in about ten quasars, roughly a dozen microlensing events towards stars in the Magellanic Clouds are on record, more than 1000 events have been measured towards stars in the Bulge. The large majority of microlensing events towards quasars and towards stars in the Magellanic Clouds or the Bulge can be explained by ordinary stellar mass objects. A small fraction of the Galactic microlensing events ($< 5 - 10$ %) could potentially be due to dark matter objects, including stellar mass Black Holes. Current and planned experiments will clarify the question whether any dark matter objects are necessary at all to explain microlensing events.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

The use of the Four College Automatic Photometric Telescope at the Fairborn Observatory in Southern Arizona with its ability to observe stars every clear night and for continuous runs has played a major role in our very successful cooperative program of Strömgren differential uvby photometry of magnetic CP (mCP) stars. Out of about 100 stars observed only a few have variable light curves. But more may be found when additional observations are made. We review the published theoretical basis and observations as well as present new results for CU Vir, V1093 Ori, MW Vul, and HR 7224. We interpret some observations as possibly requiring a more robust theory of the variability of mCP star light curves.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

The paper emphasizes the connection between observations and theory in the Lower Instability Strip. It is argued that the $\delta$ Scuti and $\gamma$ Doradus pulsators are very close relatives in the sense that the $\gamma$ Doradus phenomenon can also be seen in the slightly hotter $\delta$ Scuti variables.

New developments and recent progress are reviewed. Arguments are given why all (or almost all) stars in the Lower Instability Strip are pulsators. Furthermore, the question of the so-called missing modes is answered as very extensive photometry campaigns have revealed a large number of pulsation modes with amplitudes less than one millimag. Arguments are given that the 500+ $\ell$ = 0 to 2 modes predicted for $\delta$ Scuti stars are actually present.

The progress made in developing reliable methods of pulsation mode identification is briefly discussed. The discovery of many close modes with small amplitudes sets severe requirements concerning the length of future studies.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

We have developed a new code for measuring stellar magnetic fields by calculation using fast transforms of the Fourier cross-correlation function from spectra with reciprocal circular polarization. To test the code, spectra of stars with previously measured magnetic fields were used. In some cases, discrepancies between the field strength obtained by Babcock's classical technique and using our code occur.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

We have compiled all available measurements in the UV region for the group of $\lambda$ Bootis stars. These data are compared to those of F-weak stars and known spectroscopic binary systems. We also use synthetic photometric values from standard atmospheric models to comment on the proposed spectroscopic binary nature of members of the $\lambda$ Bootis group which can be found in the literature.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

The possibility of a quite high Li abundance in the Ap stars was first raised by Wallerstein & Merchant (1965). Since then many studies investigated the problem of Li. The more recent observations in the lithium region indicate that in some Ap stars the $\lambda 6708$ feature is variable and this variability can be explained by the existence of Li rich spots on the stellar surface. Atomic data for the Ce II $\lambda 6708.099$ were released by the D.R.E.A.M. database in 2002. The line was used to identify the prominent suspected Li-feature in post AGB stars and might as well be responsible for the absorption feature in Ap stars. Recent studies have mentioned this possibility, but it has yet to be investigated in more detail. Other physical phenomena, such as the occurrence of a partial Paschen-Back effect in the presence of magnetic fields, as well as possible hyperfine structure splitting of some Rare Earth transitions, must be taken into account to provide correct line identifications in the wavelength region around the Li-doublet at $\lambda 6708$. We discuss a possible strategy to clarify the presence of Li in Ap stars.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

We report the results of the search for rapid periodic magnetic variations in $\gamma$ Equ. Observations were collected in August 1989 with the 6-m telescope of Special Astrophysical Observatory (Russia). We analysed a 3-hour series of 1720 rapid $B_{\rm e}$ measurements of $\gamma$ Equ obtained with an average time resolution of 6.6 s. A power spectrum analysis shows a period of $P = 3.596\,{\rm min}$. That period differs from the well-known photometric period of rapid variability $P_{\rm phot} = 12.44\,{\rm min}$. We do not confirm variations of the $B_{\rm e}$ field of $\gamma$ Equ with the $P_{\rm phot}$.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

We review the scientific objectives and the present status cosmic shear studies. We discuss the future prospects and the role cosmic shear could play in a precision cosmology era.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

We report the results of the spectral monitoring of the chemically peculiar (CP2) star HD 3980 in the Li I $\lambda 6708$ spectral region. High spectral resolution observations ($R=88000$) were carried out at Mount Stromlo Observatory with the 74-inch telescope and the echelle spectrograph. HD 3980 is a late type Ap SrCrEu star with strong double-wave photometric variations in the visible and in the near infrared. We estimated the effective temperature, surface gravity, mass, luminosity, and radius of this star. The monitoring of this star shows strong spectral variations, especially in the profile of Li I $\lambda 6707.8$. We studied the variations of the equivalent width and position of the Li I line as a function of the rotating phase and compared them with the magnetic field variations and light curves taken from the literature. The synchronism of the variations of the Li I line profile, light curve, and magnetic field strength can be explained in terms of the oblique magnetic rotator model. The strong variations of position and intensity of this line with the rotation phase enable us to consider this star as a new member of the intriguing group of “Li-spotted” CP stars.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

High $S/N$, high resolution CCD spectra of 10 Am binaries with different orbital elements, rotation and atmospheric parameters obtained with the 2-m telescope of NAO Rozhen (Bulgaria) were analyzed. We carried out preliminary abundance analyses of these stars and studied the possible dependences of abundance anomalies and [Ca/Fe] on their values of effective temperature, $v\sin i$, orbital period, and eccentricity. There are weak trends of [Ca/Fe] increasing with $v\sin i$ and decreasing with eccentricity and orbital period. [Ca] seems to be increasing with effective temperature, and [Fe] decreasing with $v\sin i$ and increasing with eccentricity. However we need to study more stars to confirm and disentangle the possible trends.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

The current status of NLTE model atmosphere calculations of A type stars is reviewed. During the last decade research has concentrated on solving the restricted NLTE line formation problem for trace elements assuming LTE model atmospheres. There is a general lack of calculated NLTE line blanketed model atmospheres for A type stars, despite the availability of powerful methods and computer codes that are able to solve this task. Some directions for future model atmosphere research are suggested.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

The statistics of giant arc formation by galaxy clusters has long held promise as a cosmological tool. Progress in this field relies upon advances in modeling, and in observational samples. We introduce several new lens samples from the Red-Sequence Cluster Survey (RCS-1) which provide a significant increase in the total sample of clusters with arcs, while adding several new observational puzzles. The most significant of these observations is that the redshift distribution of clusters which form arcs is unexpectedly skewed to higher redshifts. We also discuss upcoming samples from the ongoing RCS-2 survey and other planned surveys; preliminary results from RCS-2 support this unexpected redshift distribution and deepen the disagreement between theory and observations.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

A new parametric method that approximates the radiative accelerations of bound-bound and bound-free transitions at large optical depths is presented. The parameters found in these new equations are calculated for the following trace elements: C, N, O, Ne, Na, Mg, Al, Si, S, Ar, Ca, and Fe and are made available to potential users. This method can be easily implemented in existing codes to study atomic diffusion without needing large computing resources or access to large atomic and opacity data bases.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

We present $VR$ magnification ratios of the double quasar Q0957+561AB, which are inferred from frames taken with the 2.56m Nordic Optical Telescope. From two different photometric techniques ($pho2comC$ and $psfphot$) and a reasonable range for the time delay in the system (415–430 days), we find optical continuum ratios depending on the wavelength. These chromatic ratios are consistent with either differential extinction in a dust system or gravitational microlensing in the deflector. Although the dusty scenario is only viable for a compact dust cloud in the line of sight to the A component, the possible values for the differential extinction and the ratio of total to selective extinction in the $V$ band are surprisingly reasonable. To decide on the true origin of the anomalous ratios (extinction, microlensing or a mixed scenario), we are carrying out new monitoring campaigns and planning detailed observations from the best ground–based and space telescopes.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

Using data from the new EBASIM spectrograph at the 2.1-m telescope of the Complejo Astronómico El Leoncito (CASLEO), two rather sharp-lined B stars 5 Aqr (=HD 198667, B9 III) and 30 Peg (=HD 211924, B5 IV) are being studied. The measurements are compared with those from the coudé spectrograph of the 1.22-m telescope of the Dominion Astrophysical Observatory (DAO). The equivalent width scales of the EBASIM and the DAO data are similar. As we found that the line profiles of 30 Peg are variable, we are trying to determine whether this star is a slowly pulsating B star. 5 Aqr has abundances close to those of the Sun.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html