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 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

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

Using high-dispersion spectral observations of HD 101065 (Przybylski's star) we have identified the lines of heavy radioactive elements with atomic numbers from 84 to 99 in the spectrum of the star. We found the lines of all these elements except At (Z=85) and Fr (Z=87). We try to explain a presence of such heavy species in the atmosphere of this star as a result of radioactive decay of Th and U nuclei and neutron capture process producing some transuranium isotopes. The necessary conditions for these processes to work may appear due to atomic diffusion and favorable accumulation of the thorium and uranium nuclei in the upper of the PS atmosphere.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

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 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 present the first results of the CP star observations in the spectral regions of the Li I lines at $\lambda 6708$ and $\lambda 6104$ obtained with the 6-m BTA telescope of the Special Astrophysical Observatory of the Russian Academy of Sciences.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

We present the results of observations made over three weeks using the UCT CCD Photometer on the 0.75-m telescope at the South African Astronomical Observatory. Candidate long period roAp stars were identified from their positions on the H-R diagram and observed for a typical period of 4 hr to test for the existence of pulsations, with particular emphasis on pulsations with periods in excess of 15 min. Although 13 stars were successfully observed, none exhibited significant pulsations.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

We present results of the analysis of spectroscopic time-series observations of the roAp star HD 24712 obtained at the phase of magnetic maximum. As in previous investigations of roAp stars we found that only lines of the rare-earth elements (REE) show RV pulsational variations. These pulsations in HD 24712 are characterised by gradual phase shifts between RV curves of different REE. The same pulsations of HD 24712 were observed in 2001, giving strong evidence for the stability of the pulsational mechanism in this star. NLTE calculations of Nd line depth formation in stratified atmosphere of HD 24712 allowed us to study the pulsational phase distribution with optical depth, providing strong evidence for a running wave, propagating outward with nearly constant RV amplitude.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

A rapidly oscillating Ap star pulsates in high-order p-modes under the influence of a strong magnetic field. The strong field distorts spatially the angluar and the radial pulsation amplitude (eigenfunction). To study the effect of the magnetic field on the radiative excitation and the damping of p-modes, we performed a fully nonadiabatic analysis including the effect of a dipole magnetic field. A magnetic field always tends to stabilize low order p-modes. In the presence of a magnetic field stronger than $\sim 1{\rm kG}$, all $\delta$ Scuti type pulsations are stabilized. For high-order p-modes, on the other hand, the magnetic field enhances $\kappa$-mechanism excitation in some range of the field strength, depending on the pulsation frequency.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

Spectroscopic mode identification techiques, which monitor intensity variations across an absorption line, provide the possibility of determining the quantum numbers l and m, the inclination and the intrinsic pulsation amplitude of a star. Of course, the uncertainties of the mode identification are dependent on the quality of the observations and the identification method applied. We have focused on the Pixel-by-pixel method/Direct line profile fitting (Mantegazza 2000) and the Moment method (Balona 1987, Briquet & Aerts 2003) for pinpointing mode parameters and tested the impact of various observational effects and stellar properties on the identification.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

Period04, a reworked and extended version of Period98 (Sperl 1998) and PERIOD/PERDET (Breger 1990), is a new software package especially dedicated to the statistical analysis of large astronomical data sets containing gaps. It offers tools to extract the individual frequencies from the multiperiodic content of time series and provides a flexible interface to perform multiple-frequency fits. A review of the functions of Period04 is given.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

SigSpec is a new method to compute the significance of the amplitude levels in the frequency domain, based on the false-alarm probability associated with a peak in the amplitude spectrum. The underlying probability density function (PDF) of the amplitude spectrum generated by pure noise may explicitly be derived if treated as frequency and phase-dependent. A comparison of the analytical solution with the results of extensive numerical calculatgions provides excellent agreement. In addition, the SigSpec software has already demonstrated clear advantages compared with the commonly used Fourier methods in various respects. A few examples for ground-based as well as space photometry are presented.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

We present preliminary results of a photometric multisite campaign on the $\delta$ Scuti-type Pre–Main-Sequence star IP Per. Nine telescopes have been involved in the observations, with a total of about 173 hour of observations over around 40 nights. Our current data permitted us to confirm the multiperiodic nature of this star and to determine at least 9 pulsational frequencies. A preliminary nonradial theoretical analysis seems to show that the star pulsates in a mixture of $l=0, 1, 2$ modes.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

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

Among variable stars, $\delta$ Scuti are particularly interesting since they can pulsate in different evolutionary phases and are characterized by both radial and nonradial pulsation modes. Even if the majority of $\delta$ Scuti stars are in their Main Sequence (MS) or post-MS phases, there are growing evidences in the recent years about the presence of $\delta$ Scuti variables among stars which are still in their pre-MS phase (the so-called Herbig Ae-stars). Due to the small number of the known class members, the boundaries of Instability Strip are not well constrained. In this context, we present the preliminary results of a long term project to define these limits by enlarging the sample of PMS $\delta$ Scuti stars.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

We present the results of an investigation of the magnetic Ap star HD 965 with high spectral and time resolution. We determine exact radial velocities using spectra obtained with the UVES spectrograph on the ESO VLT. Special attention is given to the spectral lines of the Rare Earth Elements which in roAp stars exhibit the strongest radial velocity variations with pulsation period. Careful time series analysis did not detect any convincing evidence of pulsation in HD 965 with an upper limit in amplitude of $15-20\,{\rm m\,s}^{-1}$.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

We derived the chemical abundances in the photosphere of $\delta$ Scuti, the prototype of the class of pulsating variables, from the analysis of a spectrum obtained at the Terskol Observatory 2-m telescope with a resolution $R=52000$ and $S/N=250$. VLT and IUE spectra were also used. The abundance pattern of $\delta$ Sct consists of 49 chemical elements. The abundances of Be, P, Ge, Nb, Mo, Ru, Er, Tb, Dy, Tm, Yb, Lu, Hf, Ta, Os, Pt, and Th were not investigated previously. The lines of third spectra of Pr and Nd also are investigated for the first time. The abundances of heavy elements show overabundances with respect to the Sun up to 1 dex. The abundance pattern of $\delta$ Sct is similar to that of the Am-Fm stars.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html