The Dushak-Erekdag survey of roAp stars

The search of roAp stars at Mt. Dushak-Erekdag Observatory was started in 1992 using the 0.8 m Odessa telescope equipped with a two-star high-speed photometer. We have observed more than a dozen stars so far and discovered HD 99563 as roAp star while BD+8087 is suspected to have rapid oscillations. Negative results of our observations for the search of rapid oscillations in four stars in NGC 752 are also discussed.     

On‐line database of photometric observations of magnetic chemically peculiar stars

We present our extensive project of the On‐line database of photometric observations of magnetic chemically peculiar stars to collect published data of photometric observations of magnetic chemically peculiar (mCP) stars in the optical and near IR regions. Now the nascent database contains more than 107 000 photometric measurements of 102 mCP stars and will be continually supplemented with published or new photometric data on these and about 150 additional mCP stars. This report describes the structure and organization of the database. Moreover, for the all included data we estimated the error of measurements and the effective amplitudes of the light curves. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Lithium in chemically peculiar CP stars with magnetic fields

The problem of lithium in chemically peculiar Ap-CP stars has been the subject of debate for many years. The main reason for this is a lack of spectral observations of Ap stars in the neighborhood of the lithium resonance doublet Li I 6708 Å. An international cooperation project on “Lithium in cool CP stars with magnetic fields” was started in 1996. Systematic observations of CP stars in spectral regions of the 6708 Å and 6103 Å lines at the ZTSh (CrAO), CAT (ESO), Feros (ESO), and the 74″ telescope of the Mount Stromlo Observatory (Australia) have been used to analyze spectra of several CP stars studied by the way the 6708 Å lithium line varies with the stars’ rotational phase. Monitoring of the spectra of the oscillating CP stars (group I) HD 83368, HD 60435, and HD 3980, for which significant Doppler shifts of the Li I 6708 Å line are observed led to the discovery of “lithium spots” on the surface of these stars whose positions are related to the magnetic field structure. Models of the surfaces of these stars with the special program “ROTATE” based on the profiles of the Li I 6708 Å line are used to estimate the size of the spots, their positions on the stars’ surface, and the lithium abundances in these spots. A detailed analysis and modelling of the spectra of slowly rotating oscillating CP stars with strong, invariant lithium 6708 Å emission, including blending with lines of the rare earth elements, reveals an enhanced lithium abundance, with the abundance determined from the lithium 6103 Å line being higher than that determined from the 6708 Å line for all the stars. This may indicate vertical stratification of lithium in the atmospheres of CP stars with an anomalous isotopic composition (⁶Li/⁷Li = 0.2–0.5). HD 101065, an ultraslow rotator (vsini ≈ 1.5) visible from the poles and with powerful oscillations which cause pulsating line broadening in its spectrum, is unique among these stars. The amount of lithium in the atmosphere of HD 101065 logN(Li) = 3.1 on a scale of logN(H) = 12.0 and the isotope ratio ⁶Li/⁷Li ≈ 0.3. The high estimates of ⁶Li/⁷Li may be explained by the production of lithium in spallation reactions and the preservation of surface ⁶Li and ⁷Li by strong magnetic fields in the upper layers of the atmosphere near the magnetic poles. __________ Translated from Astrofizika, Vol. 50, No. 3, pp. 463–492 (August 2007).     

On the behavior of the C 4267.261, 6578.052 and 6582.882 Å lines in chemically peculiar and standard stars

With the aim of investigating the possible particular behavior of carbon in a sample of chemically peculiar stars of the main sequence without turning to modeling, we performed spectroscopic observations of three important and usually prominent single ionized carbon lines: 4267.261, 6578.052 and 6582.882 Å. In addition, we observed a large number of standard stars in order to define a kind of normality strip, useful for comparing the observed trend for the peculiar stars. We paid particular attention to the problem of the determination of fundamental atmospheric parameters, especially for the chemically peculiar stars for which the abundance anomalies change the flux distribution in such a way that the classical photometric methods to infer effective temperatures and gravities parameter cannot be applied. Regarding CP stars, we found a normal carbon abundance in Hg–Mn, Si (with some exceptions) and He strong stars. He weak stars are normal too, but with a large spread out of the data around the mean value. A more complicated behavior has been noted in the group of SrCrEu stars: four out of seven show a strong overabundance, being the others normal.     

Light‐induced drift for Hg isotopes in chemically peculiar stars

In the present paper the abundance anomalies of mercury and its isotopes in the atmospheres of HgMn stars have been studied. Observations have shown strongly anomalous isotopic composition of Hg, Pt, Tl and He in the atmospheres of such CP stars. Generation of elemental abundance anomalies in quiescent atmospheres of CP stars can generally be explained by the mechanism of diffusive segregation of elements due to oppositely directed gravitational and radiative forces. It has been shown that the formation of the observed isotopic anomalies can be successfully explained by a diffusion mechanism called the light‐induced drift (LID). The observed ratios of isotopes also enable to estimate the evolutionary stages of CP stars.     

Magnetic field measurements of CP stars from hydrogen line cores

We present the results of measurements of magnetic fields of chemically peculiar (CP) stars, performed from the shifts between the circularly polarized components of metal and hydrogen lines (the Babcock method). The observations are carried out with an analyzer of circular polarization at the 6‐m telescope of the SAO RAS. We found that for the absolute majority of the objects studied (in 22 CP stars out of 23), the magnetic fields, determined from the Zeeman shifts in the hydrogen line cores, are significantly lower than those obtained from metal lines in the same spectra. This disparity varies between the stars. We show that instrumental effects can not produce the above features, and discuss the possible causes of the observed effect. The discovered condition reveals a more complicated structure of magnetic fields of CP stars than a simple dipole, in particular, a reduction of the field strength in the upper atmosphere with the vertical gradient, significantly higher than the dipole (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Variability of the HeIλ5876 Å line in early type chemically peculiar stars. II

To try to understand the behavior of helium variability in Chemically Peculiar stars, we continued our on‐going observational campaign started by Catanzaro, Leone & Catalano (1999). In this paper we present a new set of time resolved spectroscopic observations of the HeI5876 Å line for a sample of 10 stars in the spectral range B3 ‐ A2 and characterized by different overabundances. This line does not show variability in two stars: HD77350 and HD175156. It shows instead an equivalent width variation in phase with the Hipparcos light curve for two stars: HD79158 and HD196502. Antiphase variations have been found in 4 stars of our sample, namely: HD35502, HD124224, HD129174 and HD142990. Nothing we can say about HD115735 because of the constancy of Hipparcos photometric data, while no phase relation has been observed for HD90044. In the text we discuss the case of HD175156, according to photometric calibration and our spectroscopic observations we rule out the membership of this star to the main sequence chemically peculiar stars. We confirm the results obtained in the previous paper for which phase relations between light, spectral and magnetic variations are not dependent on stellar spectral type or peculiarity subclass.     

Magnetic perturbations to the acoustic modes of roAp stars

We study the effect of a large-scale surface magnetic field on the non-radial acoustic modes of roAp stars. Special attention is given to the use of a variational principle which is used for determining the shifts in the frequencies with relative ease, enabling us to avoid having to calculate the perturbed eigenfunctions. With knowledge of the frequency shifts we then estimate the eigenfunctions in a simpler, albeit approximate way. The results indicate frequency shifts of the order of few μHz, which depend on the order, degree and azimuthal order of the mode. The loss of energy through Alfvén waves is also estimated from the imaginary parts of the frequency shifts. The results indicate that the loss is particularly high near specific frequencies. This might indicate the presence of a selection effect, which could make some modes more likely to be excited than others. However, our results do not explain why the modes observed appear always to be aligned with the axis of the magnetic field. Finally, the estimated perturbed eigenfunctions contain strong components of spherical harmonics that differ from those of the original unperturbed modes.     

An elemental abundance analysis of the magnetic chemically peculiar star HR 8216

An elemental abundance analysis of the cool magnetic CP star HR 8216 (= HD204411) was performed using 2.4 Å mm⁻¹ Dominion Astrophysical Observatory Reticon exposures covering  λλ3830–4770  with a typical signal-to-noise ratio of 200 taken with the long camera of the 1.22-m telescope. The spectrograms were measured interactively with the graphics computer program reduce . The fine analysis used an ATLAS9 metal-rich model atmosphere, the predictions of which best-matched the optical region fluxes and the Hγ profile. The anomalies of HR 8216 are primarily an enhancement of many iron peak elements with the rare earths elements much less enhanced than in many similar cool magnetic CP stars. Using the results of the fine analysis the spectrum was synthesized. Comparison with the observed spectrum showed that the agreement is good but did not fully account for all of the observed line spectrum.     

On the excitation mechanism in roAp stars

We investigate a model for the excitation of high-order oscillations in roAp stars. In this model we assume that the strong concentration of magnetic field about the magnetic poles is enough to suppress convection. Thus the model considered is composed of two polar regions, in which convection is presumed to be suppressed totally, and an equatorial region, where the convection is unaffected. This model is generated by building pairs of locally spherically symmetrical equilibria to represent the polar and equatorial regions of the star, which are patched together below the base of the convection zone. Gravitational settling of heavy elements is taken into account by choosing appropriate chemical composition profiles for both the polar and equatorial regions. Our results indicate that the composite model is unstable against axisymmetric non-radial high-order modes of pulsation that are aligned with the magnetic poles. The oscillations are excited by the κ mechanism acting principally in the hydrogen ionization zones of the polar regions. The effect of the lateral inhomogeneity on the second frequency differences is also investigated; we find that the perturbation to them by the inhomogeneity is of the same order as the second differences themselves, thereby hindering potential attempts to use such differences to identify the degrees of the modes in a straightforward way.     

Magnetic fields of Ap stars as a result of the Tayler instability

Ap star magnetism is often attributed to fossil magnetic fields which have not changed much since the pre‐main‐sequence epoch of the stars. Stable magnetic field configurations are known which could persist probably for the entire mainsequence life of the star, but they may not show the complexity and diversity exhibited by the Ap stars observed. We suggest that the Ap star magnetism is not a result of stable configurations, but is the result of an instability based on strong toroidal magnetic fields buried in the stars. The highly nonaxisymmetric remainders of the instability are reminiscent of the diversity of fields seen on Ap stars. The strengths of these remnant magnetic fields are actually between a few per cent up to considerable fractions of the internal toroidal field; this means field strengths of the order of kGauss being compatible with what is observed. The magnetic fields emerge at the surface rather quickly; rough estimates deliver time‐scales of the order of a few years. Since rotation stabilizes the instability, normal A stars may still host considerable, invisible toroidal magnetic fields (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

How to drive roAp stars

Rapidly oscillating Ap stars constitute a unique class of pulsators with which to study non-radial oscillations under some — even for stars — unusual physical conditions. These stars are chemically peculiar, they have strong magnetic fields and they often pulsate in several high-order acoustic modes simultaneously. We discuss here an excitation mechanism for short-period oscillation modes based on the classical κ mechanism. We particularly stress the conditions that must be fulfilled for successful driving. Specifically, we discuss the roles of the chemical peculiarity and strong magnetic field on the oscillation modes and what separates these pulsators from δ Scuti and Am-type stars.     

Variable stars in the Bochum Galactic Disk Survey

We present a first overview of variable stars in the Bochum Galactic Disk Survey (GDS) with emphasis on eclipsing binaries (EBs). This ongoing survey is performed by a robotic twin refractor at the Universitätssternwarte Bochum located near Cerro Armazones in Chile. It comprises a mosaic of 268 fields in a stripe of Δb = ±3° along the Galactic plane observed once per month simultaneously in the Sloan r and i filters with a detection limit of r_s ∼ 16 mag and i_s ∼ 15 mag. The data from the first three years until the end of February 2014 yields a total of 41718 variable stars with variability amplitudes between 0.1–6 mag. A cross‐match with SIMBAD identified 11 465 of these variables unambiguously, while 2184 had multiple matches; most of the remaining stars could be matched with 2MASS objects. Among the SIMBAD‐listed objects with single matches, only 1982 turned out as known variables while a further 256 are suspected of variability. That leaves a total of 39480 potentially new variables. The group of known variables comprises 419 stars (21 %) that are classified as EBs while 443 (22%) are of other types; for the remaining 1120 catalogued variables (57 %) the type is unknown. Investigating variability as a function of spectral type, we find that SIMBAD provides spectral types for 2811 (25 %) of the identified stars. Spectral classes B (26 %), A (20 %), and M (25%) contain the most numerous variables, while all other classes contribute less than 10% each. More than half of the B (55 %) and A (56%) stars are designated as EBs, suggesting that hundreds of new B‐ and A‐type EBs may be contained in the GDS archive. In contrast, among the numerous M stars no EBs are known. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)