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Time-resolved photometric and spectroscopic analysis of the luminous Ap star HD103498
We present the results of the photometric and spectroscopic monitoringof the luminous Ap star HD103498. The time-series photometricobservations were carried out on 17 nights using a three-channel fastphotometer attached to the 1.04-m optical telescope at the AryabhattaResearch Institute of Observational Sciences (ARIES), Nainital. Thephotometric data from five nights in 2007 show a clear signature of15-min periodicity. However, the follow-up observations during 2007-2009did not reproduce any such periodicity. To confirm the photometric lightvariations, time-series spectroscopic observations were carried out withthe 2.56-m Nordic Optical Telescope (NOT) at La Palma on 2009 February2. No radial velocity variations were present in this data set, which isin full agreement with the photometric observations taken around thesame date. Model atmosphere and abundance analysis of HD103498 show thatthe star is evolved from the main sequence and its atmosphericabundances are similar to those of two other evolved Ap stars, HD133792and HD204411: large overabundances of Si, Cr and Fe and moderateoverabundances of the rare-earth elements. These chemical properties anda higher effective temperature distinguish HD103498 from any known roApstar.
| Asteroseismology of chemically peculiar stars
Paper published in the proceedings of the Wroc?aw HELAS Workshop``Interpretation of Asteroseismic Data'', CoAst, 157, 228 (Kochukhov2008).Pulsational variability is observed in several types of main sequencestars with anomalous chemical abundances. In this contribution Isummarize the relationship between pulsations and chemicalpeculiarities, giving special emphasis to rapid oscillations in magneticAp stars. These magneto-acoustic pulsators provide unique opportunitiesto study the interaction of pulsations, chemical inhomogeneities, andstrong magnetic fields. Time-series monitoring of rapidly oscillating Apstars using high-resolution spectrometers at large telescopes andultra-precise space photometry has led to a number of importantbreakthroughs in our understanding of these interesting objects.Interpretation of the roAp frequency spectra has allowed constrainingfundamental stellar parameters and probing poorly known properties ofthe stellar interiors. At the same time, investigation of thepulsational wave propagation in chemically stratified atmospheres ofroAp stars has been used as a novel asteroseismic tool to studypulsations as a function of atmospheric height and to map in detail thehorizontal structure of the magnetically-distorted p modes.
| Discovery of very low amplitude 9-minute multiperiodic pulsations in the magnetic Ap star HD 75445
We present our discovery of pulsational radial-velocity variations inthe cool Ap star HD 75445, an object spectroscopically similar to thebright, rapidly-oscillating Ap (roAp) star ? Equ. Based onhigh-resolution time-series spectroscopy obtained with the HARPSspectrometer at the European Southern Observatory 3.6-m telescope, wedetected oscillations in Nd II and Nd III lines with a period close to 9min and amplitudes of 20-30 m s-1. Substantial variation inthe pulsational amplitude during our 3.8 h observing run reveals thepresence of at least three excited non-radial modes. The detection ofextremely low amplitude pulsations in HD 75445 indicates that the roApexcitation mechanism produces variability in the radial velocityamplitude of between a few tens m s-1 and several kms-1. This supports the idea that many, if not all, cool Apstars occupying the roAp instability strip may harbour non-radialpulsations, which currently remain undetected due to their smallphotometric and radial-velocity amplitudes.Based on observations collected at the European Southern Observatory,Chile (ESO programs 68.D-0254, 079.D-0118).
| A search for rapid pulsations in the magnetic cool chemically peculiar star HD3980
The Ap star HD3980 appears to be a promising roAp candidate based on itsfundamental parameters, leading us to search for rapid pulsations withthe VLT UV-Visual Echelle Spectrograph (UVES). A precise Hipparcosparallax and estimated temperature of 8100K place HD3980 in the middleof the theoretical instability strip for rapidly oscillating Ap stars,about halfway through its main-sequence evolution stage. The star has astrong, variable magnetic field, as is typical of the cool magnetic Apstars. Dipole model parameters were determined from VLT observationsusing Focal Reducer and low Dispersion Spectrograph (FORS)1. FromDoppler shift measurements for individual spectral lines of rare-earthelements and the H? line core, we find no pulsations above20-30ms-1. This result is corroborated by the inspection oflines of several other chemical elements, as well as withcross-correlation for long spectral regions with the average spectrum asa template. Abundances of chemical elements were determined and showlarger than solar abundances of rare-earth elements. Further, ionizationdisequilibria for the first two ionized states of Nd and Pr aredetected. We also find that the star has a strong overabundance ofmanganese, which is typical for much hotter HgMn and other Bp stars.Line profile variability with the rotation period was detected for themajority of chemical species.Based on observations collected at the European Southern Observatory(ESO), Paranal, Chile, as part of programme 077.D-0150(A) and part ofprogrammes 074.D-0392(A) and 076.D-0535(A) in the ESO archive.E-mail: velkin@uclan.ac.uk
| Chemically peculiar stars and their temperature calibration
Aims. The determination of effective temperature for chemically peculiar(CP) stars by means of photometry is a sophisticated task due to theirabnormal colours. Standard calibrations for normal stars lead toerroneous results and, in most cases corrections are necessary. Methods: In order to specify appropriate corrections, direct temperaturedeterminations for 176 objects of the different subgroups were collectedfrom the literature. This much larger sample than in previous studiestherefore allows a more accurate investigation, mostly based on averagetemperatures. Results: For the three main photometric systems (UBV,Geneva, Strömgren {uvby}β), methods to determine effectivetemperature are presented together with a comparison with formerresults. Based on the compiled data we provide evidence that He (CP4)objects also need a considerable correction, not noticed in formerinvestigations due to their small number. Additionally, a new relationfor the bolometric correction and the capability of standardcalibrations to deduce interstellar reddening for magnetic CP stars areshown.Tables 4 to 8 are only available in electronic form at http://www.aanda.org
| Abundance analysis and searching for nonradial pulsations in the atmosphere of the chemically peculiar star HD 115708
We present the results of our abundance analysis for the magneticchemically peculiar star HD 115708 based on high-resolution spectra. Theatmospheric chemical composition of HD 115708 ( T eff = 7550K) is shown to be typical of cool Ap stars with a significant ionizationdisequilibrium for the first and second rare earth ions, which iscommonly observed in the atmospheres of pulsating Ap (roAp) stars. Ourstudy of the vertical distribution of elements has shown that Mg, Ca,Cr, and Fe concentrate in deeper atmospheric layers, with theirabundances decreasing sharply in the upper layers. The jumps inabundance are 1.5 3 orders of magnitude. Silicon is distributed in depthalmost uniformly in the atmosphere of HD 115708. The derived empiricalCr and Fe distributions agree qualitatively with the results ofdiffusion calculations. Since the atmospheric chemical peculiarities inHD 115708 correspond to roAp stars, we have performed a spectroscopicmonitoring to find nonradial pulsations. We have been able to determineonly an upper limit for the amplitude of the possible radial velocitypulsations, ˜100 m s?1, due to the insufficienttemporal resolution and instability of the main stellar spectrograph(MSS) of the 6-m telescope.
| Magnetic fields of chemically peculiar stars. I. The catalog of magnetic CP stars
This is the first paper of the series dedicated to the analysis of themagnetism of chemically peculiar (CP) stars of the upper Main Sequence.We use our own measurements and published data to compile a catalog ofmagnetic CP stars containing a total of 326 objects with confidentlydetected magnetic fields and 29 stars which are very likely to possessmagnetic field. We obtained the data on the magnetism of theoverwhelming majority of the stars solely based on the analysis oflongitudinal field component B e . The surface magneticfield, B s , has been measured for 49 objects. Our analysisshows that the number of magnetic CP stars decreases with increasingfield strength in accordance with exponential law, and stars with B e exceeding 5kG occur rarely (about 3% objects of ourlist).
| The Paschen-Back effect in the Li I 6708 Å line and the presence of lithium in cool magnetic Ap stars
Context: A number of cool magnetic Ap stars show a prominent feature atλ 6708 Å. Its identification with Li I remainscontroversial due to the lack of knowledge of the spectra of rare-earthelements that are strongly enhanced in peculiar stars so they canpotentially provide an alternative identification. Aims: We suggestinvestigating the 6708 Å line in Ap stars with strong magneticfields. In these objects, the magnetic broadening and splitting providesan additional, powerful criterium for line identification, allowing thewhole line profile to be used instead of depending on a mere coincidencein the observed and predicted wavelengths. Methods: The smallseparation of the Li I doublet components means that their magneticsplitting pattern deviates from the one expected for the Zeeman effect,even in relatively weak fields. We carried out detailed calculations ofthe transition between the Zeeman and Paschen-Back regimes in themagnetic splitting of the Li I line and computed polarised syntheticspectra for the range of field strength expected in Ap stars.Theoretical spectral synthesis is compared with the high-resolutionobservations of cool Ap stars HD 116114, HD 166473, and HD 154708, whichhave a mean field strength of 6.4, 8.6, and 24.5 kG, respectively, andshow a strong 6708 Å line. Results: High-resolution spectra forthe 6708 Å region were analysed for 17 magnetic Ap stars. Thepresence of the 6708 Å line is confirmed for 9 stars and reportedfor the first time for 6 stars. The strength of the Li I doublet doesnot correlate with the absorption features of any other element. Thestars HD 75445 and HD 201601 provide an extreme example of the twoobjects, which are dissimilar with respect to the 6708 Å line, butvery close in the atmospheric parameters and abundances of otherelements. We demonstrate that the observed profiles of the 6708 Åline in the strong field stars HD 116114, HD 166473, and HD 154708correspond fairly well to the theoretical calculations when assuming theLi I identification. Including the Paschen-Back effect improves theagreement with observations, especially for HD 154708. Conclusions:Results of our study confirm the Li I identification proposed for the6708 Å line in cool Ap stars.Based on observations collected at the European Southern Observatory,Paranal, Chile (ESO program 68.D-0254 and programs 072.D-0138,077.D-0150 retrieved through the ESO Archive).
| Isotopic anomaly and stratification of Ca in magnetic Ap stars
Aims: We have completed an accurate investigation of the Ca isotopiccomposition and stratification in the atmospheres of 23 magneticchemically peculiar (Ap) stars of different temperature and magneticfield strength. Methods: With the UVES spectrograph at the 8 mESO VLT, we have obtained high-resolution spectra of Ap stars in thewavelength range 3000-10 000 Å. Using a detailed spectrumsynthesis calculations, we have reproduced a variety of Ca lines in theoptical and ultraviolet spectral regions, inferring the overall verticaldistribution of Ca abundance, and have deduced the relative isotopiccomposition and its dependence on height using the profile of theIR-triplet Ca II line at ?8498 Å. Results: In 22out of 23 studied stars, we found that Ca is strongly stratified, beingusually overabundant by 1.0-1.5 dex below log?5000?-1, and strongly depleted above log?5000=-1.5. TheIR-triplet Ca II line at ?8498 Å reveals a significantcontribution of the heavy isotopes 46Ca and 48Ca,which represent less than 1 % of the terrestrial Ca isotopic mixture. Weconfirm our previous finding that the presence of heavy Ca isotopes isgenerally anticorrelated with the magnetic field strength. Moreover, wediscover that in Ap stars with relatively small surface magnetic fields(?4-5 kG), the light isotope 40Ca is concentrated close tothe photosphere, while the heavy isotopes are dominant in the outeratmospheric layers. This vertical isotopic separation, observed for thefirst time for any metal in a stellar atmosphere, disappears in starswith magnetic field strength above 6-7 kG. Conclusions: Wesuggest that the overall Ca stratification and depth-dependent isotopicanomaly observed in Ap stars may be attributed to a combined action ofthe radiatively-driven diffusion and light-induced drift.Based on observations collected at the European Southern Observatory,Paranal, Chile (ESO program No. 68.D-0254).
| The discovery of high-amplitude, 10.9-minute oscillations in the cool magnetic Ap star HD 115226
We present the discovery of pulsational variations in the cool magneticAp star HD 115226 - the first high-amplitude rapidly oscillating Ap(roAp) star discovered with time-series spectroscopy. Usinghigh-resolution spectra obtained with the HARPS instrument at theEuropean Southern Observatory 3.6-m telescope, we detect radial velocityvariations with a period of 10.86 min in Pr III, Nd III, Dy III lines,and in the narrow cores of hydrogen lines. Pulsational amplitudes exceed1 km s-1 in individual lines of Nd III. The presence ofrunning waves in the stellar atmosphere is inferred from a phase shiftbetween the radial velocity maxima of rare-earth and hydrogen lines. Ourabundance analysis demonstrates that HD 115226 exhibits a typical roApspectroscopic signature, notably ionization anomaly of Pr, Nd, and Dy.We discuss the discovery of pulsations in HD 115226 in the context ofrecent spectroscopic studies of roAp stars and point to the existence ofa correlation between spectroscopic pulsational amplitude and thestellar rotation rate.Based on observations collected at the European Southern Observatory, LaSilla, Chile (ESO program 079.D-0118).
| Magnetic-field dependence of chemical anomalies in CP stars
The dependence of the degree of anomaly of parameter Z of Genevaphotometry ( Z0 = Z CP ℒ Z norm.) on theaverage surface magnetic field Bs is analyzed. The Z0 value isproportional to the degree of anomaly of chemical composition. It wasfound that Bs → 0 corresponds Z0 → ‑0.010÷‑0.015, i.e., part of CP stars are virtually devoid of magneticfield, but exhibit chemical anomalies. This effect may be due toselection whereby only objects with strong chemical anomalies areclassified as CP stars, thereby producing a deficit of stars withrelatively weak anomalies. Moreover, CP stars have other sources ofstabilization of their atmospheres besides the magnetic field, e.g.,slow rotation. Formulas relating Z0 to Bs are derived.
| Heavy calcium in CP stars
Large wavelength shifts of infrared triplet lines of CaII have beenobserved in the spectra of HgMn and magnetic Ap stars. They have beenattributed to the heavy calcium isotopes, including 48Ca. Onemember of the triplet, ?8542, had been either unavailable, or ofpoor quality in earlier spectra. The present material shows conclusivelythat the stellar ?8542 shifts are consistent with aninterpretation in terms of 48Ca. We find no relation betweenisotopic shifts of the CaII triplet lines, and those of HgII?3984. There is a marginal indication that the shifts areanticorrelated with the surface field strengths of the magnetic stars.We see sparse evidence for 48Ca in other chemically peculiarstars, for example, Am stars, metal-poor stars or chemically peculiarred giants. However, the sample is still very small, and the wavelengthsof all three triplet lines, including those in the Sun, show slightpositive shifts with respect to terrestrial positions.Some profiles of the CaII infrared triplet in the magnetic stars showextensive wings beyond a well-defined core. We can obtain reasonablefits to these profiles using a stratified calcium abundance similar tothat used by previous workers. There is no indication that either thestratification or the Zeeman effect significantly disturbs themeasurement of isotope shifts.Based on observations obtained at the European Southern Observatory,Paranal and La Silla, Chile [ESO programmes 076.D-0169(A) and076.C-0172(A)].E-mail: cowley@umich.edu (CRC); shubrig@eso.org (SH);castelli@ts.astro.it (FC); fgonzalez@casleo.gov.ar (JFG); bwolff@eso.org(BW)
| Pulkovo compilation of radial velocities for 35495 stars in a common system.
Not Available
| Evolutionary state of magnetic chemically peculiar stars
Context: .The photospheres of about 5-10% of the upper main sequencestars exhibit remarkable chemical anomalies. Many of these chemicallypeculiar (CP) stars have a global magnetic field, the origin of which isstill a matter of debate. Aims: .We present a comprehensivestatistical investigation of the evolution of magnetic CP stars, aimedat providing constraints to the theories that deal with the origin ofthe magnetic field in these stars. Methods: .We have collectedfrom the literature data for 150 magnetic CP stars with accurateHipparcos parallaxes. We have retrieved from the ESO archive 142 FORS1observations of circularly polarized spectra for 100 stars. From thesespectra we have measured the mean longitudinal magnetic field, anddiscovered 48 new magnetic CP stars (five of which belonging to the rareclass of rapidly oscillating Ap stars). We have determined effectivetemperature and luminosity, then mass and position in the H-R diagramfor a final sample of 194 magnetic CP stars. Results: .We foundthat magnetic stars with M > 3 ~M_ȯ are homogeneouslydistributed along the main sequence. Instead, there are statisticalindications that lower mass stars (especially those with M ≤2~M_ȯ) tend to concentrate in the centre of the main sequence band.We show that this inhomogeneous age distribution cannot be attributed tothe effects of random errors and small number statistics. Our datasuggest also that the surface magnetic flux of CP stars increases withstellar age and mass, and correlates with the rotation period. For starswith M > 3~M_ȯ, rotation periods decrease with age in a wayconsistent with the conservation of the angular momentum, while for lessmassive magnetic CP stars an angular momentum loss cannot be ruledout. Conclusions: .The mechanism that originates and sustains themagnetic field in the upper main sequence stars may be different in CPstars of different mass.
| Temperature Behavior of Elemental Abundances in the Atmospheres of Magnetic Peculiar Stars
We analyze the temperature dependence of the abundances of the chemicalelements Si, Ca, Cr, and Fe in the atmospheres of normal, metallic-line(Am), magnetic peculiar (Ap), and pulsating magnetic peculiar (roAp)stars in the range 6000 15000 K. The Cr and Fe abundances in theatmospheres of Ap stars increase rapidly as the temperature rises from6000 to 9000 10000 K. Subsequently, the Cr abundance decreases to valuesthat exceed the solar abundance by an order of magnitude, while the Feabundance remains enhanced by approximately +1.0 dex compared to thesolar value. The temperature dependence of the abundances of theseelements in the atmospheres of normal and Am stars is similar in shape,but its maximum is several orders of magnitude lower than that observedfor Ap stars. In the range 6000 9500 K, the observed temperaturedependences for Ap stars are satisfactorily described in terms ofelement diffusion under the combined action of gravitational settlingand radiative acceleration. It may well be that diffusion also takesplace in the atmospheres of normal stars, but its efficiency is very lowdue to the presence of microturbulence. We show that the magnetic fieldhas virtually no effect on the Cr and Fe diffusion in Ap stars in therange of effective temperatures 6000 9500 K. The Ca abundance and itsvariation in the atmospheres of Ap stars can also be explained in termsof the diffusion model if we assume the existence of a stellar wind witha variable moderate rate of (2 4) × 10- 15 M ȯ yr-1.
| The calcium isotopic anomaly in magnetic CP stars
Chemically peculiar stars in the magnetic sequence can show the sameisotopic anomaly in calcium previously discovered for mercury-manganesestars in the non-magnetic sequence. In extreme cases, the dominantisotope is the exotic 48Ca. Measurements of Ca II linesarising from 3d-4p transitions reveal the anomaly by showing shifts upto 0.2 Å for the extreme cases - too large to be measurementerrors. We report measurements of miscellaneous objects, including twometal-poor stars, two apparently normal F-stars, an Am-star, and theN-star U Ant. Demonstrable anomalies are apparent only for the Ap stars.The largest shifts are found in rapidly oscillating Ap stars and in oneweakly magnetic Ap star, HD 133792. We note the possible relevance ofthese shifts for the GAIA mission.Based on observations obtained at the European Southern Observatory, LaSilla and Paranal, Chile (ESO programme Nos. 65.L-0316, 68.D-0254 and266.D-5655).
| The spectroscopic signature of roAp stars
To reliably determine the spectroscopic signature of rapidly oscillatingchemically peculiar (roAp) stars it is also necessary to investigate asample of non pulsating chemically peculiar (noAp) as well as presumably``normal'' stars. We describe in this study the sample ofspectroscopically investigated stars and comment on the techniques usedfor the analysis. In particular we discuss ionization disequilibria ofrare earths in roAp stars that distinguish them from noAp stars. In thelight of the recently discovered pulsation of β CrB we seearguments that all magnetic CP2 stars up to a transition temperature ofabout 8100 K may be pulsating.Based on observations obtained at the European Southern Observatory (LaSilla, Chile), the Canadian-French-Hawaii telescope, the South AfricaAstronomical Observatory, The Crimean Astrophysical Observatory and onnumerous SIMBAD interrogations.
| A statistical analysis of the magnetic structure of CP stars
We present the results of a statistical study of the magnetic structureof upper main sequence chemically peculiar stars. We have modelled asample of 34 stars, assuming that the magnetic morphology is describedby the superposition of a dipole and a quadrupole field, arbitrarilyoriented. In order to interpret the modelling results, we haveintroduced a novel set of angles that provides one with a convenient wayto represent the mutual orientation of the quadrupolar component, thedipolar component, and the rotation axis. Some of our results aresimilar to what has already been found in previous studies, e.g., thatthe inclination of the dipole axis to the rotation axis is usually largefor short-period stars and small for long-period ones - see Landstreet& Mathys (\cite{Landstreet2000}). We also found that forshort-period stars (approximately P<10 days) the plane containing thetwo unit vectors that characterise the quadrupole is almost coincidentwith the plane containing the stellar rotation axis and the dipole axis.Long-period stars seem to be preferentially characterised by aquadrupole orientation such that the planes just mentioned areperpendicular. There is also some loose indication of a continuoustransition between the two classes of stars with increasing rotationalperiod.
| Magnetic AP Stars in the Hertzsprung-Russell Diagram
The evolutionary state of magnetic Ap stars is rediscussed using therecently released Hipparcos data. The distribution of the magnetic Apstars of mass below 3 Msolar in the H-R diagram differs fromthat of the normal stars in the same temperature range at a high levelof significance. Magnetic stars are concentrated toward the center ofthe main-sequence band. This is shown in two forms of the H-R diagram:one where logL is plotted against logTeff and a version moredirectly tied to the observed quantities, showing the astrometry-basedluminosity (Arenou & Luri) against the (B2-G)0 index ofGeneva photometry. In particular, it is found that magnetic fieldsappear only in stars that have already completed at least approximately30% of their main-sequence lifetime. No clear picture emerges as to thepossible evolution of the magnetic field across the main sequence. Hintsof some (loose) relations between magnetic field strength and otherstellar parameters are found: stars with shorter periods tend to havestronger fields, as do higher temperature and higher mass stars. Amarginal trend of the magnetic flux to be lower in more slowly rotatingstars may possibly be seen as suggesting a dynamo origin for the field.No correlation between the rotation period and the fraction of themain-sequence lifetime completed is observed, indicating that the slowrotation in these stars must already have been achieved before theybecame observably magnetic. Based on data from the ESA Hipparcossatellite and on observations collected at the European SouthernObservatory (La Silla, Chile; ESO programs Nos. 43.7-004, 44.7-012,49.7-030, 50.7-067, 51.7-041, 52.7-063, 53.7-028, 54.E-0416, and55.E-0751), at the Observatoire de Haute-Provence (Saint-Michell'Observatoire, France), at Kitt Peak National Observatory, and at theCanada-France-Hawaii Telescope.
| Hβ photometry of southern CP2 stars: is the uvbybeta luminosity calibration also valid for peculiar stars?
We present Hβ photometry of 233 southern CP2 stars (covering themagnetic Ap stars according to the definition by Preston 1974) brighterthan V < 8.5 mag from the list of Bidelman & MacConnell (1973).Absolute magnitudes derived from this photometry together with alreadyexisting uvby photometry is confronted with Hipparcos results availablefor a common subset of 152 stars. In order to compare peculiar withnormal stars, we identified a sample of 1147 normal B to F-type starsusing their published uvbybeta and Hipparcos data. For our analysis wedivide both samples into three temperature as well as two Hipparcosparallax accuracy groups. The error distribution of both samples provedto be statistically comparable. As a result the absolute magnitudes forthe B-type CP2 stars show up to be significantly too bright by anaverage of 0.5 mag using the actual photometric calibration. On theother hand, the photometric absolute magnitudes for cool A to F-type CP2stars are up to three magnitudes fainter as compared to Hipparcos. Basedon observations at ESO-La Silla and with the Hipparcos satellite
| The HR-diagram from HIPPARCOS data. Absolute magnitudes and kinematics of BP - AP stars
The HR-diagram of about 1000 Bp - Ap stars in the solar neighbourhoodhas been constructed using astrometric data from Hipparcos satellite aswell as photometric and radial velocity data. The LM method\cite{luri95,luri96} allows the use of proper motion and radial velocitydata in addition to the trigonometric parallaxes to obtain luminositycalibrations and improved distances estimates. Six types of Bp - Apstars have been examined: He-rich, He-weak, HgMn, Si, Si+ and SrCrEu.Most Bp - Ap stars lie on the main sequence occupying the whole width ofit (about 2 mag), just like normal stars in the same range of spectraltypes. Their kinematic behaviour is typical of thin disk stars youngerthan about 1 Gyr. A few stars found to be high above the galactic planeor to have a high velocity are briefly discussed. Based on data from theESA Hipparcos astrometry satellite and photometric data collected in theGeneva system at ESO, La Silla (Chile) and at Jungfraujoch andGornergrat Observatories (Switzerland). Tables 3 and 4 are onlyavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| CP2 stars as viewed by the UVBY H_beta system
The aim of this work is to study the capacity of the uvby H_βsystem for detecting the chemically peculiar (CP) stars based on theeffect that peculiar features in the flux distribution have on all theStromgren-Crawford indices. Our study focuses on the classical magneticpeculiar stars (CP2), though Am stars (CP1) are also included forcomparison with cool CP2 stars. Satisfactory results were obtained forhot CP2 stars: the definition of a new index p, which is a linearcombination of uvby H_β colours, allowed us to separate a highpercentage of hot CP2 stars from normal stars. According to this newindex, 60 new CP2 candidates are proposed. The working sample wasextracted from The General Catalogue of Ap and Am stars by \cite[Rensonet al. (1991)]{ren91}. Photometric observations to enlarge the sample ofCP2 stars with complete uvby H_β photometry were carried out. Theseobservations are also reported in the present paper. The new index p isalso used to correct the reddening of early CP2 stars computed as ifthey were normal stars. Tables 2, 3 and 7 are also available inelectronic form from CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.128.5) or via http://cdsweb.u-strasbg.fr/Abstract.html
| The mean magnetic field modulus of AP stars
We present new measurements of the mean magnetic field modulus of asample of Ap stars with spectral lines resolved into magnetically splitcomponents. We report the discovery of 16 new stars having thisproperty. This brings the total number of such stars known to 42. Wehave performed more than 750 measurements of the mean field modulus of40 of these 42 stars, between May 1988 and August 1995. The best of themhave an estimated accuracy of 25 - 30 G. The availability of such alarge number of measurements allows us to discuss for the first time thedistribution of the field modulus intensities. A most intriguing resultis the apparent existence of a sharp cutoff at the low end of thisdistribution, since no star with a field modulus (averaged over therotation period) smaller than 2.8 kG has been found in this study. Formore than one third of the studied stars, enough field determinationswell distributed throughout the stellar rotation cycle have beenachieved to allow us to characterize at least to some extent thevariations of the field modulus. These variations are oftensignificantly anharmonic, and it is not unusual for their extrema not tocoincide in phase with the extrema of the longitudinal field (for thefew stars for which enough data exist about the latter). This, togetherwith considerations on the distribution of the relative amplitude ofvariation of the studied stars, supports the recently emerging evidencefor markedly non-dipolar geometry and fine structure of the magneticfields of most Ap stars. New or improved determinations of the rotationperiods of 9 Ap stars have been achieved from the analysis of thevariations of their mean magnetic field modulus. Tentative values of theperiod have been derived for 5 additional stars, and lower limits havebeen established for 10 stars. The shortest definite rotation period ofan Ap star with magnetically resolved lines is 3.4 deg, while thosestars that rotate slowest appear to have periods in excess of 70 or 75years. As a result of this study, the number of known Ap stars withrotation periods longer than 30 days is almost doubled. We brieflyrediscuss the slow-rotation tail of the period distribution of Ap stars.This study also yielded the discovery of radial velocity variations in 8stars. There seems to be a deficiency of binaries with short orbitalperiods among Ap stars with magnetically resolved lines. Based onobservations collected at the European Southern Observatory (La Silla,Chile; ESO programmes Nos. 43.7-004, 44.7-012, 49.7-030, 50.7-067,51.7-041, 52.7-063, 53.7-028, 54.E-0416, and 55.E-0751), at theObservatoire de Haute-Provence (Saint-Michel-l'Observatoire, France), atKitt Peak National Observatory, and at the Canada-France-HawaiiTelescope. Tables 2, 3, and 4 are also available in electronic form atthe CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html.
| Radial velocities and axial rotation for a sample of chemically peculiar stars.
As part of a systematic project we have determined radial velocities andprojected rotational velocities for a sample of 186 chemically peculiarstars which have been observed by the Hipparcos' satellite. The purposeis to provide necessary data to study the space velocities of peculiarstars.
| Hyades and Sirius supercluster members brighter than magnitude (V) 7.1. II - Right ascension six to twelve hours
The present star sample is contained in the Bright Star Catalogue andits Supplement, augmented with a further supplement of 788 stars foundduring various observing programs over the past 40 years. Accurate,four-color and H-beta, or (RI), photometry is available for most of thesupercluster members. The criteria for membership are the comparisons ofthe proper motion, radial velocity, and luminosity obtained from thesupercluster parameters with the observed motions and the luminosityderived from the photometric parameters. New proper motions, based onall available catalogs, have been derived for the additional 788 starsdiscussed here, as well as all supercluster members.
| Photoelectric photometry of peculiar and related stars. II Delta-a-photometry of 339 southern Ap-stars
Delta-a system photometry of the sample of Ap stars from Bidelman andMcConnell's (1973) catalogue is discussed. Comparing delta-ameasurements in four marginally different systems, slight modificationsof the delta-a photometric system are found to be uncritical except forthe wavelength of the depression filter g2. The reddening-free index isvery efficient for silicon and chromium stars, and the spectroscopicdetection probability for these stars decreases with decreasingtemperature. Comparison of delta-a results with the Geneva index delta(V1-G) confirms that the latter's efficiency in recognizing peculiaritydrops strongly from hot to cool Ap stars. A decrease of delta-a towardsthe hot Ap stars is found using the a vs. (u-b) diagram, implying thatb-v is an unsuitable temperature indicator for such stars. The lowaverage intrinsic variability of the delta-a in the sample, whencompared with the small variation of integrated surface magnetic fieldsin case of dipole configuration, implies that the wavelength 5200feature correlates with the surface magnetic field strength.
| Photoelectric observations of peculiar A and related stars. I - Stromgren photometry of 341 AP stars
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1979A&AS...36..477V&db_key=AST
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