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The diagnosis of the mean quadratic magnetic field of Ap stars
Aims.We assess the validity of the method of determination of the meanquadratic field modulus and we explore its limits. Methods.Weanalyse high spectral resolution, high signal-to-noise spectra of a fewAp stars, and of a superficially normal main-sequence A star, recordedover a broad wavelength range with EMMI at the NTT.Results.Weintroduce a revised form of the regression equation describing thedependence of the second-order moment of the line profiles about theircentre, in natural light, on various parameters of the correspondingtransitions. We show that interpretation of the observed dependencesallows one to determine the mean quadratic magnetic field modulus of thestudied stars, and their v sin i. We explain why the contributions tothe quadratic field of the mean square magnetic field modulus and of themean square longitudinal field cannot in general be disentangled. Forthose stars of the sample that have resolved magnetically split lines,we show that the derived values of the quadratic field are mostlyconsistent with the values of the mean longitudinal magnetic field andof the mean magnetic field modulus at the observed phase. However thereare some hints that they may occasionally slightly underestimate theactual field. This suggests that the method is unlikely to yieldspurious field detections. In addition, we illustrate the importance forthis type of analyses of using, as far as possible, samples of lines ofa single ion, and to specify in the presentation of the results whichion was used.Conclusions.The results presented in this paper lendstrong support to the validity of the quadratic field diagnostic methodto obtain a realistic quantitative characterisation of the magneticfields Ap and related stars.
| Radiative lifetimes, branching fractions and oscillator strengths in Pd I and the solar palladium abundance
Transition probabilities have been derived for 20 5s-5p transitions ofPd I from a combination of radiative lifetime measurements for 6odd-parity levels with time-resolved laser-induced fluorescencespectroscopy and of branching fraction determination using a hollowcathode discharge lamp. Additional oscillator strengths for 18transitions have been determined from measured lifetimes and theoreticalbranching fractions obtained from configuration interaction calculationswith core-polarization effects included. These new results have allowedus to refine the palladium abundance in the solar photosphere: A_Pd =1.66 ± 0.04, in the usual logarithmic scale, a result in closeagreement with the meteoritic value.
| Transition probabilities and lifetimes in singly ionized rhenium
Transition probabilities and radiative lifetimes have been obtained for45 transitions of ReII using a combination of radiative lifetime valuesobtained with the laser-induced fluorescence technique (seven levels)and theoretical branching fractions. This new set of results, concerningultraviolet and visible lines, will be useful in the future forinvestigating the composition of chemically peculiar stars.
| Statistical Constraints for Astrometric Binaries with Nonlinear Motion
Useful constraints on the orbits and mass ratios of astrometric binariesin the Hipparcos catalog are derived from the measured proper motiondifferences of Hipparcos and Tycho-2 (Δμ), accelerations ofproper motions (μ˙), and second derivatives of proper motions(μ̈). It is shown how, in some cases, statistical bounds can beestimated for the masses of the secondary components. Two catalogs ofastrometric binaries are generated, one of binaries with significantproper motion differences and the other of binaries with significantaccelerations of their proper motions. Mathematical relations betweenthe astrometric observables Δμ, μ˙, and μ̈ andthe orbital elements are derived in the appendices. We find a remarkabledifference between the distribution of spectral types of stars withlarge accelerations but small proper motion differences and that ofstars with large proper motion differences but insignificantaccelerations. The spectral type distribution for the former sample ofbinaries is the same as the general distribution of all stars in theHipparcos catalog, whereas the latter sample is clearly dominated bysolar-type stars, with an obvious dearth of blue stars. We point outthat the latter set includes mostly binaries with long periods (longerthan about 6 yr).
| The age of the Galactic thin disk from Th/Eu nucleocosmochronology. I. Determination of [Th/Eu] abundance ratios
The purpose of this work is to resume investigation of Galactic thindisk dating using nucleocosmochronology with Th/Eu stellar abundanceratios, a theme absent from the literature since 1990. A stellar sampleof 20 disk dwarfs/subgiants of F5 to G8 spectral types with-0.8≤[Fe/H]≤+0.3 was selected. In stars with such spectral typesand luminosity classes, spectral synthesis techniques must be employedif we wish to achieve acceptably accurate results. An homogeneous,self-consistent set of atmospheric parameters was determined. Effectivetemperatures were determined from photometric calibrations and Hαprofile fitting; surface gravities were obtained from Teff,stellar masses and luminosities; microturbulence velocities andmetallicities were obtained from detailed, differential spectroscopicanalysis, relative to the Sun, using equivalent widths of Fe I and Fe IIlines. Chemical abundances of the elements that contaminate the Th andEu spectral regions (Ti, V, Cr, Mn, Co, Ni, Ce, Nd, and Sm) weredetermined through spectroscopic analysis. Abundance uncertainties werethoroughly scrutinised, their average value (0.10±0.02) dex being found to be satisfactorily low. Eu and Th abundances weredetermined by spectral synthesis of one Eu II line (4129.72 Å) andone Th II line (4019.13 Å), taking into account the detailedhyperfine structures of contaminating Co lines, as well as the hyperfinestructure and isotope shift of the Eu line. Comparison of our abundanceswith literature data shows that our results exhibit a similar behaviour,but a considerably lower scatter (36% lower for Eu, and 61% lower forTh). The [Th/Eu] abundance ratios thus obtained were used, in the secondpaper of this series, to determine the age of the Galactic disk.
| A catalog of stellar magnetic rotational phase curves
Magnetized stars usually exhibit periodic variations of the effective(longitudinal) magnetic field Be caused by their rotation. Wepresent a catalog of magnetic rotational phase curves, Be vs.the rotational phase φ, and tables of their parameters for 136stars on the main sequence and above it. Phase curves were obtained bythe least squares fitting of sine wave or double wave functions to theavailable Be measurements, which were compiled from theexisting literature. Most of the catalogued objects are chemicallypeculiar A and B type stars (127 stars). For some stars we also improvedor determined periods of their rotation. We discuss the distribution ofparameters describing magnetic rotational phase curves in our sample.All tables and Appendix A are only available in electronic form athttp://www.edpsciences.org
| On the possible presence of promethium in the spectra of HD 101065 (Przybylski's star) and HD 965
% Traditional and statistical line-identification methods indicate thepresence of Pm I and II, Tc I, and perhaps Tc II in the spectrum of theroAp star HD 101065. These methods also lead to the presence of Pm IIand probably also Pm I in a related cool Ap star, HD 965. Thespectroscopic evidence is strong enough that we would declare promethiumto be present without hesitation, if any of its isotopes were stable.The longest-lived promethium isotope has a half-life of only 17.7 years.The presence of this element would mean that unrecognized processes -perhaps flare activities - are taking place in the atmospheres of thesestars. The significance of such processes for galactic chemicalevolution cannot be ruled out. We discuss the possibility that thehighly improbable wavelength coincidences are due to chance, or due tocontamination of the laboratory sources.Based on observations collected at the European Southern Observatory, LaSilla (Chile), as part of programme No. 60.E-0564, and Paranal (Chile),as part of programmes Nos. 68.D-0254 and 70.D-0470.
| Radiative lifetime, oscillator strength and Landé factor calculations in doubly ionized europium (Eu III)
A set of transition probabilities has been calculated for Eu IIItransitions of astrophysical interest particularly for the study ofchemically peculiar stars. They were obtained taking configurationinteraction and core-polarization effects into account. The accuracy ofthe new scale of oscillator strengths, which differs substantially fromprevious results, has been assessed through comparisons with recenttime-resolved laser-induced fluorescence measurements of radiativelifetimes.
| Catalogue of averaged stellar effective magnetic fields. I. Chemically peculiar A and B type stars
This paper presents the catalogue and the method of determination ofaveraged quadratic effective magnetic fields < B_e > for 596 mainsequence and giant stars. The catalogue is based on measurements of thestellar effective (or mean longitudinal) magnetic field strengths B_e,which were compiled from the existing literature.We analysed the properties of 352 chemically peculiar A and B stars inthe catalogue, including Am, ApSi, He-weak, He-rich, HgMn, ApSrCrEu, andall ApSr type stars. We have found that the number distribution of allchemically peculiar (CP) stars vs. averaged magnetic field strength isdescribed by a decreasing exponential function. Relations of this typehold also for stars of all the analysed subclasses of chemicalpeculiarity. The exponential form of the above distribution function canbreak down below about 100 G, the latter value representingapproximately the resolution of our analysis for A type stars.Table A.1 and its references are only available in electronic form atthe CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/407/631 and Tables 3 to 9are only available in electronic form at http://www.edpsciences.org
| Radiative lifetime and oscillator strength determinations in Sm III
Radiative lifetimes of 6 levels belonging to the4f5(6Hdeg,6Fdeg)5d configuration of Sm III have beenmeasured for the first time using a time-resolved laser-inducedfluorescence technique. Experimental data have been compared withsemi-empirical calculations. The agreement is excellent for 4 levelswith 6Fdeg as a parent term. Larger discrepancies(53 and 37%) are observed for 2 levels with 6Hdegand 6Fdeg, respectively, as a parent term and areexplainable by strong mixing and cancellation effects occurring in theline strength calculations which are responsible of ``instabilities''occurring in the lifetimes considered as a function of J value withinthe term. Branching ratios for the lines depopulating the levels ofinterest have been measured by Fourier transform spectroscopy. A firstset of oscillator strengths has been deduced for this ion.
| Core-polarization effects in doubly ionized cerium (Ce III) for transitions of astrophysical interest
The importance of core-polarization effects on oscillator strengthdetermination in Ce III is emphasized. The inclusion of these effects inrelativistic Hartree-Fock (HFR) calculations leads to theoreticallifetime values for some 4f6p levels of this ion in close agreement withrecent time-resolved laser-induced fluorescence measurements. These newatomic data, the most accurate to date, have allowed the refinement ofthe stellar abundance determination of cerium in the chemically peculiarstar HD 192913, but have not altered in a drastic way the conclusionsderived in recent analyses regarding the overabundance of this element.
| Experimental and theoretical studies of DyIII: radiative lifetimes and oscillator strengths of astrophysical interest
The lifetimes of three short-lived levels belonging to the4f96p configuration and of two long-lived levels of the4f95d configuration of DyIII have been measured for the firsttime using time-resolved laser-induced fluorescence techniques. They arein good agreement with theoretical values calculated within theframework of a pseudo-relativistic configuration interactionapproximation. Using the experimental lifetimes and the theoreticalbranching fractions, a first set of transition probabilities ofastrophysical interest has been obtained for DyIII.
| Rotational Velocities of B Stars
We measured the projected rotational velocities of 1092 northern B starslisted in the Bright Star Catalogue (BSC) and calibrated them againstthe 1975 Slettebak et al. system. We found that the published values ofB dwarfs in the BSC average 27% higher than those standards. Only 0.3%of the stars have rotational velocities in excess of two-thirds of thebreakup velocities, and the mean velocity is only 25% of breakup,implying that impending breakup is not a significant factor in reducingrotational velocities. For the B8-B9.5 III-V stars the bimodaldistribution in V can be explained by a set of slowly rotating Ap starsand a set of rapidly rotating normal stars. For the B0-B5 III-V starsthat include very few peculiar stars, the distributions in V are notbimodal. Are the low rotational velocities of B stars due to theoccurrence of frequent low-mass companions, planets, or disks? Therotational velocities of giants originating from late B dwarfs areconsistent with their conservation of angular momentum in shells.However, we are puzzled by why the giants that originate from the earlyB dwarfs, despite having 3 times greater radii, have nearly the samerotational velocities. We find that all B-type primaries in binarieswith periods less than 2.4 days have synchronized rotational and orbitalmotions; those with periods between 2.4 and 5.0 days are rotating withina factor 2 of synchronization or are ``nearly synchronized.'' Thecorresponding period ranges for A-type stars are 4.9 and 10.5 days, ortwice as large. We found that the rotational velocities of the primariesare synchronized earlier than their orbits are circularized. The maximumorbital period for circularized B binaries is 1.5 days and for Abinaries is 2.5 days. For stars of various ages from 107.5 to1010.2 yr the maximum circularized periods are a smoothexponential function of age.
| Multiplicity among chemically peculiar stars. II. Cool magnetic Ap stars
We present new orbits for sixteen Ap spectroscopic binaries, four ofwhich might in fact be Am stars, and give their orbital elements. Fourof them are SB2 systems: HD 5550, HD 22128, HD 56495 and HD 98088. Thetwelve other stars are: HD 9996, HD 12288, HD 40711, HD 54908, HD 65339,HD 73709, HD 105680, HD 138426, HD 184471, HD 188854, HD 200405 and HD216533. Rough estimates of the individual masses of the components of HD65339 (53 Cam) are given, combining our radial velocities with theresults of speckle interferometry and with Hipparcos parallaxes.Considering the mass functions of 74 spectroscopic binaries from thiswork and from the literature, we conclude that the distribution of themass ratio is the same for cool Ap stars and for normal G dwarfs.Therefore, the only differences between binaries with normal stars andthose hosting an Ap star lie in the period distribution: except for thecase of HD 200405, all orbital periods are longer than (or equal to) 3days. A consequence of this peculiar distribution is a deficit of nulleccentricities. There is no indication that the secondary has a specialnature, like e.g. a white dwarf. Based on observations collected at theObservatoire de Haute-Provence (CNRS), France.Tables 1 to 3 are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/394/151Appendix B is only available in electronic form athttp://www.edpsciences.org
| Rotational velocities of A-type stars in the northern hemisphere. II. Measurement of v sin i
This work is the second part of the set of measurements of v sin i forA-type stars, begun by Royer et al. (\cite{Ror_02a}). Spectra of 249 B8to F2-type stars brighter than V=7 have been collected at Observatoirede Haute-Provence (OHP). Fourier transforms of several line profiles inthe range 4200-4600 Å are used to derive v sin i from thefrequency of the first zero. Statistical analysis of the sampleindicates that measurement error mainly depends on v sin i and thisrelative error of the rotational velocity is found to be about 5% onaverage. The systematic shift with respect to standard values fromSlettebak et al. (\cite{Slk_75}), previously found in the first paper,is here confirmed. Comparisons with data from the literature agree withour findings: v sin i values from Slettebak et al. are underestimatedand the relation between both scales follows a linear law ensuremath vsin inew = 1.03 v sin iold+7.7. Finally, thesedata are combined with those from the previous paper (Royer et al.\cite{Ror_02a}), together with the catalogue of Abt & Morrell(\cite{AbtMol95}). The resulting sample includes some 2150 stars withhomogenized rotational velocities. Based on observations made atObservatoire de Haute Provence (CNRS), France. Tables \ref{results} and\ref{merging} are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.125.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/393/897
| Transition probabilities in Gd III
Theoretical lifetimes, calculated with inclusion of core-polarizationeffects, have been determined for five 4f76p levels of doublyionized gadolinium. They agree quite well with recent experimentalvalues measured by time-resolved laser-induced fluorescencespectroscopy. From this agreement, the accuracy of a first set of Gd IIItransition probabilities, calculated for4f75d-4f76p and 4f76s-4f76ptransitions of astrophysical interest, has been assessed.
| The laboratory analysis of Bi II and its application to the Bi-rich HgMn star HR 7775
The bismuth spectrum emitted from a hollow cathode discharge has beenrecorded with a Fourier Transform Spectrometer (FTS). Accuratewavelengths have been determined for 104 Bi Ii lines, and several newenergy levels have been found, while the accuracy of previously known BiIi level energies have been improved. The hyperfine structure of allobserved Bi Ii lines has been analyzed, yielding hyperfine constants Aand B for 56 Bi Ii levels. With the aid of the laboratory measurementsthe optical region spectrum of the HgMn star HR 7775 has been studiedfor all observable Bi Ii lines. The wavelengths and hfs constantsestablished from the laboratory work have been combined with theoreticalgf values to identify spectral lines and make an abundance estimation ofbismuth. It has been established that bismuth is present in HR 7775 atan enhancement level of approximately 5 orders of magnitude relative tothe meteoritic abundance, consistent with previous observations in theultraviolet region of this star. Astrophysical gf values are presentedfor a number of Bi Ii lines.
| Measurement of lifetimes by laser-induced fluorescence and determination of transition probabilities of astrophysical interest in Nd III
Selective lifetime measurements by time-resolved laser-inducedfluorescence spectroscopy for 5 levels belonging to the 4f35dconfiguration of doubly ionized neodymium provide a first and usefulexperimental test of the relativistic Hartree-Fock calculations in thision of astrophysical interest. As a consequence, the accuracy of thetransition probabilities deduced in the present work is well assessed.These new data are expected to help astrophysicists in the future torefine the analysis of the composition of chemically peculiar starswhich frequently show large overabundances of lanthanides when comparedto the solar system standards.
| Radiative lifetime measurements and oscillator strengths of astrophysical interest in Ho III
Radiative lifetimes of three long-lived levels belonging to the4f105d configuration of Ho III have been measured, for thefirst time, using the time-resolved laser-induced fluorescencetechnique. A good agreement between the experimental lifetimes andtheoretical results obtained within a multiconfigurationalpseudo-relativistic Hartree-Fock (HFR) approach includingcore-polarization (CP) effects is observed provided an adequate scalingfactor is applied to the <4f|r|5d> transition matrix element. Fromthe comparison theory-experiment, it has been possible to assess thereliability of the oscillator strengths of the 4f-5d transitions ofastrophysical interest, particularly for the lines depopulating thelevels considered in the present work. The present results fill in a gapconcerning the f-values of doubly ionized holmium and are needed for aquantitative evaluation of the holmium abundance in some peculiar stars.
| Radiative lifetime measurements and transition probabilities of astrophysical interest in Eriii
Radiative lifetimes of seven excited states of Eriii have been measuredusing time-resolved laser-induced fluorescence following two-photonexcitation. Relativistic Hartree-Fock calculations takingcore-polarization effects into account are found to be in excellentagreement with the experimental results. A large set of new calculatedtransition probabilities is presented for many transitions ofastrophysical interest. These results will be useful for investigatingthe composition of chemically peculiar stars.
| Are Stellar Rotational Axes Distributed Randomly?
Stellar line widths yield values of Vsini, but the equatorial rotationalvelocities, V, cannot be determined for individual stars withoutknowledge of their inclinations, i, relative to the lines of sight. Forlarge numbers of stars we usually assume random orientations ofrotational axes to derive mean values of V, but we wonder whether thatassumption is valid. Individual inclinations can be derived only inspecial cases, such as for eclipsing binaries where they are close to90° or for chromospherically active late-type dwarfs or spotted(e.g., Ap) stars where we have independent information about therotational periods. We consider recent data on 102 Ap stars for whichCatalano & Renson compiled rotational periods from the literatureand Abt & Morrell (primarily) obtained measures of Vsini. We findthat the rotational axes are oriented randomly within the measuringerrors. We searched for possible dependence of the inclinations onGalactic latitude or longitude, and found no dependence.
| Observing roAp Stars with WET: A Primer
We give an extensive primer on roAp stars -- introducing them, puttingthem in context and explaining terminology and jargon, and giving athorough discussion of what is known and not known about them. Thisprovides a good understanding of the kind of science WET could extractfrom these stars. We also discuss the many potential pitfalls andproblems in high-precision photometry. Finally, we suggest a WETcampaign for the roAp star HR 1217.
| 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.
| Ab initio calculations of oscillator strengths and Landé factors for Nd Iii
Theoretical calculations of oscillator strengths and Landég-factors made with the Cowan code are presented for the rare-earth ionNd Iii. The emphasis is on transitions in the optical region of thespectrum. Comparisons of previous ab initio calculations of this typefor other lanthanides (e.g., La Ii, Ce Iii, and Lu Ii) with publishedexperimental and theoretical data suggest that the accuracy of thelog(gf) values for individual transitions should be about +/-0.15 dex inthe absence of significant core polarization effects; the g-factorsshould be good to better than 5%, except in a few cases where termmixing is important. Applications of the data are made in thecalculation of neodymium abundances in the atmospheres of the chemicallypeculiar stars HD 101065 (``Przybylski's star") andHD 122970.
| Loss of angular momentum of magnetic Ap stars in the pre-main sequence phase
A model for rotation evolution of an intermediate mass star with theprimordial magnetic field in the pre-main sequence (PMS) phase wasdeveloped. It takes into account the accretion of matter along themagnetic field lines, the stellar field-disk interaction and amagnetized wind. Variations of stellar moment of inertia were includedbased on evolutionary models of PMS evolution of such stars. Stellarmass and magnetic moment were assumed constant during the PMS evolution.Values of the parameters describing the strength of the magnetic field,accretion rate and mass loss rate were taken from observations. Inaddition, the life time of the disk was varied. An equation describingthe evolution of the rotation rate of a magnetic PMS star was derivedand solved for different stellar masses. The results indicate that theinteraction of the stellar ymagnetic field with circumstellarenvironment wipes out quickly a memory of the initial rotation period.The ZAMS period depends solely on the details of this interaction.Accretion spins up a star early in its PMS life and if the diskdisappears right after that the star may keep its faster rotation untilZAMS and appear there as a Be star. A wide variety of parametersdescribing the evolution of stellar AM results in typical ZAMS rotationperiods of magnetic stars several times longer than of normal stars.This agrees well with the observations. Under special circumstances astar can reach an exceptionally long rotation period of several years(up to 100 years). This requires a long PMS life time, an existence of adisk for only a part of the PMS phase and the wind in the strongmagnetic field existing for the rest of the PMS life. The observationsconfirm indeed that extremely slowly rotating Ap stars are lower massstars with strong magnetic fields.
| Do the physical properties of Ap binaries depend on their orbital elements?
We reveal sufficient evidence that the physical characteristics of Apstars are related to binarity. The Ap star peculiarity [represented bythe Δ(V1-G) value and magnetic field strength] diminishes witheccentricity, and it may also increase with orbital period(Porb). This pattern, however, does not hold for largeorbital periods. A striking gap that occurs in the orbital perioddistribution of Ap binaries at 160-600d might well mark a discontinuityin the above-mentioned behaviour. There is also an interestingindication that the Ap star eccentricities are relatively lower thanthose of corresponding B9-A2 normal binaries for Porb>10d.All this gives serious support to the pioneering idea of Abt &Snowden concerning a possible interplay between the magnetism of Apstars and their binarity. Nevertheless, we argue instead in favour ofanother mechanism, namely that it is binarity that affects magnetism andnot the opposite, and suggest the presence of a newmagnetohydrodynamical mechanism induced by the stellar companion andstretching to surprisingly large Porb.
| Speckle Interferometry of New and Problem HIPPARCOS Binaries
The ESA Hipparcos satellite made measurements of over 12,000 doublestars and discovered 3406 new systems. In addition to these, 4706entries in the Hipparcos Catalogue correspond to double star solutionsthat did not provide the classical parameters of separation and positionangle (rho,theta) but were the so-called problem stars, flagged ``G,''``O,'' ``V,'' or ``X'' (field H59 of the main catalog). An additionalsubset of 6981 entries were treated as single objects but classified byHipparcos as ``suspected nonsingle'' (flag ``S'' in field H61), thusyielding a total of 11,687 ``problem stars.'' Of the many ground-basedtechniques for the study of double stars, probably the one with thegreatest potential for exploration of these new and problem Hipparcosbinaries is speckle interferometry. Results are presented from aninspection of 848 new and problem Hipparcos binaries, using botharchival and new speckle observations obtained with the USNO and CHARAspeckle cameras.
| The electron-impact broadening effect in CP stars: the case of La II , La II i, EU II , and EU II i lines
The electron-impact widths and shifts for six Eu ii lines and widths forthree La ii , and six La ii i multiplets have been calculated by usingthe modified semiempirical method. Estimation for Stark widths of664.506 nm (Eu ii ) and 666.634 nm (Eu ii i) lines are given as well.The influence of the electron-impact mechanism on line shapes andequivalent widths in hot star atmospheres has been considered.
| EU BT III identification and EU abundance in CP stars
We report the first identification of the Eu iii lambda 6666.347 line inoptical spectra of CP stars. This line is clearly present in the spectraof HR 4816, 73 Dra, HR 7575, beta CrB, and alpha (2) CVn, while it ismarginally present or absent in spectra of the roAp stars (rapidlyoscillating Ap stars, cf. Kurtz 1990) alpha Cir, gamma Equ, HD 203932,GZ Lib (33 Lib), and HD 24712. Careful synthetic spectrum calculationsfor the Eu ii lambda 6645.11 line taking into account hyperfine,isotopic, and magnetic splittings allow us to obtain more accurate Euabundances in the atmospheres of 9 CP stars. In most cases the derivedabundances are significantly lower than the previous results reportedfor some of the stars based on coarse analysis of the famous blue Eu iilines. Assuming an ionization balance in the stellar atmospheres we givean estimate of the astrophysical oscillator strength log (gf)=1.18 +/-0.14 for the Eu iii lambda 6666.347 line. This value is obtained withouttaking into account a possible hyperfine-splitting which is unknown forthis Eu iii line. We also provide astrophysical gf-values for Eu iiilambda lambda 7221.838, 7225.151, and 8079.071.
| Why are magnetic AP stars slowly rotating?
Observational data on rotation of Ap stars suggest that the bulk oftheir rotation rates orm a separate Maxwellian distribution with anaverage value 3-4 times lower than the normal star distribution. Noevidences for a significant angular momentum (AM) loss on the mainsequence (MS) have been found. It is thus concluded that Ap stars mustlose a large fraction of their initial angular momentum (AM) in thepre-MS phase of evolution, most probably as a result of the interactionof their primordial magnetic fields with accretion disks and stellarwinds. The observationally most acceptable values of accretion rate fromthe disk, 10^(-8) M_(Sun)/year, of mass loss rate via a magnetized wind,10^(-8) M_(Sun)/year, and of the surface magnetic field, 1 kG on theZAMS, result in the AM loss in full agreement with observations. Thereexists a separate group of extremely slowly rotating Ap stars, withperiods of the order of 10-100 years. They are too numerous to come fromthe distribution describing the bulk of Ap stars. It is conjectured thattheir extremely low rotation rates are the result of additional AM losson the MS.
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Группы:
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Наблюдательные данные и астрометрия
| Созвездие: | Андромеда |
| Прямое восхождение: | 01h38m31.70s |
| Склонение: | +45°24'00.0" |
| Видимая звёздная величина: | 6.36 |
| Расстояние: | 139.47 парсек |
| Собственное движение RA: | -17.3 |
| Собственное движение Dec: | 0.5 |
| B-T magnitude: | 6.336 |
| V-T magnitude: | 6.374 |
Каталоги и обозначения:
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INHABITED SKY
