Contents
Images
Upload your image
DSS Images Other Images
Related articles
An Atlas of K-Line Spectra for Cool Magnetic CP Stars: The Wing-Nib Anomaly (WNA)
We present a short atlas illustrating the unusual Ca II K-line profilesin upper main-sequence stars with anomalous abundances. Slopes of theprofiles for 10 cool, magnetic chemically peculiar (CP) stars changeabruptly at the very core, forming a deep ``nib.'' The nibs show thesame or nearly the same radial velocity as the other atomic lines. Thenear wings are generally more shallow than in normal stars. In threemagnetic CP stars, the K lines are too weak to show this shape, althoughthe nibs themselves are arguably present. The Ca II H lines also showdeep nibs, but the profiles are complicated by the nearby, strongHɛ absorption. The K-line structure is nearly unchanged withphase in β CrB and α Cir. Calculations, including NLTE, showthat other possibilities in addition to chemical stratification mayyield niblike cores.
| Remarkable non-dipolar magnetic field of the Bp star HD 137509
The southern magnetic Bp star HD 137509 exhibits complex rotationalmodulation of the longitudinal field and other magnetic observables.Interpretation of this magnetic variability in the framework of thelow-order multipolar field models suggests a very strong quadrupolarcomponent to dominate the surface field topology of HD 137509. I haveexamined the high-quality VLT/UVES spectra of HD 137509 and discoveredresolved Zeeman split components in some of the spectral lines. Theinferred mean surface field modulus, < B >=29 kG, agrees with themultipolar model predictions. This confirms the presence of an extremelystrong non-dipolar magnetic field in HD 137509 and establishes this staras the object with the second-largest field among magnetic chemicallypeculiar stars.
| The discovery of remarkable 5kms-1 pulsational radial velocity variations in the roAp star HD99563*
In a high-resolution spectral survey of nearly half the 34 known rapidlyoscillating Ap (roAp) stars, using the Ultraviolet-Visual EchelleSpectrograph on the Very Large Telescope, we have discovered remarkablylarge amplitude pulsations in the roAp star HD99563 with some spectrallines showing radial velocity amplitudes up to 5kms-1(10kms-1 peak-to-peak) with a pulsation period of 10.7min. Asfor many other roAp stars, we find the largest pulsation amplitudes forlines of some rare earth elements and in the core of the Hα line.The highest amplitudes of 5kms-1 are seen in rather weaklines of EuII and TmII. Stronger lines of PrIII and NdIII have pulsationamplitudes in the range 0.7 to 3.5kms-1 for different lines.In the narrow Hα core, the average amplitude is2.6kms-1, but, as is the case for other lines, the amplitudeand phase vary strongly with line depth (atmospheric height), with theamplitude of the radial velocity variations of the line bisectorreaching a maximum of 4.3kms-1 at the bottom of the core.Some other elements show pulsation amplitudes 0.1 to0.7kms-1. Variations in velocity amplitude and phase forseveral spectral lines were studied using line-bisector measurements toobtain information about the vertical structure of the pulsation modesand the stellar atmosphere.
| Probing the magnetoacoustic boundary layer in the peculiar magnetic star 33 Lib (HD 137949)*
We show unprecedented resolution of the amplitudes and phases of theprincipal 2.015-mHz mode and its 4.030-mHz harmonic as a function ofatmospheric depth in the roAp star 33 Lib using high-resolution VeryLarge Telescope (VLT) Ultraviolet-Visual Echelle Spectrograph (UVES)spectra. We show that the pulsation amplitude increases upwards into theNd III line-forming layer away from an atmospheric node, and increasesdownwards from the same node in the Nd II line-forming layer. We suggestthat the Nd III layer, which arises at, or above, τ~10-3, overlaps with the magnetoacoustic boundary layer, andthat our detailed observations of pulsation amplitude and phasevariations with depth provide a first probe of this layer.
| Pushing the ground-based limit: 14-μmag photometric precision with the definitive Whole Earth Telescope asteroseismic data set for the rapidly oscillating Ap star HR1217
HR1217 is one of the best-studied rapidly oscillating Ap (roAp) stars,with a frequency spectrum of alternating even- and odd-l modes that aredistorted by the presence of a strong, global magnetic field. Severalrecent theoretical studies have found that within the observableatmospheres of roAp stars the pulsation modes are magneto-acoustic withsignificant frequency perturbations that are cyclic with increasingfrequency. To test these theories a Whole Earth Telescope extendedcoverage campaign obtained 342 h of Johnson B data at 10-s timeresolution for the roAp star HR1217 over 35 d with a 36 per cent dutycycle in 2000 November-December. The precision of the derived amplitudesis 14 μmag, making this one of the highest precision ground-basedphotometric studies ever undertaken. Substantial support has been foundfor the new theories of the interaction of pulsation with the strongmagnetic field. In particular, the frequency jump expected as themagnetic and acoustic components cycle through 2π rad in phase hasbeen found. Additionally, comparison of the new 2000 data with anearlier 1986 multisite study shows clear amplitude modulation for somemodes between 1986 and 2000. The unique geometry of the roAp starsallows their pulsation modes to be viewed from varying aspect withrotation, yielding mode identification information in the rotationalsidelobes that is available for no other type of pulsating star. Thoserotational sidelobes in HR1217 confirm that two of the modes aredipolar, or close to dipolar; based on the frequency spacings andHipparcos parallax, three other modes must be either l= 0 or 2 modes,either distorted by the magnetic field, or a mix of m-modes of given lwhere the mixture is the result of magnetic and rotational effects. Astudy of all high-speed photometric Johnson B data from 1981 to 2000gives a rotation period Prot= 12.4572 d, as found in previouspulsation and photometric studies, but inconsistent with a differentrotation period found in magnetic studies. We suggest that this rotationperiod is correct and that zero-point shifts between magnetic data setsdetermined from different spectral lines are the probable cause of thecontroversy over the rotation period. This WET data set is likely tostand as the definitive ground-based study of HR1217. It will be thebaseline for comparison for future space studies of HR1217, particularlythe MOST satellite observations.
| 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 Åmm-1 DominionAstrophysical Observatory Reticon exposures coveringλλ3830-4770 with a typical signal-to-noise ratio of 200taken with the long camera of the 1.22-m telescope. The spectrogramswere measured interactively with the graphics computer program REDUCE.The fine analysis used an ATLAS9 metal-rich model atmosphere, thepredictions of which best-matched the optical region fluxes and theHγ profile. The anomalies of HR 8216 are primarily an enhancementof many iron peak elements with the rare earths elements much lessenhanced than in many similar cool magnetic CP stars. Using the resultsof the fine analysis the spectrum was synthesized. Comparison with theobserved spectrum showed that the agreement is good but did not fullyaccount for all of the observed line spectrum.
| HD 101065, the Most Peculiar Star: First Results from Precise Radial Velocity Study
In this paper we discuss the prospects for asteroseismology with spatialresolution and motivate studies of the most chemically peculiar roApstar HD 101065. We present the first results from a high-precisionradial velocity (RV) study of HD101065 based on data spanning fournights that were acquired using the HARPSechelle-spectrometer at theESO3.6 m telescope. The analysis of individual nights showed theamplitude and phase modulation of the dominant mode. The analysis of thewhole data set showed the presence of multi-periodic oscillations withtwo groups of equally-spaced modes. We find ν =65.2 μHz andδν =7.3 μHz for the large and the small spacing,respectively. HD 101065 is the only roAp star to show the existence oftwo groups of l =0 2 and l =1 3 excited modes.
| 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
| Radial velocity variations in pulsating Ap stars. V. 10 Aquilae
We have used precise stellar radial velocities to study the pulsationalmotion of the rapidly oscillating Ap star 10 Aql.Observations were made on three separate nights using the highresolution spectrograph of the Harlan J. Smith 2.7 m telescope atMcDonald Observatory. A high radial velocity precision was achieved byusing an iodine gas absorption cell. The integrated radial velocitymeasurements using the full wavelength region covered by iodineabsorption lines (5000-6300 Å) failed to detect any coherentpulsational velocity variations to a level of 2.5-5 m s-1. Ananalysis over a much narrower wavelength range revealed that pulsationalradial velocity variations are indeed present in 10Aql, but only in 5 spectral lines. The amplitude of thesevariations ranged from about 100 m s-1 to as high as 398 ms-1 for an unidentified feature at λ5471.40 Å.Other spectral features showing pulsational radial velocity variationsare tentatively identified as Sm II and Tm II.Based on observations collected at McDonald Observatory.
| The null result of a search for pulsational variations of the surface magnetic field in the roAp star γ Equulei
We describe an analysis of the time-resolved measurements of thesurface magnetic field in the roAp star γEqu. We have obtained ahigh-resolution and high signal-to-noise (S/N) spectroscopic timeseries, and the magnetic field was determined using Zeeman-resolvedprofiles of the FeII 6149.25 Åand FeI 6173.34 Ålines.Contrary to recent reports, we do not find any evidence of magneticvariability with pulsation phase, and derive an upper limit of 5-10 Gfor pulsational modulation of the surface magnetic field in γEqu.
| Stellar activity and magnetism studied by optical interferometry
By means of numerical simulations, we investigate the ability of opticalinterferometry, via the fringe phase observable, to address stellaractivity and magnetism. To derive abundance maps and stellar rotationaxes, we use color differential interferometry which couples highangular resolution to high spectral resolution. To constrain magneticfield topologies, we add to this spectro-interferometer a polarimetricmode. Two cases of well-known Chemically Peculiar (CP) stars (βCrBand α2CVn) are simulated to derive instrumentalrequirements to obtain 2D-maps of abundance inhomogeneities and magneticfields. We conclude that the near-infrared instrument AMBER of the VLTIwill allow us to locate abundance inhomogeneities of CP stars largerthan a fraction of milliarcsecond whereas the polarimetric mode of theFrench GI2T/REGAIN interferometer would permit one to disentanglevarious magnetic field topologies on CP stars. We emphasize the crucialneed for developing and validating inversion algorithms so that futureinstruments on optical aperture synthesis arrays can be optimally used.
| Measurements of magnetic fields over the pulsation cycle in six roAp stars with FORS 1 at the VLT
With FORS 1 at the VLT we have tried for the first time to measure themagnetic field variation over the pulsation cycle in six roAp stars tobegin the study of how the magnetic field and pulsation interact. Forthe star HD 101065, which has one of the highest photometric pulsationamplitudes of any roAp star, we found a signal at the known photometricpulsation frequency at the 3σ level in one data set; however thiscould not be confirmed by later observations. A preliminary simplecalculation of the expected magnetic variations over the pulsation cyclesuggests that they are of the same order as our current noise levels,leading us to expect that further observations with increased S/N have agood chance of achieving an unequivocal detection.Based on observations obtained at the European Southern Observatory,Paranal, Chile (ESO programmes Nos. 69.D-0210 and 270.D-5023).
| Radial velocity variations in pulsating Ap stars - II. 33 Librae
We present precise relative radial velocity (RV) measurements for therapidly oscillating Ap (roAp) star 33 Librae measured fromhigh-resolution data spanning the wavelength interval 5000-6200 Å.We find that the pulsational radial velocity amplitude determined over abroad wavelength range (~100 Å) depends on the spectral regionthat is examined and can be as high as 60 m s-1 at 5600Å and as low as 7 m s-1 in the 5900 Å region. RVmeasurements of individual spectral lines can show higher amplitudesthan results obtained using a `broad-band' measurement that includesmany spectral lines. The acoustic cross-sections of the atmosphere, i.e.the phase and amplitude of the pulsations, as a function of opticaldepth are found for spectral lines of Ca, Cr, Fe, La, Ce, Gd, Er and Nd.This analysis shows that pulsation phase is variable through theatmosphere and that Nd III lines pulsate almost 180° out of phasewith those of Nd II features and are formed significantly higher in thestellar atmosphere. This conclusively establishes the presence of atleast one radial node to the pulsations in the upper stellar atmosphere.We have estimated that this acoustic node is located above an opticaldepth log τ < -4.5 and below the level where the Nd III lines areformed. We also suspect that there may be a second atmospheric node inthe lower atmosphere below or at log τ~=-0.9 and close to continuumformation level.The histogram of pulsational phases for all individual spectral featuresshows a bimodal Gaussian distribution with 17 per cent of the lineshaving a pulsational phase ~165° out of phase with most otherspectral lines. This is also consistent with the presence of a radialnode in the stellar atmosphere. The accumulation of phase due to arunning wave component can explain the 165° phase difference as wellas the broader width (by a factor of 2) of one of the Gaussiancomponents of the phase distribution.We also found evidence for phase variations as a function of effectiveLandég-factors. This may be the influence of magnetic field andmagnetic intensification effect on depths of spectral line formation andshows that the magnetic field is controlling the pulsations. Our RVmeasurements for 33 Lib suggest that we are seeing evidence of verticalstructure to the oscillations as well as the influence of thedistribution of elements on the stellar surface.We suggest and briefly discuss a new semi-empirical tomographicprocedure for mono- and multimode roAp stars that will use acousticcross-sections obtained on different chemical elements and differentpulsation modes for restoring the abundance and acoustic profilesthroughout the stellar atmosphere and across the stellar surface.
| Radial velocity study of the roAp star α Circinus
We present results of 548 high-dispersion spectra of the roAp starα Circinus over a five-night period. The pulsational radialvelocities measured from the rare-earth elements, Nd III, Hα andHβ are easily measured and occur at the photometric period. Theamplitude is largest in Nd III and Hα, is lower in the rare earthsand Hβ, and cannot be measured in other metal lines. This behaviourcan be understood in terms of an increase of pulsational amplitude withheight in the atmosphere coupled with abundance stratification. Theradial velocities show a significant variation at the rotational periodof 4.463 d and a marginally significant periodicity at a frequency of8.16 cycle d-1. The latter may be a very low-amplitudeδ Scuti pulsation.
| New heights in asteroseismology: VLT spectroscopy of the roAp star HD 166473
We show for the first time, for any star other than the Sun, thevertical resolution of pulsation modes into standing waves in theatmosphere and running waves in the magnetoacoustic reflective boundarylayer of the upper atmosphere. This has only become possible with thenovel use of the Very Large Telescope (VLT) on a bright, exceedinglypeculiar magnetic star, HD 166473. These are the first observations thatmay directly test recent theoretical developments for magnetoacousticstellar pulsation modes in rapidly oscillating Ap stars.
| Time-series spectroscopy of pulsating sdB stars - III. Line indices of PG 1605+072
We present the detection and analysis of line index variations in thepulsating sdB star PG 1605+072. We have found a strong dependence ofline index amplitude on Balmer line order, with high-order Balmer lineamplitudes up to 10 times larger than Hβ. Using a simple model, wehave found that the line index may not only be dependent on temperature,as is usually assumed for oscillating stars, but also on surfacegravity. This information will provide another set of observables thatcan be used for mode identification of sdBs.
| High Precision with the Whole Earth Telescope: Lessons and Some Results from XCov20 for the roAp Star HR 1217
HR 1217 is a prototypical rapidly oscillating Ap star that has presenteda test to the theory of nonradial stellar pulsation. Prior observationsshowed a clear pattern of five modes with alternating frequency spacingsof 33.3 mu Hz and 34.6 mu Hz, with a sixth mode at a problematic spacingof 50.0 mu Hz (which equals 1.5 times 33.3 mu Hz) to the high-frequencyside. Asymptotic pulsation theory allowed for a frequency spacing of 34mu Hz, but Hipparcos observations rule out such a spacing. Theoreticalcalculations of magnetoacoustic modes in Ap stars by Cunha (2001)predicted that there should be a previously undetected mode 34 mu Hzhigher than the main group, with a smaller spacing between it and thehighest one. The 20th extended coverage campaign of the Whole EarthTelescope ({XCov20}) has discovered this frequency as predicted by Cunha(2001). Amplitude modulation of several of the pulsation modes betweenthe 1986 and 2000 data sets has also been discovered, while importantparameters for modelling the geometry of the pulsation modes have beenshown to be unchanged. With stringent selection of the best data fromthe WET network the amplitude spectrum shows highest peaks at only 50 mumag and formal errors on the determined amplitudes are 14 mu mag. Somelessons for future use of WET for the highest precision photometry onbright stars are discussed.
| Improved Astrometry and Photometry for the Luyten Catalog. II. Faint Stars and the Revised Catalog
We complete construction of a catalog containing improved astrometry andnew optical/infrared photometry for the vast majority of NLTT starslying in the overlap of regions covered by POSS I and by the secondincremental Two Micron All Sky Survey (2MASS) release, approximately 44%of the sky. The epoch 2000 positions are typically accurate to 130 mas,the proper motions to 5.5 mas yr-1, and the V-J colors to0.25 mag. Relative proper motions of binary components are measured to 3mas yr-1. The false-identification rate is ~1% for11<~V<~18 and substantially less at brighter magnitudes. Theseimprovements permit the construction of a reduced proper-motion diagramthat, for the first time, allows one to classify NLTT stars intomain-sequence (MS) stars, subdwarfs (SDs), and white dwarfs (WDs). We inturn use this diagram to analyze the properties of both our catalog andthe NLTT catalog on which it is based. In sharp contrast to popularbelief, we find that NLTT incompleteness in the plane is almostcompletely concentrated in MS stars, and that SDs and WDs are detectedalmost uniformly over the sky δ>-33deg. Our catalogwill therefore provide a powerful tool to probe these populationsstatistically, as well as to reliably identify individual SDs and WDs.
| The rapidly rotating delta Scuti star AV Cetei
We present results from an international spectroscopic and photometriccampaign on the delta Scuti star AV Cet. The starhas a rich and complex pulsation spectrum, and we find 7 individualfrequencies, with evidence for many more present below our detectionlimit. We investigate the prospects for mode identification in fastrotators, using several different techniques. We compare the methods andconclude that although no single technique can give unambiguous modeidentification, the collective evidence does allow some conclusions tobe drawn, suggesting the presence of one radial mode at 14.598d-1. During the campaign we found the star HD9139 to be a variable. From our photometry we find evidencefor a variability time scale around 1 day, but we cannot find anunambiguous interpretation for its light variations.Based partly on observations made with the Danish 1.5 m telescope atESO, La Silla, Chile.
| Discovery of magnetic field variations with the 12.1-minute pulsation period of the roAp star gamma Equulei
We have discovered the first magnetic field variations over thepulsation period in an roAp star. The amplitude of the magneticvariability we have found is significant at the 4.1- to 6.6 sigma levelmeasured for four strong lines of NdIII for gamma Equ with the highestamplitude found being 240 +/- 37 G for the lambda 5845.07 Å line,with a period of 12.1 min. This magnetic field variation is in goodagreement with theoretical expectations, and the period agrees well withthe known photometric periods. We have also found that the time of mostnegative effective magnetic field occurs 0.15 +/- 0.05 cycles prior tomaximum pulsation velocity of recession. There is a small butsignificant variation in the equivalent width of two of the NdIII lines,but no equivalent width variation is detectable for the other two lines.Measurements of four lines of CaI show no variations at all inequivalent width, radial velocity or effective magnetic field strength.We find a difference in the mean effective magnetic field strength offour NdIII lines and four CaI lines and speculate that this could be areal effect caused by the surface concentration of NdIII towards themagnetic pole. If true, this provides a new way to map the horizontalabundance distribution of elements in slowly rotating Ap stars for whichDoppler imaging is not possible.
| 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
| Atmospheric parameters and chemical composition of the ultra-cool roAp star HD 213637
A high-resolution spectrum obtained with the UVES instrument at the ESOVLT is used to determine atmospheric parameters and chemical compositionof the roAp star HD 213637. Resolved Zeeman split lines are discoveredin the spectrum of this star, indicating a mean field modulus of 5.5 kG.Effective temperature, Teffv{6400} determined from theprofiles of the hydrogen Hα and Hβ lines, makes HD 213637one of the coolest (ro)Ap stars known, suggesting that the class ofmagnetic chemically peculiar stars extends to lower effectivetemperatures than previously thought. Furthermore, the low temperatureof HD 213637 poses serious challenge to some excitation mechanismssuggested as an explanation of the roAp phenomenon. This peculiar staris also unique in its evolutionary stage: it is either located near thevery end of its main sequence life or has already ascended the giantbranch. Abundance analysis reveals that HD 213637 is iron-deficient andhas abundance pattern typical of the cool pulsating Ap stars. Aninconsistency between abundances derived from the weak and strongspectral lines and the lines of different ions provide a clear evidencefor prominent vertical abundance stratification of many chemicalspecies, notably Na, Mg, Ca, Fe, Pr, Nd, Tb, and Er.Based on observations obtained at the European Southern Observatory,VLT.
| Rotation and differential rotation in field F- and G-type stars
We present a detailed study of rotation and differential rotationanalyzing high resolution high S/N spectra of 142 F-, G- and earlyK-type field stars. Using Least Squares Deconvolution we obtainbroadening profiles for our sample stars and use the Fourier transformmethod to determine projected rotational velocities v sin i.Distributions of rotational velocities and periods are studied in theHR-diagram. For a subsample of 32 stars of spectral type F0-G0 we derivethe amount of differential rotation in terms of alpha = (Omega_Equator- Omega_Pole )/Omega_Equator . We find evidence for differentialrotation in ten of the 32 stars. Differential rotation seems to be morecommon in slower rotators, but deviations from rigid rotation are alsofound in some fast rotators. We search for correlations betweendifferential rotation and parameters relevant for stellar activity andshow indications against strong differential rotation in very activestars. We derive values of Delta P and Delta Omega , which support aperiod dependence of differential rotation. Derived lap times 2pi /DeltaOmega are of the order of 20 d and contradict the assumption thatconstant lap times of the order of the solar one ( ~ 130 d) are therule in stars that are thought to harbour magnetic dynamos.Based on observations collected at the European Southern Observatory, LaSilla.Tables 3 and A1 are only available at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.125.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/398/647
| Discovery of rapid radial velocity variations in the roAp star 10 Aql and possible pulsations of β CrB
We report discovery of radial velocity variations in rare earth spectrallines of the roAp star 10 Aql with amplitudes of between 30 and 130 ms-1 and periods of about 11 min. Radial velocity variationswith amplitude 70 m s-1 may also have been detected in onespectral line of Fe I in β CrB. If confirmed, our results mayindicate that all Ap stars in a certain temperature range pulsate, whichmeans that roAp stars do not exist as a separate class but are onlydistinguished by higher pulsational amplitudes.
| Radial velocity study of the roAp star HR 3831
We present results of 566 high-dispersion echelle spectra of the roApstar HR 3831. We obtain radial velocities of different ions bycross-correlation with suitable templates. The inhomogeneous abundancedistribution results in different rotational velocity curves fordifferent ions. We confirm the large pulsational amplitude in doublyionized rare-earth ions. In addition we show that, whereas Hα hasa large pulsational velocity amplitude, the pulsational velocityamplitudes in Hβ and Hγ are below the detection limit. Thissupports the idea that the pulsational amplitude is a function of heightand that there is a layer enriched with rare earths in the upperatmosphere. We present a list of lines where the pulsational velocity isthe largest. In the radial velocities of the NdIII line we are able toresolve the two frequencies of highest amplitude seen in the lightvariations. Co-added spectra indicate that one of these frequencies isan axisymmetric mode [possibly (l, m) = (2, 0)] and the other anon-axisymmetric mode [possibly (2, -2)]. This confirms recentobservations of non-axisymmetric modes in roAp stars. We suggest thatthe presence of non-axisymmetric modes is simply a consequence of thedeparture of the eigenfunction from a pure spherical harmonic inclinedto the rotational axis and is not related to the Coriolis force. Hencethe frequency splitting is precisely equal to the rotational frequency,as observed. Whereas the light variations show strong harmonics, theseare not present in the radial velocities.
| Radial velocity study of the roAp star HR 1217
We analyse 1160 high-dispersion echelle spectra of the roAp star HR 1217obtained over two 2-week runs. Radial velocities were obtained bycross-correlating with three different templates: a spectrum of a normalstar, a spectrum of HR 1217 itself and a spectrum containing only linesof rare-earth elements. We show that rotational modulation is present inthe radial velocities calculated using the normal star template and therare-earth template, but with opposite phases. The roAp pulsations arenot detected in the radial velocities from the normal star template, areweakly present in the radial velocities using a template of the staritself and are of large amplitude in the radial velocities obtainedusing a template containing only rare-earth lines. The pulsationfrequencies are, as far as we can tell, identical to those obtainedphotometrically, although only the mode of largest amplitude is detectedindependently. These observations confirm that the pulsations in roApstars are strongly concentrated at the magnetic poles, which is also thelocation where the rare-earth elements are overabundant. A strongcore-wing anomaly is present in the Hα profile of the star.
| 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
| delta Scuti stars in Praesepe. II. The STACC 1998 campaign - The spectroscopy
We present the results of a large international spectroscopic campaignon the delta Scuti star BN Cnc. Combiningobservations from five observatories taken over more than two weeks, wecalculate line indices of the Hα line. A line index is theintegrated line flux in a software filter divided by the continuum flux.We demonstrate that this can be used in combination with simultaneousphotometry to classify the oscillation modes. We recover all thefrequencies also found from photometry and assign likely modeidentifications, which differ slightly from previously published values,but are found to be consistent with simple models. The difference inidentification is found to have very little effect on the derivedluminosity and temperature.
| Abundance stratification and pulsation in the atmosphere of the roAp star boldmath gamma Equulei
We present the evidence for abundance stratification in the atmosphereof the rapidly oscillating Ap star gamma Equ. Ca, Cr, Fe, Ba, Si, Naseem to be overabundant in deeper atmospheric layers, but normal tounderabundant in the upper layers with a transition in the typical lineforming region of -1.5
| On the excitation mechanism in roAp stars
We investigate a model for the excitation of high-order oscillations inroAp stars. In this model we assume that the strong concentration ofmagnetic field about the magnetic poles is enough to suppressconvection. Thus the model considered is composed of two polar regions,in which convection is presumed to be suppressed totally, and anequatorial region, where the convection is unaffected. This model isgenerated by building pairs of locally spherically symmetricalequilibria 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 bychoosing appropriate chemical composition profiles for both the polarand equatorial regions. Our results indicate that the composite model isunstable against axisymmetric non-radial high-order modes of pulsationthat are aligned with the magnetic poles. The oscillations are excitedby the κ mechanism acting principally in the hydrogen ionizationzones of the polar regions. The effect of the lateral inhomogeneity onthe second frequency differences is also investigated; we find that theperturbation to them by the inhomogeneity is of the same order as thesecond differences themselves, thereby hindering potential attempts touse such differences to identify the degrees of the modes in astraightforward way.
|
Submit a new article
Related links
Submit a new link
Member of following groups:
|
Observation and Astrometry data
| Constellation: | Compas |
| Right ascension: | 14h42m30.40s |
| Declination: | -64°58'31.0" |
| Apparent magnitude: | 3.19 |
| Distance: | 16.402 parsecs |
| Proper motion RA: | -191.8 |
| Proper motion Dec: | -234.1 |
| B-T magnitude: | 3.474 |
| V-T magnitude: | 3.189 |
Catalogs and designations:
|
