Contents
Images
Upload your image
DSS Images Other Images
Related articles
The rapidly oscillating Ap star HD 99563 and its distorted dipole pulsation mode
We undertook a time-series photometric multisite campaign for therapidly oscillating Ap (roAp) star HD 99563 and also acquired mean lightobservations over four seasons. The pulsations of the star, which showflatter light maxima than minima, can be described with a frequencyquintuplet centred on 1557.653 μHz and some first harmonics of it.The amplitude of the pulsation is modulated with the rotation period ofthe star that we determine with 2.91179 +/- 0.00007 d from the analysisof the stellar pulsation spectrum and of the mean light data. We breakup the distorted oscillation mode into its pure spherical harmoniccomponents and find it is dominated by the l= 1 pulsation, and also hasa notable l= 3 contribution, with weak l= 0 and 2 components. Thegeometrical configuration of the star allows us to see both pulsationpoles for about the same amount of time; HD 99563 is only the fourthroAp star for which both pulsation poles are seen and only the thirdwhere the distortion of the pulsation modes has been modelled. We pointout that HD 99563 is very similar to the well-studied roAp star HR 3831.Finally, we note that the visual companion of HD 99563 is located in theδ Scuti instability strip and may thus show pulsation. We showthat if the companion was physical, the roAp star would be a2.03-Msolar, object, seen at a rotational inclination of44°, which then predicts a magnetic obliquity .
| Pulsational variability of Li I 6708 Åline profile in the spectra of roAp star gamma Equ
In the framework of the Project "Lithium in CP stars" the task ofpulsational line profile variations (LPV) for Li I 6708 Å iscarried out. The high spectral and time resolution observations wereobtained for typical roAp(CP2) star gamma Equ. Analysis of two night'sobservations shows a definite blue-to-red LPV of Li I 6708 A duringpulsational period, that could be explained by two ways: the firstsupposes the formation of shock wave in the most upper layers nearmagnetic poles and red shifts due to matter falling on star; the second- a red asymmetry of Li I line profile is explained by high isotopicratio 6Li/7Li (about 0.5) due to spallationprocesses in polar Li spot.
| 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.
| Pulsational line profile variation of the roAp star HR 3831
We report the first comprehensive investigation of the line profilevariation caused by non-radial pulsation in an oscillating magneticchemically peculiar star. Spectrum variation of the well-known roAp starHR 3831 was detected using very high-resolution high signal-to-noisespectroscopic time-series observations and was followed through thewhole rotation cycle of the star. We confirmed the outstanding diversityof pulsational behaviour of different lines in the HR 3831 spectrum andattributed this phenomenon to an interplay between extreme verticalchemical inhomogeneity of the HR 3831 atmosphere and a running pulsationwave, that is propagating towards the upper photospheric layers withincreasing amplitude. The rapid profile variation of the Nd III 6145Å line, which shows the maximum pulsational disturbances in thestudied wavelength region, is characterised by measuring changes of itsequivalent width and the first three moments. Each of these observablesexhibits almost purely sinusoidal variation through the pulsation cycle,with the amplitude and phase clearly modulated by the stellar rotation.In contrast to previous photometric pulsational measurements of roApstars, our spectroscopic observational material admits straightforwardastrophysical interpretation and, hence, opens new and excitingpossibilities for direct and accurate analysis of the roAp pulsations.We demonstrate that rotational modulation of the radial velocityoscillations cannot be fully explained by an oblique axisymmetric dipole(ℓ=1, m=0) mode, implied by the classical oblique pulsator model ofroAp stars. Pulsational variation of the higher moments, in particularthe line width, reveals substantial contribution of the high-order(ℓ=3) spherical harmonics that appear due to distortion ofpulsations in HR 3831 by the global magnetic field. We interpretedobservations with a novel numerical model of the pulsational variationand rotational modulation of the line profile moments in roAp stars.Comparison of the observed and computed radial velocity and line widthvariation was used to establish parameters of the oblique pulsator modelof HR 3831. Furthermore, definite detection of pulsational variation inlines of light and iron-peak elements allows the first 3-D mapping ofpulsations in a non-radially oscillating star.
| 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.
| A non-adiabatic analysis for axisymmetric pulsations of magnetic stars
This paper presents the results of a non-adiabatic analysis foraxisymmetric non-radial pulsations including the effect of a dipolemagnetic field. Convection is assumed to be suppressed in the stellarenvelope, and the diffusion approximation is used to radiativetransport. As in a previous adiabatic analysis, the eigenfunctions areexpanded in a series of spherical harmonics. The analysis is applied toa 1.9-Msolar, main-sequence model (logTeff=3.913). The presence of a magnetic field always stabilizes low-orderacoustic modes. All the low-order modes of the model that are excited bythe κ-mechanism in the HeII ionization zone in the absence of amagnetic field are found to be stabilized if the polar strength of thedipole magnetic field is larger than about 1 kG. For high-order p modes,on the other hand, distorted dipole and quadrupole modes excited by theκ-mechanism in the H ionization zone remain overstable, even inthe presence of a strong magnetic field. It is found, however, that allthe distorted radial high-order modes are stabilized by the effect ofthe magnetic field. Thus, our non-adiabatic analysis suggests thatdistorted dipole modes and distorted quadrupole modes are most likelyexcited in rapidly oscillating Ap stars. The latitudinal amplitudedependence is found to be in reasonable agreement with theobservationally determined one for HR 3831. Finally, the expectedamplitude of magnetic perturbations at the surface is found to be verysmall.
| 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.
| High time resolution spectroscopy and magnetic variability of the cool Ap star HD965*
We present the results of an investigation of the magnetic Ap star HD965with high spectral and time resolution. We determine precise radialvelocities using spectra obtained with the Ultraviolet-Visual EchelleSpectrograph (UVES) on the European Southern Observatory Very LargeTelescope. Special attention is given to spectral lines of rare-earthelements which in rapidly oscillating Ap (roAp) stars exhibit thestrongest radial velocity variations with pulsation period. Careful timeseries analysis did not detect any convincing evidence of pulsation inHD965 with an upper limit in amplitude of 15-20 m s-1. Allproperties of HD965 are similar to the majority of roAp stars. A likelyreason for the apparent lack of pulsation could be connected with thegeometrical structure of the magnetic field and the aspect of the starat the time of observation. Longitudinal magnetic field measurements forHD965 showed that our UVES spectral observations were carried out whenthe longitudinal field was near zero and therefore, according to theoblique rotator model, near a time when the star was viewed from themagnetic equator. For a dipole oscillation aligned with the magneticfield, as is typical of roAp stars, no variation can be detected at thisaspect. We may, therefore, expect to detect rapid oscillations in HD965in the future, when the star will present one of the magnetic poles.
| 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.
| Variation of the line profile moments for stars pulsating in distorted oblique non-radial modes
We derive expressions and develop a numerical technique for the analysisof the line profile moment variations for stars pulsating in obliquenon-radial modes. This represents an extension of the widely usedspectroscopic moment mode identification method to the oblique distortedpulsations observed in rapidly oscillating Ap stars. We demonstrate thata non-axisymmetric superposition of the pulsation and rotation velocityfields results in a qualitatively new behaviour of some of the lineprofile characteristics. It is found that for the majority of roAp starsthe second moment varies with the pulsation frequency rather than withits first harmonic even for axisymmetric modes. We also identifypulsation observables which do not change during pulsation cycle but aremodulated by the stellar rotation and can contribute to the variabilityof the stellar spectra averaged over many pulsation cycles. As anillustration of the new version of the moment technique, we computerotational modulation of the pulsational changes of the line profilemoments for the oblique axisymmetric dipolar pulsation modes withdifferent parameters. It is also shown that a distortion of the obliquedipolar modes predicted by the recent theoretical studies of the stellarmagneto-acoustic oscillations can be readily diagnosed through themoment analysis. In particular, the shape of the pulsation phasemodulation for the radial velocity and the second moment is verysensitive to non-axisymmetric pulsation components, whereas therotational modulation of the second moment amplitude is best suited torevealing axisymmetric magnetically induced distortion of pulsations.
| 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.
| Axisymmetric p-mode pulsations of stars with dipole magnetic fields
The effect of a dipole magnetic field on adiabatic axisymmetricnon-radial p-mode pulsations is studied numerically. The angulardependence of pulsation, which cannot be represented by a singlespherical harmonic in the presence of a magnetic field, is expanded intoa series of spherical harmonics with different degrees l. The presenceof a magnetic field not only shifts the pulsation frequency, thepulsations are also damped due to the generation of magnetic slow waves.In agreement with the results of Cunha & Gough, who used a differentapproach from ours, we find that the effect of a magnetic field on theintermediate-to-high-order p-modes is not monotonic but cyclic withrespect to the pulsation frequency and the magnetic field strength. Thedamping rate of a high-order p-mode becomes very small at about 3 kG and8 kG; the corresponding field strengths are higher for lower overtones.The diminished magnetic damping is favourable for the correspondingmodes, if they are excited by the classical κ-mechanism, tosurvive even in the presence of a strong magnetic field. This picturecould explain the mode selection as observed in the rapidly oscillatingAp stars. For a low-order p-mode, the damping rate increases as thestrength of the magnetic field increases. We find that in the presenceof a magnetic field of a few kG, magnetic damping seems to exceed thedriving owing to the κ-mechanism of oscillations representative ofδ Scuti variability. This may explain why δ Scuti-typeoscillations are unlikely to be seen in magnetic Ap stars. The amplitudeof a mainly dipole (or quadrupole) mode is strongly confined to themagnetic axis in the outer layers. Furthermore, horizontal motion can becomparable to radial motion even for high-order p-modes. We discuss theinfluence of the magnetic distortion of the eigenfunction on thepulsation amplitude modulation with respect to the rotation phase.
| Indirect Imaging of Nonradial Pulsations in a Rapidly Oscillating Ap Star
Many types of stars show periodic variations of radius and brightness,which are commonly referred to as ``stellar pulsations.'' Observedpulsational characteristics are determined by fundamental stellarparameters. Consequently, investigations of stellar pulsations provide aunique opportunity to verify and refine our understanding of theevolution and internal structure of stars. However, a key boundarycondition for this analysis-precise information about the geometry ofpulsations in the outer stellar envelopes-has been notoriously difficultto secure. Here we demonstrate that it is possible to solve this problemby constructing an ``image'' of the pulsation velocity field from timeseries observations of stellar spectra. This technique is applied tostudy the geometry of nonradial pulsations in a prototype magneticrapidly oscillating Ap (roAp) star, HR 3831. Our velocity map directlydemonstrates an alignment of pulsations with the axis of the globalmagnetic field and reveals a significant magnetically induced distortionof pulsations. This observation constitutes a long-sought solution ofthe problem of the pulsation geometry of roAp stars and enables verystringent tests of the recent theories of stellar magnetoacousticoscillations.Based on observations obtained at the European Southern Observatory, LaSilla, Chile.
| Observations of Nonradial Pulsations in Radio Pulsars
We introduce a model for pulsars in which nonradial oscillations of highspherical degree (l) aligned to the magnetic axis of a spinning neutronstar reproduce the morphological features of pulsar beams. In our model,rotation of the pulsar carries a pattern of pulsation nodes underneathour sight line, reproducing the longitude-stationary structure seen inaverage pulse profiles, while the associated timelike oscillationsreproduce ``drifting subpulses''-features that change their longitudebetween successive pulsar spins. We show that the presence of nodallines can account for observed 180° phase jumps in driftingsubpulses and their otherwise poor phase stability, even if the timelikeoscillations are strictly periodic. Our model can also account for the``mode changes'' and ``nulls'' observed in some pulsars asquasi-periodic changes between pulsation modes of different l or radialovertone n, analogous to pulsation mode changes observed in oscillatingwhite dwarf stars. We discuss other definitive and testable requirementsof our model and show that they are qualitatively supported by existingdata. While reserving judgment until the completion of quantitativetests, we are inspired enough by the existing observational support forour model to speculate about the excitation mechanism of the nonradialpulsations, the physics we can learn from them, and their relationshipto the period evolution of pulsars.
| Multi-element abundance Doppler imaging of the rapidly oscillating Ap star HR 3831
We investigate magnetic field geometry and surface distribution ofchemical elements in the rapidly oscillating Ap star HR 3831. Results ofthe model atmosphere analysis of the spectra of this star are combinedwith the Hipparcos parallax and evolutionary models to obtain newaccurate estimates of the fundamental stellar parameters:Teffv{7650}, log L/Lȯ=1.09,M/Mȯ= 1.77 and an inclination angle i=68° of thestellar axis of rotation. We find that the variation of the longitudinalmagnetic field of HR 3831 and the results of our analysis of themagnetic intensification of Fe I lines in the spectrum of this star areconsistent with a dipolar magnetic topology with a magnetic obliquityβ=87° and a polar strength Bp=2.5 kG. We apply amulti-element abundance Doppler imaging inversion code for the analysisof the spectrum variability of HR 3831, and recover surfacedistributions of 17 chemical elements, including Li, C, O, Na, Mg, Si,Ca, Ti, Cr, Mn, Fe, Co, Ba, Y, Pr, Nd, Eu. Our study represents the mostthorough examination of the surface chemical structure in a magnetic Apstar and provides important observational constraints for modellingradiative diffusion in magnetic stars. The exceedingly high quality ofsome of our spectroscopic data allowed us to reconstruct unprecedenteddetails of abundance distributions, demonstrating a high level ofcomplexity in the surface structure down to the resolution limit of theDoppler maps. The Doppler imaging analysis of HR 3831 forms a basis forsubsequent detailed observational investigations and theoreticalmodelling of non-radial oscillations in this star. We discuss thecompound effect of the chemical nonuniformities and pulsational velocityfield on the rapid line profile variations, and assess the possibilityof identifying pulsation modes by using spatial filtering produced by aninhomogeneous abundance distribution. The results of our study of thesurface chemical structure suggest that differences in pulsationalbehaviour of lines of different ions observed for HR 3831 are not aconsequence of horizontal atmospheric inhomogeneities, but predominantlya depth effect.Based on observations obtained at the European Southern Observatory, LaSilla, Chile.
| Doppler imaging of stellar non-radial pulsations. I. Techniques and numerical experiments
We present a novel technique aimed at the investigation of the geometryof surface velocity fluctuations in non-radially pulsating stars. Ourmethod is based on the Doppler imaging principle and enables an accuratereconstruction of a two-dimensional map of the pulsational disturbancesusing spectroscopic time-series observations. No specific assumptions onthe shape of the pulsation geometry are made. This opens a uniquepossibility to go beyond the traditional mode identification analysesand study complex rotationally and magnetically distorted non-radialpulsation modes which cannot be described by a single spherical harmonicfunction. Performance and intrinsic limitations of the new Dopplerimaging procedure are assessed with inversions of simulated data. Basedon the results of these numerical tests we conclude that a reliablerecovery of the surface pulsation velocity structures can be achievedfor all types of pulsation geometries accompanied by significant lineprofile variations. It is also demonstrated that the Doppler mapping ofthe pulsation velocity field is remarkably robust with respect to errorsin the adopted stellar parameters.
| New measurements of magnetic fields of roAp stars with FORS 1 at the VLT
Magnetic fields play a key role in the pulsations of rapidly oscillatingAp (roAp) stars since they are a necessary ingredient of all pulsationexcitation mechanisms proposed so far. This implies that the properunderstanding of the seismological behaviour of the roAp stars requiresknowledge of their magnetic fields. However, the magnetic fields of theroAp stars are not well studied. Here we present new results ofmeasurements of the mean longitudinal field of 14 roAp stars obtainedfrom low resolution spectropolarimetry with FORS 1 at the VLT.Based on observations obtained at the European Southern Observatory,Paranal, Chile (ESO programme No. 269.D-5044).
| 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).
| The next step in photometric ground-based asteroseismology: Probing stellar interiors from the Concordia Station
The low scintillation noise expected for night time observations at theConcordia station leads to the expectation of scintillation-limitedphotometric noise as much as 10 times lower than can be obtained fromany other site in the world. This is of particular interest for brighthigh frequency pulsators - solar-like pulsators, roAp stars - wherethe noise is scintillation limited. For pulsating white dwarfs stars,sub-dwarf B pulsators, and many other stars of intense asteroseismicinterest, photometric noise will also be less than for other observingsites. A single 2-m photometric telescope at Concordia will be able toproduce higher precision asteroseismic data sets than the best yetobtained by the Whole Earth Telescope (WET) - data obtained by the vasteffort of dozens of astronomers observing at sites all over the world.In just a few years of observations, the cost of such a telescope willbe less than the cost of running WET for the same length of time. The35-cm test telescope will immediately be capable of producingphotometric data sets superior to any obtained heretofore on brightstars such as the roAp stars and solar-like oscillators. Examples usingdata on the roAp stars HR 3831 from one and two sites, and on the roApstar HR 1217 using WET indicate that the level of the highest noisepeaks in the amplitude spectra of light curves obtained with the 35-cmtest telescope should be as low as 6 mu mag for a three-week run on abright roAp star. With a 2-m telescope could come down to 1 mu mag forthe brightest stars.
| 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.
| 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.
| 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
| A high-precision null result in the search for rapid oscillations in HD 965
HD 965 is a strongly magnetic peculiar A star with clear Zeemansplitting in its spectral lines, a rotation period greater than 2 yr,H-line core-wing anomaly, and strongly negative Strömgren deltac1 and delta m1 indices. All of thesecharacteristics are typical of the rapidly oscillating Ap stars, and ithas been suggested that HD 965 should be one. We have tested this starfor photometric variability under excellent conditions with lowscintillation noise using the 2-m TBL for 2.7 hr at Pic du MidiObservatory. The amplitude spectrum of the light curve is flat withnoise peaks below 0.2 mmag in the frequency range of the roAp stars.This suggests that it is a non-oscillating Ap star, although proof ofthat will need further observations each year for many years, to ruleout the possibility that rotational modulation has suppressed theobserved amplitude in 2002.
| 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.
| The pulsational behaviour of the rapidly oscillating Ap star HD 122970 during two photometric multisite campaigns
We undertook two time-series photometric multisite campaigns for therapidly oscillating Ap star HD 122970. The first one, conducted in 1998,resulted in 119h of data and in the detection of three pulsationfrequencies. The presence of possible further modes which held thepromise of deriving a mode identification motivated a second worldwidecampaign in the year 2001. This second campaign resulted in 203h ofmeasurement, but did not reveal further modes. Rather, one of thepreviously detected signals disappeared. The two modes common to bothdata sets have different spherical degree. They also showed slightfrequency modulation, and one of them varied in amplitude as well.Possible causes of the latter behaviour include intrinsic instability ofthe pulsation spectrum or precession of the pulsational axis and orbitalmotion in a binary system. Frequency analysis of the Hipparcosobservations of the star did not allow us to determine the stellarrotation period. The amplitude and phase behaviour of the two modes ofHD 122970 in the Strömgren uvby bands is quite similar to thatobserved for other roAp stars.
| Effects of Spot Structure of Lines of Rare Earths and Non-LTE Effects on Lithium Abundance Estimates for Two roAp Stars
Taking into account blending of the lithium 6108 Å line profile byadjacent rare-earth lines together with their spotted surface structuredoes not appreciably affect lithium abundance estimates for theatmospheres of HD 83368 and HD 60435 but provides a better fit of theobserved and stimulated line profiles. Our computed non-LTE correctionsreduce the lithium abundance estimates by 0.1 0.2 dex for both stars.Given the uncertainties in the lithium abundances, it is not possible tobe certain whether the lithium abundances in roAp stars, or at least intheir spots, exceed the cosmic (primordial) value.
|
Submit a new article
Related links
Submit a new link
Member of following groups:
|
Observation and Astrometry data
| Constellation: | Segel des Schiffs |
| Right ascension: | 09h36m25.30s |
| Declination: | -48°45'05.0" |
| Apparent magnitude: | 6.17 |
| Distance: | 72.464 parsecs |
| Proper motion RA: | -3.9 |
| Proper motion Dec: | -9 |
| B-T magnitude: | 6.553 |
| V-T magnitude: | 6.264 |
Catalogs and designations:
|
