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Ýlgili Makaleler
Follow-up Studies of the Pulsating Magnetic White Dwarf SDSS J142625.71+575218.3
We present a follow-up analysis of the unique magneticluminosity-variable carbon-atmosphere white dwarf SDSSJ142625.71+575218.3. This includes the results of some 106.4 hr ofintegrated light photometry which have revealed, among other things, thepresence of a new periodicity at 319.720 s which is not harmonicallyrelated to the dominant oscillation (417.707 s) previously known in thatstar. Using our photometry and available spectroscopy, we consider thesuggestion made by Montgomery et al. that the luminosity variationsin SDSS J142625.71+575218.3 may not be caused by pulsationalinstabilities, but rather by photometric activity in acarbon-transferring analog of AM CVn. This includes a detailed searchfor possible radial velocity variations due to rapid orbital motion onthe basis of Multiple Mirror Telescope spectroscopy. At the end of theexercise, we unequivocally rule out the interacting binary hypothesisand conclude instead that, indeed, the luminosity variations are causedby g-mode pulsations as in other pulsating white dwarfs. This is in linewith the preferred possibility put forward by Montgomery et al.
| A 3D study of the photosphere of HD99563 - I. Pulsation analysis
We have used high-speed spectroscopy of the rapidly oscillating Ap(roAp) star HD99563 to study the pulsation amplitude and phase behaviourof elements in its stratified atmosphere over one 2.91-d rotation cycle.We identify spectral features related to patches in the surfacedistribution of chemical elements and study the pulsation amplitudes andphases as the patches move across the stellar disc. The variations areconsistent with a distorted non-radial dipole pulsation mode. We measurea 1.6kms-1 rotational variation in the mean radial velocitiesof H? and argue that this is the first observation of H?abundance spots caused by He settling through suppression of convectionby the magnetic field on an oblique rotator, in support of a primetheory for the excitation mechanism of roAp star pulsation. Wedemonstrate that HD99563 is the second roAp star to show aspectdependence of blue-to-red running wave line profile variations in NdIIIspots.Based on observations collected at the European Southern Observatory(ESO), Paranal, Chile (programmes 072.D-0138, 078.D-0192), and at theSubaru Telescope, which is operated by the National AstronomicalObservatory of Japan (programme S07A-005).E-mail: lmfreyhammer@uclan.ac.uk
| Asteroseismology of chemically peculiar stars
Paper published in the proceedings of the Wroc?aw HELAS Workshop``Interpretation of Asteroseismic Data'', CoAst, 157, 228 (Kochukhov2008).Pulsational variability is observed in several types of main sequencestars with anomalous chemical abundances. In this contribution Isummarize the relationship between pulsations and chemicalpeculiarities, giving special emphasis to rapid oscillations in magneticAp stars. These magneto-acoustic pulsators provide unique opportunitiesto study the interaction of pulsations, chemical inhomogeneities, andstrong magnetic fields. Time-series monitoring of rapidly oscillating Apstars using high-resolution spectrometers at large telescopes andultra-precise space photometry has led to a number of importantbreakthroughs in our understanding of these interesting objects.Interpretation of the roAp frequency spectra has allowed constrainingfundamental stellar parameters and probing poorly known properties ofthe stellar interiors. At the same time, investigation of thepulsational wave propagation in chemically stratified atmospheres ofroAp stars has been used as a novel asteroseismic tool to studypulsations as a function of atmospheric height and to map in detail thehorizontal structure of the magnetically-distorted p modes.
| Photometric variability of the T Tauri star TW Hya on time-scales of hours to years
Microvariability & Oscillations of STars (MOST) and All SkyAutomated Survey (ASAS) observations have been used to characterizephotometric variability of TW Hya on time-scales from a fraction of aday to 7.5 weeks and from a few days to 8 yr, respectively. The two datasets have very different uncertainties and temporal coverage propertiesand cannot be directly combined, nevertheless, they suggest a globalvariability spectrum with `flicker-noise' properties, that is withamplitudes , over >4 decades in frequency, in the range f =0.0003-10cd-1. A 3.7d period is clearly present in thecontinuous 11d, 0.07d time resolution, observations by MOST in 2007.Brightness extrema coincide with zero-velocity crossings in periodic(3.56d) radial-velocity variability detected in contemporaneousspectroscopic observations of Setiawan et al. and interpreted as causedby a planet. The 3.56/3.7d periodicity was entirely absent in thesecond, 4 times longer MOST run in 2008, casting doubt on the planetaryexplanation. Instead, a spectrum of unstable single periods within therange of 2-9d was observed; the tendency of the periods to progressivelyshorten was well traced using the wavelet analysis. The evolvingperiodicities and the overall flicker-noise characteristics of the TWHya variability suggest a combination of several mechanisms, with thedominant ones probably related to the accretion processes from the discaround the star.Based on data from the MOST satellite, a Canadian Space Agency missionjointly operated by Dynacon Inc., the University of Toronto Institutefor Aerospace Studies and the University of British Columbia, with theassistance of the University of Vienna, and on data from the All SkyAutomated Survey (ASAS) conducted by the Warsaw University Observatory,Warsaw, Poland at the Las Campanas Observatory, Chile. E-mail: rucinski@astro.utoronto.ca
| A search for rapid pulsations in the magnetic cool chemically peculiar star HD3980
The Ap star HD3980 appears to be a promising roAp candidate based on itsfundamental parameters, leading us to search for rapid pulsations withthe VLT UV-Visual Echelle Spectrograph (UVES). A precise Hipparcosparallax and estimated temperature of 8100K place HD3980 in the middleof the theoretical instability strip for rapidly oscillating Ap stars,about halfway through its main-sequence evolution stage. The star has astrong, variable magnetic field, as is typical of the cool magnetic Apstars. Dipole model parameters were determined from VLT observationsusing Focal Reducer and low Dispersion Spectrograph (FORS)1. FromDoppler shift measurements for individual spectral lines of rare-earthelements and the H? line core, we find no pulsations above20-30ms-1. This result is corroborated by the inspection oflines of several other chemical elements, as well as withcross-correlation for long spectral regions with the average spectrum asa template. Abundances of chemical elements were determined and showlarger than solar abundances of rare-earth elements. Further, ionizationdisequilibria for the first two ionized states of Nd and Pr aredetected. We also find that the star has a strong overabundance ofmanganese, which is typical for much hotter HgMn and other Bp stars.Line profile variability with the rotation period was detected for themajority of chemical species.Based on observations collected at the European Southern Observatory(ESO), Paranal, Chile, as part of programme 077.D-0150(A) and part ofprogrammes 074.D-0392(A) and 076.D-0535(A) in the ESO archive.E-mail: velkin@uclan.ac.uk
| On the spectroscopic nature of the cool evolved Am star HD151878
Recently, Tiwari, Chaubey & Pandey detected the bright component ofthe visual binary HD151878 to exhibit rapid photometric oscillationsthrough a Johnson B filter with a period of 6min (2.78mHz) and a high,modulated amplitude up to 22mmag peak-to-peak, making this star by farthe highest amplitude rapidly oscillating Ap (roAp) star known. As a newroAp star, HD151878 is of additional particular interest as a scarceexample of the class in the northern sky, and only the second known caseof an evolved roAp star - the other being HD116114. We used theFIbre-fed Echelle Spectrograph at the Nordic Optical Telescope to obtainhigh time-resolution spectra at high dispersion to attempt to verify therapid oscillations. We show here that the star at this epoch isspectroscopically stable to rapid oscillations of no more than a fewtens of ms-1. The high-resolution spectra furthermore showthe star to be of type Am rather than Ap and we show the star lacks mostof the known characteristics for roAp stars. We conclude that this is anAm star that does not pulsate with a 6-min period. The originaldiscovery of pulsation is likely to be an instrumental artefact.Based on observations collected at the Nordic Optical Telescope as partof programme 36-418. E-mail: lfreyham@gmail.com
| The longitudinal magnetic field of the roAp star HD 99563
Based on spectropolarimetric observations, we report the first detectionof a periodic angular variation of the longitudinal magnetic field forthe roAp star HD 99563.
| New insights into the nature of the peculiar star ? Carinae
Context: ? Carinae belongs to a group of peculiar early-typestars (OBN) with enhanced nitrogen and carbon deficiency. It is alsoknown as a binary system, but it is not clear yet whether the chemicalanomalies can be explained by mass transfer between the two components.On the basis of the previously reported spectral variability of a fewmetal lines it may be expected that ? Car possesses a weakmagnetic field. Aims: A study of the physical nature of this hotmassive binary which is furthermore a well-known blue straggler lying ~2mag above the turnoff of the young open cluster IC 2602 is important tounderstand the origin of its strong chemical anomalies. Methods:We acquired high resolution spectroscopic and low resolutionspectropolarimetric observations to achieve the following goals: a) toimprove the orbital parameters to allow a more in-depth discussion onthe possibility of mass transfer in the binary system; b) to carry out anon-local thermodynamic equilibrium (NLTE) abundance analysis; and c) tosearch for the presence of a magnetic field. Results: The studyof the radial velocities using CORALIE spectra allowed us tosignificantly improve the orbital parameters. A comparative NLTEabundance analysis was undertaken for ? Car and two other earlyB-type stars with recently detected magnetic fields, ? Sco and?1 CMa. The analysis revealed significantly differentabundance patterns: a one-order-of-magnitude nitrogen overabundance andcarbon depletion was found in ? Car, while the oxygen abundanceis roughly solar. For the stars ?1 CMa and ? Sco thecarbon abundance is solar and, while an N excess is also detected, it isof much smaller amplitude (0.4-0.6 dex). Such an N overabundance istypical of the values already found for other slowly-rotating (magnetic)B-type dwarfs. For ? Car, we attribute instead the chemicalpeculiarities to a past episode of mass transfer between the two binarycomponents. The results of the search for a magnetic field using FORS 1at the VLT consisting of 26 measurements over a time span of ~1.2 h arerather inconclusive: only few measurements have a significance level of3?. Although we detect a periodicity of the order of ~8.8 min inthe dataset involving the measurements on all hydrogen Balmer lines withthe exception of the H? and H? lines, these results have tobe confirmed by additional time-resolved magnetic field observations.Based on observations collected with the CORALIE spectrograph attachedto the Euler Telescope of the Geneva Observatory located at La Silla(Chile), at the European Southern Observatory, Paranal, Chile (ESOprogrammes 67.D-0239(A), 072.D-0377(A), 078.D-0080(A) and278.D-5056(A)), and at the Complejo Astronómico El Leoncito(CASLEO), Argentina.
| Magnetic fields of chemically peculiar stars. I. The catalog of magnetic CP stars
This is the first paper of the series dedicated to the analysis of themagnetism of chemically peculiar (CP) stars of the upper Main Sequence.We use our own measurements and published data to compile a catalog ofmagnetic CP stars containing a total of 326 objects with confidentlydetected magnetic fields and 29 stars which are very likely to possessmagnetic field. We obtained the data on the magnetism of theoverwhelming majority of the stars solely based on the analysis oflongitudinal field component B e . The surface magneticfield, B s , has been measured for 49 objects. Our analysisshows that the number of magnetic CP stars decreases with increasingfield strength in accordance with exponential law, and stars with B e exceeding 5kG occur rarely (about 3% objects of ourlist).
| Pulsation properties of the rapidly oscillating Ap star 10Aquilae (HD176232)
We have studied the pulsation behaviour of radial velocities ofindividual spectral lines of different chemical elements for the wellknown, bright rapidly oscillating Ap (roAp) star 10Aql (HD176232) basedon high-quality Very Large Telescope spectra. We found that only linesof rare earth elements and the narrow H? core show detectablerapid variations of radial velocity with the known photometric pulsationperiod, 11.7min. The highest pulsation amplitudes of more than0.5kms-1 are found for rather weak lines of TbIII and DyIII.The pulsational phases obtained for lines of TbIII and DyIII differ by? rad, perhaps an indication of a radial node between theirline-forming layers. Weak lines of SmII also display amplitudes near0.5kms-1. Surprisingly, the lines of PrIII show the lowestdetected pulsation amplitude in strong contrast with the majority ofroAp stars for which this ion has some of the highest radial velocityamplitudes. Lines of NdIII with different intensities have pulsationamplitudes different to the weaker lines that are formed more deeply inthe atmosphere having higher amplitudes. Line bisectors for strong NdIIIline profiles show significant changes of phase, and even phase jumpsfor some lines, indicating complex variations in the pulsation phase asa function of atmospheric depth. The low-measured rotational velocity ofthis star indicates that there is little Doppler resolution of the lineprofiles, hence that the large range of pulsation amplitudes and phasesof radial velocities determined for line bisectors are primarily causedby atmospheric depth effects.Based on observations collected at the European Southern Observatory,Paranal, Chile, as part of programme 077.D-0150(A).E-mail: velkin@uclan.ac.uk
| A search for rapid pulsations among nine luminous Ap stars
The rapidly oscillating Ap (roAp) stars are of importance for studyingthe atmospheric structure of stars where the process of chemical elementdiffusion is significant. We have performed a survey for rapidoscillations in a sample of nine luminous Ap stars, selected from theirlocation in the colour-magnitude diagram as more evolved main-sequenceAp stars that are inside the instability strip for roAp stars. Untilrecently this region was devoid of stars with observed rapid pulsations.We used the Very Large Telescope UV-Visual Echelle Spectrograph toobtain high time resolution spectroscopy to make the first systematicspectroscopic search for rapid oscillations in this region of the roApinstability strip. We report nine null detections with upper limits forradial velocity amplitudes of 20-65ms-1 and precisions of? = 7-20ms-1 for combinations of Nd and Pr lines.Cross-correlations confirm these null results. At least six stars aremagnetic and we provide magnetic field measurements for four of them, ofwhich three are newly discovered magnetic stars. It is found that fourstars have magnetic fields smaller than ~2kG, which according totheoretical predictions might be insufficient for suppressing envelopeconvection around the magnetic poles for more evolved Ap stars.Suppression of convection is expected to be essential for the opacitymechanism acting in the hydrogen ionization zone to drive thehigh-overtone roAp pulsations efficiently. Our null results suggest thatthe more evolved roAp stars may require particularly strong magneticfields to pulsate. Three of the studied stars do, however, have magneticfields stronger than 5kG.Based on observations collected at the European Southern Observatory,Paranal, Chile, as part of programmes 075.D-0145 (A), 078.D-0080(A),072.D-0138(A) and 077.D-0150(A).E-mail: lmfreyhammer@uclan.ac.uk
| The discovery of high-amplitude, 10.9-minute oscillations in the cool magnetic Ap star HD 115226
We present the discovery of pulsational variations in the cool magneticAp star HD 115226 - the first high-amplitude rapidly oscillating Ap(roAp) star discovered with time-series spectroscopy. Usinghigh-resolution spectra obtained with the HARPS instrument at theEuropean Southern Observatory 3.6-m telescope, we detect radial velocityvariations with a period of 10.86 min in Pr III, Nd III, Dy III lines,and in the narrow cores of hydrogen lines. Pulsational amplitudes exceed1 km s-1 in individual lines of Nd III. The presence ofrunning waves in the stellar atmosphere is inferred from a phase shiftbetween the radial velocity maxima of rare-earth and hydrogen lines. Ourabundance analysis demonstrates that HD 115226 exhibits a typical roApspectroscopic signature, notably ionization anomaly of Pr, Nd, and Dy.We discuss the discovery of pulsations in HD 115226 in the context ofrecent spectroscopic studies of roAp stars and point to the existence ofa correlation between spectroscopic pulsational amplitude and thestellar rotation rate.Based on observations collected at the European Southern Observatory, LaSilla, Chile (ESO program 079.D-0118).
| Pulsation tomography of rapidly oscillating Ap stars. Resolving the third dimension in peculiar pulsating stellar atmospheres
Aims.We present detailed analysis of the vertical pulsation modecross-section in ten rapidly oscillating Ap (roAp) stars based onspectroscopic time-series observations. The aim of this analysis is toderive from observations a complete picture of how the amplitude andphase of magnetoacoustic waves depend on depth. Methods: We use theunique properties of roAp stars, in particular chemical stratification,to resolve the vertical structure of p-modes. Our approach consists ofcharacterising pulsational behaviour of a carefully chosen, butextensive sample of spectral lines. We analyse the resultingamplitude-phase diagrams and interpret observations in terms ofpulsation wave propagation. Results: We find common features in thepulsational behaviour of roAp stars. Within a sample of representativeelements the lowest amplitudes are detected for Eu ii (and Fe in 33 Liband in HD 19918), then pulsations go through the layers where Hαcore, Nd, and Pr lines are formed. There RV amplitude reaches itsmaximum, and after that decreases in most stars. The maximum RV of thesecond REE ions is always delayed relative to the first ions. Thelargest phase shifts are detected in Tb iii and Th iii lines.Pulsational variability of the Th iii lines is detected here for thefirst time. The Y ii lines deviate from this picture, showing even loweramplitudes than Eu ii lines but half a period phase shift relative toother weakly pulsating lines. We measured an extra broadening,equivalent to a macroturbulent velocity from 4 to 11-12 kms-1 (where maximum values are observed for Tb iii and Th iiilines), for pulsating REE lines. The surface magnetic field strength isderived for the first time for three roAp stars: HD 9289 (2 kG), HD12932 (1.7 kG), and HD 19918 (1.6 kG). Conclusions: The roAp starsexhibit similarity in the depth-dependence of pulsation phase andamplitude, indicating similar chemical stratification and comparablevertical mode cross-sections. In general, pulsations waves arerepresented by a superposition of the running and standing wavecomponents. In the atmospheres of roAp stars with the pulsationfrequency below the acoustic cut-off frequency, pulsations have astanding-wave character in the deeper layers and behave like a runningwave in the outer layers. Cooler roAp stars develop a running wavehigher in the atmosphere. In stars with pulsation frequency close to theacoustic cut-off one, pulsation waves have a running character startingfrom deep layers. The transition from standing to running wave isaccompanied by an increase in the turbulent broadening of spectrallines.Based on observations made with the SAO 6-m telescope, with theCanada-France-Hawaii Telescope, and the ESO VLT (DDT programme274.D-5011 and programme 072.D-0138, retrieved through the ESO archive).Table 3 and Figs. 5, 6, 8-10, 12, 13 and 15-18 are only available inelectronic form at http://www.aanda.org
| The detection of the very low amplitude 16.2-min pulsation in individual lines of first ionization stage of rare earth elements in the roAp star ?CrB†
With high precision spectroscopic data obtained with Ultraviolet andVisual Echelle Spectrograph (UVES) on the Very Large Telescope (VLT) wedetect radial velocity pulsation with a period of 16.2min and withamplitudes of about 30ms-1 in individual lines of the firstionization stage of rare earth elements, and in H?, with typicalprecision in the range 2-8ms-1 for a single line, and betterthan 1ms-1 for some ensembles of lines. The second ionizationstages of rare earth elements show weak or no detectable pulsation,while lines of FeI or FeII show no signal at high precision. Anabundance analysis shows the rare earth elements to be overabundant byabout 1dex, rather than the 2-3dex often seen in other rapidlyoscillating Ap (roAp) stars, and it shows the presence of the ionizationdisequilibrium for Nd and Pr that is typical of most roAp stars. Thepulsational radial velocity curve reaches maximum for the lines of therare earth elements before it does for H? which is opposite to thecase for most other roAp stars with implications for the stratificationlevel of the rare earth elements. Similarities between ?CrB and thelongest period roAp star, HD116114, suggest that many more low-amplituderoAp stars await detection, with implications for understanding thepulsation driving mechanism and mode selection mechanism in these stars.
| Pulsation in the presence of a strong magnetic field: the roAp star HD166473
Phase-resolved high-resolution, high signal-to-noise ratio (S/N)observations of the strongly magnetic roAp star HD166473 are analysed.HD166473 was selected as the target of this study because it has one ofthe strongest magnetic fields of all the roAp stars known with resolvedmagnetically split lines. Indeed, we show that enhanced pulsationdiagnosis can be achieved from consideration of the different pulsationbehaviour of the π and σ Zeeman components of the resolvedspectral lines. This study is based on a time-series of high spectralresolution observations obtained with the Ultraviolet and Visual EchelleSpectrograph of the Very Large Telescope of the European SouthernObservatory. Radial velocity variations due to pulsation are observed inrare earth lines, with amplitudes up to 110ms-1. Thevariations occur with three frequencies, already detected in photometry,but which can in this work be determined with better precision: 1.833,1.886 and 1.928mHz. The pulsation amplitudes and phases observed in therare earth element lines vary with atmospheric height, as is the case inother roAp stars studied in detail. Lines of Fe and of other (mostlynon-rare earth) elements do not show any variation to very highprecision (1.5ms-1 in the case of Fe). The low amplitudes ofthe observed variations do not allow the original goal of studyingdifferences between the behaviour of the resolved Zeeman line componentsto be reached; the S/N achieved in the radial velocity determinations isinsufficient to establish definitely the possible existence of suchdifferences. Yet the analysis provides a tantalizing hint at theoccurrence of variations of the mean magnetic field modulus with thepulsation frequency, with an amplitude of 21 +/- 5G.Based on observations collected at the European Southern Observatory,Paranal, Chile, as part of programme 067.D-0272.E-mail: gmathys@eso.org
| Line profile variations in rapidly oscillating Ap stars: resolution of the enigma
We have carried out the first survey of the pulsational line profilevariability in rapidly oscillating Ap (roAp) stars. We analysed highsignal-to-noise ratio time-series observations of 10 sharp-lined roApstars obtained with the high-resolution spectrographs attached to theVery Large Telescope and Canada-France-Hawaii Telescope. We investigatedin detail the variations of PrIII, NdII, NdIII and TbIII lines anddiscovered a prominent change of the profile variability pattern withheight in the atmospheres of all studied roAp stars. We show that, inevery investigated star, profile variability of at least one rare-earthion is characterized by unusual blue-to-red moving features, which wepreviously discovered in the time-resolved spectra of the roAp starγ Equ. This behaviour is common in rapidly rotating non-radialpulsators but is inexplicable in the framework of the standard obliquepulsator model of slowly rotating roAp stars. Using analysis of the lineprofile moments and spectrum synthesis calculations, we demonstrate thatunusual oscillations in spectral lines of roAp stars arise from thepulsational modulation of linewidths. This variation occursapproximately in quadrature with the radial velocity changes, and itsamplitude rapidly increases with height in stellar atmosphere. Wepropose that the linewidth modulation is a consequence of the periodicexpansion and compression of turbulent layers in the upper atmospheresof roAp stars. Thus, the line profile changes observed in slowlyrotating magnetic pulsators should be interpreted as a superposition oftwo types of variability: the usual time-dependent velocity field due toan oblique low-order pulsation mode and an additional linewidthmodulation, synchronized with the changes of stellar radius. Ourexplanation of the line profile variations of roAp stars solves thelong-standing observational puzzle and opens new possibilities forconstraining geometric and physical properties of the stellarmagnetoacoustic pulsations.Based on observations collected at the European Southern Observatory(Paranal, La Silla), at the Canada-France-Hawaii Telescope and on dataretrieved from the ESO Science Archive.E-mail: oleg@astro.uu.se
| Pulsation in the atmosphere of the roAp star HD 24712. I. Spectroscopic observations and radial velocity measurements
Aims:We have investigated the structure of the pulsating atmosphere ofone of the best studied rapidly oscillating Ap stars, HD 24712. Methods: For this purpose we analyzed spectra collected during2001-2004. An extensive data set was obtained in 2004 simultaneouslywith the photometry of the Canadian MOST mini-satellite. This allows usto connect directly atmospheric dynamics observed as radial velocityvariations with light variations seen in photometry. Results: Wedirectly derived for the first time and for different chemical elements,respectively ions, phase shifts between photometric and radial velocitypulsation maxima indicating, as we suggest, different line formationdepths in the atmosphere. This allowed us to estimate for the first timethe propagation velocity of a pulsation wave in the outer stellaratmosphere of a roAp star to be slightly lower than the sound speed. Weconfirm large pulsation amplitudes (150-400 m s-1) for REElines and the H? core, while spectral lines of the other elements(Mg, Si, Ca, and Fe-peak elements) have nearly constant velocities. Wedid not find different pulsation amplitudes and phases for the lines ofrare-earth elements before and after the Balmer jump, which supports thehypothesis of REE concentration in the upper atmosphere above thehydrogen line-forming layers. We also discuss radial velocity amplitudesand phases measured for individual spectral lines as tools for a 3Dtomography of the atmosphere of HD 24712.Based on observations collected at the Canada-France-Hawaii Telescope(CFHT), at the Nordic Optical Telescope (NOT), at the European SouthernObservatory, Paranal, Chile, (DDT-274.D-5011), at the TelescopioNazionale Galileo (TNG), and from MOST, a Canadian Space Agency missionoperated jointly by Dynacon, Inc., the University of Toronto Instituteof Aerospace Studies, and the University of British Columbia, withassistance from the University of Vienna. Tables 4, 5 and Fig. 9 areonly available in electronic form at http://www.aanda.org
| Pulkovo compilation of radial velocities for 35495 stars in a common system.
Not Available
| On the roAp star status of β Coronae Borealis
β CrB is one of the best-studied of the magnetic Ap stars. Threeindependent investigations have suggested that this star is pulsatingwith a period of either 6.1 min, 11.5 min or 16.2 min, making this arapidly oscillating Ap star. The presence of pulsations in β CrBhas important implications for the understanding of pulsation drivingand damping in roAp stars, and each study has called for additionalobservations to confirm the suggested pulsations. New high timeresolution, high spectral resolution, high signal-to-noise spectra ofβ CrB obtained with the high resolution spectrograph SARG on the3.55-m Telescopio Nazionale Galileo are unable to confirm any of thesuggested periods. There is no indication of any variability with aperiod near 6.1 min. Studies of Fe i lines suggest that the 11.5-minperiod is spurious. Studies of Ce ii lines do not find the 16.2-minperiod suggested for one Ce ii line, but are not precise enough to testthe finding of 16.2-min oscillations for a large section of spectrum,hence the case for the 16.2-min period is still good. An extensiveinvestigation of β CrB is needed to resolve the issue.
| The discovery of 8.0-min radial velocity variations in the strongly magnetic cool Ap star HD154708, a new roAp star
HD154708 has an extraordinarily strong magnetic field of 24.5kG. Using2.5h of high time resolution Ultraviolet and Visual Echelle Spectrograph(UVES) spectra we have discovered this star to be an roAp star with apulsation period of 8min. The radial velocity amplitudes in the rareearth element lines of NdII, NdIII and PrIII are unusually low -~60ms-1 - for an roAp star. Some evidence suggests that roApstars with stronger magnetic fields have lower pulsation amplitudes.Given the central role that the magnetic field plays in the obliquepulsator model of the roAp stars, an extensive study of the relation ofmagnetic field strength to pulsation amplitude is desirable.Based on observations collected at the European Southern Observatory,Paranal, Chile, as part of programme 075.D-0145.E-mail: dwkurtz@uclan.ac.uk
| The discovery of a new type of upper atmospheric variability in the rapidly oscillating Ap stars with VLT high-resolution spectroscopy
In a high-resolution spectroscopic survey of rapidly oscillating Ap(roAp) stars with the Ultraviolet and Visual Echelle Spectrograph on theVery Large Telescope of the European Southern Observatory, we find thatalmost all stars show significant variation of the radial velocityamplitudes - on a time-scale of a few pulsation cycles - for lines ofthe rare earth ion PrIII and in the core of the Hα line. Thesevariations in the radial velocity amplitudes are described by newfrequencies in the amplitude spectra that are not seen in broad-bandphotometric studies of the same stars. The PrIII lines form high in theatmosphere of these stars at continuum optical depths oflogτ5000 <= -5 and tend to be concentrated towards themagnetic poles in many stars, and the core of the Hα line forms atcontinuum optical depths -5 <= logτ5000 <= -2,whereas the photometry samples the atmosphere on average at continuumoptical depths closer to logτ5000 = 0 and averages overthe visible hemisphere of the star. Therefore, there are three possibleexplanations for the newly discovered frequencies: (1) there are modeswith nodes near to the level where the photometry samples that can beeasily detected at the higher level of formation of the PrIII lines; or(2) there are higher degree, l, non-radial oblique pulsation modes thatare detectable in the spectroscopy because the PrIII is concentratedtowards the magnetic poles where such modes have their highestamplitudes, but average out over the visible hemisphere in thephotometry which samples the star's surface more uniformly; or (3) thereis significant growth and decay of the principal mode amplitudes on atime-scale of just a few pulsation cycles at the high level of formationof the PrIII lines and core of the Hα line. The third hypothesisimplies that this level is within the magneto-acoustic boundary layerwhere energy is being dissipated by both outward acoustic running wavesand inward magnetic slow waves. We suggest observations that candistinguish among these three possibilities. We propose that strongchanges in pulsation phase seen with atmospheric height in roAp stars,in some cases more than π rad from the top to the bottom of a singlespectral line, strongly affect the pulsation phases seen in photometryin various bandpasses which explains why phase differences betweenbandpasses for roAp stars have never been explicable with standardtheories that assume single spherical harmonics within the observableatmosphere. We also discuss the photometric amplitude variations as afunction of bandpass, and suggest that these are primarily caused bycontinuum variations, rather than by variability in the rare earthelement lines. We propose further tests of this suggestion.Based on observations collected at the European Southern Observatory,Paranal, Chile, as part of programme 072.D-0138.E-mail: dwkurtz@uclan.ac.uk
| Evolution of magnetic fields in stars across the upper main sequence: I. Catalogue of magnetic field measurements with FORS 1 at the VLT
To properly understand the physics of Ap and Bp stars it is particularlyimportant to identify the origin of their magnetic fields. For that, anaccurate knowledge of the evolutionary state of stars that have ameasured magnetic field is an important diagnostic. Previous resultsbased on a small and possibly biased sample suggest that thedistribution of magnetic stars with mass below 3 M_ȯ in the H-Rdiagram differs from that of normal stars in the same mass range (Hubriget al. 2000). In contrast, higher mass magnetic Bp stars may well occupythe whole main-sequence width (Hubrig, Schöller & North 2005b).In order to rediscuss the evolutionary state of upper main sequencemagnetic stars, we define a larger and bias-free sample of Ap and Bpstars with accurate Hipparcos parallaxes and reliably determinedlongitudinal magnetic fields. We used FORS 1 at the VLT in itsspectropolarimetric mode to measure the magnetic field in chemicallypeculiar stars where it was unknown or poorly known as yet. In thisfirst paper we present our results of the mean longitudinal magneticfield measurements in 136 stars. Our sample consists of 105 Ap and Bpstars, two PGa stars, 17 HgMn stars, three normal stars, and nine SPBstars. A magnetic field was for the first time detected in 57 Ap and Bpstars, in four HgMn stars, one PGa star, one normal B-type star and fourSPB stars.
| Evolutionary state of magnetic chemically peculiar stars
Context: .The photospheres of about 5-10% of the upper main sequencestars exhibit remarkable chemical anomalies. Many of these chemicallypeculiar (CP) stars have a global magnetic field, the origin of which isstill a matter of debate. Aims: .We present a comprehensivestatistical investigation of the evolution of magnetic CP stars, aimedat providing constraints to the theories that deal with the origin ofthe magnetic field in these stars. Methods: .We have collectedfrom the literature data for 150 magnetic CP stars with accurateHipparcos parallaxes. We have retrieved from the ESO archive 142 FORS1observations of circularly polarized spectra for 100 stars. From thesespectra we have measured the mean longitudinal magnetic field, anddiscovered 48 new magnetic CP stars (five of which belonging to the rareclass of rapidly oscillating Ap stars). We have determined effectivetemperature and luminosity, then mass and position in the H-R diagramfor a final sample of 194 magnetic CP stars. Results: .We foundthat magnetic stars with M > 3 ~M_ȯ are homogeneouslydistributed along the main sequence. Instead, there are statisticalindications that lower mass stars (especially those with M ≤2~M_ȯ) tend to concentrate in the centre of the main sequence band.We show that this inhomogeneous age distribution cannot be attributed tothe effects of random errors and small number statistics. Our datasuggest also that the surface magnetic flux of CP stars increases withstellar age and mass, and correlates with the rotation period. For starswith M > 3~M_ȯ, rotation periods decrease with age in a wayconsistent with the conservation of the angular momentum, while for lessmassive magnetic CP stars an angular momentum loss cannot be ruledout. Conclusions: .The mechanism that originates and sustains themagnetic field in the upper main sequence stars may be different in CPstars of different mass.
| 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 .
| Rapid radial velocity variations in the cool roAp star HD 99563.
In a high resolution spectroscopic study of the rapidly oscillating Ap(roAp) star HD 99563 with the Ultraviolet-Visual Echelle Spectrograph(UVES) on the Very Large Telescope (VLT) we have discovered remarkablylarge amplitude pulsations with some spectral lines showing radialvelocity amplitudes up to 5 km s-1 (10 km s-1peak-to-peak) with a pulsation period of 10.7 min.Based on observations obtained at ESO, as part of programme 072.D-0138.
| 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.
| The Dushak-Erekdag Survey of roAp Stars
The search of roAp stars at Mt. Dushak-Erekdag Observatory was startedin 1992 using the 0.8 m Odessa telescope equipped with a two-starhigh-speed photometer. We have observed more than a dozen stars so farand discovered HD 99563 as roAp star while BD +8087 is suspected to haverapid oscillations. Negative results of our observations for the searchof rapid oscillations in four stars in NGC 752 are also discussed.
| Asteroseismology: Past, Present and Future
Asteroseismology studies stars with a wide variety of interior andsurface conditions. For two decades asteroseismic techniques have beenapplied to many pulsating stars across the HR diagram. Asteroseismologyis now a booming field of research with stunning new discoveries; Ihighlight a personal selection of these in this review, many of whichare discussed in more detail elsewhere in these proceedings. For manyyears the Nainital-Cape Survey for northern roAp stars has been runningat ARIES, so I emphasise new spectroscopic results for roAp stars andpoint out the outstanding prospects for the planned ARIES 3-m telescopeat Devastal. High precision spectroscopy has revolutionised theasteroseismic study of some types of stars - particularly solar-likeoscillators and roAp stars - while photometry is still the best way tostudy the frequency spectra that are the basic data of asteroseismology.New telescopes, new photo-meters and space missions are revolutionisingasteroseismic photometry. In addition to the ground-based potential ofasteroseismic spectroscopy, India has the knowledge and capability forspace-based asteroseismic photometry. The future for asteroseismology isbright indeed, especially for Indian astronomers.
| 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.
| 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).
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| Takýmyýldýz: | Kupa |
| Sað Açýklýk: | 11h27m16.64s |
| Yükselim: | -08°52'08.2" |
| Görünürdeki Parlaklýk: | 8.677 |
| Uzaklýk: | 234.192 parsek |
| özdevim Sað Açýklýk: | -43.4 |
| özdevim Yükselim: | 8.7 |
| B-T magnitude: | 8.949 |
| V-T magnitude: | 8.7 |
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