Contenidos
Imágenes
Subir su imagen
DSS Images Other Images
Artículos relacionados
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
| The diagnosis of the mean quadratic magnetic field of Ap stars
Aims.We assess the validity of the method of determination of the meanquadratic field modulus and we explore its limits. Methods.Weanalyse high spectral resolution, high signal-to-noise spectra of a fewAp stars, and of a superficially normal main-sequence A star, recordedover a broad wavelength range with EMMI at the NTT.Results.Weintroduce a revised form of the regression equation describing thedependence of the second-order moment of the line profiles about theircentre, in natural light, on various parameters of the correspondingtransitions. We show that interpretation of the observed dependencesallows one to determine the mean quadratic magnetic field modulus of thestudied stars, and their v sin i. We explain why the contributions tothe quadratic field of the mean square magnetic field modulus and of themean square longitudinal field cannot in general be disentangled. Forthose stars of the sample that have resolved magnetically split lines,we show that the derived values of the quadratic field are mostlyconsistent with the values of the mean longitudinal magnetic field andof the mean magnetic field modulus at the observed phase. However thereare some hints that they may occasionally slightly underestimate theactual field. This suggests that the method is unlikely to yieldspurious field detections. In addition, we illustrate the importance forthis type of analyses of using, as far as possible, samples of lines ofa single ion, and to specify in the presentation of the results whichion was used.Conclusions.The results presented in this paper lendstrong support to the validity of the quadratic field diagnostic methodto obtain a realistic quantitative characterisation of the magneticfields Ap and related stars.
| Lithium and the 6Li-7Li isotope ratio in the atmospheres of some sharp-lined roAp stars
The λ 6708 Å and 6103 Å lithium lines in thehigh-resolution spectra of some sharp-lined roAp stars are analyzedusing three spectral-synthesis codes STARSP, ZEEMAN2, and SYNTHM. Thelines from the VALD database were supplemented with lines of rare-earthelements from the DREAM database and new lines calculated using the NISTenergy levels. Our synthetic-spectrum calculations take into accountmagnetic splitting and other line-broadening effects. Lithiumoverabundances were found in the atmospheres of the stars based on ouranalysis of both lithium lines, along with high values of the 6Li-7Liisotope ratio (0.2 0.5). This can be explained if lithium is produced inspallation reactions and the surface 6Li and 7Li is preserved by strongmagnetic fields in the upper layers of the stellar atmospheres, aroundthe poles of the dipole field. The asymmetry of lithium lines to the redmay be due to the action of shocks in the optically thin upperatmosphere, with the shocked material subsequently falling onto thestar.
| 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.
| 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.
| 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.
| 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.
| NLTE ionization equilibrium of Nd II and Nd III in cool A and Ap stars
We investigate the formation of Nd ii- iii lines in the atmospheres ofA-type stars with a comprehensive atomic model including 1651 levels ofNd ii, 607 levels of Nd iii and the ground state of Nd iv. NLTE leads tooverionization of Nd ii which weakens the Nd ii lines relative to thecorresponding LTE line strengths at mild neodymium overabundance ([Nd/H]< 2.5) and amplifies them at higher [Nd/H] values. NLTE abundancecorrections grow with the effective temperature and reach 0.6 dex atT_eff = 9500 K for [Nd/H] = 2.5. The Nd iii lines are strengthenedcompared with LTE in all cases, and NLTE abundance corrections liebetween -0.3 dex and -0.2 dex for T_eff between 7500 K and 9500 K. NLTEeffects are larger for an inhomogeneous vertical abundance distributioncompared with a homogeneous one resulting in positive NLTE abundancecorrection up to 1.3 dex for the Nd ii lines and in negative ones downto -0.5 dex for the Nd iii lines. The neodymium distribution in theatmospheres of roAp stars γ Equ and HD 24712 is deduced from NLTEanalysis of the Nd ii and Nd iii lines and a strong evidence is foundfor the existence of enhanced Nd abundance layers abovelogτ5000 = -3.
| New Photometry of the roAp Star 33 Lib
New photometric observations of 33 Lib show that a new frequencydetected spectroscopically in radial velocity variations is definitelynot present in broad-band photometric variations.
| 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.
| 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.
| Temperature Behavior of Elemental Abundances in the Atmospheres of Magnetic Peculiar Stars
We analyze the temperature dependence of the abundances of the chemicalelements Si, Ca, Cr, and Fe in the atmospheres of normal, metallic-line(Am), magnetic peculiar (Ap), and pulsating magnetic peculiar (roAp)stars in the range 6000 15000 K. The Cr and Fe abundances in theatmospheres of Ap stars increase rapidly as the temperature rises from6000 to 9000 10000 K. Subsequently, the Cr abundance decreases to valuesthat exceed the solar abundance by an order of magnitude, while the Feabundance remains enhanced by approximately +1.0 dex compared to thesolar value. The temperature dependence of the abundances of theseelements in the atmospheres of normal and Am stars is similar in shape,but its maximum is several orders of magnitude lower than that observedfor Ap stars. In the range 6000 9500 K, the observed temperaturedependences for Ap stars are satisfactorily described in terms ofelement diffusion under the combined action of gravitational settlingand radiative acceleration. It may well be that diffusion also takesplace in the atmospheres of normal stars, but its efficiency is very lowdue to the presence of microturbulence. We show that the magnetic fieldhas virtually no effect on the Cr and Fe diffusion in Ap stars in therange of effective temperatures 6000 9500 K. The Ca abundance and itsvariation in the atmospheres of Ap stars can also be explained in termsof the diffusion model if we assume the existence of a stellar wind witha variable moderate rate of (2 4) × 10- 15 M ȯ yr-1.
| 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.
| The discovery of a luminous, rapidly oscillating Ap star, HD 116114, with a 21-minute pulsation period*
The discovery of a new rapidly oscillating Ap star, HD 116114, with apulsation period of 21 min, using high-resolution spectra obtained withthe Ultraviolet-Visual Echelle Spectrograph at the European SouthernObservatory's Very Large Telescope, is presented. The highest amplitudesof the radial velocity variations are between 50 and 125 ms-1 visible in the EuII lines. The spectral lines of LaII andthe core of the Hα line have amplitudes of about 30 ms-1. The frequency obtained for the oscillations is in goodagreement with theoretical predictions of longer-period, evolved roApstars. The distinction in luminosity between the roAp and noAp stars,and the suggestion that in all roAp stars the abundance of the secondions of Pr and Nd, relative to the abundance of the first ions, isanomalously high, need to be revised in the light of this discovery.
| 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.
| 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.
| The calcium isotopic anomaly in magnetic CP stars
Chemically peculiar stars in the magnetic sequence can show the sameisotopic anomaly in calcium previously discovered for mercury-manganesestars in the non-magnetic sequence. In extreme cases, the dominantisotope is the exotic 48Ca. Measurements of Ca II linesarising from 3d-4p transitions reveal the anomaly by showing shifts upto 0.2 Å for the extreme cases - too large to be measurementerrors. We report measurements of miscellaneous objects, including twometal-poor stars, two apparently normal F-stars, an Am-star, and theN-star U Ant. Demonstrable anomalies are apparent only for the Ap stars.The largest shifts are found in rapidly oscillating Ap stars and in oneweakly magnetic Ap star, HD 133792. We note the possible relevance ofthese shifts for the GAIA mission.Based on observations obtained at the European Southern Observatory, LaSilla and Paranal, Chile (ESO programme Nos. 65.L-0316, 68.D-0254 and266.D-5655).
| 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 5200-Åflux depression of chemically peculiar stars - II. The cool chemically peculiar and λ Bootis stars
After establishing the synthetic Δa photometric system in thefirst paper of this series, we now present model atmospheres computedwith individual abundances for a representative sample of chemicallypeculiar (CP) stars and either confirm or redetermine their inputparameters through comparisons with photometric, spectrophotometric andhigh-resolution spectroscopic data. The final models obtained from thisprocedure were used to compute synthetic Δa indices which werecompared with observations. The observed behaviour of Δa isreproduced for several types of CP stars: models for Am stars shownegligible (or marginally positive) values of a few mmag, while forλ Bootis stars - and for metal deficient A stars in general - weobtain negative values (as low as -12 mmag in one case). For the coolestCP2 stars with effective temperatures below about 8500 K, we obtain mild(~+10 mmag) to moderately large (~+30 mmag) flux depressions inagreement with observations. However, Δa values for slightlyhotter members of the CP2 group (for which still Teff <10000 K) are underestimated from these new models. The effect of themicroturbulence parameter on the Δa index is revisited and itsdifferent role in various types of CP stars for reproducing the fluxdepression at 5200 Åis explained. We also provide reasons whymodels based on enhanced microturbulence and scaled solar abundancecould not explain the observed flux depression for all types of CPstars. We discuss potential improvements of the current models includingthe possibility of still missing line opacities (unidentified andautoionization lines), modifications due to an explicit account of aglobal stellar magnetic field, and the effect of vertical abundancestratification.
| The spectroscopic signature of roAp stars
To reliably determine the spectroscopic signature of rapidly oscillatingchemically peculiar (roAp) stars it is also necessary to investigate asample of non pulsating chemically peculiar (noAp) as well as presumably``normal'' stars. We describe in this study the sample ofspectroscopically investigated stars and comment on the techniques usedfor the analysis. In particular we discuss ionization disequilibria ofrare earths in roAp stars that distinguish them from noAp stars. In thelight of the recently discovered pulsation of β CrB we seearguments that all magnetic CP2 stars up to a transition temperature ofabout 8100 K may be pulsating.Based on observations obtained at the European Southern Observatory (LaSilla, Chile), the Canadian-French-Hawaii telescope, the South AfricaAstronomical Observatory, The Crimean Astrophysical Observatory and onnumerous SIMBAD interrogations.
| Probable detection of radial magnetic field gradients in the atmospheres of Ap stars
For the first time the possible presence of radial gradients of magneticfields in the atmospheres of three magnetic Ap stars has been criticallyexamined by measurements of the mean magnetic field modulus fromspectral lines resolved into magnetically split components lying on thedifferent sides of the Balmer jump. A number of useful diagnostic linesbelow and above the Balmer discontinuity, only slightly affected byblends, with simple doublet and triplet Zeeman pattern have beenidentified from the comparison between synthetic spectra computed withthe SYNTHMAG code and the high resolution and S/N spectra obtained inunpolarized light with the ESO VLT UVES spectrograph. For all threestars of our sample, HD 965, HD 116114 and 33 Lib, an increase of themagnetic field strength of the order of a few hundred Gauss has beendetected bluewards of the Balmer discontinuity. These results should betaken into account in future modelling of the geometric structure of Apstar magnetic fields and the determination of the chemical abundances inAp stars with strong magnetic fields.Based on observations obtained at the European Southern Observatory,Paranal, Chile (ESO program No. 70.D-0470).
| Radial velocity variations in pulsating Ap stars - III. The discovery of 16.21-min oscillations in β CrB
We present the analysis of 3 h of a rapid time series of precisestellar radial velocity (RV) measurements (σ= 4.5 ms-1) of the cool Ap star β CrB. The integrated RVmeasurements spanning the wavelength interval 5000-6000 Åshowsignificant variations (false alarm probability = 10-5) witha period of 16.21 min (ν= 1028.17 μHz) and an amplitude of 3.54+/- 0.56 m s-1. The RV measured over a much narrowerwavelength interval reveals one spectral feature at λ6272.0Åpulsating with the same 16.21-min period and an amplitude of 138+/- 23 m s-1. These observations establish β CrB to be alow-amplitude rapidly oscillating Ap star.
| 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.
| 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
| Magnetic field measurements of Ap stars. Discovery of a strong magnetic field in HD 18610
HD 18610 was included in our project on abundance determination ofrapidly oscillating (roAp) and non-oscillating (noAp) cool chemicallypeculiar stars. Already a preliminary investigation of the raw spectraindicated the presence of an intense magnetic field which was confirmedby a more detailed analysis.To determine the magnetic field strength we primarily used the Fe IIline at 6149.26 Å, a strong line which is split in a anomalousZeeman doublet. HD 18610 is a Cr-Eu-Sr non-pulsating Ap star with aTeff and log g typical for roAp stars. The classification ofthis star as a CP2 star (Preston \cite{Preston}) was confirmed withDelta a photometry by Maitzen & Vogt (\cite{MV83}). In the presentinvestigation we derived a mean magnetic field modulus of |vec {B}|=5700 +/- 200 G.
|
Enviar un nuevo artículo
Enlaces relacionados
- - No se han encontrado enlaces -
En viar un nuevo enlace
Miembro de los siguientes grupos:
|
Datos observacionales y astrométricos
| Constelación: | Libra |
| Ascensión Recta: | 15h29m34.74s |
| Declinación: | -17°26'27.4" |
| Magnitud Aparente: | 6.675 |
| Distancia: | 89.206 parsecs |
| Movimiento Propio en Ascensión Recta: | -69.1 |
| Movimiento Propio en Declinación: | 5.7 |
| B-T magnitude: | 7.126 |
| V-T magnitude: | 6.713 |
Catálogos y designaciones:
|
