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Reduction of time-resolved space-based CCD photometry developed for MOST Fabry Imaging data*
The MOST (Microvariability and Oscillations of Stars) satellite obtainsultraprecise photometry from space with high sampling rates and dutycycles. Astronomical photometry or imaging missions in low Earth orbits,like MOST, are especially sensitive to scattered light from Earthshine,and all these missions have a common need to extract target informationfrom voluminous data cubes. They consist of upwards of hundreds ofthousands of two-dimensional CCD frames (or subrasters) containing fromhundreds to millions of pixels each, where the target information,superposed on background and instrumental effects, is contained only ina subset of pixels (Fabry Images, defocused images, mini-spectra). Wedescribe a novel reduction technique for such data cubes: resolvinglinear correlations of target and background pixel intensities. Thisstep-wise multiple linear regression removes only those targetvariations which are also detected in the background. The advantage ofregression analysis versus background subtraction is the appropriatescaling, taking into account that the amount of contamination may differfrom pixel to pixel. The multivariate solution for all pairs oftarget/background pixels is minimally invasive of the raw photometrywhile being very effective in reducing contamination due to, e.g. straylight. The technique is tested and demonstrated with both simulatedoscillation signals and real MOST photometry.
| Improved pulsating models of magnetic Ap stars - I. Exploring different magnetic field configurations
Magnetic perturbations to the frequencies of low-degree, high radialorder, axisymmetric pulsations in stellar models permeated bylarge-scale magnetic fields are presented. Magnetic fields with dipolar,quadrupolar and a superposition of aligned dipolar and quadrupolarcomponents are considered. The results confirm that the magnetic fieldmay produce strong anomalies in the power spectra of roAp stars. It isshown for the first time that anomalies may result both from a suddendecrease or a sudden increase of a mode frequency. Moreover, the resultsindicate that the anomalies depend essentially on the geometry of theproblem, that is, on the configuration of the magnetic field and on thedegree of the modes. This dependence opens the possibility of usingthese anomalies as a tool to learn about the magnetic fieldconfiguration in the magnetic boundary layer of pulsating starspermeated by large-scale magnetic fields.
| Spectroscopy of roAp star pulsation: HD 24712 .
We present results of the radial velocity (RV) analysis of spectroscopictime-series observations of the roAp star HD 24712 (HR 1217) which werecarried out simultaneously with the Canadian MOST mini-satellitephotometry. Only lines of the rare-earth elements (REE) show substantialamplitudes of RV pulsations. Based on new Zeeman measurements we founddifferent shapes of the magnetic curves derived by using Fe-peak and REEseparately. Frequency analysis of the spectroscopic data showed that thehighest amplitude frequencies are the same in photometry andspectroscopy. Photometric and spectroscopic pulsation curves are shiftedin phase, and the phase shift depends on the atomic species. Theobserved distribution of RV pulsation amplitudes and phases with theoptical depth as well as the observed phase lag between luminosity andradius variations are explained satisfactorily by the model ofnonadiabatic nonradial pulsations of a magnetic star.
| Nonadiabatic p-mode pulsations of magnetic stars .
Properties of nonadiabatic axisymmetric nonradial pulsations of magneticstars are discussed. Frequencies and the amplitude modulations of theroAp star HR1217 are compared with theoretical ones.
| 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.
| Searching for links between magnetic fields and stellar evolution. I. A survey of magnetic fields in open cluster A- and B-type stars with FORS1
Context: .About 5% of upper main sequence stars are permeated by astrong magnetic field, the origin of which is still matter of debate. Aims: . With this work we provide observational material to studyhow magnetic fields change with the evolution of stars on the mainsequence, and to constrain theory explaining the presence of magneticfields in A and B-type stars. Methods: . Using FORS1 inspectropolarimetric mode at the ESO VLT, we have carried out a survey ofmagnetic fields in early-type stars belonging to open clusters andassociations of various ages. Results: . We have measured themagnetic field of 235 early-type stars with a typical uncertainty of 100 G. In our sample, 97 stars are Ap or Bp stars. For thesetargets, the median error bar of our field measurements was 80 G.A field has been detected in about 41 of these stars, 37 of which werenot previously known as magnetic stars. For the 138 normal A and B-typestars, the median error bar was 136 G, and no field was detected in anyof them.
| Probing the magnetoacoustic boundary layer in the peculiar magnetic star 33 Lib (HD 137949)*
We show unprecedented resolution of the amplitudes and phases of theprincipal 2.015-mHz mode and its 4.030-mHz harmonic as a function ofatmospheric depth in the roAp star 33 Lib using high-resolution VeryLarge Telescope (VLT) Ultraviolet-Visual Echelle Spectrograph (UVES)spectra. We show that the pulsation amplitude increases upwards into theNd III line-forming layer away from an atmospheric node, and increasesdownwards from the same node in the Nd II line-forming layer. We suggestthat the Nd III layer, which arises at, or above, τ~10-3, overlaps with the magnetoacoustic boundary layer, andthat our detailed observations of pulsation amplitude and phasevariations with depth provide a first probe of this layer.
| Pushing the ground-based limit: 14-μmag photometric precision with the definitive Whole Earth Telescope asteroseismic data set for the rapidly oscillating Ap star HR1217
HR1217 is one of the best-studied rapidly oscillating Ap (roAp) stars,with a frequency spectrum of alternating even- and odd-l modes that aredistorted by the presence of a strong, global magnetic field. Severalrecent theoretical studies have found that within the observableatmospheres of roAp stars the pulsation modes are magneto-acoustic withsignificant frequency perturbations that are cyclic with increasingfrequency. To test these theories a Whole Earth Telescope extendedcoverage campaign obtained 342 h of Johnson B data at 10-s timeresolution for the roAp star HR1217 over 35 d with a 36 per cent dutycycle in 2000 November-December. The precision of the derived amplitudesis 14 μmag, making this one of the highest precision ground-basedphotometric studies ever undertaken. Substantial support has been foundfor the new theories of the interaction of pulsation with the strongmagnetic field. In particular, the frequency jump expected as themagnetic and acoustic components cycle through 2π rad in phase hasbeen found. Additionally, comparison of the new 2000 data with anearlier 1986 multisite study shows clear amplitude modulation for somemodes between 1986 and 2000. The unique geometry of the roAp starsallows their pulsation modes to be viewed from varying aspect withrotation, yielding mode identification information in the rotationalsidelobes that is available for no other type of pulsating star. Thoserotational sidelobes in HR1217 confirm that two of the modes aredipolar, or close to dipolar; based on the frequency spacings andHipparcos parallax, three other modes must be either l= 0 or 2 modes,either distorted by the magnetic field, or a mix of m-modes of given lwhere the mixture is the result of magnetic and rotational effects. Astudy of all high-speed photometric Johnson B data from 1981 to 2000gives a rotation period Prot= 12.4572 d, as found in previouspulsation and photometric studies, but inconsistent with a differentrotation period found in magnetic studies. We suggest that this rotationperiod is correct and that zero-point shifts between magnetic data setsdetermined from different spectral lines are the probable cause of thecontroversy over the rotation period. This WET data set is likely tostand as the definitive ground-based study of HR1217. It will be thebaseline for comparison for future space studies of HR1217, particularlythe MOST satellite observations.
| Canada's Little Space Telescope That Could: Another Year of Scientific Surprises From the MOST Microsatellite
At CASCA 2004 in Winnipeg, I announced the first scientific results fromthe MOST (Microvariability & Oscillations of STars) mission,launched in summer 2003. These included the controversial null detectionof acoustic oscillations in the light output of Procyon, at odds withtheory and groundbased spectroscopy, and the first direct measurement ofdifferential rotation in a star other than the Sun, kappa 1 Ceti. A yearlater, by CASCA 2005 in Montreal, I'll be able to share even moreexciting astrophysics, including: ultraprecise photometric studies ofexoplanetary systems like 51 Pegasi, tau Bootis, and HD 209458; newperspectives on massive stars like zeta Oph and the Wolf-Rayet star WR123; the definitive eigenfrequency spectrum of a pulsating chemicallypeculiar star (HR 1217), which provides an acid test for theories ofmagneto-acoustic coupling; and seismology of pulsating protostars in theopen cluster NGC 2264. I'll also return to Procyon, shedding more lighton the oscillation controversy by showing how MOST observations compareto 3-D hydrodynamical simulations of granulation in that star. Andbetween February and May, MOST will monitor a G dwarf and a K dwarf tosearch for true analogues of the solar 5-min oscillations. Too much tofit into 50 minutes, but I'll try to convey a broad flavour of thediverse MOST science (and, as usual, I'll talk really, really fast).
| High Resolution Stellar Spectroscopy with VBT Echelle Spectrometer
The optical design and performance of the recently commissioned fiberfed echelle spectrometer of 2.34 meter Vainu Bappu Telescope aredescribed. The use of it for stellar spectroscopic studies is discussed.
| Results from the Whole Earth Telescope - Indian Contributions
The Indian team at ISRO has been part of the Whole Earth Telescope (WET)team since 1988 when we first participated in the campaign on V471 Tau.We have been part of many other observing campaigns over the lastdecade. This presentation traces the circumstances leading to ourjoining the WET team and how useful the coverage from the Indianlongitude has been. The results of several pulsators from the WET runsduring which we participated are also described. These includePG1159-035 the prototype of the GWVir type of stars, RE J 0751 +14 acataclysmic variable, PG 1336-018 a binary with an sdB pulsator andfinally HR 1217 a roAp star. The paper concludes with what thelimitations are in our obser-vations and how we can overcome them in thefuture.
| 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.
| Statistical Constraints for Astrometric Binaries with Nonlinear Motion
Useful constraints on the orbits and mass ratios of astrometric binariesin the Hipparcos catalog are derived from the measured proper motiondifferences of Hipparcos and Tycho-2 (Δμ), accelerations ofproper motions (μ˙), and second derivatives of proper motions(μ̈). It is shown how, in some cases, statistical bounds can beestimated for the masses of the secondary components. Two catalogs ofastrometric binaries are generated, one of binaries with significantproper motion differences and the other of binaries with significantaccelerations of their proper motions. Mathematical relations betweenthe astrometric observables Δμ, μ˙, and μ̈ andthe orbital elements are derived in the appendices. We find a remarkabledifference between the distribution of spectral types of stars withlarge accelerations but small proper motion differences and that ofstars with large proper motion differences but insignificantaccelerations. The spectral type distribution for the former sample ofbinaries is the same as the general distribution of all stars in theHipparcos catalog, whereas the latter sample is clearly dominated bysolar-type stars, with an obvious dearth of blue stars. We point outthat the latter set includes mostly binaries with long periods (longerthan about 6 yr).
| 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.
| 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).
| 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.
| Radial velocity variations in pulsating Ap stars. IV. First results on HR 1217
In this paper we present the first results from a high-precision radialvelocity (RV) study of the rapidly oscillating Ap (roAp) starHR 1217. Data spanning a complete rotation periodwere acquired on 9 nights in late 1997 and early 1998 using the HarlanJ. Smith 2.7 m telescope at McDonald Observatory. The RVs were measuredusing the wavelength range of each spectral order (≈100 Å) ofthe ``2dcoude'' echelle spectrograph. Most of the pulsational modes canbe seen in all spectral regions but the amplitude varies by factors often or more between the different wavelength intervals. A detailedanalysis of the spectral order centered on 5825 Å, which had thelargest amplitudes, showed the presence of all published photometricfrequencies f1-f7. The multi-mode oscillationswere also found in the RV variations of the high amplitude Nd III 5294Å feature. Our RV data reveal two new excited modes at frequenciesf0 = 223.37 cd-1 (2585.3 μHz) andf-1 = 220.58 cd-1 (2553.0 μHz) that follow theodd and even alternative mode spacing with Δ ν0/2≃ 34 μHz. An analysis of individual nights shows strongamplitude and phase modulation of all excited modes, including the twonewly discovered ones. The amplitudes of the f2 = 229.21cd-1 (2652.9 μHz) and f4 = 235.08cd-1 (2720.85 μHz) modes are modulated with the publishedmagnetic field variations and reach their maximum and minimum atmagnetic extrema. However, the phase variability is in disagreement withthat expected from the standard oblique pulsator model. The phase jumpsfor the f2 mode occur exactly at magnetic maximum and closeto the phase of magnetic minimum, while the f3 andf4 modes have a continuous change. The peculiar phasevariability is attributed to strong vertical phase changes in theline-forming layers of atmosphere. An echelle-diagram for all knownexcited modes in HR 1217 is constructed. We interpretthe f7 = 242.41 cd-1 (2805.7 μHz) mode with the``peculiar'' spacing as due to a mode of degree l = 4 that is the onlyobserved member of another system of equally-spaced frequencies. Wepredict the existence of modes belonging to this system that should beequally spaced at 68 μHz with f7.Based on observations made at McDonald Observatory.
| Rotational periods of four roAp stars
Forty-five new measurements of the mean longitudinal magnetic fields andmean equivalent widths of 4 roAp stars have obtained using the MuSiCoSspectropolarimeter at Pic du Midi observatory. These new high-precisiondata have been combined with archival measurements in order to constrainthe rotational periods of HD 12098, HD 24712 = HR 1217, HD 122970 and HD176232 = 10 Aql. We report a revised rotational period for HD 24712(Prot = 12.45877 ± 0.00016 d, crucial forinterpretation of upcoming MOST observations of this star), newrotational periods for HD 12098 and HD 122970 (Prot=5.460± 0.001 d and Prot=3.877 ± 0.001 d,respectively) and evidence for an extremely long period for HD 176232.Table 1 is only available in electonic form at the CDS via anonymous ftpto cdsarc.u-strasbg.fr (130.79.128.5) or via http: / /cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/429/L55
| 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.
| Radial velocity pulsations in the atmosphere of the roAp star HD 24712
We present results of the analysis of spectroscopic time-seriesobservations of the roAp star HD 24712 obtained at the phase of magneticmaximum. As in previous investigations of roAp stars we found that onlylines of the rare-earth elements (REE) show RV pulsational variations.These pulsations in HD 24712 are characterised by gradual phase shiftsbetween RV curves of different REE. The same pulsations of HD 24712 wereobserved in 2001, giving strong evidence for the stability of thepulsational mechanism in this star. NLTE calculations of Nd line depthformation in stratified atmosphere of HD 24712 allowed us to study thepulsational phase distribution with optical depth, providing strongevidence for a running wave, propagating outward with nearly constant RVamplitude.
| Vertical and horizontal abundance structures of the roAp star HD 24712
High-resolution spectroscopic and spectropolarimetric data of therapidly oscillating Ap star HD 24712 (HR 1217, DO Eri) has been analysedincluding modelling the vertical elemental abundance structures. Westudy the interaction and the relation of the vertical (stratification)and the horizontal (spots) abundance characteristics of Fe and thestellar magnetic field. By this synopsis and the relation of our resultsto the analysis of high resolution and high time resolved observations(Sachkov et al. 2005) we are likely to gain new insights about theatmospheric structure and the geometry, the origin, and the evolution ofthe magnetic fields of roAp stars.
| The effect of the surface distribution of elements on measuring the magnetic field of chemically peculiar stars. The case of the roAp star HD 24712
A non-homogeneous distribution over the stellar surface of the chemicalelements and the presence of a large-scale magnetic field are invoked inorder to explain the periodic line strength, photometric and magneticvariability of magnetic chemically peculiar stars. In such a framework,the variability period is identical to the rotational period. Inmagnetic chemically peculiar stars, the so-called effective magneticfield Heff, the average over the visible stellar disk of thelongitudinal field component weighted by the local line strength, isroutinely measured from Stokes I and V profiles of selected spectrallines. In spite of evidence that the distribution of the variouschemical elements over the stellar surface can be inhomogeneous anddifferent from element to element, Heff values obtained fromlines of different elements are often statistically combined to improvethe accuracy of effective field measurements. Similarly, mean high S/Nprofiles are obtained from the profiles of lines of different elements.We have established, by means of R = 115 000 circular spectropolarimetryof the magnetic chemically peculiar star HD 24712 in the 4700-7000Å range, the dependence of the measurements of Heff onthe atomic weight of 24 elements (from carbon to erbium, if possible atdifferent ionisation states). At all 3 rotational phases considered,Heff values derived from different elements can differ by upto 800 G. We find an overall increase in Heff with atomicnumber and a maximum near Z=60. The behaviour of sodium is quitesingular inasmuch as it always exhibits a negative value of the field,peaking at -0.39 kG when the rare earths give Heff = 1.2 kG.Under the assumption of a dipolar field, we conclude that the elementsgiving the largest values of Heff are concentrated near thepositive polar region and that the other elements are more homogeneouslydistributed over the stellar surface or concentrated in belts around themagnetic equator. Sodium seems to be localised in the negative magnetichemisphere only. This picture is corroborated by the equivalent widthvariability of the lines: up to iron, equivalent widths changesout-of-phase with respect to the Heff variability; elementsheavier than iron present equivalent widths that are variable in-phase.We do not find any relation between the respective amplitudes of theequivalent width variations and the atomic numbers of the elements. Foriron, the observed equivalent width variability does not seem to besimply related to any non-homogeneous distribution over the stellarsurface. We conclude that measurements originating from differentelements cannot in general be combined to improve the precision ofHeff measurements. Indeed, any modelling attempt based on theperiodic variations in Heff is subject to the risk that thesampling of the magnetic field over the stellar surface by the lines ofa given chemical element is uneven or incomplete.In memory of Horace Babcock.
| 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.
| 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 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.
| 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.
| Seismic tests of the structure of rapidly oscillating Ap stars: HR 1217
HR 1217 is probably the best studied multiperiodic rapidly oscillatingAp star, which makes this star a test case for the seismic analysis ofthese complex stars. In this paper, we combine the observational dataavailable for HR 1217 and use these data to constrain theoretical modelsappropriate for this star. In particular, we try to establish whichunknown model parameters are the most relevant to reproduce the seismicdata. We find that the interior chemical composition has more impact inthe theoretical oscillation spectrum of HR 1217 than parameters relatedto the modelling of convection. Models with lower abundances of heavyelements and larger abundances of helium produce oscillation spectrathat are closer to that observed. However, like Matthews et al., we showthat it is difficult to find a plausible model that can reproduce theseismic observations of HR 1217, when the uncertainties in theobservables are not taken into account, unless a complementary effect isadded in order to account for a decrease in the expected large frequencyseparations.
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Osservazione e dati astrometrici
| Costellazione: | Eridano |
| Ascensione retta: | 03h55m16.30s |
| Declinazione: | -12°05'57.0" |
| Magnitudine apparente: | 6 |
| Distanza: | 48.996 parsec |
| Moto proprio RA: | -75.2 |
| Moto proprio Dec: | -28.3 |
| B-T magnitude: | 6.371 |
| V-T magnitude: | 6.029 |
Cataloghi e designazioni:
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