내용
사진
사진 업로드
DSS Images Other Images
관련 글
Radiative decay of the 4d5(6S)5p z5,7Po states in TcII: comparison along the homologous and isoelectronic sequences. Application to astrophysics
Using three independent theoretical approaches (CA, HFR + CP,AUTOSTRUCTURE), oscillator strengths have been calculated for a set ofTcII transitions of astrophysical interest and the reliability of theirabsolute scale has been assessed. The examination of the spectra emittedby some Ap stars has allowed the identification of TcII transitions inHD 125248. This TcII detection should however await confirmation fromspectral synthesis relying on dedicated model atmospheres. New partitionfunctions are also provided for TcI, TcII and TcIII for temperaturesranging between 4000 and 13000 K.
| 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
| Rare-earth elements in the atmosphere of the magnetic chemically peculiar star HD 144897. New classification of the Nd III spectrum
Context: . The chemically peculiar stars of the upper main sequencerepresent a natural laboratory for the study of rare-earth elements(REE). Aims: . We want to check the reliability of the energylevels and atomic line parameters for the second REE ions currentlyavailable in the literature, and obtained by means of experiments andtheoretical calculations. Methods: . We have obtained a UVESspectrum of a slowly rotating strongly magnetic Ap star, HD 144897, thatexhibits very large overabundances of rare-earth elements. Here wepresent a detailed spectral analysis of this object, taking effects ofnon-uniform vertical distribution (stratification) of chemical elementsinto account. Results: . We determined the photosphericabundances of 40 ions. For seven elements (Mg, Si, Ca, Ti, Cr, Mn, Fe),we obtained a stratification model that allows us to produce asatisfactory fit to the observed profiles of spectral lines of variousstrengths. All the stratified elements but Cr show a steep decrease inconcentration toward the upper atmospheric layers; for Cr the transitionfrom high to low concentration regions appears smoother than for theother elements. The REEs abundances, which have been determined for thefirst time from the lines of the first and second ions, have been foundtypically four dex higher than solar abundances. Our analysis of REEspectral lines provides strong support for the laboratory lineclassification and determination of the atomic parameters. The onlyremarkable exception is Nd iii, for which spectral synthesis was foundto be inconsistent with the observations. We therefore performed arevision of the Nd iii classification. We confirmed the energies for 11out of 24 odd energy levels that were classified previously, and derivedthe energies for additional 24 levels of Nd iii, thereby substantiallyincreasing the number of classified Nd iii lines with correctedwavelengths and atomic parameters.
| 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
| Astrophysics in 2005
We bring you, as usual, the Sun and Moon and stars, plus some galaxiesand a new section on astrobiology. Some highlights are short (the newlyidentified class of gamma-ray bursts, and the Deep Impact on Comet9P/Tempel 1), some long (the age of the universe, which will be found tohave the Earth at its center), and a few metonymic, for instance theterm ``down-sizing'' to describe the evolution of star formation rateswith redshift.
| 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.
| Improved oscillator strengths and wavelengths in Hf II, with applications to stellar elemental abundances
Aims. We present new and improved radiative lifetimes for eight levelsin Hf I and 18 levels in Hf II, along with oscillator strengths andwavelengths for 195 transitions in Hf II. With these data we determinethe abundance of hafnium in two chemically peculiar stars: the hot-Amstar HR 3383 and the HgMn star χ Lupi, and discuss the implicationsof the new data to the hafnium abundance for the Sun and the metal-poorgalactic halo stars CS 22892-052 and CS 31082-001. Methods: . Theoscillator strengths are derived by combining radiative lifetimesmeasured with the laser induced fluorescence technique and branchingfractions determined from intensity calibrated Fourier transformspectra. The hafnium abundance in the two sharp-lined peculair stars isdetermined by comparison of spectra obtained from instruments onboardthe Hubble Space Telescope with synthetic spectra, while the abundanceof hafnium in the solar photosphere and the metal-poor halo stars isdiscussed in terms of rescaling previous investigations using the new gfvalues. Results: . The abundance enhancement of hafnium has beendetermined in HR 3383 to be +1.7 dex and that for χ Lupi A is +1.3dex. In the course of the analysis we have also determined an abundanceenhancement for molybdenum in HR 3383 to be +1.2 dex, which is similarto that known for χ Lupi A. The abundances in the metal-poor halostars CS 31082-001 and CS 22892-052 were rescaled to log ɛ(Hf) =-0.75 and -0.82 respectively, with smaller 1σ uncertainty. Thishas the effect of improving the theoretical fits of r-processnucleosynthesis to abundance data for heavy elements. The change of gfvalues also implies that the hafnium abundance in the solar photosphereshould be reduced by up to 0.2 dex, thereby inducing a discrepancy withthe meteoritic hafnium abundance.
| Abundances of vanadium and bromine in 3 Centauri A. Additional odd-Z anomalies
We report abundance excesses of 1.2 and 2.6 dex, respectively, forvanadium and bromine in the hot, peculiar star 3 Cen A. Abundances forthese two odd-Z elements have not been previously reported for thisstar. Taken with previous work, they strengthen the case of the originof the abundance peculiarities by diffusion.
| 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.
| Radiative parameters for some transitions in the spectrum of AgII
Radiative parameters for transitions depopulating the levels belongingto the 4d85s2, 4d96s and4d95d configurations of AgII have been obtained from acombination of theoretical lifetimes and experimental branchingfractions. On the experimental side, a laser-produced plasma was used asa source of Ag+ ions. The light emitted by the plasma wasanalysed by a grating monochromator coupled with a time-resolved opticalmultichannel analyser system. Spectral response calibration of theexperimental system was performed using a deuterium lamp in thewavelength range from 200 to 400 nm, and a standard tungsten lamp in therange from 350 to 600 nm. The transition probabilities were obtainedfrom measured branching ratios and theoretical radiative lifetimes ofthe corresponding states calculated with a relativistic Hartree-Fockapproach including core-polarization effects and configurationinteraction in an extensive way. Theoretical and experimental data havebeen compared and the new data have also been compared with the fewprevious results available in the literature.
| Oscillator strength calculations in neutral technetium
Ab initio multiconfiguration calculations are performed for theoscillator strengths of the λ= 4238, 4262 and 4297 ÅTcIresonance lines of astrophysical interest. Electron correlation istreated through multiconfiguration expansions built from elaboratecorrelation models, while relativistic effects are introduced in theperturbation Breit-Pauli approximation or in the multiconfigurationDirac-Fock-Breit variational approach. The calculated gf-values aresensitively lower (~30 per cent) than the values obtained with thepseudo-relativistic Hartree-Fock wavefunctions calculated from aparametric analysis of TcI by Palmeri & Wyart. The inclusion of acore-polarization potential in the latter approach confirms the presentab initio results when different ionic cores are used for the differenttransition arrays. The strong lines of TcI are revisited adopting thismodel, giving rise to a systematic reduction in the oscillator strengthscale due to core polarization. The astrophysical implications arediscussed.
| Detection of an extraordinarily large magnetic field in the unique ultra-cool Ap star HD 154708
We have discovered an extraordinarily large mean longitudinal magneticfield of 7.5 kG in the ultra-cool low mass Ap starHD 154708 using FORS 1 in spectropolarimetric mode. FromUVES spectra, we have measured a mean magnetic field modulus of24.5 kG. This is the second-largest mean magnetic field modulusever measured in an Ap star. Furthermore, it is very likely thatthis star is one of the coolest and least massive among the Ap stars andis located in the H-R diagram in the same region in which rapidlyoscillating Ap stars have been detected. We note that all known roApstars have much smaller magnetic fields, by at least a factor of three.
| A New Isotopic Abundance Anomaly in Chemically Peculiar Stars
Not Available
| 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.
| 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.
| 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).
| The Enigma of Przybylski's Star
Not Available
| 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.
| On the possible presence of promethium in the spectra of HD 101065 (Przybylski's star) and HD 965
% Traditional and statistical line-identification methods indicate thepresence of Pm I and II, Tc I, and perhaps Tc II in the spectrum of theroAp star HD 101065. These methods also lead to the presence of Pm IIand probably also Pm I in a related cool Ap star, HD 965. Thespectroscopic evidence is strong enough that we would declare promethiumto be present without hesitation, if any of its isotopes were stable.The longest-lived promethium isotope has a half-life of only 17.7 years.The presence of this element would mean that unrecognized processes -perhaps flare activities - are taking place in the atmospheres of thesestars. The significance of such processes for galactic chemicalevolution cannot be ruled out. We discuss the possibility that thehighly improbable wavelength coincidences are due to chance, or due tocontamination of the laboratory sources.Based on observations collected at the European Southern Observatory, LaSilla (Chile), as part of programme No. 60.E-0564, and Paranal (Chile),as part of programmes Nos. 68.D-0254 and 70.D-0470.
| The inclusion of magnetic splitting of lines in the determination of the lithium abundance and isotopic ratio in the atmosphere of the roAp-star HD 101065
Calculations of synthetic spectra with the Zeeman magnetic splitting oflithium and rare earth (Ce II, Nd II, Sm II and others) lines for theroAp-star HD 101065 (Przybylski's star) in a spectral region of 670.58to 670.87 nm were carried out to fit to an observed spectrum near thelithium line λ = 670.8 nm. The hyperfine structure of lithiumlines are taken into account in detailed calculations. This allowed usto evaluate the influence of magnetic splitting on estimates of thelithium abundance in the star atmosphere and a value of the isotopicratio 6Li/7Li.
| 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 spectrum of the roAp star HD 101065 (Przybylski's star) in the Li I 6708 Åspectral region
We carried out a detailed analysis of spectra of the unique roAp star HD101065 (Przybylski's star) near the resonance doublet Li I 6708 Å,using a most complete line list including all possible transitionsbetween REE levels of the NIST database. Our model calculations wereperformed under two assumptions: a blend of Li and REE lines, and ablend of REE lines only. They prove that Li lines are present in therange 6707.72-6708.02 Å, and that the resulting Li abundance is3.1 dex (in the scale log N(H) = 12.0), while the isotopic ratio6Li/7Li is near to 0.3.Based on observations collected at the European Southern Observatory, LaSilla, Chile (programme 56.E-0640).
|
새 글 등록
관련 링크
새 링크 등록
다음 그룹에 속해있음:
|
관측 및 측정 데이터
천체목록:
|
