Contenuti
Immagini
Carica la tua immagine
DSS Images Other Images
Articoli relazionati
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 rapidly oscillating Ap star HD 99563 and its distorted dipole pulsation mode
We undertook a time-series photometric multisite campaign for therapidly oscillating Ap (roAp) star HD 99563 and also acquired mean lightobservations over four seasons. The pulsations of the star, which showflatter light maxima than minima, can be described with a frequencyquintuplet centred on 1557.653 μHz and some first harmonics of it.The amplitude of the pulsation is modulated with the rotation period ofthe star that we determine with 2.91179 +/- 0.00007 d from the analysisof the stellar pulsation spectrum and of the mean light data. We breakup the distorted oscillation mode into its pure spherical harmoniccomponents and find it is dominated by the l= 1 pulsation, and also hasa notable l= 3 contribution, with weak l= 0 and 2 components. Thegeometrical configuration of the star allows us to see both pulsationpoles for about the same amount of time; HD 99563 is only the fourthroAp star for which both pulsation poles are seen and only the thirdwhere the distortion of the pulsation modes has been modelled. We pointout that HD 99563 is very similar to the well-studied roAp star HR 3831.Finally, we note that the visual companion of HD 99563 is located in theδ Scuti instability strip and may thus show pulsation. We showthat if the companion was physical, the roAp star would be a2.03-Msolar, object, seen at a rotational inclination of44°, which then predicts a magnetic obliquity .
| 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.
| Evolution of magnetic fields in stars across the upper main sequence: I. Catalogue of magnetic field measurements with FORS 1 at the VLT
To properly understand the physics of Ap and Bp stars it is particularlyimportant to identify the origin of their magnetic fields. For that, anaccurate knowledge of the evolutionary state of stars that have ameasured magnetic field is an important diagnostic. Previous resultsbased on a small and possibly biased sample suggest that thedistribution of magnetic stars with mass below 3 M_ȯ in the H-Rdiagram differs from that of normal stars in the same mass range (Hubriget al. 2000). In contrast, higher mass magnetic Bp stars may well occupythe whole main-sequence width (Hubrig, Schöller & North 2005b).In order to rediscuss the evolutionary state of upper main sequencemagnetic stars, we define a larger and bias-free sample of Ap and Bpstars with accurate Hipparcos parallaxes and reliably determinedlongitudinal magnetic fields. We used FORS 1 at the VLT in itsspectropolarimetric mode to measure the magnetic field in chemicallypeculiar stars where it was unknown or poorly known as yet. In thisfirst paper we present our results of the mean longitudinal magneticfield measurements in 136 stars. Our sample consists of 105 Ap and Bpstars, two PGa stars, 17 HgMn stars, three normal stars, and nine SPBstars. A magnetic field was for the first time detected in 57 Ap and Bpstars, in four HgMn stars, one PGa star, one normal B-type star and fourSPB stars.
| 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.
| Evolutionary state of magnetic chemically peculiar stars
Context: .The photospheres of about 5-10% of the upper main sequencestars exhibit remarkable chemical anomalies. Many of these chemicallypeculiar (CP) stars have a global magnetic field, the origin of which isstill a matter of debate. Aims: .We present a comprehensivestatistical investigation of the evolution of magnetic CP stars, aimedat providing constraints to the theories that deal with the origin ofthe magnetic field in these stars. Methods: .We have collectedfrom the literature data for 150 magnetic CP stars with accurateHipparcos parallaxes. We have retrieved from the ESO archive 142 FORS1observations of circularly polarized spectra for 100 stars. From thesespectra we have measured the mean longitudinal magnetic field, anddiscovered 48 new magnetic CP stars (five of which belonging to the rareclass of rapidly oscillating Ap stars). We have determined effectivetemperature and luminosity, then mass and position in the H-R diagramfor a final sample of 194 magnetic CP stars. Results: .We foundthat magnetic stars with M > 3 ~M_ȯ are homogeneouslydistributed along the main sequence. Instead, there are statisticalindications that lower mass stars (especially those with M ≤2~M_ȯ) tend to concentrate in the centre of the main sequence band.We show that this inhomogeneous age distribution cannot be attributed tothe effects of random errors and small number statistics. Our datasuggest also that the surface magnetic flux of CP stars increases withstellar age and mass, and correlates with the rotation period. For starswith M > 3~M_ȯ, rotation periods decrease with age in a wayconsistent with the conservation of the angular momentum, while for lessmassive magnetic CP stars an angular momentum loss cannot be ruledout. Conclusions: .The mechanism that originates and sustains themagnetic field in the upper main sequence stars may be different in CPstars of different mass.
| The 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.
| An elemental abundance analysis of the magnetic chemically peculiar star HR 8216
An elemental abundance analysis of the cool magnetic CP star HR 8216 (=HD204411) was performed using 2.4 Åmm-1 DominionAstrophysical Observatory Reticon exposures coveringλλ3830-4770 with a typical signal-to-noise ratio of 200taken with the long camera of the 1.22-m telescope. The spectrogramswere measured interactively with the graphics computer program REDUCE.The fine analysis used an ATLAS9 metal-rich model atmosphere, thepredictions of which best-matched the optical region fluxes and theHγ profile. The anomalies of HR 8216 are primarily an enhancementof many iron peak elements with the rare earths elements much lessenhanced than in many similar cool magnetic CP stars. Using the resultsof the fine analysis the spectrum was synthesized. Comparison with theobserved spectrum showed that the agreement is good but did not fullyaccount for all of the observed line spectrum.
| 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.
| 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.
| 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
| 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.
| 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).
| 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.
| The 77th Name-List of Variable Stars
We present the next regular Name-List of variable stars containinginformation on 1152 variable stars recently designated in the system ofthe General Catalogue ofVariable Stars. New designations are alsointroduced for two ``old'' variables whose corrected coordinates bringthem to other constellations.
| On the influence of Stark broadening on Si I lines in stellar atmospheres
We study the influence of Stark broadening and stratification effects onSi I lines in the rapidly oscillating (roAp) star 10 Aql, where the SiI,6142.48 Å and 6155.13 Å lines are asymmetrical andshifted. First we have calculated Stark broadening parameters using thesemiclassical perturbation method for three Si I lines: 5950.2 Å,6142.48 Å and 6155.13 Å. We revised the synthetic spectrumcalculation code taking into account both Stark width and shift forthese lines. From the comparison of our calculations with theobservations we found that Stark broadening + the stratification effectcan explain asymmetry of the Si I 6142.48 Å and 6155.13 Ålines in the atmospere of roAp star 10 Aql.
| Atmospheric parameters and chemical composition of the ultra-cool roAp star HD 213637
A high-resolution spectrum obtained with the UVES instrument at the ESOVLT is used to determine atmospheric parameters and chemical compositionof the roAp star HD 213637. Resolved Zeeman split lines are discoveredin the spectrum of this star, indicating a mean field modulus of 5.5 kG.Effective temperature, Teffv{6400} determined from theprofiles of the hydrogen Hα and Hβ lines, makes HD 213637one of the coolest (ro)Ap stars known, suggesting that the class ofmagnetic chemically peculiar stars extends to lower effectivetemperatures than previously thought. Furthermore, the low temperatureof HD 213637 poses serious challenge to some excitation mechanismssuggested as an explanation of the roAp phenomenon. This peculiar staris also unique in its evolutionary stage: it is either located near thevery end of its main sequence life or has already ascended the giantbranch. Abundance analysis reveals that HD 213637 is iron-deficient andhas abundance pattern typical of the cool pulsating Ap stars. Aninconsistency between abundances derived from the weak and strongspectral lines and the lines of different ions provide a clear evidencefor prominent vertical abundance stratification of many chemicalspecies, notably Na, Mg, Ca, Fe, Pr, Nd, Tb, and Er.Based on observations obtained at the European Southern Observatory,VLT.
| A theoretical instability strip for rapidly oscillating Ap stars
The mechanism responsible for exciting high-order acoustic oscillationsin rapidly oscillating Ap stars is still an open issue. Recently,Balmforth et al. (hereafter BCDGV) proposed a model according to whichhigh-frequency oscillations may be excited in roAp stars if theintensities of the magnetic fields present in these stars aresufficiently large to suppress convection at least in some region oftheir envelopes. Using models similar to those proposed by BCDGV, wepredict the theoretical edges of the instability strip appropriate toroAp stars and compare them with the observations. Moreover, we discussour results in the light of some of the systematic differences foundbetween roAp stars, noAp stars and Ap stars in general. We suggest thata combination of intrinsic differences between these types of stars anda bias related to the frequencies of the unstable oscillations mighthold the explanation to some of the differences observed.
| The pulsational behaviour of the rapidly oscillating Ap star HD 122970 during two photometric multisite campaigns
We undertook two time-series photometric multisite campaigns for therapidly oscillating Ap star HD 122970. The first one, conducted in 1998,resulted in 119h of data and in the detection of three pulsationfrequencies. The presence of possible further modes which held thepromise of deriving a mode identification motivated a second worldwidecampaign in the year 2001. This second campaign resulted in 203h ofmeasurement, but did not reveal further modes. Rather, one of thepreviously detected signals disappeared. The two modes common to bothdata sets have different spherical degree. They also showed slightfrequency modulation, and one of them varied in amplitude as well.Possible causes of the latter behaviour include intrinsic instability ofthe pulsation spectrum or precession of the pulsational axis and orbitalmotion in a binary system. Frequency analysis of the Hipparcosobservations of the star did not allow us to determine the stellarrotation period. The amplitude and phase behaviour of the two modes ofHD 122970 in the Strömgren uvby bands is quite similar to thatobserved for other roAp stars.
| Measurement of lifetimes by laser-induced fluorescence and determination of transition probabilities of astrophysical interest in Nd III
Selective lifetime measurements by time-resolved laser-inducedfluorescence spectroscopy for 5 levels belonging to the 4f35dconfiguration of doubly ionized neodymium provide a first and usefulexperimental test of the relativistic Hartree-Fock calculations in thision of astrophysical interest. As a consequence, the accuracy of thetransition probabilities deduced in the present work is well assessed.These new data are expected to help astrophysicists in the future torefine the analysis of the composition of chemically peculiar starswhich frequently show large overabundances of lanthanides when comparedto the solar system standards.
| Abundance stratification and pulsation in the atmosphere of the roAp star boldmath gamma Equulei
We present the evidence for abundance stratification in the atmosphereof the rapidly oscillating Ap star gamma Equ. Ca, Cr, Fe, Ba, Si, Naseem to be overabundant in deeper atmospheric layers, but normal tounderabundant in the upper layers with a transition in the typical lineforming region of -1.5
| A Study of Rare Earth Elements in the Atmospheres of Chemically Peculiar Stars. Pr III and Nd III Lines
We determine the abundances of Pr and Nd in the atmospheres of magneticand non-magnetic chemically peculiar stars from the lines of rare earthelements in the first and second ionization states. The computations forthe magnetic stars take into account the influence of the magnetic fieldon line formation. We studied the influence of errors in thestellar-atmosphere parameters and the atomic parameters of the spectrallines on the accuracy of abundance determinations. Within the derivedaccuracy, ionization equilibrium is satisfied in the atmospheres ofnon-pulsating magnetic and non-magnetic stars (so that abundancesderived separately from lines of first and second ions agree). For allthe pulsating magnetic (roAp) stars studied, the abundances derived fromlines of second ions are 1.0 to 1.7 dex higher than those derived fromfirst ions. The violation of ionization equilibrium in the atmospheresof pulsating stars is probably due to, first, considerable enrichment ofPr and Nd in the uppermost atmospheric layers, and second, a higherlocation for the layer of enhanced elemental abundance in roAp starsthan in non-pulsating stars. Two objects from the list of non-pulsatingmagnetic stars, HD 62140 and HD 115708, exhibit anomalies of their Prand Nd lines characteristic of roAp stars. The differences in the rareearth anomalies for the pulsating and non-pulsating peculiar stars canbe used as a selection criterion for candidate roAp stars.
| The Wolfgang and Amadeus Automatic Photoelectric Telescopes. A ``Kleine-Nacht-Musik'' during the first five years of routine operation
We present a summary of five years of continuous operation of theUniversity of Vienna twin Automatic Photoelectric Telescopes (APTs) --Wolfgang and Amadeus. These two telescopes are part of the FairbornObservatory facility located in the Sonoran desert close to WashingtonCamp in southern Arizona. The detection and distinction procedurebetween weather-induced data-quality loss and systematic data-qualityloss turned out to be a crucial task. Therefore, special emphasis islaid on the data quality monitoring tools developed throughout theyears. Furthermore, we summarize the scientific highlights from thefirst five years of operation
| The core-wing anomaly of cool Ap stars. Abnormal Balmer Profiles
The profiles of Hα in a number of cool Ap stars are anomalous.Broad wings, indicative of temperatures in the range 7000-8000 K endabruptly in narrow cores. The widths of these cores are compatible withthose of dwarfs with temperatures of 6000 K or lower. This profile hasbeen known for Przybylski's star, but it is seen in other cool Ap stars.The Hβ profile in several of these stars shows a similar core-winganomaly (CWA). In Przybylski's star, the CWA is probably present athigher Balmer members. We are unable to account for these profileswithin the context of LTE and normal dwarf atmospheres. We conclude thatthe atmospheres of these stars are not ``normal''. This is contrary to anotion that has long been held. Based on observations obtained at theEuropean Southern Observatory, Paranal, Chile (ESO programme No.65.I-0644) and the South African Astronomical Observatory (SAAO).}
| Abundances in Przybylski's star
We have derived abundances for 54 elements in the extreme roAp starHD101065. ESO spectra with a resolution of about 80000, and S/N of 200or more were employed. The adopted model has Teff=6600K, andlog(g)=4.2. Because of the increased line opacity and consequent low gaspressure, convection plays no significant role in the temperaturestructure. Lighter elemental abundances through the iron group scatterabout standard abundance distribution (SAD) (solar) values. Iron andnickel are about one order of magnitude deficient while cobalt isenhanced by 1.5dex. Heavier elements, including the lanthanides,generally follow the solar pattern but enhanced by 3 to 4dex. Odd-Zelements are generally less abundant than their even-Z neighbours. Witha few exceptions (e.g. Yb), the abundance pattern among the heavyelements is remarkably coherent, and resembles a displaced solardistribution.
| A Proposal for Multicolor WET Photometric Campaign on roAp Stars.
A review of the asteroseismology of the rapidly oscillating Ap (roAp)stars based on the simultaneous or multicolor photometry is presented.So far, multicolor photometric studies of roAp stars are based on singlefrequencies. In this paper we propose simultaneous multicolorphotometric study on multi-mode roAp stars such as HR 1217. HR 1217 hassix modes, three of which are of comparable and large enough amplitudesto be detectable in several wavelengths. With a WET campaign on a starlike HR 1217, we could resolve the individual modes, and monitor each indifferent colors for amplitude and phase information. This study is alsowell suited for mode-identification.
| Observing roAp Stars with WET: A Primer
We give an extensive primer on roAp stars -- introducing them, puttingthem in context and explaining terminology and jargon, and giving athorough discussion of what is known and not known about them. Thisprovides a good understanding of the kind of science WET could extractfrom these stars. We also discuss the many potential pitfalls andproblems in high-precision photometry. Finally, we suggest a WETcampaign for the roAp star HR 1217.
| Ab initio calculations of oscillator strengths and Landé factors for Nd Iii
Theoretical calculations of oscillator strengths and Landég-factors made with the Cowan code are presented for the rare-earth ionNd Iii. The emphasis is on transitions in the optical region of thespectrum. Comparisons of previous ab initio calculations of this typefor other lanthanides (e.g., La Ii, Ce Iii, and Lu Ii) with publishedexperimental and theoretical data suggest that the accuracy of thelog(gf) values for individual transitions should be about +/-0.15 dex inthe absence of significant core polarization effects; the g-factorsshould be good to better than 5%, except in a few cases where termmixing is important. Applications of the data are made in thecalculation of neodymium abundances in the atmospheres of the chemicallypeculiar stars HD 101065 (``Przybylski's star") andHD 122970.
|
Sottometti un nuovo articolo
Link relazionati
Sottometti un nuovo link
Membro dei seguenti gruppi:
|
Osservazione e dati astrometrici
| Costellazione: | Vergine |
| Ascensione retta: | 14h04m48.94s |
| Declinazione: | +05°24'51.5" |
| Magnitudine apparente: | 8.297 |
| Distanza: | 129.199 parsec |
| Moto proprio RA: | -64.7 |
| Moto proprio Dec: | 3.8 |
| B-T magnitude: | 8.778 |
| V-T magnitude: | 8.337 |
Cataloghi e designazioni:
|
INHABITED SKY
