Оглавление
Изображения
Загрузить ваше изображение
DSS Images Other Images
Публикации по объекту
Magnetic fields of chemically peculiar stars. II: Magnetic fields and rotation of stars with strong and weak anomalies in the continuum energy distribution
We make a comparative analysis of magnetic fields and rotationparameters of magnetic CP stars with strong and weak anomalies in thespectral energy distribution. Stars with strong depressions in thecontinuum at 5200 Å are shown to have significantly strongerfields (the mean longitudinal component of the fields of these stars is< B e> = 1341 ± 98 G) compared to objects withweaker depressions (< B e> = 645 ± 58 G). Starswith stronger depressions are also found to occur more commonly amongslow rotators. Their rotation periods are, on the average, about 10 dayslong, three times longer than these of stars with weak depressions(about three days). This fact is indicative of a decrease of the degreeof anomality of the magnetic stars continuum spectrum with increasingrotational velocity. Yet another proof has been obtained suggesting thatslow rotation is one of the crucial factors contributing to thedevelopment of the phenomenon of magnetic chemically peculiar stars.Magnetic CP stars with weak depressions at 5200 Å are intermediateobjects between stars with strong depressions and normal A- and B-typestars both in terms of field strength and rotational velocity.
| Asteroseismology of chemically peculiar stars
Paper published in the proceedings of the Wroc?aw HELAS Workshop``Interpretation of Asteroseismic Data'', CoAst, 157, 228 (Kochukhov2008).Pulsational variability is observed in several types of main sequencestars with anomalous chemical abundances. In this contribution Isummarize the relationship between pulsations and chemicalpeculiarities, giving special emphasis to rapid oscillations in magneticAp stars. These magneto-acoustic pulsators provide unique opportunitiesto study the interaction of pulsations, chemical inhomogeneities, andstrong magnetic fields. Time-series monitoring of rapidly oscillating Apstars using high-resolution spectrometers at large telescopes andultra-precise space photometry has led to a number of importantbreakthroughs in our understanding of these interesting objects.Interpretation of the roAp frequency spectra has allowed constrainingfundamental stellar parameters and probing poorly known properties ofthe stellar interiors. At the same time, investigation of thepulsational wave propagation in chemically stratified atmospheres ofroAp stars has been used as a novel asteroseismic tool to studypulsations as a function of atmospheric height and to map in detail thehorizontal structure of the magnetically-distorted p modes.
| Discovery of very low amplitude 9-minute multiperiodic pulsations in the magnetic Ap star HD 75445
We present our discovery of pulsational radial-velocity variations inthe cool Ap star HD 75445, an object spectroscopically similar to thebright, rapidly-oscillating Ap (roAp) star ? Equ. Based onhigh-resolution time-series spectroscopy obtained with the HARPSspectrometer at the European Southern Observatory 3.6-m telescope, wedetected oscillations in Nd II and Nd III lines with a period close to 9min and amplitudes of 20-30 m s-1. Substantial variation inthe pulsational amplitude during our 3.8 h observing run reveals thepresence of at least three excited non-radial modes. The detection ofextremely low amplitude pulsations in HD 75445 indicates that the roApexcitation mechanism produces variability in the radial velocityamplitude of between a few tens m s-1 and several kms-1. This supports the idea that many, if not all, cool Apstars occupying the roAp instability strip may harbour non-radialpulsations, which currently remain undetected due to their smallphotometric and radial-velocity amplitudes.Based on observations collected at the European Southern Observatory,Chile (ESO programs 68.D-0254, 079.D-0118).
| New light on the driving mechanism in roAp stars. I. Effects of metallicity
Context: Observations suggest that a relationship exists between thedriving mechanism of roAp star pulsations and the heavy elementdistribution in these stars. Aims: We attempt to study theeffects of local and global metallicity variations on the excitationmechanism of high order p-modes in A star models. Methods: Wedeveloped stellar evolutionary models to describe magnetic A stars withdifferent global metallicity or local metal accumulation profiles. Thesemodels were computed with CLES (“Code Liègeoisd'évolution stellaire”), and the stability of our modelswas assessed with the non-adiabatic oscillation code MAD. Results: Our models reproduce the blue edge of the roAp starinstability strip, but generate a red edge hotter than the observed one,regardless of metallicity. Surprisingly, we find that an increase inopacity inside the driving region can produce a lower amount of driving,which we refer to as the “inverse ?-mechanism”.
| A search for rapid pulsations in the magnetic cool chemically peculiar star HD3980
The Ap star HD3980 appears to be a promising roAp candidate based on itsfundamental parameters, leading us to search for rapid pulsations withthe VLT UV-Visual Echelle Spectrograph (UVES). A precise Hipparcosparallax and estimated temperature of 8100K place HD3980 in the middleof the theoretical instability strip for rapidly oscillating Ap stars,about halfway through its main-sequence evolution stage. The star has astrong, variable magnetic field, as is typical of the cool magnetic Apstars. Dipole model parameters were determined from VLT observationsusing Focal Reducer and low Dispersion Spectrograph (FORS)1. FromDoppler shift measurements for individual spectral lines of rare-earthelements and the H? line core, we find no pulsations above20-30ms-1. This result is corroborated by the inspection oflines of several other chemical elements, as well as withcross-correlation for long spectral regions with the average spectrum asa template. Abundances of chemical elements were determined and showlarger than solar abundances of rare-earth elements. Further, ionizationdisequilibria for the first two ionized states of Nd and Pr aredetected. We also find that the star has a strong overabundance ofmanganese, which is typical for much hotter HgMn and other Bp stars.Line profile variability with the rotation period was detected for themajority of chemical species.Based on observations collected at the European Southern Observatory(ESO), Paranal, Chile, as part of programme 077.D-0150(A) and part ofprogrammes 074.D-0392(A) and 076.D-0535(A) in the ESO archive.E-mail: velkin@uclan.ac.uk
| Chemically peculiar stars and their temperature calibration
Aims. The determination of effective temperature for chemically peculiar(CP) stars by means of photometry is a sophisticated task due to theirabnormal colours. Standard calibrations for normal stars lead toerroneous results and, in most cases corrections are necessary. Methods: In order to specify appropriate corrections, direct temperaturedeterminations for 176 objects of the different subgroups were collectedfrom the literature. This much larger sample than in previous studiestherefore allows a more accurate investigation, mostly based on averagetemperatures. Results: For the three main photometric systems (UBV,Geneva, Strömgren {uvby}β), methods to determine effectivetemperature are presented together with a comparison with formerresults. Based on the compiled data we provide evidence that He (CP4)objects also need a considerable correction, not noticed in formerinvestigations due to their small number. Additionally, a new relationfor the bolometric correction and the capability of standardcalibrations to deduce interstellar reddening for magnetic CP stars areshown.Tables 4 to 8 are only available in electronic form at http://www.aanda.org
| On the spectroscopic nature of the cool evolved Am star HD151878
Recently, Tiwari, Chaubey & Pandey detected the bright component ofthe visual binary HD151878 to exhibit rapid photometric oscillationsthrough a Johnson B filter with a period of 6min (2.78mHz) and a high,modulated amplitude up to 22mmag peak-to-peak, making this star by farthe highest amplitude rapidly oscillating Ap (roAp) star known. As a newroAp star, HD151878 is of additional particular interest as a scarceexample of the class in the northern sky, and only the second known caseof an evolved roAp star - the other being HD116114. We used theFIbre-fed Echelle Spectrograph at the Nordic Optical Telescope to obtainhigh time-resolution spectra at high dispersion to attempt to verify therapid oscillations. We show here that the star at this epoch isspectroscopically stable to rapid oscillations of no more than a fewtens of ms-1. The high-resolution spectra furthermore showthe star to be of type Am rather than Ap and we show the star lacks mostof the known characteristics for roAp stars. We conclude that this is anAm star that does not pulsate with a 6-min period. The originaldiscovery of pulsation is likely to be an instrumental artefact.Based on observations collected at the Nordic Optical Telescope as partof programme 36-418. E-mail: lfreyham@gmail.com
| Discovery of 17 new sharp-lined Ap stars with magnetically resolved lines
Chemically peculiar A stars (Ap) are extreme examples of the interactionof atomic element diffusion processes with magnetic fields in stellaratmospheres. The rapidly oscillating Ap stars provide a means forstudying these processes in three dimensions and are at the same timeimportant for studying the pulsation excitation mechanism in A stars. Aspart of the first comprehensive, uniform, high-resolution spectroscopicsurvey of Ap stars, which we are conducting in the Southern hemispherewith the Michigan Spectral Catalogues as the basis of target selection,we report here the discovery of 17 new magnetic Ap stars havingspectroscopically resolved Zeeman components from which we derivemagnetic field moduli in the range 3-30kG. Among these are (1) thecurrent second strongest known magnetic A star, (2) a double-lined Apbinary with a magnetic component and (3) an A star with particularlypeculiar and variable abundances. Polarimetry of these stars is neededto constrain their field geometries and to determine their rotationperiods. We have also obtained an additional measurement of the magneticfield of the Ap star HD92499.Based on observations collected at the European Southern Observatory,Paranal, Chile, as part of programmes 078.D-0080(A), 078.D-0192(A),072.D-0138(A).E-mail: lmfreyhammer@uclan.ac.uk
| Magnetic fields of chemically peculiar stars. I. The catalog of magnetic CP stars
This is the first paper of the series dedicated to the analysis of themagnetism of chemically peculiar (CP) stars of the upper Main Sequence.We use our own measurements and published data to compile a catalog ofmagnetic CP stars containing a total of 326 objects with confidentlydetected magnetic fields and 29 stars which are very likely to possessmagnetic field. We obtained the data on the magnetism of theoverwhelming majority of the stars solely based on the analysis oflongitudinal field component B e . The surface magneticfield, B s , has been measured for 49 objects. Our analysisshows that the number of magnetic CP stars decreases with increasingfield strength in accordance with exponential law, and stars with B e exceeding 5kG occur rarely (about 3% objects of ourlist).
| ?Sct pulsation in magnetic Ap stars: the discovery of ?Sct pulsations in HD218994AB and measurement of the magnetic fields of HD218994A and HD21190
We have discovered that at least one component of the visual binary starHD218994AB is a ?Sct star. The primary star of this system is aknown rapidly oscillating Ap (roAp) star; therefore, it is possible thatit is both an roAp star and a ?Sct star - a hybrid. There are noconfirmed such hybrid roAp-?Sct stars, and they are predictedtheoretically not to exist. More probably, HD218994B is the ?Sctstar, as it is 0.54mag fainter than the primary and also lies in the?Sct instability strip. It then offers the potential to determineTeff and L for the primary which appears to be one of themore evolved roAp stars and an important test the theoreticallycalculated roAp star instability strip. In this case, the system also isimportant since the stars are a co-eval pair of very similar mass, yetone is a magnetic roAp star and the other is probably a non-magnetic?Sct star with a normal spectrum. We detect a significantlongitudinal magnetic field strength of 440 +/- 23G in the roAp starHD218994A. We also present the first measurement of the longitudinalmagnetic field strength for the ?Sct star HD21190, themost-evolved Ap star known. We obtain a formally significant value of 47+/- 13G. If confirmed, this will make HD21190 the first magnetic?Sct star known.Based on observations collected at the European Southern Observatory,Paranal, Chile, as part of programme 079.D-0241.E-mail: dwkurtz@uclan.ac.uk
| Pulsation properties of the rapidly oscillating Ap star 10Aquilae (HD176232)
We have studied the pulsation behaviour of radial velocities ofindividual spectral lines of different chemical elements for the wellknown, bright rapidly oscillating Ap (roAp) star 10Aql (HD176232) basedon high-quality Very Large Telescope spectra. We found that only linesof rare earth elements and the narrow H? core show detectablerapid variations of radial velocity with the known photometric pulsationperiod, 11.7min. The highest pulsation amplitudes of more than0.5kms-1 are found for rather weak lines of TbIII and DyIII.The pulsational phases obtained for lines of TbIII and DyIII differ by? rad, perhaps an indication of a radial node between theirline-forming layers. Weak lines of SmII also display amplitudes near0.5kms-1. Surprisingly, the lines of PrIII show the lowestdetected pulsation amplitude in strong contrast with the majority ofroAp stars for which this ion has some of the highest radial velocityamplitudes. Lines of NdIII with different intensities have pulsationamplitudes different to the weaker lines that are formed more deeply inthe atmosphere having higher amplitudes. Line bisectors for strong NdIIIline profiles show significant changes of phase, and even phase jumpsfor some lines, indicating complex variations in the pulsation phase asa function of atmospheric depth. The low-measured rotational velocity ofthis star indicates that there is little Doppler resolution of the lineprofiles, hence that the large range of pulsation amplitudes and phasesof radial velocities determined for line bisectors are primarily causedby atmospheric depth effects.Based on observations collected at the European Southern Observatory,Paranal, Chile, as part of programme 077.D-0150(A).E-mail: velkin@uclan.ac.uk
| The Paschen-Back effect in the Li I 6708 Å line and the presence of lithium in cool magnetic Ap stars
Context: A number of cool magnetic Ap stars show a prominent feature atλ 6708 Å. Its identification with Li I remainscontroversial due to the lack of knowledge of the spectra of rare-earthelements that are strongly enhanced in peculiar stars so they canpotentially provide an alternative identification. Aims: We suggestinvestigating the 6708 Å line in Ap stars with strong magneticfields. In these objects, the magnetic broadening and splitting providesan additional, powerful criterium for line identification, allowing thewhole line profile to be used instead of depending on a mere coincidencein the observed and predicted wavelengths. Methods: The smallseparation of the Li I doublet components means that their magneticsplitting pattern deviates from the one expected for the Zeeman effect,even in relatively weak fields. We carried out detailed calculations ofthe transition between the Zeeman and Paschen-Back regimes in themagnetic splitting of the Li I line and computed polarised syntheticspectra for the range of field strength expected in Ap stars.Theoretical spectral synthesis is compared with the high-resolutionobservations of cool Ap stars HD 116114, HD 166473, and HD 154708, whichhave a mean field strength of 6.4, 8.6, and 24.5 kG, respectively, andshow a strong 6708 Å line. Results: High-resolution spectra forthe 6708 Å region were analysed for 17 magnetic Ap stars. Thepresence of the 6708 Å line is confirmed for 9 stars and reportedfor the first time for 6 stars. The strength of the Li I doublet doesnot correlate with the absorption features of any other element. Thestars HD 75445 and HD 201601 provide an extreme example of the twoobjects, which are dissimilar with respect to the 6708 Å line, butvery close in the atmospheric parameters and abundances of otherelements. We demonstrate that the observed profiles of the 6708 Åline in the strong field stars HD 116114, HD 166473, and HD 154708correspond fairly well to the theoretical calculations when assuming theLi I identification. Including the Paschen-Back effect improves theagreement with observations, especially for HD 154708. Conclusions:Results of our study confirm the Li I identification proposed for the6708 Å line in cool Ap stars.Based on observations collected at the European Southern Observatory,Paranal, Chile (ESO program 68.D-0254 and programs 072.D-0138,077.D-0150 retrieved through the ESO Archive).
| A search for rapid pulsations among nine luminous Ap stars
The rapidly oscillating Ap (roAp) stars are of importance for studyingthe atmospheric structure of stars where the process of chemical elementdiffusion is significant. We have performed a survey for rapidoscillations in a sample of nine luminous Ap stars, selected from theirlocation in the colour-magnitude diagram as more evolved main-sequenceAp stars that are inside the instability strip for roAp stars. Untilrecently this region was devoid of stars with observed rapid pulsations.We used the Very Large Telescope UV-Visual Echelle Spectrograph toobtain high time resolution spectroscopy to make the first systematicspectroscopic search for rapid oscillations in this region of the roApinstability strip. We report nine null detections with upper limits forradial velocity amplitudes of 20-65ms-1 and precisions of? = 7-20ms-1 for combinations of Nd and Pr lines.Cross-correlations confirm these null results. At least six stars aremagnetic and we provide magnetic field measurements for four of them, ofwhich three are newly discovered magnetic stars. It is found that fourstars have magnetic fields smaller than ~2kG, which according totheoretical predictions might be insufficient for suppressing envelopeconvection around the magnetic poles for more evolved Ap stars.Suppression of convection is expected to be essential for the opacitymechanism acting in the hydrogen ionization zone to drive thehigh-overtone roAp pulsations efficiently. Our null results suggest thatthe more evolved roAp stars may require particularly strong magneticfields to pulsate. Three of the studied stars do, however, have magneticfields stronger than 5kG.Based on observations collected at the European Southern Observatory,Paranal, Chile, as part of programmes 075.D-0145 (A), 078.D-0080(A),072.D-0138(A) and 077.D-0150(A).E-mail: lmfreyhammer@uclan.ac.uk
| Discovery of pulsational line profile variations in the ? Scuti star HD21190 and in the Ap Sr star HD218994
Asteroseismology has the potential to provide new insights into thephysics of stellar interiors. We have obtained Ultraviolet and VisualEchelle Spectrograph high time-resolution observations of the ?Scuti star HD21190 and of the Ap Sr star HD218994 to search forpulsational line profile variations. We report the discovery of a newroAp star, HD218994, with a pulsation period of 14.2min. This is one ofthe most evolved roAp stars. No rapid pulsations have been found in thespectra of the cool Ap star - ? Scuti star HD21190. However, wedetect with unprecedented clarity for a ?Scuti star moving peaksin the cores of spectral lines that indicate the presence of high-degreenon-radial pulsations in this star.Based on observations collected at the European Southern Observatory,Paranal, Chile [ESO programm 78.D-0341(A)].E-mail: fgonzalez@casleo.gov.ar
| Isotopic anomaly and stratification of Ca in magnetic Ap stars
Aims: We have completed an accurate investigation of the Ca isotopiccomposition and stratification in the atmospheres of 23 magneticchemically peculiar (Ap) stars of different temperature and magneticfield strength. Methods: With the UVES spectrograph at the 8 mESO VLT, we have obtained high-resolution spectra of Ap stars in thewavelength range 3000-10 000 Å. Using a detailed spectrumsynthesis calculations, we have reproduced a variety of Ca lines in theoptical and ultraviolet spectral regions, inferring the overall verticaldistribution of Ca abundance, and have deduced the relative isotopiccomposition and its dependence on height using the profile of theIR-triplet Ca II line at ?8498 Å. Results: In 22out of 23 studied stars, we found that Ca is strongly stratified, beingusually overabundant by 1.0-1.5 dex below log?5000?-1, and strongly depleted above log?5000=-1.5. TheIR-triplet Ca II line at ?8498 Å reveals a significantcontribution of the heavy isotopes 46Ca and 48Ca,which represent less than 1 % of the terrestrial Ca isotopic mixture. Weconfirm our previous finding that the presence of heavy Ca isotopes isgenerally anticorrelated with the magnetic field strength. Moreover, wediscover that in Ap stars with relatively small surface magnetic fields(?4-5 kG), the light isotope 40Ca is concentrated close tothe photosphere, while the heavy isotopes are dominant in the outeratmospheric layers. This vertical isotopic separation, observed for thefirst time for any metal in a stellar atmosphere, disappears in starswith magnetic field strength above 6-7 kG. Conclusions: Wesuggest that the overall Ca stratification and depth-dependent isotopicanomaly observed in Ap stars may be attributed to a combined action ofthe radiatively-driven diffusion and light-induced drift.Based on observations collected at the European Southern Observatory,Paranal, Chile (ESO program No. 68.D-0254).
| The discovery of high-amplitude, 10.9-minute oscillations in the cool magnetic Ap star HD 115226
We present the discovery of pulsational variations in the cool magneticAp star HD 115226 - the first high-amplitude rapidly oscillating Ap(roAp) star discovered with time-series spectroscopy. Usinghigh-resolution spectra obtained with the HARPS instrument at theEuropean Southern Observatory 3.6-m telescope, we detect radial velocityvariations with a period of 10.86 min in Pr III, Nd III, Dy III lines,and in the narrow cores of hydrogen lines. Pulsational amplitudes exceed1 km s-1 in individual lines of Nd III. The presence ofrunning waves in the stellar atmosphere is inferred from a phase shiftbetween the radial velocity maxima of rare-earth and hydrogen lines. Ourabundance analysis demonstrates that HD 115226 exhibits a typical roApspectroscopic signature, notably ionization anomaly of Pr, Nd, and Dy.We discuss the discovery of pulsations in HD 115226 in the context ofrecent spectroscopic studies of roAp stars and point to the existence ofa correlation between spectroscopic pulsational amplitude and thestellar rotation rate.Based on observations collected at the European Southern Observatory, LaSilla, Chile (ESO program 079.D-0118).
| The detection of the very low amplitude 16.2-min pulsation in individual lines of first ionization stage of rare earth elements in the roAp star ?CrB†
With high precision spectroscopic data obtained with Ultraviolet andVisual Echelle Spectrograph (UVES) on the Very Large Telescope (VLT) wedetect radial velocity pulsation with a period of 16.2min and withamplitudes of about 30ms-1 in individual lines of the firstionization stage of rare earth elements, and in H?, with typicalprecision in the range 2-8ms-1 for a single line, and betterthan 1ms-1 for some ensembles of lines. The second ionizationstages of rare earth elements show weak or no detectable pulsation,while lines of FeI or FeII show no signal at high precision. Anabundance analysis shows the rare earth elements to be overabundant byabout 1dex, rather than the 2-3dex often seen in other rapidlyoscillating Ap (roAp) stars, and it shows the presence of the ionizationdisequilibrium for Nd and Pr that is typical of most roAp stars. Thepulsational radial velocity curve reaches maximum for the lines of therare earth elements before it does for H? which is opposite to thecase for most other roAp stars with implications for the stratificationlevel of the rare earth elements. Similarities between ?CrB and thelongest period roAp star, HD116114, suggest that many more low-amplituderoAp stars await detection, with implications for understanding thepulsation driving mechanism and mode selection mechanism in these stars.
| Pulsation in the presence of a strong magnetic field: the roAp star HD166473
Phase-resolved high-resolution, high signal-to-noise ratio (S/N)observations of the strongly magnetic roAp star HD166473 are analysed.HD166473 was selected as the target of this study because it has one ofthe strongest magnetic fields of all the roAp stars known with resolvedmagnetically split lines. Indeed, we show that enhanced pulsationdiagnosis can be achieved from consideration of the different pulsationbehaviour of the π and σ Zeeman components of the resolvedspectral lines. This study is based on a time-series of high spectralresolution observations obtained with the Ultraviolet and Visual EchelleSpectrograph of the Very Large Telescope of the European SouthernObservatory. Radial velocity variations due to pulsation are observed inrare earth lines, with amplitudes up to 110ms-1. Thevariations occur with three frequencies, already detected in photometry,but which can in this work be determined with better precision: 1.833,1.886 and 1.928mHz. The pulsation amplitudes and phases observed in therare earth element lines vary with atmospheric height, as is the case inother roAp stars studied in detail. Lines of Fe and of other (mostlynon-rare earth) elements do not show any variation to very highprecision (1.5ms-1 in the case of Fe). The low amplitudes ofthe observed variations do not allow the original goal of studyingdifferences between the behaviour of the resolved Zeeman line componentsto be reached; the S/N achieved in the radial velocity determinations isinsufficient to establish definitely the possible existence of suchdifferences. Yet the analysis provides a tantalizing hint at theoccurrence of variations of the mean magnetic field modulus with thepulsation frequency, with an amplitude of 21 +/- 5G.Based on observations collected at the European Southern Observatory,Paranal, Chile, as part of programme 067.D-0272.E-mail: gmathys@eso.org
| Magnetic-field dependence of chemical anomalies in CP stars
The dependence of the degree of anomaly of parameter Z of Genevaphotometry ( Z0 = Z CP ℒ Z norm.) on theaverage surface magnetic field Bs is analyzed. The Z0 value isproportional to the degree of anomaly of chemical composition. It wasfound that Bs → 0 corresponds Z0 → ‑0.010÷‑0.015, i.e., part of CP stars are virtually devoid of magneticfield, but exhibit chemical anomalies. This effect may be due toselection whereby only objects with strong chemical anomalies areclassified as CP stars, thereby producing a deficit of stars withrelatively weak anomalies. Moreover, CP stars have other sources ofstabilization of their atmospheres besides the magnetic field, e.g.,slow rotation. Formulas relating Z0 to Bs are derived.
| Evolution of magnetic fields in stars across the upper main sequence: II. Observed distribution of the magnetic field geometry
We re-discuss the evolutionary state of upper main sequence magneticstars using a sample of Ap and Bp stars with accurate Hipparcosparallaxes and definitely determined longitudinal magnetic fields. Weconfirm our previous results obtained from the study of Ap and Bp starswith accurate measurements of the mean magnetic field modulus and meanquadratic magnetic fields that magnetic stars of mass {M}<3 M_ȯare concentrated towards the centre of the main-sequence band. Incontrast, stars with masses {M}>3 M_ȯ seem to be concentratedcloser to the ZAMS. The study of a few known members of nearby openclusters with accurate Hipparcos parallaxes confirms these conclusions.Stronger magnetic fields tend to be found in hotter, younger and moremassive stars, as well as in stars with shorter rotation periods. Thelongest rotation periods are found only in stars which spent alreadymore than 40% of their main sequence life, in the mass domain between1.8 and 3 M_ȯ and with log g values ranging from 3.80 to 4.13. Noevidence is found for any loss of angular momentum during themain-sequence life. The magnetic flux remains constant over the stellarlife time on the main sequence. An excess of stars with largeobliquities β is detected in both higher and lower mass stars. It isquite possible that the angle β becomes close to 0o inslower rotating stars of mass {M}>3 M_ȯ too, analog to thebehaviour of angles β in slowly rotating stars of {M}<3 M_ȯ.The obliquity angle distribution as inferred from the distribution ofr-values appears random at the time magnetic stars become observable onthe H-R diagram. After quite a short time spent on the main sequence,the obliquity angle β tends to reach values close to either90o or 0o for {M}<3 M_ȯ. The evolution ofthe obliquity angle β seems to be somewhat different for low andhigh mass stars. While we find a strong hint for an increase of βwith the elapsed time on the main sequence for stars with {M}>3M_ȯ, no similar trend is found for stars with {M}<3 M_ȯ.However, the predominance of high values of β at advanced ages inthese stars is notable. As the physics governing the processes takingplace in magnetised atmospheres remains poorly understood, magneticfield properties have to be considered in the framework of dynamo orfossil field theories.
| Heavy calcium in CP stars
Large wavelength shifts of infrared triplet lines of CaII have beenobserved in the spectra of HgMn and magnetic Ap stars. They have beenattributed to the heavy calcium isotopes, including 48Ca. Onemember of the triplet, ?8542, had been either unavailable, or ofpoor quality in earlier spectra. The present material shows conclusivelythat the stellar ?8542 shifts are consistent with aninterpretation in terms of 48Ca. We find no relation betweenisotopic shifts of the CaII triplet lines, and those of HgII?3984. There is a marginal indication that the shifts areanticorrelated with the surface field strengths of the magnetic stars.We see sparse evidence for 48Ca in other chemically peculiarstars, for example, Am stars, metal-poor stars or chemically peculiarred giants. However, the sample is still very small, and the wavelengthsof all three triplet lines, including those in the Sun, show slightpositive shifts with respect to terrestrial positions.Some profiles of the CaII infrared triplet in the magnetic stars showextensive wings beyond a well-defined core. We can obtain reasonablefits to these profiles using a stratified calcium abundance similar tothat used by previous workers. There is no indication that either thestratification or the Zeeman effect significantly disturbs themeasurement of isotope shifts.Based on observations obtained at the European Southern Observatory,Paranal and La Silla, Chile [ESO programmes 076.D-0169(A) and076.C-0172(A)].E-mail: cowley@umich.edu (CRC); shubrig@eso.org (SH);castelli@ts.astro.it (FC); fgonzalez@casleo.gov.ar (JFG); bwolff@eso.org(BW)
| Medium-resolution Isaac Newton Telescope library of empirical spectra - II. The stellar atmospheric parameters
We present a homogeneous set of stellar atmospheric parameters(Teff, logg, [Fe/H]) for MILES, a new spectral stellarlibrary covering the range λλ 3525-7500Å at2.3Å (FWHM) spectral resolution. The library consists of 985 starsspanning a large range in atmospheric parameters, from super-metal-rich,cool stars to hot, metal-poor stars. The spectral resolution, spectraltype coverage and number of stars represent a substantial improvementover previous libraries used in population synthesis models. Theatmospheric parameters that we present here are the result of aprevious, extensive compilation from the literature. In order toconstruct a homogeneous data set of atmospheric parameters we have takenthe sample of stars of Soubiran, Katz & Cayrel, which has very welldetermined fundamental parameters, as the standard reference system forour field stars, and have calibrated and bootstrapped the data fromother papers against it. The atmospheric parameters for our clusterstars have also been revised and updated according to recent metallicityscales, colour-temperature relations and improved set of isochrones.
| Pulkovo compilation of radial velocities for 35495 stars in a common system.
Not Available
| On the roAp star status of β Coronae Borealis
β CrB is one of the best-studied of the magnetic Ap stars. Threeindependent investigations have suggested that this star is pulsatingwith a period of either 6.1 min, 11.5 min or 16.2 min, making this arapidly oscillating Ap star. The presence of pulsations in β CrBhas important implications for the understanding of pulsation drivingand damping in roAp stars, and each study has called for additionalobservations to confirm the suggested pulsations. New high timeresolution, high spectral resolution, high signal-to-noise spectra ofβ CrB obtained with the high resolution spectrograph SARG on the3.55-m Telescopio Nazionale Galileo are unable to confirm any of thesuggested periods. There is no indication of any variability with aperiod near 6.1 min. Studies of Fe i lines suggest that the 11.5-minperiod is spurious. Studies of Ce ii lines do not find the 16.2-minperiod suggested for one Ce ii line, but are not precise enough to testthe finding of 16.2-min oscillations for a large section of spectrum,hence the case for the 16.2-min period is still good. An extensiveinvestigation of β CrB is needed to resolve the issue.
| The discovery of 8.0-min radial velocity variations in the strongly magnetic cool Ap star HD154708, a new roAp star
HD154708 has an extraordinarily strong magnetic field of 24.5kG. Using2.5h of high time resolution Ultraviolet and Visual Echelle Spectrograph(UVES) spectra we have discovered this star to be an roAp star with apulsation period of 8min. The radial velocity amplitudes in the rareearth element lines of NdII, NdIII and PrIII are unusually low -~60ms-1 - for an roAp star. Some evidence suggests that roApstars with stronger magnetic fields have lower pulsation amplitudes.Given the central role that the magnetic field plays in the obliquepulsator model of the roAp stars, an extensive study of the relation ofmagnetic field strength to pulsation amplitude is desirable.Based on observations collected at the European Southern Observatory,Paranal, Chile, as part of programme 075.D-0145.E-mail: dwkurtz@uclan.ac.uk
| Medium-resolution Isaac Newton Telescope library of empirical spectra
A new stellar library developed for stellar population synthesismodelling is presented. The library consists of 985 stars spanning alarge range in atmospheric parameters. The spectra were obtained at the2.5-m Isaac Newton Telescope and cover the range λλ3525-7500 Å at 2.3 Å (full width at half-maximum) spectralresolution. The spectral resolution, spectral-type coverage,flux-calibration accuracy and number of stars represent a substantialimprovement over previous libraries used in population-synthesis models.
| 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 Nainital-Cape Survey. II. Report for pulsation in five chemically peculiar A-type stars and presentation of 140 null results
Aims.We search for photometric variability in chemically peculiar A typestars in the northern hemisphere. Methods: .High-speed photometricobservations of Ap and Am star candidates have been carried out fromARIES (Manora Peak, Nainital) using a three-channel fast photometerattached to the ARIES 104-cm Sampurnanand telescope. Results:.This paper presents three new variables: HD 113878, HD 118660 and HD207561. During the time span of the survey (1999 December to 2004January) pulsations of the δ Sct type were also found for the twoevolved Am stars HD 102480 and HD 98851, as reported in Joshi et al.(2002, 2003). Additionally, we present 140 null results of the surveyfor this time span. Conclusions: .The star HD 113878 pulsates witha period of 2.31 h, which is typical of δ Sct stars. HD 118660exhibits multi-periodic variability with a prominent period of nearly 1h. These periods need to be investigated and make HD 118660 aparticularly interesting target for further observations. For HD 207561,a star classified as Am, a probable pulsation with a period of 6 min wasfound in the light curves obtained on two consecutive nights. Both HD102480 and HD 98851 exhibit unusual alternating high and low amplitudemaxima, with a period ratio of 2:1. The analysis of the null resultsconfirms the photometric quality of the Nainital site.
| 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.
| 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.
|
Добавить новую статью
Внешние ссылки
- - Внешних ссылок не найдено -
Добавить внешнюю ссылку
Группы:
|
Наблюдательные данные и астрометрия
| Созвездие: | Дева |
| Прямое восхождение: | 13h21m46.31s |
| Склонение: | -18°44'31.6" |
| Видимая звёздная величина: | 7.03 |
| Расстояние: | 140.449 парсек |
| Собственное движение RA: | -49.9 |
| Собственное движение Dec: | -11 |
| B-T magnitude: | 7.403 |
| V-T magnitude: | 7.061 |
Каталоги и обозначения:
|
