目录
图像
上传图像
DSS Images Other Images
相关文章
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
| 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).
| 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).
| 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).
| On the nature of the upper atmospheric variability in the rapidly oscillating Ap star HD134214
Recently, we discovered an entirely new type of variability in the upperatmospheres of rapidly oscillating Ap (roAp) stars. This manifestsitself in amplitude modulation of the radial velocities that has notbeen previously detected in photometric studies of the same stars. Tostudy this new variability further we obtained a full night, 8.85h, ofhigh time resolution (70s), high spectral resolution (R = 105000), highsignal-to-noise ratio (on average S/N ~ 130) data with Ultraviolet andVisual Echelle Spectrograph (UVES) on the Very Large Telescope (VLT) forthe roAp star HD134214. We also obtained 4.2h of new photometric data inJohnson B with the South African Astronomical Observatory (SAAO) 0.5-mtelescope 2d later. HD134214 has been known for years to be singlyperiodic with a relatively stable amplitude in photometry; it has thehighest pulsation frequency of any roAp star of 2.949mHz (P = 5.65min).Our new UVES data show this principal frequency, plus five otherfrequencies in amplitude spectra of rare earth elements lines and theHα line. The new frequencies are stable over the 8.85h ofobservation, and the highest of them ν2 = 2.782mHz is thesame as found in data taken 2yr earlier with UVES presented in thediscovery paper. The amplitudes of the new frequencies drop faster withatmospheric depth than does the amplitude of the principal frequency,hence explaining why they are generally not seen in broad-bandphotometric measurements that sample on average more deeply in theatmosphere. Our new photometric measurements also detectν2 for the first time in photometric data. Our analysissuggests that the new frequencies are associated with pulsation modes,but the nature of those modes and why they increase in amplitude withatmospheric height more strongly than the principal frequencies is notyet known.Based on observations collected at the European Southern Observatory,Paranal, Chile, as part of programme 077.D-0149.E-mail: dwkurtz@uclan.ac.uk
| Asteroseismology and interferometry
Asteroseismology provides us with a unique opportunity to improve ourunderstanding of stellar structure and evolution. Recent developments,including the first systematic studies of solar-like pulsators, haveboosted the impact of this field of research within astrophysics andhave led to a significant increase in the size of the researchcommunity. In the present paper we start by reviewing the basicobservational and theoretical properties of classical and solar-likepulsators and present results from some of the most recent andoutstanding studies of these stars. We centre our review on thoseclasses of pulsators for which interferometric studies are expected toprovide a significant input. We discuss current limitations toasteroseismic studies, including difficulties in mode identification andin the accurate determination of global parameters of pulsating stars,and, after a brief review of those aspects of interferometry that aremost relevant in this context, anticipate how interferometricobservations may contribute to overcome these limitations. Moreover, wepresent results of recent pilot studies of pulsating stars involvingboth asteroseismic and interferometric constraints and look into thefuture, summarizing ongoing efforts concerning the development of futureinstruments and satellite missions which are expected to have an impactin this field of research.
| 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.
| Lithium in chemically peculiar CP stars with magnetic fields
The problem of lithium in chemically peculiar Ap-CP stars has been thesubject of debate for many years. The main reason for this is a lack ofspectral observations of Ap stars in the neighborhood of the lithiumresonance doublet Li I 6708 Å. An international cooperationproject on “Lithium in cool CP stars with magnetic fields”was started in 1996. Systematic observations of CP stars in spectralregions of the 6708 Å and 6103 Å lines at the ZTSh (CrAO),CAT (ESO), Feros (ESO), and the 74″ telescope of the Mount StromloObservatory (Australia) have been used to analyze spectra of several CPstars studied by the way the 6708 Å lithium line varies with thestars’ rotational phase. Monitoring of the spectra of theoscillating CP stars (group I) HD 83368, HD 60435, and HD 3980, forwhich significant Doppler shifts of the Li I 6708 Å line areobserved led to the discovery of “lithium spots” on thesurface of these stars whose positions are related to the magnetic fieldstructure. Models of the surfaces of these stars with the specialprogram “ROTATE” based on the profiles of the Li I 6708Å line are used to estimate the size of the spots, their positionson the stars’ surface, and the lithium abundances in these spots.A detailed analysis and modelling of the spectra of slowly rotatingoscillating CP stars with strong, invariant lithium 6708 Åemission, including blending with lines of the rare earth elements,reveals an enhanced lithium abundance, with the abundance determinedfrom the lithium 6103 Å line being higher than that determinedfrom the 6708 Å line for all the stars. This may indicate verticalstratification of lithium in the atmospheres of CP stars with ananomalous isotopic composition (6Li/7Li = 0.20.5). HD 101065, an ultraslow rotator (vsini ≈ 1.5) visible from thepoles and with powerful oscillations which cause pulsating linebroadening in its spectrum, is unique among these stars. The amount oflithium in the atmosphere of HD 101065 logN(Li) = 3.1 on a scale oflogN(H) = 12.0 and the isotope ratio 6Li/7Li ≈0.3. The high estimates of 6Li/7Li may beexplained by the production of lithium in spallation reactions and thepreservation of surface 6Li and 7Li by strongmagnetic fields in the upper layers of the atmosphere near the magneticpoles.
| 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)
| Line profile variations in rapidly oscillating Ap stars: resolution of the enigma
We have carried out the first survey of the pulsational line profilevariability in rapidly oscillating Ap (roAp) stars. We analysed highsignal-to-noise ratio time-series observations of 10 sharp-lined roApstars obtained with the high-resolution spectrographs attached to theVery Large Telescope and Canada-France-Hawaii Telescope. We investigatedin detail the variations of PrIII, NdII, NdIII and TbIII lines anddiscovered a prominent change of the profile variability pattern withheight in the atmospheres of all studied roAp stars. We show that, inevery investigated star, profile variability of at least one rare-earthion is characterized by unusual blue-to-red moving features, which wepreviously discovered in the time-resolved spectra of the roAp starγ Equ. This behaviour is common in rapidly rotating non-radialpulsators but is inexplicable in the framework of the standard obliquepulsator model of slowly rotating roAp stars. Using analysis of the lineprofile moments and spectrum synthesis calculations, we demonstrate thatunusual oscillations in spectral lines of roAp stars arise from thepulsational modulation of linewidths. This variation occursapproximately in quadrature with the radial velocity changes, and itsamplitude rapidly increases with height in stellar atmosphere. Wepropose that the linewidth modulation is a consequence of the periodicexpansion and compression of turbulent layers in the upper atmospheresof roAp stars. Thus, the line profile changes observed in slowlyrotating magnetic pulsators should be interpreted as a superposition oftwo types of variability: the usual time-dependent velocity field due toan oblique low-order pulsation mode and an additional linewidthmodulation, synchronized with the changes of stellar radius. Ourexplanation of the line profile variations of roAp stars solves thelong-standing observational puzzle and opens new possibilities forconstraining geometric and physical properties of the stellarmagnetoacoustic pulsations.Based on observations collected at the European Southern Observatory(Paranal, La Silla), at the Canada-France-Hawaii Telescope and on dataretrieved from the ESO Science Archive.E-mail: oleg@astro.uu.se
| 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
| 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.
| 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.
| 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.
| 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.
| 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.
| 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 discovery of a luminous, rapidly oscillating Ap star, HD 116114, with a 21-minute pulsation period*
The discovery of a new rapidly oscillating Ap star, HD 116114, with apulsation period of 21 min, using high-resolution spectra obtained withthe Ultraviolet-Visual Echelle Spectrograph at the European SouthernObservatory's Very Large Telescope, is presented. The highest amplitudesof the radial velocity variations are between 50 and 125 ms-1 visible in the EuII lines. The spectral lines of LaII andthe core of the Hα line have amplitudes of about 30 ms-1. The frequency obtained for the oscillations is in goodagreement with theoretical predictions of longer-period, evolved roApstars. The distinction in luminosity between the roAp and noAp stars,and the suggestion that in all roAp stars the abundance of the secondions of Pr and Nd, relative to the abundance of the first ions, isanomalously high, need to be revised in the light of this discovery.
| Pushing the ground-based limit: 14-μmag photometric precision with the definitive Whole Earth Telescope asteroseismic data set for the rapidly oscillating Ap star HR1217
HR1217 is one of the best-studied rapidly oscillating Ap (roAp) stars,with a frequency spectrum of alternating even- and odd-l modes that aredistorted by the presence of a strong, global magnetic field. Severalrecent theoretical studies have found that within the observableatmospheres of roAp stars the pulsation modes are magneto-acoustic withsignificant frequency perturbations that are cyclic with increasingfrequency. To test these theories a Whole Earth Telescope extendedcoverage campaign obtained 342 h of Johnson B data at 10-s timeresolution for the roAp star HR1217 over 35 d with a 36 per cent dutycycle in 2000 November-December. The precision of the derived amplitudesis 14 μmag, making this one of the highest precision ground-basedphotometric studies ever undertaken. Substantial support has been foundfor the new theories of the interaction of pulsation with the strongmagnetic field. In particular, the frequency jump expected as themagnetic and acoustic components cycle through 2π rad in phase hasbeen found. Additionally, comparison of the new 2000 data with anearlier 1986 multisite study shows clear amplitude modulation for somemodes between 1986 and 2000. The unique geometry of the roAp starsallows their pulsation modes to be viewed from varying aspect withrotation, yielding mode identification information in the rotationalsidelobes that is available for no other type of pulsating star. Thoserotational sidelobes in HR1217 confirm that two of the modes aredipolar, or close to dipolar; based on the frequency spacings andHipparcos parallax, three other modes must be either l= 0 or 2 modes,either distorted by the magnetic field, or a mix of m-modes of given lwhere the mixture is the result of magnetic and rotational effects. Astudy of all high-speed photometric Johnson B data from 1981 to 2000gives a rotation period Prot= 12.4572 d, as found in previouspulsation and photometric studies, but inconsistent with a differentrotation period found in magnetic studies. We suggest that this rotationperiod is correct and that zero-point shifts between magnetic data setsdetermined from different spectral lines are the probable cause of thecontroversy over the rotation period. This WET data set is likely tostand as the definitive ground-based study of HR1217. It will be thebaseline for comparison for future space studies of HR1217, particularlythe MOST satellite observations.
| 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).
| Asteroseismic signatures of helium gradients in main-sequence A stars; application to the roAp star HD 60435
Asteroseismology is found to be a excellent tool for detectingdiffusion-induced helium gradients inside main-sequence A stars. Modelshave been computed for 1.6 and 2.0 M? stars with purehelium diffusion, at different ages, so that the helium gradient lies atdifferent depths inside the star. The adiabatic oscillation frequencieshave been analysed and compared with those of a model without diffusion.Clear signatures of the diffusion-induced helium gradient are found inthe so-called ``second differences'': these frequency differencespresent modulations due to the partial reflexion of the sound waves onthe layer where the helium gradient occurs. A tentative application tothe roAp star HD 60435, which presents enough detected oscillationfrequencies for the test to be possible, is very encouraging. Theresults suggest the presence of a helium gradient inside the star, whichis consistent with the idea that the triggering of the oscillations isdue to the hydrogen ?-mechanism.
| 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.
| High Precision with the Whole Earth Telescope: Lessons and Some Results from XCov20 for the roAp Star HR 1217
HR 1217 is a prototypical rapidly oscillating Ap star that has presenteda test to the theory of nonradial stellar pulsation. Prior observationsshowed a clear pattern of five modes with alternating frequency spacingsof 33.3 mu Hz and 34.6 mu Hz, with a sixth mode at a problematic spacingof 50.0 mu Hz (which equals 1.5 times 33.3 mu Hz) to the high-frequencyside. Asymptotic pulsation theory allowed for a frequency spacing of 34mu Hz, but Hipparcos observations rule out such a spacing. Theoreticalcalculations of magnetoacoustic modes in Ap stars by Cunha (2001)predicted that there should be a previously undetected mode 34 mu Hzhigher than the main group, with a smaller spacing between it and thehighest one. The 20th extended coverage campaign of the Whole EarthTelescope ({XCov20}) has discovered this frequency as predicted by Cunha(2001). Amplitude modulation of several of the pulsation modes betweenthe 1986 and 2000 data sets has also been discovered, while importantparameters for modelling the geometry of the pulsation modes have beenshown to be unchanged. With stringent selection of the best data fromthe WET network the amplitude spectrum shows highest peaks at only 50 mumag and formal errors on the determined amplitudes are 14 mu mag. Somelessons for future use of WET for the highest precision photometry onbright stars are discussed.
| The WET Director's Report
This review summarizes several short contributions that I made in therole of WET (co-)Director. I review the progress of the WET since ourlast workshop in 1999. I discuss the diversification of our targets,the growth of the CCD component of our instrumentation, and theevolution of our funding situation. We also face new opportunities andnew challenges with respect to our changing and growing membership.
|
提交文章
相关链接
提交链接
下列团体成员
|
观测天体数据
目录:
|
INHABITED SKY
