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The Geneva-Copenhagen survey of the solar neighbourhood. III. Improved distances, ages, and kinematics
Context: Ages, chemical compositions, velocity vectors, and Galacticorbits for stars in the solar neighbourhood are fundamental test datafor models of Galactic evolution. The Geneva-Copenhagen Survey of theSolar Neighbourhood (Nordström et al. 2004; GCS), amagnitude-complete, kinematically unbiased sample of 16 682 nearby F andG dwarfs, is the largest available sample with complete data for starswith ages spanning that of the disk. Aims: We aim to improve theaccuracy of the GCS data by implementing the recent revision of theHipparcos parallaxes. Methods: The new parallaxes yield improvedastrometric distances for 12 506 stars in the GCS. We also use theparallaxes to verify the distance calibration for uvby? photometryby Holmberg et al. (2007, A&A, 475, 519; GCS II). We add newselection criteria to exclude evolved cool stars giving unreliableresults and derive distances for 3580 stars with large parallax errorsor not observed by Hipparcos. We also check the GCS II scales of T_effand [Fe/H] and find no need for change. Results: Introducing thenew distances, we recompute MV for 16 086 stars, and U, V, W,and Galactic orbital parameters for the 13 520 stars that also haveradial-velocity measurements. We also recompute stellar ages from thePadova stellar evolution models used in GCS I-II, using the new valuesof M_V, and compare them with ages from the Yale-Yonsei andVictoria-Regina models. Finally, we compare the observed age-velocityrelation in W with three simulated disk heating scenarios to show thepotential of the data. Conclusions: With these revisions, thebasic data for the GCS stars should now be as reliable as is possiblewith existing techniques. Further improvement must await consolidationof the T_eff scale from angular diameters and fluxes, and the Gaiatrigonometric parallaxes. We discuss the conditions for improvingcomputed stellar ages from new input data, and for distinguishingdifferent disk heating scenarios from data sets of the size andprecision of the GCS.Full Table 1 is only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/501/941
| 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”.
| 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
| 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).
| 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).
| 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
| 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.
| 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)
| Pulkovo compilation of radial velocities for 35495 stars in a common system.
Not Available
| The discovery of 8.0-min radial velocity variations in the strongly magnetic cool Ap star HD154708, a new roAp star
HD154708 has an extraordinarily strong magnetic field of 24.5kG. Using2.5h of high time resolution Ultraviolet and Visual Echelle Spectrograph(UVES) spectra we have discovered this star to be an roAp star with apulsation period of 8min. The radial velocity amplitudes in the rareearth element lines of NdII, NdIII and PrIII are unusually low -~60ms-1 - for an roAp star. Some evidence suggests that roApstars with stronger magnetic fields have lower pulsation amplitudes.Given the central role that the magnetic field plays in the obliquepulsator model of the roAp stars, an extensive study of the relation ofmagnetic field strength to pulsation amplitude is desirable.Based on observations collected at the European Southern Observatory,Paranal, Chile, as part of programme 075.D-0145.E-mail: dwkurtz@uclan.ac.uk
| Rare-earth elements in the atmosphere of the magnetic chemically peculiar star HD 144897. New classification of the Nd III spectrum
Context: . The chemically peculiar stars of the upper main sequencerepresent a natural laboratory for the study of rare-earth elements(REE). Aims: . We want to check the reliability of the energylevels and atomic line parameters for the second REE ions currentlyavailable in the literature, and obtained by means of experiments andtheoretical calculations. Methods: . We have obtained a UVESspectrum of a slowly rotating strongly magnetic Ap star, HD 144897, thatexhibits very large overabundances of rare-earth elements. Here wepresent a detailed spectral analysis of this object, taking effects ofnon-uniform vertical distribution (stratification) of chemical elementsinto account. Results: . We determined the photosphericabundances of 40 ions. For seven elements (Mg, Si, Ca, Ti, Cr, Mn, Fe),we obtained a stratification model that allows us to produce asatisfactory fit to the observed profiles of spectral lines of variousstrengths. All the stratified elements but Cr show a steep decrease inconcentration toward the upper atmospheric layers; for Cr the transitionfrom high to low concentration regions appears smoother than for theother elements. The REEs abundances, which have been determined for thefirst time from the lines of the first and second ions, have been foundtypically four dex higher than solar abundances. Our analysis of REEspectral lines provides strong support for the laboratory lineclassification and determination of the atomic parameters. The onlyremarkable exception is Nd iii, for which spectral synthesis was foundto be inconsistent with the observations. We therefore performed arevision of the Nd iii classification. We confirmed the energies for 11out of 24 odd energy levels that were classified previously, and derivedthe energies for additional 24 levels of Nd iii, thereby substantiallyincreasing the number of classified Nd iii lines with correctedwavelengths and atomic parameters.
| The discovery of a new type of upper atmospheric variability in the rapidly oscillating Ap stars with VLT high-resolution spectroscopy
In a high-resolution spectroscopic survey of rapidly oscillating Ap(roAp) stars with the Ultraviolet and Visual Echelle Spectrograph on theVery Large Telescope of the European Southern Observatory, we find thatalmost all stars show significant variation of the radial velocityamplitudes - on a time-scale of a few pulsation cycles - for lines ofthe rare earth ion PrIII and in the core of the Hα line. Thesevariations in the radial velocity amplitudes are described by newfrequencies in the amplitude spectra that are not seen in broad-bandphotometric studies of the same stars. The PrIII lines form high in theatmosphere of these stars at continuum optical depths oflogτ5000 <= -5 and tend to be concentrated towards themagnetic poles in many stars, and the core of the Hα line forms atcontinuum optical depths -5 <= logτ5000 <= -2,whereas the photometry samples the atmosphere on average at continuumoptical depths closer to logτ5000 = 0 and averages overthe visible hemisphere of the star. Therefore, there are three possibleexplanations for the newly discovered frequencies: (1) there are modeswith nodes near to the level where the photometry samples that can beeasily detected at the higher level of formation of the PrIII lines; or(2) there are higher degree, l, non-radial oblique pulsation modes thatare detectable in the spectroscopy because the PrIII is concentratedtowards the magnetic poles where such modes have their highestamplitudes, but average out over the visible hemisphere in thephotometry which samples the star's surface more uniformly; or (3) thereis significant growth and decay of the principal mode amplitudes on atime-scale of just a few pulsation cycles at the high level of formationof the PrIII lines and core of the Hα line. The third hypothesisimplies that this level is within the magneto-acoustic boundary layerwhere energy is being dissipated by both outward acoustic running wavesand inward magnetic slow waves. We suggest observations that candistinguish among these three possibilities. We propose that strongchanges in pulsation phase seen with atmospheric height in roAp stars,in some cases more than π rad from the top to the bottom of a singlespectral line, strongly affect the pulsation phases seen in photometryin various bandpasses which explains why phase differences betweenbandpasses for roAp stars have never been explicable with standardtheories that assume single spherical harmonics within the observableatmosphere. We also discuss the photometric amplitude variations as afunction of bandpass, and suggest that these are primarily caused bycontinuum variations, rather than by variability in the rare earthelement lines. We propose further tests of this suggestion.Based on observations collected at the European Southern Observatory,Paranal, Chile, as part of programme 072.D-0138.E-mail: dwkurtz@uclan.ac.uk
| 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.
| Evolutionary state of magnetic chemically peculiar stars
Context: .The photospheres of about 5-10% of the upper main sequencestars exhibit remarkable chemical anomalies. Many of these chemicallypeculiar (CP) stars have a global magnetic field, the origin of which isstill a matter of debate. Aims: .We present a comprehensivestatistical investigation of the evolution of magnetic CP stars, aimedat providing constraints to the theories that deal with the origin ofthe magnetic field in these stars. Methods: .We have collectedfrom the literature data for 150 magnetic CP stars with accurateHipparcos parallaxes. We have retrieved from the ESO archive 142 FORS1observations of circularly polarized spectra for 100 stars. From thesespectra we have measured the mean longitudinal magnetic field, anddiscovered 48 new magnetic CP stars (five of which belonging to the rareclass of rapidly oscillating Ap stars). We have determined effectivetemperature and luminosity, then mass and position in the H-R diagramfor a final sample of 194 magnetic CP stars. Results: .We foundthat magnetic stars with M > 3 ~M_ȯ are homogeneouslydistributed along the main sequence. Instead, there are statisticalindications that lower mass stars (especially those with M ≤2~M_ȯ) tend to concentrate in the centre of the main sequence band.We show that this inhomogeneous age distribution cannot be attributed tothe effects of random errors and small number statistics. Our datasuggest also that the surface magnetic flux of CP stars increases withstellar age and mass, and correlates with the rotation period. For starswith M > 3~M_ȯ, rotation periods decrease with age in a wayconsistent with the conservation of the angular momentum, while for lessmassive magnetic CP stars an angular momentum loss cannot be ruledout. Conclusions: .The mechanism that originates and sustains themagnetic field in the upper main sequence stars may be different in CPstars of different mass.
| An Atlas of K-Line Spectra for Cool Magnetic CP Stars: The Wing-Nib Anomaly (WNA)
We present a short atlas illustrating the unusual Ca II K-line profilesin upper main-sequence stars with anomalous abundances. Slopes of theprofiles for 10 cool, magnetic chemically peculiar (CP) stars changeabruptly at the very core, forming a deep ``nib.'' The nibs show thesame or nearly the same radial velocity as the other atomic lines. Thenear wings are generally more shallow than in normal stars. In threemagnetic CP stars, the K lines are too weak to show this shape, althoughthe nibs themselves are arguably present. The Ca II H lines also showdeep nibs, but the profiles are complicated by the nearby, strongHɛ absorption. The K-line structure is nearly unchanged withphase in β CrB and α Cir. Calculations, including NLTE, showthat other possibilities in addition to chemical stratification mayyield niblike cores.
| An X-Ray Search for Compact Central Sources in Supernova Remnants. II. Six Large-Diameter SNRs
We present the second in a series of studies in which we have searchedfor undiscovered neutron stars in supernova remnants (SNRs). This paperdeals with the six largest SNRs in our sample, too large for Chandra orXMM-Newton to cover in a single pointing. These SNRs are nearby, withtypical distances of <1 kpc. We therefore used the ROSAT BrightSource Catalog and past observations in the literature to identify X-raypoint sources in and near the SNRs. Out of 54 sources, we wereimmediately able to identify optical/IR counterparts to 41 from existingdata. We obtained Chandra snapshot images of the remaining 13 sources.Of these, 10 were point sources with readily identified counterparts,two were extended, and one was not detected in the Chandra observationbut is likely a flare star. One of the extended sources may be a pulsarwind nebula, but if so it is probably not associated with the nearbySNR. We are then left with no identified neutron stars in these six SNRsdown to luminosity limits of ~1032 ergs s-1. Theselimits are generally less than the luminosities of typical neutron starsof the same ages, but are compatible with some lower luminosity sourcessuch as the neutron stars in the SNRs CTA 1 and IC 443.
| New Photometry of the roAp Star 33 Lib
New photometric observations of 33 Lib show that a new frequencydetected spectroscopically in radial velocity variations is definitelynot present in broad-band photometric variations.
| Detection of an extraordinarily large magnetic field in the unique ultra-cool Ap star HD 154708
We have discovered an extraordinarily large mean longitudinal magneticfield of 7.5 kG in the ultra-cool low mass Ap starHD 154708 using FORS 1 in spectropolarimetric mode. FromUVES spectra, we have measured a mean magnetic field modulus of24.5 kG. This is the second-largest mean magnetic field modulusever measured in an Ap star. Furthermore, it is very likely thatthis star is one of the coolest and least massive among the Ap stars andis located in the H-R diagram in the same region in which rapidlyoscillating Ap stars have been detected. We note that all known roApstars have much smaller magnetic fields, by at least a factor of three.
| Asteroseismology: Past, Present and Future
Asteroseismology studies stars with a wide variety of interior andsurface conditions. For two decades asteroseismic techniques have beenapplied to many pulsating stars across the HR diagram. Asteroseismologyis now a booming field of research with stunning new discoveries; Ihighlight a personal selection of these in this review, many of whichare discussed in more detail elsewhere in these proceedings. For manyyears the Nainital-Cape Survey for northern roAp stars has been runningat ARIES, so I emphasise new spectroscopic results for roAp stars andpoint out the outstanding prospects for the planned ARIES 3-m telescopeat Devastal. High precision spectroscopy has revolutionised theasteroseismic study of some types of stars - particularly solar-likeoscillators and roAp stars - while photometry is still the best way tostudy the frequency spectra that are the basic data of asteroseismology.New telescopes, new photo-meters and space missions are revolutionisingasteroseismic photometry. In addition to the ground-based potential ofasteroseismic spectroscopy, India has the knowledge and capability forspace-based asteroseismic photometry. The future for asteroseismology isbright indeed, especially for Indian astronomers.
| Temperature Behavior of Elemental Abundances in the Atmospheres of Magnetic Peculiar Stars
We analyze the temperature dependence of the abundances of the chemicalelements Si, Ca, Cr, and Fe in the atmospheres of normal, metallic-line(Am), magnetic peculiar (Ap), and pulsating magnetic peculiar (roAp)stars in the range 6000 15000 K. The Cr and Fe abundances in theatmospheres of Ap stars increase rapidly as the temperature rises from6000 to 9000 10000 K. Subsequently, the Cr abundance decreases to valuesthat exceed the solar abundance by an order of magnitude, while the Feabundance remains enhanced by approximately +1.0 dex compared to thesolar value. The temperature dependence of the abundances of theseelements in the atmospheres of normal and Am stars is similar in shape,but its maximum is several orders of magnitude lower than that observedfor Ap stars. In the range 6000 9500 K, the observed temperaturedependences for Ap stars are satisfactorily described in terms ofelement diffusion under the combined action of gravitational settlingand radiative acceleration. It may well be that diffusion also takesplace in the atmospheres of normal stars, but its efficiency is very lowdue to the presence of microturbulence. We show that the magnetic fieldhas virtually no effect on the Cr and Fe diffusion in Ap stars in therange of effective temperatures 6000 9500 K. The Ca abundance and itsvariation in the atmospheres of Ap stars can also be explained in termsof the diffusion model if we assume the existence of a stellar wind witha variable moderate rate of (2 4) × 10- 15 M ȯ yr-1.
| Pushing the ground-based limit: 14-μmag photometric precision with the definitive Whole Earth Telescope asteroseismic data set for the rapidly oscillating Ap star HR1217
HR1217 is one of the best-studied rapidly oscillating Ap (roAp) stars,with a frequency spectrum of alternating even- and odd-l modes that aredistorted by the presence of a strong, global magnetic field. Severalrecent theoretical studies have found that within the observableatmospheres of roAp stars the pulsation modes are magneto-acoustic withsignificant frequency perturbations that are cyclic with increasingfrequency. To test these theories a Whole Earth Telescope extendedcoverage campaign obtained 342 h of Johnson B data at 10-s timeresolution for the roAp star HR1217 over 35 d with a 36 per cent dutycycle in 2000 November-December. The precision of the derived amplitudesis 14 μmag, making this one of the highest precision ground-basedphotometric studies ever undertaken. Substantial support has been foundfor the new theories of the interaction of pulsation with the strongmagnetic field. In particular, the frequency jump expected as themagnetic and acoustic components cycle through 2π rad in phase hasbeen found. Additionally, comparison of the new 2000 data with anearlier 1986 multisite study shows clear amplitude modulation for somemodes between 1986 and 2000. The unique geometry of the roAp starsallows their pulsation modes to be viewed from varying aspect withrotation, yielding mode identification information in the rotationalsidelobes that is available for no other type of pulsating star. Thoserotational sidelobes in HR1217 confirm that two of the modes aredipolar, or close to dipolar; based on the frequency spacings andHipparcos parallax, three other modes must be either l= 0 or 2 modes,either distorted by the magnetic field, or a mix of m-modes of given lwhere the mixture is the result of magnetic and rotational effects. Astudy of all high-speed photometric Johnson B data from 1981 to 2000gives a rotation period Prot= 12.4572 d, as found in previouspulsation and photometric studies, but inconsistent with a differentrotation period found in magnetic studies. We suggest that this rotationperiod is correct and that zero-point shifts between magnetic data setsdetermined from different spectral lines are the probable cause of thecontroversy over the rotation period. This WET data set is likely tostand as the definitive ground-based study of HR1217. It will be thebaseline for comparison for future space studies of HR1217, particularlythe MOST satellite observations.
| The calcium isotopic anomaly in magnetic CP stars
Chemically peculiar stars in the magnetic sequence can show the sameisotopic anomaly in calcium previously discovered for mercury-manganesestars in the non-magnetic sequence. In extreme cases, the dominantisotope is the exotic 48Ca. Measurements of Ca II linesarising from 3d-4p transitions reveal the anomaly by showing shifts upto 0.2 Å for the extreme cases - too large to be measurementerrors. We report measurements of miscellaneous objects, including twometal-poor stars, two apparently normal F-stars, an Am-star, and theN-star U Ant. Demonstrable anomalies are apparent only for the Ap stars.The largest shifts are found in rapidly oscillating Ap stars and in oneweakly magnetic Ap star, HD 133792. We note the possible relevance ofthese shifts for the GAIA mission.Based on observations obtained at the European Southern Observatory, LaSilla and Paranal, Chile (ESO programme Nos. 65.L-0316, 68.D-0254 and266.D-5655).
| The 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.
| The Geneva-Copenhagen survey of the Solar neighbourhood. Ages, metallicities, and kinematic properties of 14 000 F and G dwarfs
We present and discuss new determinations of metallicity, rotation, age,kinematics, and Galactic orbits for a complete, magnitude-limited, andkinematically unbiased sample of 16 682 nearby F and G dwarf stars. Our63 000 new, accurate radial-velocity observations for nearly 13 500stars allow identification of most of the binary stars in the sampleand, together with published uvbyβ photometry, Hipparcosparallaxes, Tycho-2 proper motions, and a few earlier radial velocities,complete the kinematic information for 14 139 stars. These high-qualityvelocity data are supplemented by effective temperatures andmetallicities newly derived from recent and/or revised calibrations. Theremaining stars either lack Hipparcos data or have fast rotation. Amajor effort has been devoted to the determination of new isochrone agesfor all stars for which this is possible. Particular attention has beengiven to a realistic treatment of statistical biases and errorestimates, as standard techniques tend to underestimate these effectsand introduce spurious features in the age distributions. Our ages agreewell with those by Edvardsson et al. (\cite{edv93}), despite severalastrophysical and computational improvements since then. We demonstrate,however, how strong observational and theoretical biases cause thedistribution of the observed ages to be very different from that of thetrue age distribution of the sample. Among the many basic relations ofthe Galactic disk that can be reinvestigated from the data presentedhere, we revisit the metallicity distribution of the G dwarfs and theage-metallicity, age-velocity, and metallicity-velocity relations of theSolar neighbourhood. Our first results confirm the lack of metal-poor Gdwarfs relative to closed-box model predictions (the ``G dwarfproblem''), the existence of radial metallicity gradients in the disk,the small change in mean metallicity of the thin disk since itsformation and the substantial scatter in metallicity at all ages, andthe continuing kinematic heating of the thin disk with an efficiencyconsistent with that expected for a combination of spiral arms and giantmolecular clouds. Distinct features in the distribution of the Vcomponent of the space motion are extended in age and metallicity,corresponding to the effects of stochastic spiral waves rather thanclassical moving groups, and may complicate the identification ofthick-disk stars from kinematic criteria. More advanced analyses of thisrich material will require careful simulations of the selection criteriafor the sample and the distribution of observational errors.Based on observations made with the Danish 1.5-m telescope at ESO, LaSilla, Chile, and with the Swiss 1-m telescope at Observatoire deHaute-Provence, France.Complete Tables 1 and 2 are only available in electronic form at the CDSvia anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/418/989
| New measurements of magnetic fields of roAp stars with FORS 1 at the VLT
Magnetic fields play a key role in the pulsations of rapidly oscillatingAp (roAp) stars since they are a necessary ingredient of all pulsationexcitation mechanisms proposed so far. This implies that the properunderstanding of the seismological behaviour of the roAp stars requiresknowledge of their magnetic fields. However, the magnetic fields of theroAp stars are not well studied. Here we present new results ofmeasurements of the mean longitudinal field of 14 roAp stars obtainedfrom low resolution spectropolarimetry with FORS 1 at the VLT.Based on observations obtained at the European Southern Observatory,Paranal, Chile (ESO programme No. 269.D-5044).
| Measurements of magnetic fields over the pulsation cycle in six roAp stars with FORS 1 at the VLT
With FORS 1 at the VLT we have tried for the first time to measure themagnetic field variation over the pulsation cycle in six roAp stars tobegin the study of how the magnetic field and pulsation interact. Forthe star HD 101065, which has one of the highest photometric pulsationamplitudes of any roAp star, we found a signal at the known photometricpulsation frequency at the 3σ level in one data set; however thiscould not be confirmed by later observations. A preliminary simplecalculation of the expected magnetic variations over the pulsation cyclesuggests that they are of the same order as our current noise levels,leading us to expect that further observations with increased S/N have agood chance of achieving an unequivocal detection.Based on observations obtained at the European Southern Observatory,Paranal, Chile (ESO programmes Nos. 69.D-0210 and 270.D-5023).
| Discovery of magnetic field variations with the 12.1-minute pulsation period of the roAp star gamma Equulei
We have discovered the first magnetic field variations over thepulsation period in an roAp star. The amplitude of the magneticvariability we have found is significant at the 4.1- to 6.6 sigma levelmeasured for four strong lines of NdIII for gamma Equ with the highestamplitude found being 240 +/- 37 G for the lambda 5845.07 Å line,with a period of 12.1 min. This magnetic field variation is in goodagreement with theoretical expectations, and the period agrees well withthe known photometric periods. We have also found that the time of mostnegative effective magnetic field occurs 0.15 +/- 0.05 cycles prior tomaximum pulsation velocity of recession. There is a small butsignificant variation in the equivalent width of two of the NdIII lines,but no equivalent width variation is detectable for the other two lines.Measurements of four lines of CaI show no variations at all inequivalent width, radial velocity or effective magnetic field strength.We find a difference in the mean effective magnetic field strength offour NdIII lines and four CaI lines and speculate that this could be areal effect caused by the surface concentration of NdIII towards themagnetic pole. If true, this provides a new way to map the horizontalabundance distribution of elements in slowly rotating Ap stars for whichDoppler imaging is not possible.
| Catalogue of averaged stellar effective magnetic fields. I. Chemically peculiar A and B type stars
This paper presents the catalogue and the method of determination ofaveraged quadratic effective magnetic fields < B_e > for 596 mainsequence and giant stars. The catalogue is based on measurements of thestellar effective (or mean longitudinal) magnetic field strengths B_e,which were compiled from the existing literature.We analysed the properties of 352 chemically peculiar A and B stars inthe catalogue, including Am, ApSi, He-weak, He-rich, HgMn, ApSrCrEu, andall ApSr type stars. We have found that the number distribution of allchemically peculiar (CP) stars vs. averaged magnetic field strength isdescribed by a decreasing exponential function. Relations of this typehold also for stars of all the analysed subclasses of chemicalpeculiarity. The exponential form of the above distribution function canbreak down below about 100 G, the latter value representingapproximately the resolution of our analysis for A type stars.Table A.1 and its references are only available in electronic form atthe CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/407/631 and Tables 3 to 9are only available in electronic form at http://www.edpsciences.org
| 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.
| Interpretation of the Core-Wing Anomaly of Balmer Line Profiles of Cool Ap Stars
A number of cool magnetic chemically peculiar stars exhibit abnormalprofiles of hydrogen Balmer lines. This anomaly, which is most clearlyvisible in Hα, consists of a sharp transition between broad Starkwings and an unusually narrow Doppler core. Although the core-winganomaly is a clear indication of an abnormal structure of theatmospheres of cool Ap stars, it has so far eluded even qualitativeinterpretation. In this Letter we report results of an attempt toreproduce the core-wing anomaly of Balmer lines by empiricalmodification of the thermal atmospheric structure. We find that it ispossible to obtain a very good fit to the inner and outer wings as wellas to reproduce the abrupt core-wing transition and widths of bothHα and Hβ by increasing the temperature by 500-1000 K atintermediate atmospheric layers (-4<=logτ5000<=-1).Thus, detailed analysis of hydrogen lines provides a very useful methodfor revealing the atmospheric structure of cool Ap stars and shouldserve as a crucial test for future self-consistent model atmospheres ofpeculiar stars. Based on observations obtained at the European SouthernObservatory, Very Large Telescope.
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לינקים קשורים
הכנס לינק חדש
משמש של הקבוצה הבאה
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תצפית ומידע אסטרומטרי
| קבוצת-כוכבים: | עגור |
| התרוממות ימנית: | 23h01m46.82s |
| סירוב: | -44°50'26.9" |
| גודל גלוי: | 7.529 |
| מרחק: | 95.329 פארסק |
| תנועה נכונה: | -91.7 |
| תנועה נכונה: | -47.8 |
| B-T magnitude: | 8.084 |
| V-T magnitude: | 7.575 |
קטלוגים וכינוים:
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