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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
| 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.
| High time resolution spectroscopy and magnetic variability of the cool Ap star HD965*
We present the results of an investigation of the magnetic Ap star HD965with high spectral and time resolution. We determine precise radialvelocities using spectra obtained with the Ultraviolet-Visual EchelleSpectrograph (UVES) on the European Southern Observatory Very LargeTelescope. Special attention is given to spectral lines of rare-earthelements which in rapidly oscillating Ap (roAp) stars exhibit thestrongest radial velocity variations with pulsation period. Careful timeseries analysis did not detect any convincing evidence of pulsation inHD965 with an upper limit in amplitude of 15-20 m s-1. Allproperties of HD965 are similar to the majority of roAp stars. A likelyreason for the apparent lack of pulsation could be connected with thegeometrical structure of the magnetic field and the aspect of the starat the time of observation. Longitudinal magnetic field measurements forHD965 showed that our UVES spectral observations were carried out whenthe longitudinal field was near zero and therefore, according to theoblique rotator model, near a time when the star was viewed from themagnetic equator. For a dipole oscillation aligned with the magneticfield, as is typical of roAp stars, no variation can be detected at thisaspect. We may, therefore, expect to detect rapid oscillations in HD965in the future, when the star will present one of the magnetic poles.
| Probable detection of radial magnetic field gradients in the atmospheres of Ap stars
For the first time the possible presence of radial gradients of magneticfields in the atmospheres of three magnetic Ap stars has been criticallyexamined by measurements of the mean magnetic field modulus fromspectral lines resolved into magnetically split components lying on thedifferent sides of the Balmer jump. A number of useful diagnostic linesbelow and above the Balmer discontinuity, only slightly affected byblends, with simple doublet and triplet Zeeman pattern have beenidentified from the comparison between synthetic spectra computed withthe SYNTHMAG code and the high resolution and S/N spectra obtained inunpolarized light with the ESO VLT UVES spectrograph. For all threestars of our sample, HD 965, HD 116114 and 33 Lib, an increase of themagnetic field strength of the order of a few hundred Gauss has beendetected bluewards of the Balmer discontinuity. These results should betaken into account in future modelling of the geometric structure of Apstar magnetic fields and the determination of the chemical abundances inAp stars with strong magnetic fields.Based on observations obtained at the European Southern Observatory,Paranal, Chile (ESO program No. 70.D-0470).
| On the possible presence of promethium in the spectra of HD 101065 (Przybylski's star) and HD 965
% Traditional and statistical line-identification methods indicate thepresence of Pm I and II, Tc I, and perhaps Tc II in the spectrum of theroAp star HD 101065. These methods also lead to the presence of Pm IIand probably also Pm I in a related cool Ap star, HD 965. Thespectroscopic evidence is strong enough that we would declare promethiumto be present without hesitation, if any of its isotopes were stable.The longest-lived promethium isotope has a half-life of only 17.7 years.The presence of this element would mean that unrecognized processes -perhaps flare activities - are taking place in the atmospheres of thesestars. The significance of such processes for galactic chemicalevolution cannot be ruled out. We discuss the possibility that thehighly improbable wavelength coincidences are due to chance, or due tocontamination of the laboratory sources.Based on observations collected at the European Southern Observatory, LaSilla (Chile), as part of programme No. 60.E-0564, and Paranal (Chile),as part of programmes Nos. 68.D-0254 and 70.D-0470.
| A Tight Upper Limit on Oscillations in the Ap Star ɛ Ursae Majoris from WIRE Photometry
Observations of ɛ UMa obtained with the star tracker on the WideField Infrared Explorer satellite during a month in mid-2000 areanalyzed. This is some of the most precise photometry of an Ap star. Theamplitude spectrum is used to set an upper limit of 75 parts per millionfor the amplitude of stellar pulsations in this star unless bycoincidence it oscillates with a single mode at the satellite orbit, itsharmonics, or their 1 day-1 aliases. This is the tightestlimit put on the amplitude of oscillations in an Ap star. As therotation period of ɛ UMa is relatively short (5.1 days), it cannotbe argued that the observations were made at a wrong rotational phase.Our results thus support the idea that some Ap stars do not pulsate atall.
| Scattered Light from Close-in Extrasolar Planets: Prospects of Detection with the MOST Satellite
The ultraprecise photometric space satellite MOST (Microvariability andOscillations of STars) will provide the first opportunity to measure thealbedos and scattered light curves from known short-period extrasolarplanets. Because of the changing phases of an extrasolar planet as itorbits its parent star, the combined light of the planet-star systemwill vary on the order of tens of micromagnitudes. The amplitude andshape of the resulting light curve is sensitive to the planet's radiusand orbital inclination, as well as the composition and sizedistribution of the scattering particles in the planet's atmosphere. Topredict the capabilities of MOST and other planned space missions, wehave constructed a series of models of such light curves, improving uponearlier work by incorporating more realistic details such as limbdarkening of the star, intrinsic granulation noise in the star itself,tidal distortion and back-heating, higher angular resolution of thelight scattering from the planet, and exploration of the significance ofthe angular size of the star as seen from the planet. We use photometricperformance simulations of the MOST satellite, with the light-curvemodels as inputs, for one of the mission's primary targets, τBoötis. These simulations demonstrate that, even adopting a veryconservative signal detection limit of 4.2 μmag in amplitude (notpower), we will be able to either detect the τ Boötis planetlight curve or put severe constraints on possible extrasolar planetatmospheric models.
| A high-precision null result in the search for rapid oscillations in HD 965
HD 965 is a strongly magnetic peculiar A star with clear Zeemansplitting in its spectral lines, a rotation period greater than 2 yr,H-line core-wing anomaly, and strongly negative Strömgren deltac1 and delta m1 indices. All of thesecharacteristics are typical of the rapidly oscillating Ap stars, and ithas been suggested that HD 965 should be one. We have tested this starfor photometric variability under excellent conditions with lowscintillation noise using the 2-m TBL for 2.7 hr at Pic du MidiObservatory. The amplitude spectrum of the light curve is flat withnoise peaks below 0.2 mmag in the frequency range of the roAp stars.This suggests that it is a non-oscillating Ap star, although proof ofthat will need further observations each year for many years, to ruleout the possibility that rotational modulation has suppressed theobserved amplitude in 2002.
| 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.
| The core-wing anomaly of cool Ap stars. Abnormal Balmer Profiles
The profiles of Hα in a number of cool Ap stars are anomalous.Broad wings, indicative of temperatures in the range 7000-8000 K endabruptly in narrow cores. The widths of these cores are compatible withthose of dwarfs with temperatures of 6000 K or lower. This profile hasbeen known for Przybylski's star, but it is seen in other cool Ap stars.The Hβ profile in several of these stars shows a similar core-winganomaly (CWA). In Przybylski's star, the CWA is probably present athigher Balmer members. We are unable to account for these profileswithin the context of LTE and normal dwarf atmospheres. We conclude thatthe atmospheres of these stars are not ``normal''. This is contrary to anotion that has long been held. Based on observations obtained at theEuropean Southern Observatory, Paranal, Chile (ESO programme No.65.I-0644) and the South African Astronomical Observatory (SAAO).}
| Radial velocities. Measurements of 2800 B2-F5 stars for HIPPARCOS
Radial velocities have been determined for a sample of 2930 B2-F5 stars,95% observed by the Hipparcos satellite in the north hemisphere and 80%without reliable radial velocity up to now. Observations were obtainedat the Observatoire de Haute Provence with a dispersion of 80Ä,mm(-1) with the aim of studying stellar and galactic dynamics.Radial velocities have been measured by correlation with templates ofthe same spectral class. The mean obtained precision is 3.0 km s(-1)with three observations. A new MK spectral classification is estimatedfor all stars. Based on observations made at the Haute ProvenceObservatory, France and on data from The Hipparcos Catalogue, ESA.Tables 4, 5 and 6 are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr or viahttp://cdsweb.u-strasbg.fr/Abstract.htm
| The HR-diagram from HIPPARCOS data. Absolute magnitudes and kinematics of BP - AP stars
The HR-diagram of about 1000 Bp - Ap stars in the solar neighbourhoodhas been constructed using astrometric data from Hipparcos satellite aswell as photometric and radial velocity data. The LM method\cite{luri95,luri96} allows the use of proper motion and radial velocitydata in addition to the trigonometric parallaxes to obtain luminositycalibrations and improved distances estimates. Six types of Bp - Apstars have been examined: He-rich, He-weak, HgMn, Si, Si+ and SrCrEu.Most Bp - Ap stars lie on the main sequence occupying the whole width ofit (about 2 mag), just like normal stars in the same range of spectraltypes. Their kinematic behaviour is typical of thin disk stars youngerthan about 1 Gyr. A few stars found to be high above the galactic planeor to have a high velocity are briefly discussed. Based on data from theESA Hipparcos astrometry satellite and photometric data collected in theGeneva system at ESO, La Silla (Chile) and at Jungfraujoch andGornergrat Observatories (Switzerland). Tables 3 and 4 are onlyavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| The mean magnetic field modulus of AP stars
We present new measurements of the mean magnetic field modulus of asample of Ap stars with spectral lines resolved into magnetically splitcomponents. We report the discovery of 16 new stars having thisproperty. This brings the total number of such stars known to 42. Wehave performed more than 750 measurements of the mean field modulus of40 of these 42 stars, between May 1988 and August 1995. The best of themhave an estimated accuracy of 25 - 30 G. The availability of such alarge number of measurements allows us to discuss for the first time thedistribution of the field modulus intensities. A most intriguing resultis the apparent existence of a sharp cutoff at the low end of thisdistribution, since no star with a field modulus (averaged over therotation period) smaller than 2.8 kG has been found in this study. Formore than one third of the studied stars, enough field determinationswell distributed throughout the stellar rotation cycle have beenachieved to allow us to characterize at least to some extent thevariations of the field modulus. These variations are oftensignificantly anharmonic, and it is not unusual for their extrema not tocoincide in phase with the extrema of the longitudinal field (for thefew stars for which enough data exist about the latter). This, togetherwith considerations on the distribution of the relative amplitude ofvariation of the studied stars, supports the recently emerging evidencefor markedly non-dipolar geometry and fine structure of the magneticfields of most Ap stars. New or improved determinations of the rotationperiods of 9 Ap stars have been achieved from the analysis of thevariations of their mean magnetic field modulus. Tentative values of theperiod have been derived for 5 additional stars, and lower limits havebeen established for 10 stars. The shortest definite rotation period ofan Ap star with magnetically resolved lines is 3.4 deg, while thosestars that rotate slowest appear to have periods in excess of 70 or 75years. As a result of this study, the number of known Ap stars withrotation periods longer than 30 days is almost doubled. We brieflyrediscuss the slow-rotation tail of the period distribution of Ap stars.This study also yielded the discovery of radial velocity variations in 8stars. There seems to be a deficiency of binaries with short orbitalperiods among Ap stars with magnetically resolved lines. Based onobservations collected at the European Southern Observatory (La Silla,Chile; ESO programmes Nos. 43.7-004, 44.7-012, 49.7-030, 50.7-067,51.7-041, 52.7-063, 53.7-028, 54.E-0416, and 55.E-0751), at theObservatoire de Haute-Provence (Saint-Michel-l'Observatoire, France), atKitt Peak National Observatory, and at the Canada-France-HawaiiTelescope. Tables 2, 3, and 4 are also available in electronic form atthe CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html.
| Observations d'étoiles AP et AM
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1970A&AS....1....1F&db_key=AST
| Catalogue et bibliographie des étoiles A à spectre particulier - Deuxième supplément
Not Available
| Rotational velocities of faint stars of early spectral types.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1963AJ.....68..428S&db_key=AST
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