Оглавление
Изображения
Загрузить ваше изображение
DSS Images Other Images
Публикации по объекту
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
| 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
| On-line database of photometric observations of magnetic chemically peculiar stars
We present our extensive project of the On-line database ofphotometric observations of magnetic chemically peculiar stars tocollect published data of photometric observations of magneticchemically peculiar (mCP) stars in the optical and near IR regions. Nowthe nascent database contains more than 107 000 photometric measurementsof 102 mCP stars and will be continually supplemented with published ornew photometric data on these and about 150 additional mCP stars. Thisreport describes the structure and organization of the database.Moreover, for the all included data we estimated the error ofmeasurements and the effective amplitudes of the light curves.
| 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
| 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.
| 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.
| 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.
| 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.
| The calcium isotopic anomaly in magnetic CP stars
Chemically peculiar stars in the magnetic sequence can show the sameisotopic anomaly in calcium previously discovered for mercury-manganesestars in the non-magnetic sequence. In extreme cases, the dominantisotope is the exotic 48Ca. Measurements of Ca II linesarising from 3d-4p transitions reveal the anomaly by showing shifts upto 0.2 Å for the extreme cases - too large to be measurementerrors. We report measurements of miscellaneous objects, including twometal-poor stars, two apparently normal F-stars, an Am-star, and theN-star U Ant. Demonstrable anomalies are apparent only for the Ap stars.The largest shifts are found in rapidly oscillating Ap stars and in oneweakly magnetic Ap star, HD 133792. We note the possible relevance ofthese shifts for the GAIA mission.Based on observations obtained at the European Southern Observatory, LaSilla and Paranal, Chile (ESO programme Nos. 65.L-0316, 68.D-0254 and266.D-5655).
| Radial velocity variations in pulsating Ap stars. V. 10 Aquilae
We have used precise stellar radial velocities to study the pulsationalmotion of the rapidly oscillating Ap star 10 Aql.Observations were made on three separate nights using the highresolution spectrograph of the Harlan J. Smith 2.7 m telescope atMcDonald Observatory. A high radial velocity precision was achieved byusing an iodine gas absorption cell. The integrated radial velocitymeasurements using the full wavelength region covered by iodineabsorption lines (5000-6300 Å) failed to detect any coherentpulsational velocity variations to a level of 2.5-5 m s-1. Ananalysis over a much narrower wavelength range revealed that pulsationalradial velocity variations are indeed present in 10Aql, but only in 5 spectral lines. The amplitude of thesevariations ranged from about 100 m s-1 to as high as 398 ms-1 for an unidentified feature at λ5471.40 Å.Other spectral features showing pulsational radial velocity variationsare tentatively identified as Sm II and Tm II.Based on observations collected at McDonald Observatory.
| Radial velocity variations in pulsating Ap stars - II. 33 Librae
We present precise relative radial velocity (RV) measurements for therapidly oscillating Ap (roAp) star 33 Librae measured fromhigh-resolution data spanning the wavelength interval 5000-6200 Å.We find that the pulsational radial velocity amplitude determined over abroad wavelength range (~100 Å) depends on the spectral regionthat is examined and can be as high as 60 m s-1 at 5600Å and as low as 7 m s-1 in the 5900 Å region. RVmeasurements of individual spectral lines can show higher amplitudesthan results obtained using a `broad-band' measurement that includesmany spectral lines. The acoustic cross-sections of the atmosphere, i.e.the phase and amplitude of the pulsations, as a function of opticaldepth are found for spectral lines of Ca, Cr, Fe, La, Ce, Gd, Er and Nd.This analysis shows that pulsation phase is variable through theatmosphere and that Nd III lines pulsate almost 180° out of phasewith those of Nd II features and are formed significantly higher in thestellar atmosphere. This conclusively establishes the presence of atleast one radial node to the pulsations in the upper stellar atmosphere.We have estimated that this acoustic node is located above an opticaldepth log τ < -4.5 and below the level where the Nd III lines areformed. We also suspect that there may be a second atmospheric node inthe lower atmosphere below or at log τ~=-0.9 and close to continuumformation level.The histogram of pulsational phases for all individual spectral featuresshows a bimodal Gaussian distribution with 17 per cent of the lineshaving a pulsational phase ~165° out of phase with most otherspectral lines. This is also consistent with the presence of a radialnode in the stellar atmosphere. The accumulation of phase due to arunning wave component can explain the 165° phase difference as wellas the broader width (by a factor of 2) of one of the Gaussiancomponents of the phase distribution.We also found evidence for phase variations as a function of effectiveLandég-factors. This may be the influence of magnetic field andmagnetic intensification effect on depths of spectral line formation andshows that the magnetic field is controlling the pulsations. Our RVmeasurements for 33 Lib suggest that we are seeing evidence of verticalstructure to the oscillations as well as the influence of thedistribution of elements on the stellar surface.We suggest and briefly discuss a new semi-empirical tomographicprocedure for mono- and multimode roAp stars that will use acousticcross-sections obtained on different chemical elements and differentpulsation modes for restoring the abundance and acoustic profilesthroughout the stellar atmosphere and across the stellar surface.
| 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
| 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.
| Multiplicity among chemically peculiar stars. II. Cool magnetic Ap stars
We present new orbits for sixteen Ap spectroscopic binaries, four ofwhich might in fact be Am stars, and give their orbital elements. Fourof them are SB2 systems: HD 5550, HD 22128, HD 56495 and HD 98088. Thetwelve other stars are: HD 9996, HD 12288, HD 40711, HD 54908, HD 65339,HD 73709, HD 105680, HD 138426, HD 184471, HD 188854, HD 200405 and HD216533. Rough estimates of the individual masses of the components of HD65339 (53 Cam) are given, combining our radial velocities with theresults of speckle interferometry and with Hipparcos parallaxes.Considering the mass functions of 74 spectroscopic binaries from thiswork and from the literature, we conclude that the distribution of themass ratio is the same for cool Ap stars and for normal G dwarfs.Therefore, the only differences between binaries with normal stars andthose hosting an Ap star lie in the period distribution: except for thecase of HD 200405, all orbital periods are longer than (or equal to) 3days. A consequence of this peculiar distribution is a deficit of nulleccentricities. There is no indication that the secondary has a specialnature, like e.g. a white dwarf. Based on observations collected at theObservatoire de Haute-Provence (CNRS), France.Tables 1 to 3 are 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/394/151Appendix B is only available in electronic form athttp://www.edpsciences.org
| On the excitation mechanism in roAp stars
We investigate a model for the excitation of high-order oscillations inroAp stars. In this model we assume that the strong concentration ofmagnetic field about the magnetic poles is enough to suppressconvection. Thus the model considered is composed of two polar regions,in which convection is presumed to be suppressed totally, and anequatorial region, where the convection is unaffected. This model isgenerated by building pairs of locally spherically symmetricalequilibria to represent the polar and equatorial regions of the star,which are patched together below the base of the convection zone.Gravitational settling of heavy elements is taken into account bychoosing appropriate chemical composition profiles for both the polarand equatorial regions. Our results indicate that the composite model isunstable against axisymmetric non-radial high-order modes of pulsationthat are aligned with the magnetic poles. The oscillations are excitedby the κ mechanism acting principally in the hydrogen ionizationzones of the polar regions. The effect of the lateral inhomogeneity onthe second frequency differences is also investigated; we find that theperturbation to them by the inhomogeneity is of the same order as thesecond differences themselves, thereby hindering potential attempts touse such differences to identify the degrees of the modes in astraightforward way.
| uvby photometry of the magnetic CP stars 20 Eridani, HR 1297, 12 Canis Majoris, and HD 134214
Differential Strömgren uvby observations from the Four CollegeAutomated Photoelectric Telescope (FCAPT) are presented for the mCPstars 20 Eri, HR 1297, 12 CMa, and HD 134214. Adelman & Boyce foundsuggestions that the light curves of 20 Eri change between observingseasons. Observations for two additional seasons confirm this behaviorand lead to an improved period of 1.928890 days. For HR 1297 a newperiod of 15.7490 days was derived. The uvby light curves now appear tobe the same for different observing seasons. 12 CMa is found to be aphotometric variable with the 2.18010 day period determined frommagnetic measurements by Bohlender et al. Observations of the rapidlyoscillating Ap star HD 134214 confirm the lack of photometricvariability which is not associated with the rapid oscillations. Tables2, 3, 4 and 5 are 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/Abstract.html
| Magnetic AP Stars in the Hertzsprung-Russell Diagram
The evolutionary state of magnetic Ap stars is rediscussed using therecently released Hipparcos data. The distribution of the magnetic Apstars of mass below 3 Msolar in the H-R diagram differs fromthat of the normal stars in the same temperature range at a high levelof significance. Magnetic stars are concentrated toward the center ofthe main-sequence band. This is shown in two forms of the H-R diagram:one where logL is plotted against logTeff and a version moredirectly tied to the observed quantities, showing the astrometry-basedluminosity (Arenou & Luri) against the (B2-G)0 index ofGeneva photometry. In particular, it is found that magnetic fieldsappear only in stars that have already completed at least approximately30% of their main-sequence lifetime. No clear picture emerges as to thepossible evolution of the magnetic field across the main sequence. Hintsof some (loose) relations between magnetic field strength and otherstellar parameters are found: stars with shorter periods tend to havestronger fields, as do higher temperature and higher mass stars. Amarginal trend of the magnetic flux to be lower in more slowly rotatingstars may possibly be seen as suggesting a dynamo origin for the field.No correlation between the rotation period and the fraction of themain-sequence lifetime completed is observed, indicating that the slowrotation in these stars must already have been achieved before theybecame observably magnetic. Based on data from the ESA Hipparcossatellite and on observations collected at the European SouthernObservatory (La Silla, Chile; ESO programs 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, and55.E-0751), at the Observatoire de Haute-Provence (Saint-Michell'Observatoire, France), at Kitt Peak National Observatory, and at theCanada-France-Hawaii Telescope.
| ICCD Speckle Observations of Binary Stars. XXIII. Measurements during 1982-1997 from Six Telescopes, with 14 New Orbits
We present 2017 observations of 1286 binary stars, observed by means ofspeckle interferometry using six telescopes over a 15 year period from1982 April to 1997 June. These measurements constitute the 23dinstallment in CHARA's speckle program at 2 to 4 m class telescopes andinclude the second major collection of measurements from the MountWilson 100 inch (2.5 m) Hooker Telescope. Orbital elements are alsopresented for 14 systems, seven of which have had no previouslypublished orbital analyses.
| Rapidly oscillating Ap stars versus non-oscillating Ap stars
The positions in the HR diagram and the kinematic characteristics ofrapidly oscillating and non-oscillating chemically peculiar stars areobtained using new Hipparcos proper motions and parallaxes, and our ownradial velocity measurements. We find that rapidly oscillating stars, asa group, are (-0.47 +/- 0.34) mag above the zero-age main sequence(ZAMS), while the non-oscillating stars are (-1.20 +/- 0.65) mag abovethe ZAMS and so appear slightly more evolved on average. From thecomparison of the kinematical characteristics, we conclude that bothgroups are very similar. The results of radial velocity measurementsindicate that there is a real deficiency of binaries among rapidlyoscillating stars. Presently, no such star is known to be aspectroscopic binary.
| Observing roAp Stars with WET: A Primer
We give an extensive primer on roAp stars -- introducing them, puttingthem in context and explaining terminology and jargon, and giving athorough discussion of what is known and not known about them. Thisprovides a good understanding of the kind of science WET could extractfrom these stars. We also discuss the many potential pitfalls andproblems in high-precision photometry. Finally, we suggest a WETcampaign for the roAp star HR 1217.
| Lithium on the surface of cool magnetic CP stars I. Summary of spectroscopic observations with three telescopes
We present new results of observations of the 6708 Ä line in twelveCP stars made with three telescopes: the ESO CAT telescope, the CrimeanObservatory (CrAO) 2.6-m telescope, and the Nordic Optical Telescope(NOT). Unique profile and wavelength variations of the line at 6708Ä were discovered in two stars, HD 60435 and HD 83368. We presentarguments in favour of the identification of this feature with theresonant Li i line. According to the incidence and variability of thisline, spectra of twelve stars were divided in four groups: Group 1 - theline is remarkably variable in intensity and wavelength; this groupincludes the stars HD 83386 and HD 60435; Group 2 - the line is variableto a lesser extent; includes beta CrB and HD 188041; Group 3 - the lineis not variable; includes 33 Lib, HD 134214, HD 166473 and gamma Equ;Group 4 - the line is not present; includes HD 42659, HD 80316, HD118022 and HD 128898. The behaviour in Groups 1-3 can be explained bythe existence of Li rich spots on the star's surface using the obliquerotator model with different inclination and magnetic obliquity for eachstar. For HD 83368 we have found a tight correlation between thevisibility of the spots and the orientation of the dipole pulsationmode. Based on observations collected at the European SouthernObservatory, La Silla, Chile, the Nordic Optical Telescope, La Palma,Spainand the Shajn Telescope, Crimean Astrophysical Observatory,Ukraine}
| How to drive roAp stars
Rapidly oscillating Ap stars constitute a unique class of pulsators withwhich to study non-radial oscillations under some - even for stars -unusual physical conditions. These stars are chemically peculiar, theyhave strong magnetic fields and they often pulsate in several high-orderacoustic modes simultaneously. We discuss here an excitation mechanismfor short-period oscillation modes based on the classical kappamechanism. We particularly stress the conditions that must be fulfilledfor successful driving. Specifically, we discuss the roles of thechemical peculiarity and strong magnetic field on the oscillation modesand what separates these pulsators from delta Scuti and Am-type stars.
| On the HIPPARCOS photometry of chemically peculiar B, A, and F stars
The Hipparcos photometry of the Chemically Peculiar main sequence B, A,and F stars is examined for variability. Some non-magnetic CP stars,Mercury-Manganese and metallic-line stars, which according to canonicalwisdom should not be variable, may be variable and are identified forfurther study. Some potentially important magnetic CP stars are noted.Tables 1, 2, and 3 are available only in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| Determining temperature amplitude as a function of depth in the atmospheres of rapidly oscillating AP stars
We present numerical models based on realistic treatment of theintensity spectrum (from model atmospheres), and demonstrate that theyare consistent with Kurtz and Medupe's recent formula in showing thatlimb darkening is too small an effect to explain the observed sharpdecline of pulsation light amplitude with wavelength in rapidlyoscillating Ap stars. Kurtz and Medupe's formula is shown to be aspecial form of Watson's earlier general formula for non-radial lightvariations of a star pulsating in any mode (lm). Using a techniquesuggested by Kurtz and Medupe we derive temperature semi-amplitude as afunction of depth in the atmospheres of alpha Cir and HR 3831, assumingthat we can neglect non-adiabatic effects.
| 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 acoustic cut-off frequency of roAp stars
Some of the rapidly oscillating (roAp) stars, have frequencies which arelarger than the acoustic cut-off frequency determined from publishedstellar models which usually assume a grey atmosphere. As the cut-offfrequency depends on the T(tau ) relation, we have computed models andadiabatic frequencies for pulsating Ap stars with more realisticatmospheres which include a frequency dependent treatment of radiativetransfer, take blanketing effects into account, and which have a bettertreatment of the radiative pressure. In addition, we are using opacitydistribution functions specific to the atmospheric composition. Withthese improvements over the classical stellar models the theoreticalacoustic cut-off frequency for roAp stars are increased by about 200 muHz, which brings them close to the observations. We restrict thecomparison of our computations with observations to those two`pathological' roAp stars for which more reliable astrophysicalparameters are available, HD 24712 and alpha Cir, and comment briefly ona third one, HD 134214. For alpha Cir we find models which have indeed acut-off frequency beyond the largest observed frequency and which arewell within the Teff - L/L_ȯ error box. For HD 24712only models which are hotter by about 100 K and less luminous by nearly10% than what is actually the most probable value derived byspectroscopy would have an acoustic cut-off frequency large enough. HD134214 fits our models best, however, the error box for Teff- L/L_ȯ is the largest of all three stars. One may thus speculatethat the old controversy about a mismatch between observed largestfrequencies and theoretical cut-off frequencies of roAp star models isresolved. Based on hipparcos data.
| Observations of roAp stars at the Mt. Dushak-Erekdag station of Odessa Astronomical Observatory
Since 1992, observations of roAp stars have been carried out using thedual-channel photometer attached to the 0.8m telescope, which issituated in Central Asia, at the Mt. Dushak-Erekdag station of OdessaAstronomical Observatory. Some results of observations of gamma Equ andof HD 134214 are presented. 5 stars were investigated as roApcandidates. The Fourier spectra of 4 stars did not show any variabilityin the high-frequency region. The Fourier spectrum of HD 99563 revealeda peak at a frequency f=128.9 c/d and with a semi-amplitude of 3.98mmag.
| Atmospheric structure and acoustic cut-off frequency of roAp stars
Some of the rapidly oscillating (CP2) stars, have frequencies which arelarger than the theoretical acoustic cut-off frequency. As the cut-offfrequency depends on the T(tau) relation in the atmosphere, we havecomputed models and adiabatic frequencies for pulsating Ap stars withT(tau) laws based on Kurucz model atmospheres and on Hopf's purelyradiative relation. The frequency-dependent treatment of radiativetransfer as well as an improved calculation of the radiative pressure inKurucz model atmospheres increase the theoretical acoustic cut-offfrequency by about 200 microHz, which is closer to the observations. Foralpha Cir we find models with Kurucz atmospheres which have indeed acut-off frequency beyond the largest observed frequency and which arewell within the T(eff) - L error box. For HD 24712 only models which arehotter by about 100 K and less luminous by nearly 10% than what isactually the most probable value would have an acoustic cut-offfrequency large enough. One may thus speculate that the old controversyabout a mismatch between observed largest frequencies and theoreticalcut-off frequencies of roAp star models is resolved. However, theobservational errors for the astrophysical fundamental parameters haveto be reduced further and the model atmospheres refined.
|
Добавить новую статью
Внешние ссылки
- - Внешних ссылок не найдено -
Добавить внешнюю ссылку
Группы:
|
Наблюдательные данные и астрометрия
| Созвездие: | Весы |
| Прямое восхождение: | 15h09m02.40s |
| Склонение: | -13°59'58.7" |
| Видимая звёздная величина: | 7.471 |
| Расстояние: | 91.575 парсек |
| Собственное движение RA: | -48.2 |
| Собственное движение Dec: | 12.5 |
| B-T magnitude: | 7.908 |
| V-T magnitude: | 7.508 |
Каталоги и обозначения:
|
