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Ýlgili Makaleler
Deep Astrometric Standards and Galactic Structure
The advent of next-generation imaging telescopes, such as the LargeSynoptic Survey Telescope (LSST) and the Panoramic Survey Telescope andRapid Response System (Pan-STARRS), has revitalized the need for deepand precise reference frames. The proposed weak-lensing observationswith these facilities put the highest demands on image quality over wideangles on the sky. It is particularly difficult to achieve asubarcsecond point-spread function on stacked images, where preciseastrometry plays a key role. Current astrometric standards areinsufficient to achieve the science goals of these facilities. We thuspropose the establishment of a few selected deep (V=25) astrometricstandards (DAS). These will enable a reliable geometric calibration ofsolid-state mosaic detectors in the focal plane of large ground-basedtelescopes, and will make a substantial contribution to ourunderstanding of stellar populations in the Milky Way. In this paper weexamine the need for such standards and discuss the strategy forselecting them and their acquisition and reduction techniques. Thefeasibility of DAS is demonstrated by a pilot study around the opencluster NGC 188, using the Kitt Peak National Observatory 4 m CCD Mosaiccamera, and by Subaru Suprime-Cam observations. The goal of reaching anaccuracy of 5-10 mas in positions and obtaining absolute proper motionsgood to 2 mas yr-1 over a several square-degree area ischallenging, but reachable with the NOAO 4 m telescopes and CCD mosaicimagers, or a similar setup. Our proposed DAS aims to establish fourfields near the Galactic plane, at widely separated coordinates. Inaddition to their utilitarian purpose for DAS, the data we will obtainin these fields will enable fundamental Galactic science in their ownright. The positions, proper motions, and VI photometry of faint starswill address outstanding questions of Galactic disk formation andevolution, stellar buildup, and mass assembly via merger events.
| On the Spectroscopic Determination of Atmospheric Parameters and O/Fe Abundances of RR Lyrae Stars
In order to study how the conventional spectroscopic method based on theequivalent widths of the Fe I and Fe II lines effectively applies todetermining the atmospheric parameters (Teff, log g,vt, [Fe/H]) of RR Lyrae variables and how accurately theabundances can be established from such constructed model atmospheres,we analyzed 15 high-dispersion spectrograms of RR Lyr, DX Del, DH Peg,and VY Ser taken at several different phases by using the HDSspectrograph of the Subaru Telescope, and examined the consistency ofthe resulting phase-to-phase abundances. Taking oxygen as the targetelement along with Fe, we determined its non-LTE abundance from the O I6155-8 and 7771-5 triplets. It was found that consistent abundances wereobtained for O as well as Fe to a level of 0.1dex, irrespective ofthe pulsation phase, except for the special near-maximumhigh-temperature phase. This suggests that classical model atmospheresare reliably applicable to abundance determinations of RR Lyrae stars inmost cases. While the oxygen abundances derived from O I 7771-5 wellcorrelate with those from O I 6155-8, the former tends to besystematically larger by 0.1-0.2dex than the latter, which may beinterpreted as being due to the depth-dependence of the microturbulenceincreasing with height; i.e., the vt value derived fromdeep-forming Fe lines of weak/medium-strength may not be simply appliedto higher forming strong lines, such as O I 7771-5.
| Modelling eclipsing binaries with pulsating components: Phase dependence of observed pulsation amplitudes
We investigate the phase dependence of the light variations due topulsations, as modulated with light variations due to the eclipses. Thestudy is based, both on a simplified model and on the rigorous lightcurve (LC) synthesis software of Wilson and Devinney. We model AB Cas asa prototype, using both of these approaches. The phase dependence of thedifferential light variations for total systems is also discussed .
| A catalogue of RR Lyrae stars from the Northern Sky Variability Survey
A search for RR Lyrae stars has been conducted in the publicly availabledata of the Northern Sky Variability Survey. Candidates have beenselected by the statistical properties of their variation; the standarddeviation, skewness and kurtosis with appropriate limits determined froma sample 314 known RRab and RRc stars listed in the General Catalogue ofVariable Stars. From the period analysis and light-curve shape of over3000 candidates 785 RR Lyrae have been identified of which 188 arepreviously unknown. The light curves were examined for the Blazhkoeffect and several new stars showing this were found. Six double-mode RRLyrae stars were also found of which two are new discoveries. Somepreviously known variables have been reclassified as RR Lyrae stars andsimilarly some RR Lyrae stars have been found to be other types ofvariable, or not variable at all.
| Blazhko effect and double-mode RR Lyrae stars .
There are two kinds of multi-periodic RR Lyrae stars. The Blazhko effectis a modulation of the primary light cycle that takes place on a typicaltimescale of tens of days. Double-mode RR Lyrae stars pulsatesimultaneously in the fundamental and first overtone radial modes. Thestate of our knowledge of these multi-periodic variables is reviewed.
| How good are RR Lyrae and Cepheids really as distance indicators? . The observational approach
A number of recent technical developments, including the Hipparcossatellite, the Hubble Space Telescope fine guidance sensors and longbase line near-IR interferometry has made it possible to employ severallargely geometrical methods to determine direct distances to RR Lyraestars and Cepheids. The distance scale now rests on a much firmer basisand the significant differences between the distances based on RR Lyraestars (short) and Cepheids (long) to the LMC have been largelyeliminated. The effects of metallicity on the RR Lyrae period-luminosity(PL) relation in the K-band as well as on the Cepheid PL relationappears to be the main remaining issues but even here empirical resultsare beginning to show convergence. I review here some of these recentdevelopments seen from the perspective of the near-IR surface brightnessmethod.
| Photoelectric Minima of Selected Eclipsing Binaries and Maxima of Pulsating Stars
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| Beobachtungskampagne SW Boo, was kam heraus?
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| Beobachtungsergebnisse Bundesdeutsche Arbeitsgemeinschaft fur Veranderliche Sterne e.V.
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| A New Explanation of the Blazhko Effect in RR Lyrae Stars
An interpretation of the modulating Blazhko effect in RR Lyrae stars ispresented here, in which turbulent convection inside the hydrogen andhelium ionization zones becomes cyclically weakened and strengthenedowing to the presence of a transient magnetic field that is generated insitu by either a turbulent or a rotational dynamo mechanism. Successfulpredictions, both qualitative and quantitative, are made for the smallchanges of the primary pulsation period, the fluctuations of light andvelocity amplitudes, the slow periodicity and irregularity of theBlazhko cycles, the restricted ranges of effective temperature for theRRab and RRc Blazhko variables, and the complex correlations between theprimary period change, amplitude change, and mean effective temperature.Characteristic features of the predicted light and velocity curves athigh and low amplitudes, even though they are based on radiative stellarmodels, agree well with the observed features, for the most part. Thepresent theory of the Blazhko effect is simple enough that it does notrequire any basic change in our current understanding of RRab and RRcstars as being purely fundamental-mode and first-overtone radialpulsators. It also accounts naturally for the observed fact that hotterand cooler classes of periodic variable stars do not exhibit the Blazhkoeffect.
| On the CO Near-Infrared Band and the Line-splitting Phenomenon in the Yellow Hypergiant ρ Cassiopeiae
We report on multiepoch optical and near-infrared spectroscopy aroundthe first-overtone rovibrational band of CO in the pulsating yellowhypergiant ρ Cas, one of the most massive stars in the Galaxy and acandidate SN II progenitor. We argue that the double cores of the COabsorption lines, which have previously been attributed to separatecircumstellar shells expelled during its recurrent outbursts, result infact from a superposition of a wide absorption line and a narrow centralemission line. The CO line doubling returns over subsequent pulsationcycles, where the superposed line emission assumes its largest intensitynear phases of maximum light. We find that the morphology and behaviorof the CO band closely resemble the remarkable ``line-splittingphenomenon'' also observed in optical low-excitation atomic lines. Basedon radiative transport calculations, we present a simplified model ofthe near-infrared CO emission emerging from cooler atmospheric layers inthe immediate vicinity of the photosphere. We speculate that the kinetictemperature minimum in our model results from a periodicpulsation-driven shock wave. We further discuss a number of alternativeexplanations for the origin of the ubiquitous emission-line spectrum,possibly due to a quasi-chromosphere or a steady shock wave at theinterface of a fast expanding wind and the interstellar medium. Wepresent a number of interesting spectroscopic similarities between ρCas and other types of cool variable supergiants, such as the RV Tau andR CrB stars. We further propose a possibly common mechanism for theenigmatic outburst behavior of these luminous pulsating cool stars.
| TY Gruis: A Metal-Poor Carbon and Neutron-Capture-Rich RR Lyrae Star
We have conducted a photometric and high-resolution spectroscopicanalysis of the high-latitude (l=1deg, b=-55deg)metal-poor RR Lyrae star TY Gru (=CS 22881-071). We find this star tohave large overabundances of carbon and neutron-capture elements. Masstransfer from an asymptotic giant branch binary companion prior to itsRR Lyrae evolutionary state is the simplest explanation of thiscircumstance. Unfortunately, TY Gru is afflicted by the Blazhko effect,which greatly complicates accurate measurement of the motion of itscenter of mass. We have not yet detected the small orbital motion thatwould be produced by a distant degenerate companion of TY Gru.This paper includes data gathered with the 6.5 m Magellan Telescopeslocated at Las Campanas Observatory, Chile.
| On the Predicted and Observed Color Boundaries of the RR Lyrae Instability Strip as a Function of Metallicity
The purpose of this paper is to predict the temperature at thefundamental blue edge (FBE) of the instability strip for RR Lyrae (RRL)variables from the pulsation equation that relates temperature toperiod, luminosity, and mass. Modern data for the correlations betweenperiod, luminosity, and metallicity at the FBE for field and cluster RRLstars are used for the temperature calculation. The predictedtemperatures are changed to B-V colors using an adopted colortransformation. The predicted temperatures at the FBE become hotter as[Fe/H] changes from 0 to -1.5, and thereafter cooler as the metallicitydecreases to -2.5. The temperature range over this interval ofmetallicity is ΔlogTe=0.04, or 640 K at 6900 K. Thepredicted color variation is at the level of 0.03 mag in B-V over mostof this range. The predictions are compared with the observed RRL colorsat the FBE for both the field and cluster variables, showing generalagreement at the level of 0.02 mag in (B-V)0, which, however,is the uncertainty of the reddening corrections. The focus of theproblem is then reversed by fitting a better envelope to the observedFBE relation between color and metallicity for metallicities smallerthan -1.8, which, when inserted in the pulsation equation, gives anonlinear calibration of the absolute magnitude of the average evolvedlevel of the horizontal branch (HB) ofMV=1.109+0.600([Fe/H])+0.140([Fe/H])2, where thezero point has been set by the observed RR Lyrae stars in the LMC at0=19.064 by Clementini et al. for [Fe/H]=-1.5, andusing an adopted LMC modulus of (m-M)0=18.54 from Tammann etal. that is independent of the LMC Cepheids. This equation givesMV=0.52 at [Fe/H]=-1.5.However, the calibration fails for the extreme second-parameter clustersNGC 6388 and NGC 6441. Proof that the MV absolute magnitudesfor their RR Lyrae variables are ~0.4 mag brighter than the calibrationequation predicts is from the unusually long periods for givenamplitudes at their high metallicities of [Fe/H]~-0.5. Allsecond-parameter clusters are believed to violate the equation, but lessseverely than these two extreme examples. An additional complication inusing RRL stars as distance indicators at the 0.1 mag level is shown bythe difference of ΔlogP=0.029+/-0.007 in the position of theenvelope locus at the shortest periods for the observedperiod-metallicity correlation between the field and cluster variables.The field variables have shorter periods than cluster variables at theenvelope. This requires the cluster RRL stars to be 0.09 mag brighterthan the field variables at the same temperature and mass, or to have atemperature difference of ΔlogTe=0.008 at fixedluminosity and mass. The field and cluster variables also differ in thenear absence of cluster RR Lyrae stars in the -1.7>[Fe/H]>-2.0metallicity interval, whereas the field variables show no such gap. Atest is proposed for different origins for the field and the clustervariables by comparing the morphology of the HBs in the local dwarfspheroidal galaxies with that in the Galactic globular clusters in theinner halo and by relating the differences with the relevantsecond-parameter indicators.
| The Blazhko effect of RR Lyrae in 2003-2004
Aims.Extensive photometry of RR Lyr was obtained over a 421-day intervalin 2003-2004, covering more than 10 Blazhko cycles in a multisitecampaign. The length and density of this data set allow for a detailedanalysis. Methods: .We used Fourier techniques to study RR Lyr'sbehavior over the pulsation and the Blazhko cycle. We propose atwo-frequency model for decomposing the frequency spectrum.Results: .The light variations were fitted with the main radialfrequency, its harmonics up to 11th order, and the detected tripletfrequencies. No significant quintuplet components were found in thefrequency spectrum. Given the total time span of the measurements, wecan now unambiguously conclude that the Blazhko period has becomenotably shorter than the previously known value of 40.8 days, whereasthe main pulsation period remained roughly the same. Changes in themodulation period have been reported for other well-studied Blazhkovariables. They challenge the explanations for the Blazhko effect whichlink the modulation period directly to the rotation period. The newphotometry reveals an interval in the pulsation cycle of RR Lyr duringwhich the star's intensity barely changes over the Blazhko cycle. Thisinterval occurs during the infalling motion and between the supposedphases of the early and the main shock. The data also permit a moredetailed study of the light curve shape at different phases in theBlazhko period through Fourier parameters.
| A new method for the spectroscopic identification of stellar non-radial pulsation modes. I. The method and numerical tests
Aims.We present the Fourier parameter fit method, a new method forspectroscopically identifying stellar radial and non-radial pulsationmodes based on the high-resolution time-series spectroscopy ofabsorption-line profiles. In contrast to previous methods this onepermits a quantification of the statistical significance of the computedsolutions. The application of genetic algorithms in seeking solutionsmakes it possible to search through a large parameter space.Methods: .The mode identification is carried out by minimizing χ^2,using the observed amplitude and phase across the line profile and theirmodeled counterparts. Computations of the theoretical line profiles arebased on a stellar displacement field, which is described assuperposition of spherical harmonics and that includes the first ordereffects of the Coriolis force. Results: .We made numerical testsof the method on a grid of different mono- and multi-mode models for 0≤ ℓ ≤ 4 in order to explore its capabilities andlimitations. Our results show that whereas the azimuthal order m can beunambiguously identified for low-order modes, the error of ℓ is inthe range of ± 1. The value of m can be determined with higherprecision than with other spectroscopic mode identification methods.Improved values for the inclination can be obtained from the analysis ofnon-axisymmetric pulsation modes. The new method is ideally suited tointermediatley rotating δ Scuti and β Cephei stars.
| [Fe/H] derived from the light curves of RR Lyrae stars in the Galactic halo
Context: .The iron abundance of halo RR Lyrae stars can provideimportant information about the formation history of the Galactichalo. Aims: .We determine the [Fe/H] of the sample of halo RRabstars by using the P-ϕ31-[Fe/H] relation developed byJurcsik & Kovács based on their light curves. We need toextend the relation from the V band to our unfiltered CCD band. Methods: .To do this, we use the low-dispersion spectroscopic [Fe/H] ofliteratures and the photometric data released by the first-generationRobotic Optical Transient Search Experiment (ROTSE-I) project. We doregression analyses for the calibrating sample using a linear functionand test its validity by comparing of the predicted [Fe/H] with thespectroscopic [Fe/H]. In general, the fit accuracy for the two different[Fe/H] is better than 0.19 dex. Results: . We derive an empiricalP-ϕ31-[Fe/H] linear relation for the unfiltered CCD band(ROTSE-I), i.e. [ Fe/H]=-3.766-5.350P+1.044ϕ31. In ourtest, the P-ϕ31-[Fe/H] relation is also fit for ourunfiltered CCD band. In addition, another linear relation,ϕ31_V=0.882+0.792ϕ31_W, is also derivedfor the transformation between the V and W bands. We present thepredicted [Fe/H] of the sample (the 31 halo RRab stars) in a catalog. Conclusions: . The mean [Fe/H] of the sample is -1.63 with dispersionof 0.45 dex in distribution, which is consistent with the resultsderived from the blue horizontal branch star candidates by Kinnman etal. (2000, A&A, 364, 102). The mean [Fe/H] values of the RRab starsin the range of 1 kpc, 2 kpc, and 3 kpc from the star 91 (a double-modeRR Lyrae star), are all lower than that of the background halo stars.These values are consistent with that of star 91 suggested by Wu et al.(2005, AJ, 130, 1640), which indicates they might have a common origin.
| On the Distribution of the Modulation Amplitudes of Blazhko Type RRab Stars
Using all available data on the modulation properties of Blazhko RRabvariables connection has been found between the pulsation period and theamplitude of the modulation. The possible largest value of themodulation amplitude, defined as the sum of the Fourier amplitudes ofthe first four modulation frequency components, increases towardsshorter period variables.
| Photoelectric Minima of Selected Eclipsing Binaries and Maxima of Pulsating Stars
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| The GEOS RR Lyr Survey
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| RR-Lyr-Sterne im Kugelsternhaufen M 5.
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| Wer beobachtet mit? RR Lyrae.
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| Bericht uber die Veranderlichen-Beobachtungswoche an der VdS-Sternwarte in Kirchheim.
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| Einige allgemeine Bemerkungen zum Blazhko-Effekt.
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| Beobachtungssergebnisse Bundesdeutsche Arbeitsgemeinschaft fuer Veraenderliche Sterne e.V.
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| Predicted and Empirical Radii of RR Lyrae Stars
We present new theoretical period-radius-metallicity relations for RRLyrae stars. Current predictions are based on a large set of nonlinear,convective models that cover a broad range of chemical abundances andinput parameters. We also provide new and homogeneous estimates ofangular diameters for a sample of field RR Lyrae stars using a recentcalibration of the Barnes-Evans surface brightness relation. Predictedand empirical radii are, within the errors, in reasonable agreement, butin the short-period range the latter present a larger scatter. As aworking hypothesis we suggest that this discrepancy might be due to theoccurrence either of nonlinear features such as bumps or a steep risingbranch. A new distance determination for RR Lyr itself is in very goodagreement with the Hubble Space Telescope trigonometric parallax andwith the pulsation parallax.
| Metallicity Dependence of the Blazhko Effect
The microlensing surveys, such as OGLE or MACHO, have led to thediscovery of thousands of RRLyr stars in the Galactic bulge and in theMagellanic Clouds, allowing for detailed investigation of these stars,especially the still mysterious Blazhko phenomenon. Higher incidencerate of Blazhko (BL) variables in the more metal-rich Galactic bulgethan in the LMC, suggests that occurrence of Blazhko effect correlateswith metallicity. To investigate this problem, we calibrate thephotometric method of determining the metallicity of RRab stars in theI-band and apply it to the OGLE Galactic bulge and LMC data. In bothsystems, metallicities of non Blazhko and Blazhko variables are close toeach other. The LMC Blazhko pulsators prefer slightly lowermetallicities. The different metallicities of the Galactic bulge and theLMC, cannot explain the observed incidence rates.As a by-product of our metallicity estimates, we investigate theluminosity-metallicity relation, finding a steep dependence of theluminosity on [Fe/H].
| Proper identification of RR Lyrae stars brighter than 12.5 mag
RR Lyrae stars are of great importance for investigations of Galacticstructure. However, a complete compendium of all RR-Lyraes in the solarneighbourhood with accurate classifications and coordinates does notexist to this day. Here we present a catalogue of 561 local RR-Lyraestars (V_max ≤ 12.5 mag) according to the magnitudes given in theCombined General Catalogue of Variable Stars (GCVS) and 16 fainter ones.The Tycho2 catalogue contains ≃100 RR Lyr stars. However, manyobjects have inaccurate coordinates in the GCVS, the primary source ofvariable star information, so that a reliable cross-identification isdifficult. We identified RR Lyrae from both catalogues based on anintensive literature search. In dubious cases we carried out photometryof fields to identify the variable. Mennessier & Colome (2002,A&A, 390, 173) have published a paper with Tyc2-GCVSidentifications, but we found that many of their identifications arewrong.
| RR Lyrae stars: kinematics, orbits and z-distribution
RR Lyrae stars in the Milky Way are good tracers to study the kinematicbehaviour and spatial distribution of older stellar populations. Arecently established well documented sample of 217 RR Lyr stars withV<12.5 mag, for which accurate distances and radial velocities aswell as proper motions from the Hipparcos and Tycho-2 catalogues areavailable, has been used to reinvestigate these structural parameters.The kinematic parameters allowed to calculate the orbits of the stars.Nearly 1/3 of the stars of our sample have orbits staying near the MilkyWay plane. Of the 217 stars, 163 have halo-like orbits fulfilling one ofthe following criteria: Θ < 100 km s-1, orbiteccentricity >0.4, and normalized maximum orbital z-distance>0.45. Of these stars roughly half have retrograde orbits. Thez-distance probability distribution of this sample shows scale heightsof 1.3±0.1 kpc for the disk component and 4.6±0.3 kpc forthe halo component. With our orbit statistics method we found a(vertical) spatial distribution which, out to z=20 kpc, is similar tothat found with other methods. This distribution is also compatible withthe ones found for blue (HBA and sdB) halo stars. The circular velocityΘ, the orbit eccentricity, orbit z-extent and [Fe/H] are employedto look for possible correlations. If any, it is that the metal poorstars with [Fe/H] <1.0 have a wide symmetric distribution aboutΘ=0, thus for this subsample on average a motion independent ofdisk rotation. We conclude that the Milky Way possesses a halo componentof old and metal poor stars with a scale height of 4-5 kpc having randomorbits. The presence in our sample of a few metal poor stars (thus partof the halo population) with thin disk-like orbits is statistically notsurprising. The midplane density ratio of halo to disk stars is found tobe 0.16, a value very dependent on proper sample statistics.
| A catalog of stellar magnetic rotational phase curves
Magnetized stars usually exhibit periodic variations of the effective(longitudinal) magnetic field Be caused by their rotation. Wepresent a catalog of magnetic rotational phase curves, Be vs.the rotational phase φ, and tables of their parameters for 136stars on the main sequence and above it. Phase curves were obtained bythe least squares fitting of sine wave or double wave functions to theavailable Be measurements, which were compiled from theexisting literature. Most of the catalogued objects are chemicallypeculiar A and B type stars (127 stars). For some stars we also improvedor determined periods of their rotation. We discuss the distribution ofparameters describing magnetic rotational phase curves in our sample.All tables and Appendix A are only available in electronic form athttp://www.edpsciences.org
| The Blazhko behaviour of RR Geminorum I. CCD photometric results in 2004
Extended CCD monitoring of RR Gem revealed that it isa Blazhko type RRab star with the shortest Blazhko period (7.23°)and smallest modulation amplitude (Δ Mmax<0.1 mag)currently known. The short period of the modulation cycle enabled us toobtain complete phase coverage of the pulsation at each phase of themodulation. This is the first multicolour observation of a Blazhko starwhich is extended enough to define accurate mean magnitudes and coloursof the variable at different Blazhko phases. Small, but real, changes inthe intensity mean colours at different Blazhko phases have beendetected. The Fourier analysis of the light curves shows that, in spiteof the mmag and smaller order of the amplitudes, the triplet structureis noticeable up to about the 14th harmonic. The modulation isconcentrated to a very narrow, 0.2 phase range of the pulsation, centredon the supposed onset of the H emission during rising light. Theseobservational results raise further complications for theoreticalexplanation of the long known but poorly understood Blazhko phenomenon.Tables 1-4 are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http ://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/430/1049
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| Takýmyýldýz: | Calgi |
| Sað Açýklýk: | 19h25m27.91s |
| Yükselim: | +42°47'03.7" |
| Görünürdeki Parlaklýk: | 7.878 |
| Uzaklýk: | 228.311 parsek |
| özdevim Sað Açýklýk: | -109 |
| özdevim Yükselim: | -195.2 |
| B-T magnitude: | 8.391 |
| V-T magnitude: | 7.921 |
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