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Asteroseismic constraints on the Pleiades distance
The global parameters of the Pleiades cluster are derived from thecomparison of observed and computed frequencies of six multi-periodicdelta Scuti stars. The best frequency fits lead to a distance modulusof 5.70 in good agreement with the pre-Hipparcos MS fitting methods.
| A seismological analysis of δ Scuti stars in the Pleiades cluster
A comparison between the oscillation frequencies of six multi-periodicδ Scuti stars of the Pleiades cluster and the eigenfrequencies ofrotating stellar models that match the corresponding stellar parametershas been carried out. The assumption that all the stars considered havesome common parameters, such as metallicity, distance or age, is imposedas a constraint. As a result, we have a best fit solution associatedwith a cluster metallicity of [Fe/H]≃ 0.067, an age between 70× 106 and 100 × 106 yr and a distancemodulus of mV-MV=5.60-5.70 mag. All the stars werefound to oscillate mainly in non-radial, low degree, low order p modes.Estimates of mass and rotation rates for each star are also obtained.
| XMM-Newton and the Pleiades - I. Bright coronal sources and the X-ray emission from intermediate-type stars
We present results of X-ray spectral and timing analyses of solar-like(spectral types F5-K8) and intermediate-type (B4-F4) Pleiads observed ina 40-ks XMM-Newton EPIC exposure, probing X-ray luminosities(LX) up to a factor 10 fainter than previous studies usingthe ROSAT PSPC. All eight solar-like members have`quasi-steady'LX>~ 1029erg s-1,consistent with the known rotation-activity relation and four exhibitflares. Using a hydrodynamic modelling technique, we derive loophalf-lengths for the two strongest flares, on H II 1032 and H II 1100.Near the beginning of its flare, the light curve of H II 1100 shows afeature with a profile suggestive of a total occultation of the flaringloop. Eclipse by a substellar companion in a close orbit is possible butwould seem an extraordinarily fortuitous event; absorption by afast-moving cloud of cool material requires NH at least twoorders of magnitude greater than any solar or stellar prominence. Anoccultation may have been mimicked by the coincidence of two flares,though the first, with its decay time being shorter than its rise timeand suggestive of , would be unusual.Spectral modelling of the quasi-steady emission shows a rising trend incoronal temperature from F and slowly rotating G stars to K stars tofast-rotating G stars, and a preference for low coronal metallicity.These features are consistent with those of nearby solar-like stars,although none of the three stars showing `saturated' emission bears thesignificant component at 2 keV seen in the saturated coronae of AB Dorand 47 Cas. Of five intermediate-type stars, two are undetected(LX < 4 × 1027erg s-1) andthree show X-ray emission with a spectrum and LX consistentwith origin from an active solar-like companion.
| Chandra Observations of the Pleiades Open Cluster: X-Ray Emission from Late B- to Early F-Type Binaries
We present the analysis of a 38.4 and 23.6 ks observation of the core ofthe Pleiades open cluster. The Advanced CCD Imaging Spectrometer onboard the Chandra X-Ray Observatory detected 99 X-ray sources in a17'×17' region, including 18 of 23 Pleiadesmembers. Five candidate Pleiades members have also been detected,confirming their cluster membership. Fifty-seven sources have no opticalor near-infrared counterparts to limiting magnitudes V=22.5 and J=14.5.The unidentified X-ray sources are probably background active galacticnuclei and not stars. The Chandra field of view contains sevenintermediate-mass cluster members. Five of these, HII 980 (B6+G), HII956 (A7+F6), HII 1284 (A9+K), HII 1338 (F3+F6), and HII 1122 (F4+K), aredetected in this study. All but HII 1284 have high X-ray luminosity andsoft X-ray spectra. HII 1284 has X-ray properties comparable tononflaring K-type stars. Since all five stars are visual orspectroscopic binaries with X-ray properties similar to F-G stars, thelate-type binary companions are probably producing the observed coronalX-ray emission. Strengthening this conclusion is the nondetection byChandra of two A stars, HII 1362 (A7, no known companion) and HII 1375(A0+A SB) with X-ray luminosity upper limits 27-54 times smaller thanHII 980 and HII 956, the B6-A7 stars with cooler companions. Despite thelow number statistics, the Chandra data appear to confirm theexpectation that late B and A stars are not strong intrinsic X-raysources. The ACIS spectra and hardness ratios suggest a gradual increasein coronal temperature with decreasing mass from F4 to K. M stars appearto have somewhat cooler coronae than active K stars.
| A study of correlation between the oscillation amplitude and stellar parameters of delta Scutis in open clusters. Toward selection rules for delta Scuti star oscillations
In the present work, we study correlations between stellar fundamentalparameters and the oscillation amplitude for delta Scuti stars. Wepresent this study for a sample of 17 selected delta Scutis belonging to5 young open clusters. Taking advantage of properties of delta Scutis inclusters, we correct the photometric parameters of our objects for theeffects of fast rotation. We confirm the benefit of applying suchcorrections in this kind of studies. In addition, the technique used forthis correction allows us to obtain an estimate of stellar parameterssuch as the angle of inclination and the rotation rate, usually notaccessible. A significant correlation between the parameteri'(estimation of the angle of inclination of the star) andthe oscillation amplitude is found. A discussion and interpretation ofthese a priori surprising results is proposed, in terms of a possibleselection rule for oscillation modes of delta Scuti stars.
| Pulsational frequencies in the delta Scuti stars V624 Tauri and HD 23194. Results of the STEPHI X campaign on the Pleiades cluster
The results of the tenth multi-site campaign of the STEPHI network arereported. The delta Scuti stars V624 Tau (HD 23156) and HD 23194,belonging to the Pleiades cluster, were observed photometrically for 34days on three continents during 1999 November-December. An overall runof 343 hours of data was collected. Seven frequencies for V624 Tau andtwo frequencies for HD 23194 have been found above a 99% confidencelevel. These results greatly improve those found in previous studieswith much less data. A preliminary comparison of observed andtheoretical frequencies suggests that both stars may oscillate withradial and non-radial p modes of radial orders typical among delta Scutistars.
| The Physical Basis of Luminosity Classification in the Late A-, F-, and Early G-Type Stars. II. Basic Parameters of Program Stars and the Role of Microturbulence
Paper I of this series presented precise MK spectral types for 372 lateA-, F-, and early G-type stars with the aim of understanding the natureof luminosity classification on the MK spectral classification systemfor this range of spectral types. In this paper, a multidimensionaldownhill simplex technique is introduced to determine the basicparameters of the program stars from fits of synthetic spectra andfluxes with observed spectra and fluxes from Strömgren uvbyphotometry. This exercise yields useful calibrations of the MK spectralclassification system but, most importantly, gives insight into thephysical nature of luminosity classification on the MK spectralclassification system. In particular, we find that in this range ofspectral types, microturbulence appears to be at least as important asgravity in determining the MK luminosity type.
| The Physical Basis of Luminosity Classification in the Late A-, F-, and Early G-Type Stars. I. Precise Spectral Types for 372 Stars
This is the first in a series of two papers that address the problem ofthe physical nature of luminosity classification in the late A-, F-, andearly G-type stars. In this paper, we present precise spectralclassifications of 372 stars on the MK system. For those stars in theset with Strömgren uvbyβ photometry, we derive reddenings andpresent a calibration of MK temperature types in terms of the intrinsicStrömgren (b-y)0 index. We also examine the relationshipbetween the luminosity class and the Strömgren c1 index,which measures the Balmer jump. The second paper will address thederivation of the physical parameters of these stars, and therelationships between these physical parameters and the luminosityclass. Stars classified in this paper include one new λ Bootisstar and 10 of the F- and G-type dwarfs with recently discoveredplanets.
| Internal kinematics and binarity of X-ray stars in the Pleiades open cluster
The classical convergent point analysis is implemented for the Pleiadesstars with proper motions in the Tycho-2 Catalogue and X-ray fluxesmeasured by the ROSAT satellite. It is demonstrated that, with thestandard astrometric errors as given in Tycho-2, strong X-ray sources inthe cluster (log L_X > 29.1, where L_X is in erg s-1)exhibit a velocity dispersion in one component of only 0.20 kms-1, while the distributions of velocity components ofmoderate (log LX < 29.1) sources and stars not detected byROSAT at all are consistent with a velocity dispersion of 0.64 kms-1. The difference is statistically significant at the levelof 1.6sigma , or 0.95 confidence limit. This result is a clue to thekinematics/X-ray luminosity segregation, similar to that previouslydiscovered in the Hyades open cluster. It is discussed that thesegregation may be caused by a wide spread of ages of the member stars.The occurrence of high X-ray luminosities is found to correlate verywell with visual binarity and multiplicity (separations > 10 AU).
| Catalogue of Apparent Diameters and Absolute Radii of Stars (CADARS) - Third edition - Comments and statistics
The Catalogue, available at the Centre de Données Stellaires deStrasbourg, consists of 13 573 records concerning the results obtainedfrom different methods for 7778 stars, reported in the literature. Thefollowing data are listed for each star: identifications, apparentmagnitude, spectral type, apparent diameter in arcsec, absolute radiusin solar units, method of determination, reference, remarks. Commentsand statistics obtained from CADARS are given. The Catalogue isavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcar?J/A+A/367/521
| delta Scuti and related stars: Analysis of the R00 Catalogue
We present a comprehensive analysis of the properties of the pulsatingdelta Scuti and related variables based mainly on the content of therecently published catalogue by Rodríguez et al.(\cite{retal00a}, hereafter R00). In particular, the primaryobservational properties such as visual amplitude, period and visualmagnitude and the contributions from the Hipparcos, OGLE and MACHOlong-term monitoring projects are examined. The membership of thesevariables in open clusters and multiple systems is also analyzed, withspecial attention given to the delta Scuti pulsators situated ineclipsing binary systems. The location of the delta Scuti variables inthe H-R diagram is discussed on the basis of HIPPARCOS parallaxes anduvbybeta photometry. New borders of the classical instability arepresented. In particular, the properties of the delta Scuti pulsatorswith nonsolar surface abundances (SX Phe, lambda Boo, rho Pup, delta Deland classical Am stars subgroups) are examined. The Hipparcos parallaxesshow that the available photometric uvbybeta absolute magnitudecalibrations by Crawford can be applied correctly to delta Scutivariables rotating faster than v sin i ~ 100 km s{-1} withnormal spectra. It is shown that systematic deviations exist for thephotometrically determined absolute magnitudes, which correlate with vsin i and delta m1. The photometric calibrations are found tofit the lambda Boo stars, but should not be used for the group ofevolved metallic-line A stars. The related gamma Dor variables and thepre-main-sequence delta Scuti variables are also discussed. Finally, thevariables catalogued with periods longer than 0fd 25 are examined on astar-by-star basis in order to assign them to the proper delta Scuti, RRLyrae or gamma Dor class. A search for massive, long-period delta Scutistars similar to the triple-mode variable AC And is also carried out.
| Chromospheric activity in the late A- and early F-type stars of open clusters - II. Pleiades and Alpha Persei
We report observations of the Hei λ5876 (D3) line in the late A-and early F-type stars in the Pleiades and Alpha Persei star clustersused to determine chromospheric activity levels. This represents thefirst sample of young stars in this temperature range with chromosphericactivity measurements. We find the same average activity level in theyoung early F stars as in Hyades-age stars and field stars. In addition,the young star sample shows the same large star-to-star variation inactivity as seen in the older stars. Thus, as a whole, chromosphericactivity in this photospheric temperature range remains the same overnearly a factor of 100 in stellar age (50Myr to 3Gyr), in strikingcontrast to the behaviour of later-type stars. In the five late A starswe find three certain detections of D3 and one likely detection. Thisincludes the bluest star yet observed with a chromospheric D3 line,Pleiades star HII 1362 at (B-V)0=0.22, making it one of theearliest stars with an observed chromosphere. The late A stars have D3equivalent widths comparable to the weakest early F stars. However, whencomparing D3 measurements in the young late A stars with older late Astars, we find evidence for a slight decrease in activity with age basedon the large number of non-detections in the older stars. We find anapparently linear relationship between the activity upper limit and B-Vover our entire range of B-V. Extrapolated blueward, this relationshippredicts that the chromospheric D3 line would disappear for all stars atB-V ~0.13.
| A revised catalogue of delta Sct stars
An extensive and up-dated list of delta Sct stars is presented here.More than 500 papers, published during the last few years, have beenrevised and 341 new variables have been added to our last list, sixyears ago. This catalogue is intended to be a comprehensive review onthe observational characteristics of all the delta Sct stars known untilnow, including stars contained in earlier catalogues together with othernew discovered variables, covering information published until January2000. In summary, 636 variables, 1149 references and 182 individualnotes are presented in this new list. Tables 1 and 2 will be accessibleonly 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
| Multi-site observation of the delta Scuti star V647 Tau. Result of the eighth STEPHI campaign.
Not Available
| 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
| Statistics of binaries in the Pleiades cluster
We present a statistical analysis of binaries in galactic clusters,based on photometric properties. Synthetic clusters are used to solvethe deconvolution problem. If the colour-magnitude relation for singlestars is given and if triple stars are treated as binaries, thedistribution of systems in a colour-magnitude diagram (CMD) can be usedto determine cluster properties. They include the maximum number ofsingle stars N1max which is compatible with the distributionof systems, a standard deviation sigma (describing the scatter of starson the main sequence), a critical mass ratio q_c, and the mass ratiodistribution in the case of N_1=N1max. In the general case(N_1<= N1max) the mass ratio distribution can bedetermined for q>q_c, and in favourable cases (if N_1 is sufficientlylarge) for all mass ratios. A first application concerns thePleiades cluster in the colour range 0.2<= B-V<0.98. The concentration of systems near the main sequence in the CMDis used to derive an approximation for the colour-magnitude relation. Asmall positive number c is involved as a parameter. The mass ratiodistribution depends sensitively on c and increases towards small massratios, at least up to q =~ 0.5 and probably up to q =~ 0.3. Thedistribution is bimodal, with a peak at q=1. Photometric arguments showthat c<~ 0.02. A binary frequency of 60-70% as expected from clustersimulations (Kroupa \cite{krou}) requires c<~ 0.03. An adjustment oftwo parameters (c=0.02 and a 70% binary frequency) is sufficient toreproduce the mass ratio distribution for binaries in the galacticfield. This suggests that the mass ratio distribution in the Pleiades issimilar to the distribution in the field, in accordance with aconjecture of Bouvier et al. (\cite{brn}, BRN).
| Amplitude investigation of delta Scuti variables in open clusters
We report here the results of a statistical analysis of 28 delta Scutistars from five open clusters, alpha Persei, Pleiades, Hyades, Praesepeand Coma. These delta Scuti variables, most of which are on the mainsequence, tend to show a positive correlation between oscillationamplitude and absolute luminosity. No correlation is found betweenamplitude and effective temperature.
| Deep ROSAT HRI observations of the Pleiades
In a deep X-ray survey of the Pleiades open cluster, we use the ROSATHigh Resolution Imager to explore a region of the cluster formerlysurveyed with the PSPC. These new observations substantially improveupon both the sensitivity and the spatial resolution for this region ofthe Pleiades, allowing us to detect 18 cluster members not detectedbefore and 16 members not included in the catalogs used in previoussurveys. The high sensitivity of the present observations permits us toobtain more stringent upper limits for 72 additional members and alsoprovides sufficient numbers of stars to enable us to explore thedependence of L_x on stellar rotation for the slow rotators of thePleiades. Using the new high sensitivity X-ray observations and therecent rotational measurements we discuss the activity-rotationrelationship in the Pleiades solar type stars. We also present newphotometric observations of optical counterparts of a number of X-raysources detected in previous surveys but not yet identified. Table~2 isavailable in electronic form at CDS via ftp 130.79.128.5
| Atmospheric chemical composition of Am stars in the Pleiades
Based on new spectroscopic observational data (high-resolution CCDspectrograms obtained with the coude spectrograph of the 2.6-mtelescope), we perform a model-atmosphere analysis of six A stars in thePleiades cluster which are classified as metallic-line stars. Wedetermine the atmospheric parameters of these stars and estimate theirmasses, radii, and luminosities from a comparison with evolutionarycalculations. Acomparison of the observed and synthetic spectra hasyielded the atmospheric abundances of O, Si, Ca, Fe, and Ba in thestars. Using the currently available oscillator strengths, we determinethe iron abundance in the atmospheres of the stars by themodel-atmosphere method from published measurements of line equivalentwidths. These results and the results of our previous abundance analysisof Coma stars lend support to the conclusion that the differences in theatmospheric abundances of chemically peculiar stars in the same clusterare real.
| The Pleiades and alpha Persei Clusters
The upper-main-sequence members of the Pleiades and alpha Perseiclusters, considered as members of the Local Association, yield meanparallaxes that are only 4% larger than the mean values from Hipparcosobservations. The (log T_eff, M_V) diagram reveals that in thetemperature range from 6000 to 8000 K, the Hyades and alpha Perseimain-sequence members are nearly identical and several tenths of amagnitude brighter than similar stars on the Pleiades main sequence. Thedeparture of the Pleiades main sequence cannot be traced to either ageor heavy-element abundance differences in the range thought to apply tothese clusters. A 50% increase in the helium abundance of Pleiades overHyades stars could account for the luminosity difference. Alternativeexplanations are that the Pleiades cluster is rejected from superclustermembership and/or that the Hipparcos parallax results for the Pleiadesare in error by some 10%.
| Pulsation, binarity and close frequency spacing in $delta Scuti ta$ Scuti stars: BQ and BW Cancri. Results of the STEPHI VI campaign in Praesepe Cluster
New pulsation modes in two delta Scuti stars of Praesepe cluster, BQ andBW Cnc, have been detected during the STEPHI VI campaign in 1995. Inparticular, 3 frequencies for BQ Cnc and 9 frequencies for BW Cnc havebeen found above a 99% confidence level. The possibility of the presenceof a g-mode present in BQ Cnc is discussed, considering its binarity.The effect of mutual interference between very close detectedfrequencies in BW Cnc during the observations, is also considered. Thislast effect reveals the necessity of long-period observing runs, inorder to avoid its influence in the final number of detected modes. Insuch situation, studies of secular amplitude changes can be stronglyaffected.
| Metal abundances of A-type stars in galactic clusters. II. Pleiades, Coma Berenices, Hyades, and Praesepe
A study of chemical composition for 23 A-type stars in four nearby openclusters (Pleiades, Coma, Hyades and Praesepe) has derived detailedabundances for Mg, Ca, Sc, Cr, Fe, and Ni from high resolutionspectroscopy. These results are discussed using the microscopicdiffusion model, which yields time-dependent element stratifications asin the case of Am stars. For the Pleiades, the youngest cluster, we findseveral atypical abundance patterns, which may be transient phases ofthe Am phenomenon. The members of the older clusters show globally moreclassical patterns. Based on observations collected at the Observatoirede Haute-Provence (France)
| Chemical Abundances of Galactic Cepheid Variables that Calibrate the P-L Relation
We have completed spectroscopic studies of 23 Galactic cepheids todetermine possible metallicity effects on measuring the zero point ofthe P-L relation. We find a spread of 0.4 dex in [Fe/H], which is notthe result of either observational scatter or of a metallicity gradientwithin the Galactic disk. We find a smaller, less significant spread of0.2 dex in [alpha /Fe]. Our results are robust, determined by constancyof [Fe/H] derived at differing temperatures throughout the pulsationalcycle, and abundances of two dwarfs compared to U Sgr in the cluster M25. We discuss briefly the effect of metallicity on the P-L relation. Wealso argue that EV Sct and QZ Nor are overtone pulsators, and that thelatter is not a member of the cluster NGC 6067.
| A catalogue of [Fe/H] determinations: 1996 edition
A fifth Edition of the Catalogue of [Fe/H] determinations is presentedherewith. It contains 5946 determinations for 3247 stars, including 751stars in 84 associations, clusters or galaxies. The literature iscomplete up to December 1995. The 700 bibliographical referencescorrespond to [Fe/H] determinations obtained from high resolutionspectroscopic observations and detailed analyses, most of them carriedout with the help of model-atmospheres. The Catalogue is made up ofthree formatted files: File 1: field stars, File 2: stars in galacticassociations and clusters, and stars in SMC, LMC, M33, File 3: numberedlist of bibliographical references The three files are only available inelectronic form at the Centre de Donnees Stellaires in Strasbourg, viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5), or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| The Pleiades open cluster: abundances of Li, Al, Si, S, Fe, Ni, and EU in normal A and AM stars.
In the first of a series of papers on the A stars in open clusters,normal A and Am stars in the Pleiades were observed with theCanada-France-Hawaii telescope at high spectral resolution and highsignal-to-noise ratios. Photospheric abundances have been determined forLi, Al, Si, S, Fe, Ni, and Eu from model atmosphere abundance analysis.The A stars of the Pleiades are at the beginning of their Main Sequenceevolution. At this stage, Li is clearly deficient in the Am starscompared with the normal A stars (-0.65 dex), and the abundance of Fe isthe same for both stellar groups, twice its original solar value asgiven by the Pleiades F stars. These Fe results are unexpected since,firstly, normal A stars are thought to have normal abundances and,secondly, Am stars are classically said to be overabundant in Fecompared with normal A stars. The maximum Li abundance of the cluster isfound in the normal A stars with log N (Li)=3.55+/-0.1 on the scale logN(H)=12.0. These stars seem to have preserved their original Li betterthan any other cooler stars of the Pleiades cluster. The behavior of Liin the slow-rotator Am stars versus the rapid-rotator normal A stars inthe Pleiades is exactly comparable with that found in the "low v sin i"versus "high v sin i" stars with similar masses in the Orionassociation. Could it be that Li differentiation between Am and normal Astars takes place during their pre-Main-Sequence evolution? The Liresults in the Am stars challenge predictions from model envelopescoupling diffusion and evolution (plus mass loss) in non rotating starssince only strong underabundances are expected at the age of thePleiades.
| ROSAT Observations of the Pleiades. I. X-Ray Characteristics of a Coeval Stellar Population
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1996ApJS..102...75M&db_key=AST
| Determinations of Metal Abundances from ROSAT Observations
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1996ApJ...457..382B&db_key=AST
| High-precision positions and proper motions of 441 stars in the Pleiades astrometric standard region.
Not Available
| Blue Stragglers in the Solar Vicinity: Newborn or Reborn
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1995AJ....110..823E&db_key=AST
| Vitesses radiales. Catalogue WEB: Wilson Evans Batten. Subtittle: Radial velocities: The Wilson-Evans-Batten catalogue.
We give a common version of the two catalogues of Mean Radial Velocitiesby Wilson (1963) and Evans (1978) to which we have added the catalogueof spectroscopic binary systems (Batten et al. 1989). For each star,when possible, we give: 1) an acronym to enter SIMBAD (Set ofIdentifications Measurements and Bibliography for Astronomical Data) ofthe CDS (Centre de Donnees Astronomiques de Strasbourg). 2) the numberHIC of the HIPPARCOS catalogue (Turon 1992). 3) the CCDM number(Catalogue des Composantes des etoiles Doubles et Multiples) byDommanget & Nys (1994). For the cluster stars, a precise study hasbeen done, on the identificator numbers. Numerous remarks point out theproblems we have had to deal with.
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