The problem of the Pleiades distance. Constraints from Strömgren photometry of nearby field stars
The discrepancy between the Pleiades cluster distance based on Hipparcosparallaxes and main sequence fitting is investigated on the basis ofStrömgren photometry of F-type stars. Field stars with the samemetallicity as the Pleiades have been selected from the m1index and a technique has been developed to locate the ZAMS of thesefield stars in color-magnitude diagrams based on the color/temperatureindices b-y, v-y, and beta . Fitting the Pleiades to these ZAMSrelations results in a distance modulus of 5.61+/-0.03 mag in contrastto the Hipparcos modulus of 5.36+/-0.06 mag. Hence, we cannot confirmthe recent claim by Grenon (\cite{grenon1999}) that the distance problemis solved by adopting a low metallicity of the Pleiades(Fe/H]Pleiades=-0.11) as determined from Geneva photometry.The metallicity sensitivity of the ZAMS determined by the field stars isinvestigated, and by combining this sensitivity in all threecolor/temperature indices b-y, v-y, and beta we get a independent testof the Pleiades distance modulus which support our value of 5.61 mag.Furthermore, the field star sample used for the comparison is testedagainst theoretical isochrones of different ages to show thatevolutionary effects in the field star sample are not biasing ourdistance modulus estimate significantly. Possible explanations of thePleiades distance problem are discussed and it is suggested that thediscrepancy in the derived moduli may be linked to a non-spherical shapeof the cluster.
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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).
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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
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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).
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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
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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%.
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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
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High-precision positions and proper motions of 441 stars in the Pleiades astrometric standard region.
Not Available
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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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Spectral and Temporal Characteristics of X-Ray--bright Stars in the Pleiades
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1995ApJ...450..217G&db_key=AST
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Membership probabilities in the Pleiades field.
A catalogue of proper motions and photographic B, V magnitudes for starsup to B=19mag within a region centered near Alcyone is presented. Thecatalogue is based on MAMA measurements of 8 plates taken with theTautenburg Schmidt telescope. The survey includes ca. 14500 stars andcovers a total field of about 9 square degrees. For stars brighter thanB=18mag an internal accuracy of 0.05mag and 2.5mas/year has beenestimated for photometric data and proper motions, respectively.Membership probabilities, proper motions and B, V magnitudes are listedfor 442 stars up to B=19mag in the Pleiades field.
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A deep imaging survey of the Pleiades with ROSAT
We have obtained deep ROSAT images of three regions within the Pleiadesopen cluster. We have detected 317 X-ray sources in these ROSAT PositionSensitive Proportional Counter (PSPC) images, 171 of which we associatewith certain or probable members of the Pleiades cluster. We detectnearly all Pleiades members with spectral types later than G0 and within25 arcminutes of our three field centers where our sensitivity ishighest. This has allowed us to derive for the first time the luminosityfunction for the G, K, amd M dwarfs of an open cluster without the needto use statistical techniques to account for the presence of upperlimits in the data sample. Because of our high X-ray detection frequencydown to the faint limit of the optical catalog, we suspect that some ofour unidentified X-ray sources are previously unknown, very low-massmembers of Pleiades. A large fraction of the Pleiades members detectedwith ROSAT have published rotational velocities. Plots ofLX/LBol versus spectroscopic rotational velocityshow tightly correlated `saturation' type relations for stars with ((B -V)0) greater than or equal to 0.60. For each of several colorranges, X-ray luminosities rise rapidly with increasing rotation rateuntil c sin i approximately equal to 15 km/sec, and then remainsessentially flat for rotation rates up to at least v sin i approximatelyequal to 100 km/sec. The dispersion in rotation among low-mass stars inthe Pleiades is by far the dominant contributor to the dispersion inLX at a given mass. Only about 35% of the B, A, and early Fstars in the Pleiades are detected as X-ray sources in our survey. Thereis no correlation between X-ray flux and rotation for these stars. TheX-ray luminosity function for the early-type Pleiades stars appears tobe bimodal -- with only a few exceptions, we either detect these starsat fluxes in the range found for low-mass stars or we derive X-raylimits below the level found for most Pleiades dwarfs. The X-ray spectrafor the early-type Pleiades stars detected by ROSAT areindistinguishable from the spectra of the low-mass Pleiades members. Webelieve that the simplest explanation for this behavior is that theearly-type Pleiades stars are not themselves intrinsic X-ray sources andthat the X-ray emission actually arises from low-mass companions tothese stars.
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The evolution of the lithium abundances of solar-type stars. III - The Pleiades
New measurements of lithium in more than 100 Pleiades F, G, and K dwarfsare reported. Abundances are determined from spectrum synthesis fits tothe data as well as from use of new covers of growth from the Li 6708-Afeature. It is argued that most Late-F and early-G dwarfs in thePleiades are consistent with the tight N(Li) vs mass relation seen inthe Hyades in the same mass range. Most Li-rich stars have abundances ator near the primordial level for Population I, and none exceed thatlevel by a significant amount. At any given color the stars that rotatefast have the most Li and have the strongest chromospheric activity.Ways in which an apparent spread in N(Li) could arise from anintrinsically tight n(Li)-mass relation are considered, and it isconcluded that the spread is probably real and is not an artifact ofline formation conditions or inhomogeneous atmospheres on the stars.
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Rotation and chromospheric emission among F, G, and K dwarfs of the Pleiades
High-resolution echelle spectra of more than 100 F, G, and K dwarfs inthe Pleiades are reported. Chromospheric activity in these stars ismeasured via comparisons of the profiles of H-alpha and the Ca II IRtriplet to chromospherically inactive field stars. Consistent dereddenedcolors are determined from the available photometry and temperatures arederived. Most G and K dwarfs in the Pleiades rotate slowly, but about 20percent of the stars are ultrafast rotators (UFRs). That fraction ofUFRs is independent of color, and the highest rotation rates are foundamong the K dwarfs. The Pleiades exhibit a broad range in the strengthof chromospheric emission at any one color. Most G and K dwarfs in thePleiades show H-alpha and the IR triple in absorption, with filling inof the line cores.
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Investigation of the Pleiades cluster. II - Binary stars in the F5-K0 spectral region
Coravel observations of 100 F5-K0 stars in the Pleiades cluster yielded13 spectroscopic binary stars, and 11 orbits were determined. All 11periods are shorter than 1000 days and the longest circular period is7.05 days. One single-lined spectroscopic binary belongs to a triplesystem, the orbital motion of which has been detected. Based on acomplete sample in the color range B-V = 0.40 - 0.90, (88 stars), thepercentage of spectroscopic binaries with P less than 1000 days is 13percent. The number of single:binary:triple stars is 56:30:2.
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Radial velocity measurements in the Pleiades
High-resolution radial velocities for 71 stars of both early and latespectral types in the Pleiades cluster have been obtained in order toinvestigate the possibility of establishing an early-type star velocitysystem by bootstrapping late-type velocity standards to early-type onesin young clusters. It is shown that in the absence of early-typevelocity standards the velocity scale of the early-type spectra in thecluster can be tied into the IAU velocity system by examining thevelocity distributions of both the early-type and the late-type stars,since the internal velocity dispersion in an open cluster is less thanabout 1 km/s. It is found that the cluster velocity of the Pleiades is6.0 km/s, with an observed velocity dispersion of 1 km/s. Based on thesevelocity results, a list of potentially constant velocity B and A starsis proposed for further studies to confirm the constancy of theirvelocities.
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X-ray studies of coeval star samples. II - The Pleiades cluster as observed with the Einstein Observatory
Coronal X-ray emission of the Pleiades stars is investigated, andmaximum likelihood, integral X-ray luminosity functions are computed forPleiades members in selected color-index ranges. A detailed search isconducted for long-term variability in the X-ray emission of those starsobserved more than once. An overall comparison of the survey resultswith those of previous surveys confirms the ubiquity of X-ray emissionin the Pleiades cluster stars and its higher rate of emission withrespect to older stars. It is found that the X-ray emission from dA andearly dF stars cannot be proven to be dissimilar to that of Hyades andfield stars of the same spectral type. The Pleiades cluster members showa real rise of the X-ray luminosity from dA stars to early dF stars.X-ray emission for the young, solarlike Pleiades stars is about twoorders of magnitude more intense than for the nearby solarlike stars.
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Lithium in old open clusters - NGC 188
Echelle spectra which include the Li I line at 6707 A are reported forseven main-sequence stars and one subgiant in NGC 188. The Li I line isdetected in five of the six dwarfs which are highly probable clustermembers. The derived atmospheric Li/H ratios exceed the solar value byfactors ranging approximately from 10 to 40, although these apparentlyclosely solarlike stars are about twice as old as the sun. The variationof the lithium abundance with stellar mass along the main sequences ofthe Pleiades, the Hyades, NGC 752, and NGC 188 are compared. Theresulting evolutionary pattern indicates that the lithium fraction inthe Galactic gas has shown no appreciable change from Li/H of roughly 10to the -9th since the birth of NGC 188 about 10 Gyr ago, except that theabundance could have been higher by an uncertain but possiblyappreciable factor at the beginning of that epoch.
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Determination of the Proper Motions for Stars in the Region of the Pleiades - Part One
Not Available
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The abundance of lithium in pleiades F stars
The abundance of lithium has been determined for 18 stars in thePleiades cluster with spectral types from A7V to G0V. The pronounced dipin the lithium abundance among the mid-F stars which has been reportedfor other, older star clusters is not present in the Pleiades. Theremoval of lithium from the surfaces of middle-F dwarfs therefore occursprincipally after about 100 Myr on the main sequence.
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Interstellar extinction in the direction of the Merope dark cloud in Pleiades
The dependence of interstellar extinction on distance in the directionof the dark cloud south of Merope is determined using photoelectricphotometry of 93 stars in the Vilnius photometric system. The cloudfront edge is detected at 120-130 pc from the sun, and the distance ofthe Pleiades cluster is found to be 127 pc. Mean extinction, A(V), inthe Merope cloud is of the order of 1.0 mag. There is no evidence ofextinction at distances exceeding the Merope cloud distance. Thevariable extinction method yields R = A(V)/E(B-V) = 3.6, while themaximum polarization wavelength method gives the value 3.4. SomePleiades stars are suspected to be unresolved binaries.
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The Pleiades cluster. III - Polarization, reddening, and the unusual distribution of interstellar matter
A polarization survey of Pleiades cluster members and nonmembers hasbeen carried out together with a determination of interstellar reddeningfor cluster members. The ISM in front of and inside the cluster leads tovariable polarization and reddening across the cluster. The optical datacorrelate well with available radio and millimeter features. Twodistinct ISM concentrations are isolated: (1) a uniform foreground sheetin front of the whole cluster which is seen as constant reddening andpolarization in the eastern part of the cluster; and (2) additionalintracluster material, located mainly in the western part of thecluster.
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A VBLUW photometric survey of the Pleiades cluster
Photometric data are presented for 390 known or suspected members of thePleiades cluster. The data were obtained in 1979 at ESO using theWalraven VBLUW photometer and the Dutch 91-cm telescope. A comparisonwas made with subsets obtained with the same telescope andinstrumentation at the former Leiden Southern Station at the SAAO annexduring 1976 and 1977, and with data obtained at ESO during 1980 and1981. The much improved performance of the telescope and the photometerat their new site is obvious from these comparisons. The stars measuredcomprise the selection of possible members by Hertzsprung (1947)brighter than m(pg) = 14.5 and a selection of possible members in theouter region of the cluster by Pels et al. (1975). Of the starsselected, 66 were found not to be members. A few of these are possiblyescaping members. The present data set provides a well determined mainsequence over the range K2V to B9V, as well as data on some of theprobably premain-sequence K2-to-K5 and postmain-sequence B8-to-B6 stars.Finding charts for the stars selected by Pels are presented in anappendix.
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Rotational studies of lower main sequence stars in nearby open galactic clusters. I - Velocity distributions and age dependence
More than 300 new v sin i values are determined by means of thespectrometer CORAVEL in the following four nearby open galacticclusters: Pleiades, Hyades, Praesepe, and Coma Ber. After excluding allvariable stars and known binaries the sample still contains about 200stars. A determination of the velocity distribution in each cluster ispresented which shows that for the Hyades the velocity distribution isconsistent with a Dirac function. By comparing the mean rotationalvelocity of the different clusters it is shown that there is equal timedependence of rotation for stars of spectral type F8 V to K3 V and thatthe t to the -1/2 law for the rotational decay does not fit well thedata. The v sin i determined for some photometric variable K dwarfs ofthe Pleiades seem to be in disagreement with a rotational modulation ofluminosity.
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Refined Data for Parallax Stars
Not Available
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Metal abundance and microturbulence in F0-G2 stars and the calibration of the Stromgren m1 index
The strengths of two narrow groups of metal lines are measuredphotoelectrically by means of an echelle spectrometer for 16 F0-G2 giantfield stars and for 12 Hyades main-sequence stars. A model-atmosphereanalysis of these observations and similar observations of main-sequencefield stars observed earlier results in the determination of the metalabundance for 179 stars. In addition, the microturbulence parameter isdetermined for 73 of these stars. The internal accuracy of the resultsis estimated to be plus or minus 0.08 for the logarithmicmetal-to-hydrogen ratio and plus or minus 0.2 km/sec for themicroturbulence parameter. The metal abundances are found to agree verysatisfactorily with values of the logarithmic iron-to-hydrogen ratiodetermined from classical coude spectroscopy regarding both zero pointand scale. It is found that the microturbulence parameter is a functionof the effective temperature and the surface gravity. It increases from1.2 km/sec for solar type stars to approximately 3.0 km/sec for earlyF-type giants.
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Apparent radii and other parameters for 416 B5 V-F5 V stars of the catalogue of the Geneva Observatory
Apparent radius, visual brightness, effective temperature and absoluteradius for 416 B5 v-F5 v stars of the catalogue of the GenevaObservatory (Rufener, 1976) have been determined. Twenty-eight stars,anomalous in log a" versus (m~)o diagrams, have been singled out. A goodcorrelation for seven stars, in common with the list of Hanbury Brown etal. (1974), has been found. Similar parameters determined for 279 B5v-F5 v stars of two preceding papers (Fracassini et al., 1973, 1975)have allowed us to determine the averaged diagrams , and versus (B -V)0 for 695 B5 v-F5 v stars. Moreover, in the present paper a goodcorrelation versus and carefulrelation = -7.40 + 3.31 for B5v-F5 V stars have been determined. Plain correlations between log R/R0and blanketing parameter m2 for some spectral types seem to point outthat there are real differences in the absolute radii of stars of thesame spectral type, in agreement with recent researches on the HRdiagram (Houck and Fesen, 1978). Systematic differences between double(spectroscopic and visual) and single stars are found. In particular,the averaged relation versus shows that A2v-F5 v double stars may have a higher metallicity index m2 and smallerabsolute radii than single stars. Finally, the diagram log v sin iversus log R/R0 confirms some properties of binary systems found byother researchers (Huang, 1966; Plavec, 1970; Levato, 1974; Kitamura andKondo, 1978)
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Wide binaries in the Pleiades
The numbers and distributions of wide binary stars in the Pleiades starcluster are compared with those of the solar neighborhood. The numbersof distances between stars in Hertzsprung's (1947) catalog of stars inthe inner region of the Pleiades lying within given intervals werecounted and compared with the expected numbers of optical pairs for arandom distribution of stars in order to obtain the number of visualbinaries. For both a parametric representation of stars in the solarneighborhood and a selection of the nearby binaries with magnitudesgreater than that corresponding to the limit of 16 at the distance ofthe Pleiades, it is found that the magnitude and distance statistics ofthe two regions are in general agreement. The only significant deviationis the deficit of systems with fainter companions in the Pleiadescompared with local stars.
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Spectral types in the Pleiades.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1978PASP...90..201A&db_key=AST
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Four-color and H-beta photometry for open clusters. XI - The Pleiades
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1976AJ.....81..419C&db_key=AST
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