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The Formation and Evolution of Planetary Systems: Placing Our Solar System in Context with Spitzer
We provide an overview of the Spitzer Legacy Program, Formation andEvolution of Planetary Systems, that was proposed in 2000, begun in2001, and executed aboard the Spitzer Space Telescope between 2003 and2006. This program exploits the sensitivity of Spitzer to carry outmid-infrared spectrophotometric observations of solar-type stars. With asample of ~328 stars ranging in age from ~3 Myr to ~3 Gyr, we trace theevolution of circumstellar gas and dust from primordial planet-buildingstages in young circumstellar disks through to older collisionallygenerated debris disks. When completed, our program will help define thetimescales over which terrestrial and gas giant planets are built,constrain the frequency of planetesimal collisions as a function oftime, and establish the diversity of mature planetary architectures. Inaddition to the observational program, we have coordinated a concomitanttheoretical effort aimed at understanding the dynamics of circumstellardust with and without the effects of embedded planets, dust spectralenergy distributions, and atomic and molecular gas line emission.Together with the observations, these efforts will provide anastronomical context for understanding whether our solar system-and itshabitable planet-is a common or a rare circumstance. Additionalinformation about the FEPS project can be found on the team Web site.
| Spitzer Space Telescope Observations of G Dwarfs in the Pleiades: Circumstellar Debris Disks at 100 Myr Age
Fluxes and upper limits in the wavelength range from 3.6 to 70 μmfrom the Spitzer Space Telescope are provided for 20 solar-mass Pleiadesmembers. One of these stars shows a probable mid-IR excess, and twoothers have possible excesses, presumably due to circumstellar debrisdisks. For the star with the largest, most secure excess flux at MIPS(Multiband Imaging Photometer for Spitzer) wavelengths, HII 1101, wederive log(Ldust/L*)~-3.8 and an estimated debrisdisk mass of 4.2×10-5 M♁ for anassumed uniform dust grain size of 10 μm. If the stars with detectedexcesses are interpreted as stars with relatively recent, largecollisional events producing a transient excess of small dust particles,the frequency of such disk transients is ~10% for our ~100 Myr, PleiadesG dwarf sample. For the stars without detected 24-70 μm excesses, theupper limits to their fluxes correspond to approximate 3 σ upperlimits to their disk masses of 6×10-6M♁ using the MIPS 24 μm upper limit or2×10-4 M♁ using the MIPS 70 μmlimit. These upper limit disk masses (for ``warm'' and ``cold'' dust,respectively) are roughly consistent with, but somewhat lower than,predictions of a heuristic model for the evolution of an ``average''solar-mass star's debris disk based on extrapolation backward in timefrom current properties of the Sun's Kuiper Belt.This work is based (in part) on observations made with the Spitzer SpaceTelescope, which is operated by the Jet Propulsion Laboratory,California Institute of Technology, under NASA contract 1407.This publication makes use of data products from the Two Micron All SkySurvey, which is a joint project of the University of Massachusetts andthe Infrared Processing and Analysis Center, California Institute ofTechnology, funded by the National Aeronautics and Space Administrationand the National Science Foundation.
| The Pleiades, Map-based Trigonometric Parallaxes of Open Clusters. V.
The Multichannel Astrometric Photometer and Thaw Refractor (Thaw/MAP) ofthe University of Pittsburgh's Allegheny Observatory have been used todetermine the trigonometric parallax of the Pleiades star cluster. Theparallax determined, 0.00764" with a standard error of +/-0.00043"(corresponding to a distance modulus of 5.59+/-0.12 mag) places thecluster significantly further away than indicated by the mean parallaxof cluster members drawn from the Hipparcos catalog. The distancederived here is in general agreement with values based on main-sequencefitting, indicating that cluster members are not subluminous assuggested by the Hipparcos-based results. The current study combines thedata from our initial study of this cluster with new observations ofthat region and of a second Pleiades region in an overlappingconfiguration. It thus supersedes our first determination of theparallax of the Pleiades cluster. A third Pleiades field is beingselected for future measurement of the cluster's trigonometric parallax,and assistance with the luminosity classification of reference stars issought.
| 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).
| The rotational velocity of low-mass stars in the Pleiades cluster
We present new {vsin i} measurements for 235 low-mass stars in thePleiades. The differential rotational broadening has been resolved forall the stars in our sample. These results, combined with previouslypublished measurements, provide a complete and unbiased rotation dataset for stars in the mass range from 0.6 to 1.2{Msun}.Applying a numerical inversion technique on the {vsin i} distributions,we derive the distributions of equatorial velocities for low-massPleiades members. We find that half of the Pleiades dwarfs with a massbetween 0.6 to 1 {Msun} have rotation rates lower than 10{ kms(-1) }. Comparison of the rotational distributions of low-mass membersbetween IC 2602/2391 (~ 35 Myr) and the Pleiades (~ 100 Myr) suggeststhat G dwarfs behave like solid-bodies and follow Skumanich's law duringthis time span. However, comparison between Pleiades and older clusters-M34 (~ 200 Myr) and Hyades (~ 600 Myr)- indicates that the braking ofslow rotators on the early main sequence is weaker than predicted by anasymptotical Skumanich's law. This strongly supports the view thatangular momentum tapped in the radiative core of slow rotators on thezero age main sequence (ZAMS) resurfaces into the convective envelopebetween Pleiades and Hyades age. For the G-dwarfs, we derive acharacteristic coupling time scale between the core and the envelope ofabout 100-200 Myr, which accounts for the observed evolution of surfacerotation from the ZAMS to the Hyades. The relationship between rotationand coronal activity in the Pleiades is in agreement with previousobservations in other clusters and field stars. We show that the Rossbydiagram provides an excellent description of the X-ray activity for allstars in the mass domain studied. The Pleiades data for slow andmoderate rotators fills the gap between the X-ray-rotation correlationfound for slow rotators and the X-ray ``saturation plateau'' observedfor young fast rotators. The transition between increasing X-ray fluxwith rotation and X-ray saturation is observed at log (P/tau)=0.8+/-0.1. These results strengthen the hypothesis that the``saturation'' of the angular momentum loss process depends on thestellar mass. Based on observations collected at the Observatoire deHaute-Provence with ELODIE at the 193cm telescope and with CORAVEL atthe 1m-swiss telescope
| Investigation of the Pleiades cluster. IV. The radial structure
On the basis of the best available member list and duplicityinformation, we have studied the radial distribution of 270 stars andmultiple systems earlier than K0 in the Pleiades. Five new long periodspectroscopic binaries have been identified from the CORAVELobservations. We have found a clear mass segregation between binariesand single stars, which is explained by the greater average mass of themultiple systems. The mass function of the single stars and primariesappears to be significantly different. While the central part of thecluster is spherical, the outer part is clearly elliptical, with anellipticity of 0.17. The various parameters describing the Pleiades are(for a distance of 125 pc): core radius rc = 0fdg6 (1.4 pc),tidal radius rt = 7fdg4 (16 pc), half mass radius r_{m/2} =0fdg88 (1.9 pc), harmonic radius /line{r} = 1fdg82 (4 pc). Low-massstars (later than K0) probably extend further out and new proper motionand radial velocity surveys over a larger area and to fainter magnitudeswould be very important to improve the description of the clusterstructure and complete mass function. Based on observations collected atthe Haute-Provence Observatory (France). Table~3 is available only inelectronic form at CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.128.5) or via http://cdsweb.u-strasbg.fr/Abstract.html
| 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
| Pre-Main-Sequence Stars in the Pleiades Supercluster
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1995AJ....110.1749E&db_key=AST
| 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
| 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.
| The Henry Draper Extension Charts: A catalogue of accurate positions, proper motions, magnitudes and spectral types of 86933 stars
The Henry Draper Extension Charts (HDEC), published in the form offinding charts, provide spectral classification for some 87000 starsmostly between 10th and 11th magnitude. This data, being highlyvaluable, as yet was practically unusable for modern computer-basedastronomy. An earlier pilot project (Roeser et al. 1991) demonstrated apossibility to convert this into a star catalogue, using measurements ofcartesian coordinates of stars on the charts and positions of theAstrographic Catalogue (AC) for subsequent identification. We presenthere a final HDEC catalogue comprising accurate positions, propermotions, magnitudes and spectral classes for 86933 stars of the HenryDraper Extension Charts.
| 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.
| The integrated spectra of M32 and of 47 Tuc: A comparative study at high spectral resolution
Integrated spectra have been obtained for the elliptical galaxy M32 andfor the 'metal-rich' Galactic globular cluster 47 Tuc. The spectra coverthe wavelength interval lambda lambda 3800-4400 A at a resolution of 2.5A full width at half maximum (FWHM) and S/N ratio of approximately100:1. Similar data have been acquired for a library of 191 individualstars, and, to support the 47 Tuc observations, integrated spectra offour additional metal-rich Galactic globular clusters have beenobtained. These observations are used to compare in detail theintegrated spectra of M32 (the most extensively studied ellipticalgalaxy) and 47 Tuc (the best-studied metal-rich Galactic globularcluster). Although M32 and 47 Tuc have similar optical broadband colorsand overall spectral types, when viewed at 2.5 A resolution spectranumerous subtle differences between their integrated are clearlyvisible. A system of 13 spectral indices, many of them originallydefined in Rose (1984), has been used to quantify these differences.Altogether twelve diagnostic diagrams are presented to illustrate themanner in which the integrated spectrum of M32 differs from that of 47Tuc. These diagrams are used to place several strong constraints on thestellar populations in these two systems.
| 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.
| 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.
| A catalogue of Fe/H determinations - 1991 edition
A revised version of the catalog of Fe/H determinations published by G.Cayrel et al. (1985) is presented. The catalog contains 3252 Fe/Hdeterminations for 1676 stars. The literature is complete up to December1990. The catalog includes only Fe/H determinations obtained from highresolution spectroscopic observations based on detailed spectroscopicanalyses, most of them carried out with model atmospheres. The catalogcontains a good number of Fe/H determinations for stars from open andglobular clusters and for some supergiants in the Magellanic Clouds.
| Photometry of astrometric reference stars
UBVRI, DDO, and uvby, H-beta photometry of astrometric reference starsis presented. Spectral types and luminosity classifications made fromthe colors are used to determine their spectroscopic parallaxes. In thispaper, colors for 309 stars in 25 regions are given, and classificationsfor 210 stars have been made. These stars form reference frames in theAllegheny Observatory Multichannel Astrometric Photometer astrometricprogram, and in the Praesepe cluster reduced by Russell (1976). It isfound that the present photometric spectral types are reliable to within2.5 spectral subclasses.
| Chemical abundances and ages of open clusters
The metal content of nine open clusters is discussed with the help ofresults from detailed spectral analyses of main-sequence and slightlyevolved stars, belonging to those clusters. This small sample ofclusters, out of about 1200 known, has, however, the advantage ofrepresenting the whole span of ages of open clusters with two young(Alpha Per, the Pleiades, with five intermediate age, including U Ma,Hyades, Coma, Praesepe, and NGC 752) and with two old (M 67 and NGC 188)clusters. It has also the advantage that the observations of almost allthe stars in these different clusters have been carried out on similar,when not identical telescopes, on similar high-resolution spectrographs,and similar detectors. The high signal/noise spectra of these clusterstars have been reduced with similar procedures, and interpreted withsimilar model atmosphere analyses. In taking into account the error barsattributed to the mean metal content parameter, Fe/H, of each cluster,one may conclude that only the Hyades cluster has a metal content whichdiffers significantly from that of the sun, being between 30 and 40percent higher than that of the sun.
| Very active chromosphere stars and the age, chromosphere activity relation
The wide range of objects in stellar superclusters and groups near thesun is used in an attempt to define normal stellar chromosphericbehavior, with which 'very active chromosphere' (VAC) stars can beisolated. Two chromosphere signatures, soft X-ray flux, and Mg II h andk line flux, are discussed. The X-ray flux is of limited use in thisregard because of a wide range of variation with time, although themaximum values are well defined for stars of a given age. The VAC stars,which have an excessive X-ray flux, also have an excessive bolometricflux. The ratio of the Mg II h and k (and Ca II H and K) line emissionand the total bolometric emission, R(hk) decreases with bolometricluminosity. The rate of decrease in log R(hk), 0.175 (35.00 - logLBOL), is not very dependent on stellar age, but the zeropoint at log LBOL = 35.00 decades varies from log R(hk) =-4.45 decades for main-sequence stars in the Pleiades supercluster(200-300 million yr) to - 4.91 decades for those in the HR 1614 group (8billion yr). The age dependence of R(hk) is clearly demonstrated notonly by the supercluster and group stars but by the 'Mount Wilsonsample' of objects that have been observed in Ca II H and K for over 15years (e.g., Noyes et al., 1984). Several determinations (e.g.,Soderblom and Clements, 1987) have established a linear relation betweenR(hk) and R(HK). Some 50 stars in the Mount Wilson sample havewell-established luminosities and space motions (e.g., Eggen, 1989) thatallow the separation of young and old disk stars in velocity space(Eggen, 1989). The young disk stars all have values of R(hk) thatidentify them as having ages less than the youngest Hyades superclustermembers (about 600 million yr).
| The distribution of rotational velocities for low-mass stars in the Pleiades
The available spectral type and color data for late-type Pleiadesmembers have been reanalyzed, and new reddening estimates are obtained.New photometry for a small number of stars and a compilation of H-alphaequivalent widths for Pleiades dwarfs are presented. These data are usedto examine the location of the rapid rotators in color-magnitudediagrams and the correlation between chromospheric activity androtation. It is shown that the wide range of angular momenta exhibitedby Pleiades K and M dwarfs is not necessarily produced by a combinationof main-sequence spin-downs and a large age spread; it can also resultfrom a plausible spread in initial angular momenta, coupled with initialmain-sequence spin-down rates that are only weakly dependent onrotation. The new reddening estimates confirm Breger's (1985) finding oflarge extinctions confined to a small region in the southern portion ofthe Merope nebula.
| Chromospheric activity and ages of solar-type stars
Observations of 15 solar-type stars in the intermediate-age open clusterNGC 752 are reported. A lower resolution analog of the Mount Wilson Sindex is shown to yield absolute chromospheric surface flux values forthese stars with about 60 percent of the sensitivity of the Mount Wilsonsystem. Absolute chromospheric surface fluxes of solar-type stars ineight clusters ranging from 10 million yrs to six billion or more yearsin age are presented. Two heuristic forms are shown to fit the dataabout equally well, with no indication of a discontinuity atintermediate ages. These relations can yield chromospheric ages for anyG-type dwarf or subgiant with a Mount Wilson S index. The usefulness ofthis lower resolution approach for studies of chemical and dynamicalevolution of the Galaxy as well as of the stellar birth rate is pointedout.
| 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.
| 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.
| Red horizontal-branch stars in the galactic disk
A quantitative, three-dimensional spectral classification systemdeveloped by Rose (1984), which uses 2.5-A resolution spectra in theblue, has been used to identify a class of red horizontal branch (RHB)stars in the Galactic disk that are similar to those in the 'metal rich'globular cluster M 71. The RHB are denoted as evolved stars by their SrII 4077 line, and are distinguished from post-main sequence starsevolving through the same region of the HR diagram on the basis of theunique appearance of their CN 3883 and 4216 A bands. The RHB starsconsitute at least 5 percent of the entire giant branch population ofthe disk.
| A catalogue of Fe/H determinations, 1984 edition
The present version of the Cayrel de Strobel et al. (1981) catalog ofFe/H abundance ratio determinations contains 1921 values for 1035 stars,which represents an augmentation over the previous publication of 48 and47 percent, respectively. In addition, the literature search conductedis complete up to December, 1983. Stellar metal abundance, effectivetemperature, spectroscopic gravity, spectral type, and photometricindices are covered.
| Spectral anomalies in the Hyades and Pleiades and in field stars with active chromospheres
Widened photographic image-tube spectra at 50 A/mm dispersion have beenobtained for a large number of late-type field dwarfs and giants withwell-determined atmospheric parameters and for 35 Hyades dwarfs and 31Pleiades dwarfs. The observations and data analysis are discussed, and asummary of the quantitative spectral classification system is given.This system is used to compare the Hyades and Pleiades with field stars.The cluster stars are found to exhibit several anomalies with respect tothe field stars which are not readily explained either by abundance orsurface gravity peculiarities, or by the presence of cool starspots onthe strong Ca II emission stars, or by cool companions. It is proposedthat active regions on stars with strong Ca II emission reversals haveseriously affected many of the principal spectral lines.
| Lithium abundance and age spread in the Pleiades
Surface lithium abundance, a parameter which usually decreases with agein late-type stars, has been determined for Pleiades stars as cool asT(e) approximately 4700 K and Hyades stars as cool as T(e) approximately5000 K. A large abundance spread is seen among the coolest Pleiadesstars, consistent with their having been formed over an interval as longas 0.4 x 10 to the 9th yr, several times longer than the nuclear age ofthe cluster. Observations of the strength of the Ca II H and K lineemission in three of the Pleiades stars also indicate a significantspread in age. Some Pleiades stars as cool as T(e) approximately 5200 Khave primordial Li abundances, in conflict with the predictions ofpre-main-sequence stellar evolution calculations, indicating that suchcalculations probably overestimate the importance of convection.
| The Color-Magnitude Diagram of the Pleiades Cluster. II.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1958ApJ...128...31J&db_key=AST
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Наблюдательные данные и астрометрия
| Созвездие: | Телец |
| Прямое восхождение: | 03h46m38.78s |
| Склонение: | +24°57'34.7" |
| Видимая звёздная величина: | 10.318 |
| Собственное движение RA: | 19.3 |
| Собственное движение Dec: | -47 |
| B-T magnitude: | 10.967 |
| V-T magnitude: | 10.372 |
Каталоги и обозначения:
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