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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.
| Evolution of the Lithium Abundance of Solar-Type Stars. X. Does Accretion Affect the Lithium Dispersion in the Pleiades?
Sixteen G and K dwarfs in the Pleiades cluster were analyzed forevidence that accretion of hydrogen-depleted planetesimals hascontributed to the lithium dispersion observed in cluster stars ofsimilar effective temperature. The HiRes spectrograph on the Keck 10 mtelescope was used to obtain spectra of slowly rotating Pleiades starshaving similar effective temperatures but different lithium abundances.The high-resolution echelle spectra were analyzed for a possibledispersion in the metal abundances correlated with the observeddispersion in lithium abundance. The existence of such a correlationwould support the possibility that young stars can replenish theirsurface lithium by accreting hydrogen-depleted material from acircumstellar disk. The narrow dispersion in the metal abundances of theobserved stars makes it unlikely that accretion plays a role in thedispersion in lithium abundances.
| 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).
| 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
| Pleiades low-mass binaries: do companions affect the evolution of protoplanetary disks?
We have observed 144 G and K dwarf members of the Pleiades cluster tosearch for close multiple systems using CFHT's Adaptive Optics adaptorin the near-IR. We detected 22 binary systems and 3 triples, with aseparation between 0.08 and 6.9arcsec (11-910AU). After correction forincompleteness, we derive a binary frequency in the orbital period rangefrom 4.2 to 7.1 log days, of 28+/-4% for G and K Pleiades dwarfs,similar to that of field G-type dwarfs (27%). The distributions of bothorbital periods and mass-ratios of the Pleiades systems also appearsimilar to those of G dwarf binaries of the field. The binary frequencyin the 100Myr-old Pleiades cluster is much lower than that observed forMyr-old pre-main sequence (PMS) stars in the Taurus-Auriga cloud. Weargue that this difference does not result from the evolution of thebinary systems during the pre-main sequence. Instead, we suggest thatthe low Pleiades binary frequency is typical of stellar populationsformed in dense protoclusters, while the higher binary frequencyobserved among Tau-Aur PMS stars is more typical of loose Tassociations. The implication is that most field stars are born in denseprotostellar clusters. All 144 surveyed stars have known rotationalvelocities. Based on the current beliefs that i) the rotation rate ofPleiades late-type dwarfs is largely dictated by the lifetime of theirpre-main sequence circumstellar disks and that ii) the evolution of thedisks is affected by the presence of a close companion, we searched fora relationship between rotational velocity and binarity among Pleiades Gand K dwarfs. We find no significant difference between the distributionof rotational velocities of single and binary stars. Unless currentmodels of PMS angular momentum evolution are flawed, this indicates thatthe presence of a companion within a distance of 10-1000AU does notprevent accretion from occurring onto the primary at a rate similar tothat observed for single PMS stars. For the closest systems, thisimplies that accretion must proceed from the circumbinary disk onto thecentral stars. For slightly wider systems, it suggests that thetruncated circumstellar disks of the primary and of the secondary arefed by an external (circumbinary) reservoir of mass.
| Formation et evolution des systèmes dinaires: une "grosse" de Pléiades
Not Available
| 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.
| Chromospheric and coronal activity of low-mass stars in the Pleiades
We present the results of a study of the chromospheric and coronalproperties of a large sample of Pleiades low-mass stars. Hαobservations have been obtained for 34 proper motion members of thecluster and are combined with previously published measurements.Additionally we have analysed X-ray data from three ROSAT PSPC pointingsin the cluster. Both Hα and X-ray luminosities have beencalculated and the ratios L_Hα/L_bol and L_X/L_bol have been usedas indicators of chromospheric and coronal activity respectively. Usingsurvival analysis techniques we form cumulative distribution functions(CDFs) for these activity indicators, and show that the means of theCDFs turn over at M_I~8-9 (M~0.4-0.3 M_solar), consistent with the pointat which stars are expected to become fully convective. However, thedetection of a number of very X-ray-active fully convective starsimplies that at least some of these objects can support coronae that areas active as those observed in stars with radiative cores.
| 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 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.
| 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.
| 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.
| Determination of the Proper Motions for Stars in the Region of the Pleiades - Part One
Not Available
| 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.
| Optical and infrared photometry of late-type stars in the Pleiades
New optical photometry has been obtained for 130 stars in the Pleiadesfainter than than V approximately equal to 10 mag. New infraredphotometry for 35 of those stars has also been obtained. Analysis of theoptical data indicates that the photometric binary frequency forlate-type stars in the cluster is 26 percent. That frequency isconsistent with both the binary frequency for field G stars derived byAbt and Levy (1976) and the photometric binary frequency for high-massstars in the Pleiades derived by Bettis (1975). The latter resultindicates that binary frequency is nearly independent of the mass of theprimary star over the range of M/solar mass between 0.4 and 4.0. Thelate G and K stars in the Pleiades fall systematically below a nominalmain sequence derived from photometry of Hyades and Praesepe stars.Evidence is presented to ascribe that displacement to a metallicitydifference between the Pleiades and Hyades stars, to slight IR excessesfor the late-type Pleiades stars, and to an incorrect calibration of thecluster differential distance moduli. If the V approximately equal to12-13 mag Pleiades stars are forced to lie on the zero-age mainsequence, the Vandenberg et al. (1983) isochrones suggest a contractionage for low-mass stars in the Pleiades of slightly more than 100 millionyears. The difference between the nuclear age and the contraction age ofthe Pleiades would then be greater than a few times 10 million years, inagreement with the age spread of star formation estimated fromrotational velocity data presented elsewhere.
| 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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