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The impact of stellar model spectra in disc detection
We present a study of the impact of different model groups in thedetection of circumstellar debris discs. Almost all previous studies inthis field have used KURUCZ (ATLAS9) model spectra to predict thestellar contribution to the flux at the wavelength of observation, thusdetermining the existence of a disc excess. Only recently have othermodel groups or families like MARCS and NEXTGEN (PHOENIX) becomeavailable to the same extent as ATLAS9. This study aims to determinewhether the predicted stellar flux of a disc target can change with thechoice of model family - can a disc excess be present in the use of onemodel family whilst being absent from another. A simple comparison ofKURUCZ model spectra with MARCS and NEXTGEN model spectra of identicalstellar parameters was conducted and differences were present atnear-infrared wavelengths. Model spectra often do not extend inwavelength to that of observation and therefore extrapolation of thespectrum is required. In extrapolation of model spectra to the SpitzerMIPS passbands, prediction of the stellar contribution differed by 5 percent at 70μm for F, G and early K spectral types with differencesincreasing to 15 per cent for early M dwarfs. Analysis of the SpitzerMIPS 24-μm observations of 37 F, G and K solar-like stars in thePleiades cluster was conducted. In using KURUCZ model spectra, sevendisc excesses were detected while only three and four excesses weredetected in using MARCS and NEXTGEN (PHOENIX) model spectra,respectively.
| Debris Disks of Members of the Blanco 1 OPen Cluster
We have used the Spitzer Space Telescope to obtain Multiband ImagingPhotometer for Spitzer (MIPS) 24 ?m photometry for 37 members of the~100 Myr old open cluster Blanco 1. For the brightest 25 of these stars(where we have 3? uncertainties less than 15%), we findsignificant mid-IR excesses for eight stars, corresponding to a debrisdisk detection frequency of about 32%. The stars with excesses includetwo A stars, four F dwarfs, and two G dwarfs. The most significantlinkage between 24 ?m excess and any other stellar property for ourBlanco 1 sample of stars is with binarity. Blanco 1 members that arephotometric binaries show few or no detected 24 ?m excesses whereas aquarter of the apparently single Blanco 1 members do have excesses. Wehave examined the MIPS data for two other clusters of similar age toBlanco 1—NGC 2547 and the Pleiades. The AFGK photometric binarystar members of both of these clusters also show a much lower frequencyof 24 ?m excesses compared to stars that lie near the single-starmain sequence. We provide a new determination of the relation betweenthe V - K s color and K s - [24] color for mainsequence photospheres based on Hyades members observed with MIPS. As aresult of our analysis of the Hyades data, we identify three low massHyades members as candidates for having debris disks near the MIPSdetection limit.
| Debris Disks around Solar-type Stars: Observations of the Pleiades with the Spitzer Space Telescope
We present Spitzer MIPS observations at 24 ?m of 37 solar-type starsin the Pleiades and combine them with previous observations to obtain asample of 71 stars. We report that 23 stars, or 32% ± 6.8%, haveexcesses at 24 ?m at least 10% above their photospheric emission. Wecompare our results with studies of debris disks in other open clustersand with a study of A stars to show that debris disks around solar-typestars at 115 Myr occur at nearly the same rate as around A-type stars.We analyze the effects of binarity and X-ray activity on the excessflux. Stars with warm excesses tend not to be in equal-mass binarysystems, possibly due to clearing of planetesimals by binary companionsin similar orbits. We find that the apparent anti-correlations in theincidence of excess and both the rate of stellar rotation and also thelevel of activity as judged by X-ray emission are statistically weak.
| Near- and Mid-Infrared Photometry of the Pleiades and a New List of Substellar Candidate Members
We make use of new near- and mid-IR photometry of the Pleiades clusterin order to help identify proposed cluster members. We also use the newphotometry with previously published photometry to define thesingle-star main-sequence locus at the age of the Pleiades in a varietyof color-magnitude planes. The new near- and mid-IR photometry extendeffectively 2 mag deeper than the 2MASS All-Sky Point Source catalog,and hence allow us to select a new set of candidate very low-mass andsubstellar mass members of the Pleiades in the central square degree ofthe cluster. We identify 42 new candidate members fainter thanKs=14 (corresponding to 0.1 Msolar). Thesecandidate members should eventually allow a better estimate of thecluster mass function to be made down to of order 0.04Msolar. We also use new IRAC data, in particular the imagesobtained at 8 ?m, in order to comment briefly on interstellar dust inand near the Pleiades. We confirm, as expected, that-with oneexception-a sample of low-mass stars recently identified as having 24?m excesses due to debris disks do not have significant excesses atIRAC wavelengths. However, evidence is also presented that several ofthe Pleiades high-mass stars are found to be impacting with localcondensations of the molecular cloud that is passing through thePleiades at the current epoch.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.
| Pulkovo compilation of radial velocities for 35495 stars in a common system.
Not Available
| Proper motion surveys of the young open clusters Alpha Persei and the Pleiades
In this paper we present surveys of two open clusters using photometryand accurate astrometry from the SuperCOSMOS microdensitometer. Theseuse plates taken by the Palomar Oschin Schmidt Telescope giving a widefield (5° from the cluster centre in both cases),accurate positions and a long time baseline for the proper motions.Distribution functions are fitted to proper motion vector point diagramsyielding formal membership probabilities. Luminosity and mass functionsare then produced along with a catalogue of high probability members.Background star contamination limited the depth of the study of AlphaPer to R=18. Due to this the mass function found for this cluster couldonly be fitted with a power law (ξ(m) = m-α) withα=0.86+0.14-0.19. However with the betterseperation of the Pleiades' cluster proper motion from the fieldpopulation results were obtained down to R=21. As the mass functionproduced for this cluster extends to lower masses it is possible to seethe gradient becoming increasingly shallow. This mass function is wellfitted by a log normal distribution.Complete tables from Appendix B are only available in electronic form atthe CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/416/125
| Computing the Parallax of the Pleiades from the Hipparcos Intermediate Astrometry Data: An Alternative Approach
The inconsistency between the mean parallax of the Pleidaes open clusterfrom the Hipparcos catalog and that obtained from the stellar evolutiontheory and photometric measurements is probed by recomputing theHipparcos data in a different way that reduces the propagation of thealong-scan attitude errors. This is achieved by coupling observations ofstars made nearly simultaneously in the two separate fields of view ofthe telescope. A direct calculation of astrometric quantities of 54Pleiades members by the new method, based on the Intermediate AstrometryData, provides a correction of -0.71+/-0.14 mas to the weighted meanparallax of the cluster. The mean corrected parallax of the Pleiades is7.75+/-0.20 mas.
| Astrometric radial velocities. III. Hipparcos measurements of nearby star clusters and associations
Radial motions of stars in nearby moving clusters are determined fromaccurate proper motions and trigonometric parallaxes, without any use ofspectroscopy. Assuming that cluster members share the same velocityvector (apart from a random dispersion), we apply a maximum-likelihoodmethod on astrometric data from Hipparcos to compute radial and spacevelocities (and their dispersions) in the Ursa Major, Hyades, ComaBerenices, Pleiades, and Praesepe clusters, and for theScorpius-Centaurus, alpha Persei, and ``HIP 98321'' associations. Theradial motion of the Hyades cluster is determined to within 0.4 kms-1 (standard error), and that of its individual stars towithin 0.6 km s-1. For other clusters, Hipparcos data yieldastrometric radial velocities with typical accuracies of a few kms-1. A comparison of these astrometric values withspectroscopic radial velocities in the literature shows a good generalagreement and, in the case of the best-determined Hyades cluster, alsopermits searches for subtle astrophysical differences, such as evidencefor enhanced convective blueshifts of F-dwarf spectra, and decreasedgravitational redshifts in giants. Similar comparisons for the ScorpiusOB2 complex indicate some expansion of its associations, albeit slowerthan expected from their ages. As a by-product from the radial-velocitysolutions, kinematically improved parallaxes for individual stars areobtained, enabling Hertzsprung-Russell diagrams with unprecedentedaccuracy in luminosity. For the Hyades (parallax accuracy 0.3 mas), itsmain sequence resembles a thin line, possibly with wiggles in it.Although this main sequence has underpopulated regions at certaincolours (previously suggested to be ``Böhm-Vitense gaps''), suchare not visible for other clusters, and are probably spurious. Futurespace astrometry missions carry a great potential for absoluteradial-velocity determinations, insensitive to the complexities ofstellar spectra. Based on observations by the ESA Hipparcos satellite.Extended versions of Tables \ref{tab1} and \ref{tab2} are available inelectronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.125.8) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/381/446
| Open clusters with Hipparcos. I. Mean astrometric parameters
New memberships, mean parallaxes and proper motions of all 9 openclusters closer than 300 pc (except the Hyades) and 9rich clusters between 300 and 500 pc have been computed using Hipparcosdata. Precisions, ranging from 0.2 to 0.5 mas for parallaxes and 0.1 to0.5 mas/yr for proper motions, are of great interest for calibratingphotometric parallaxes as well as for kinematical studies. Carefulinvestigations of possible biases have been performed and no evidence ofsignificant systematic errors on the mean cluster parallaxes has beenfound. The distances and proper motions of 32 more distant clusters,which may be used statistically, are also indicated. Based onobservations made with the ESA Hipparcos astrometry satellite
| 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
| 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. I - Radial velocities of corona stars
Coravel radial velocities of 83 stars ranging in spectral types from F5to K0 selected frozen van Leeuwen's et al. (1986) survey of the coronaof the Pleiades show that only 56 are actually members. Sixspectroscopic binaries among the members and two among the nonmembershave been found. The large extent of the cluster over the sky alsoinduces a radial-velocity gradient from which a convergent point hasbeen determined. Since the cluster's depth is so large, the width of themain sequence is increased. The present results lead to a star densityof 1.1 stars per sq deg for r between 2.5 and 3.5 deg. However, westress that this is only a preliminary result, since the sample of starsin the corona is incomplete.
| The search for faint members of the Pleiades. I - A proper motion membership study of the Pleiades to M(V) of about 12.5
New probable Pleiades cluster members are identified, based on anunbiased survey of stars within 4 x 4-deg region centered near Alcyone.The faint magnitude limit of the survey is about V = 18, and the brightlimit is about V = 14. In order to accurately define the luminosityfunction of the Pleiades, new proper motions are derived for a largenumber of stars previously suggested as possible Pleiades members.Accurate coordinates, V magnitudes, and membership probabilities areprovided for these stars.
| 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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| Сазвежђа: | Бик |
| Ректацензија: | 03h38m56.86s |
| Deклинација: | +24°34'11.3" |
| Apparent магнитуда: | 10.549 |
| Proper motion RA: | 18.1 |
| Proper motion Dec: | -43.8 |
| B-T magnitude: | 11.32 |
| V-T magnitude: | 10.613 |
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INHABITED SKY
