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Direct Distances to Cepheids in the Large Magellanic Cloud: Evidence for a Universal Slope of the Period-Luminosity Relation up to Solar Abundance
We have applied the infrared surface brightness (ISB) technique toderive distances to 13 Cepheid variables in the LMC that span a periodrange from 3 to 42 days. From the absolute magnitudes of the variablescalculated from these distances, we find that the LMC Cepheids definetight period-luminosity (PL) relations in the V, I, W, J, and K bandsthat agree exceedingly well with the corresponding Galactic PL relationsderived from the same technique and are significantly steeper than theLMC PL relations in these bands observed by the OGLE-II Project in V, I,and W and by Persson and coworkers in J and K. We find that the LMCCepheid distance moduli we derive, after correcting them for the tilt ofthe LMC bar, depend significantly on the period of the stars, in thesense that the shortest period Cepheids have distance moduli near 18.3,whereas the longest period Cepheids are found to lie near 18.6. Sincesuch a period dependence of the tilt-corrected LMC distance modulishould not exist, there must be a systematic, period-dependent error inthe ISB technique not discovered in previous work. We identify as themost likely culprit the p-factor, which is used to convert the observedCepheid radial velocities into their pulsational velocities. Bydemanding (1) a zero slope on the distance modulus versus period diagramand (2) a zero mean difference between the ISB and ZAMS fitting distancemoduli of a sample of well-established Galactic cluster Cepheids, wefind that p=1.58(+/-0.02)-0.15(+/-0.05)logP, with the p-factor dependingmore strongly on Cepheid period (and thus luminosity) than indicated bypast theoretical calculations. When we recalculate the distances of theLMC Cepheids with the revised p-factor law suggested by our data, we notonly obtain consistent distance moduli for all stars but also decreasethe slopes in the various LMC PL relations (and particularly in thereddening-independent K and W bands) to values that are consistent withthe values observed by OGLE-II and Persson and coworkers. From our 13Cepheids, we determine the LMC distance modulus to be 18.56+/-0.04 mag,with an additional estimated systematic uncertainty of ~0.1 mag. Usingthe same corrected p-factor law to redetermine the distances of theGalactic Cepheids, the new Galactic PL relations are also foundconsistent with the observed optical and near-infrared PL relations inthe LMC. Our main conclusion from the ISB analysis of the LMC Cepheidsample is that, within current uncertainties, there seems to be nosignificant difference between the slopes of the PL relations in theMilky Way and LMC. With literature data on more metal-poor systems, itseems now possible to conclude that the slope of the Cepheid PL relationis independent of metallicity in the broad range in [Fe/H] from -1.0 dexto solar abundance, within a small uncertainty. The new evidence fromthe first ISB analysis of a sizable sample of LMC Cepheids suggests thatthe previous, steeper Galactic PL relations obtained from this techniquewere caused by an underestimation of the period dependence in themodel-based p-factor law used in the previous work. We emphasize,however, that our current results must be substantiated by newtheoretical models capable of explaining the steeper period dependenceof the p-factor law, and we will also need data on more LMC fieldCepheids to rule out remaining concerns about the validity of ourcurrent interpretation.
| Decay of Planetary Debris Disks
We report new Spitzer 24 μm photometry of 76 main-sequence A-typestars. We combine these results with previously reported Spitzer 24μm data and 24 and 25 μm photometry from the Infrared SpaceObservatory and the Infrared Astronomy Satellite. The result is a sampleof 266 stars with mass close to 2.5 Msolar, all detected toat least the ~7 σ level relative to their photospheric emission.We culled ages for the entire sample from the literature and/orestimated them using the H-R diagram and isochrones; they range from 5to 850 Myr. We identified excess thermal emission using an internallyderived K-24 (or 25) μm photospheric color and then compared allstars in the sample to that color. Because we have excluded stars withstrong emission lines or extended emission (associated with nearbyinterstellar gas), these excesses are likely to be generated by debrisdisks. Younger stars in the sample exhibit excess thermal emission morefrequently and with higher fractional excess than do the older stars.However, as many as 50% of the younger stars do not show excessemission. The decline in the magnitude of excess emission, for thosestars that show it, has a roughly t0/time dependence, witht0~150 Myr. If anything, stars in binary systems (includingAlgol-type stars) and λ Boo stars show less excess emission thanthe other members of the sample. Our results indicate that (1) there issubstantial variety among debris disks, including that a significantnumber of stars emerge from the protoplanetary stage of evolution withlittle remaining disk in the 10-60 AU region and (2) in addition, it islikely that much of the dust we detect is generated episodically bycollisions of large planetesimals during the planet accretion end game,and that individual events often dominate the radiometric properties ofa debris system. This latter behavior agrees generally with what we knowabout the evolution of the solar system, and also with theoreticalmodels of planetary system formation.
| Confirmation of Errors in Hipparcos Parallaxes from Hubble Space Telescope Fine Guidance Sensor Astrometry of the Pleiades
We present absolute trigonometric parallaxes and relative proper motionsfor three members of the Pleiades, obtained with the Hubble SpaceTelescope's Fine Guidance Sensor 1r, a white-light interferometer. Weestimate spectral types and luminosity classes of the stars comprisingthe astrometric reference frame from R~2000 spectra, VJHK photometry,and reduced proper motions. From these we derive estimates of absoluteparallaxes and introduce them into our model as observations with error.We constrain the three cluster members to have a 1 σ dispersion indistance less than 6.4 pc and find an averageπabs=7.43+/-0.17+/-0.20 mas, where the second error issystematic due to member placement within the cluster. This parallaxcorresponds to a distance of 134.6+/-3.1 pc or a distance modulus ofm-M=5.65+/-0.05 for these three Pleiades stars, presuming a centrallocation. This result agrees with three other independent determinationsof the Pleiades distance. Presuming that the cluster depth systematicerror can be significantly reduced because of the random placement ofthese many members within the cluster, these four independent measuresyield a best-estimate Pleiades distance ofπabs=7.49+/-0.07 mas, corresponding to a distance of133.5+/-1.2 pc or a distance modulus of m-M=5.63+/-0.02. This resolvesthe dispute between the main-sequence fitting and the Hipparcos distancemoduli in favor of main-sequence fitting.Based on observations made with the NASA/ESA Hubble Space Telescope,obtained at the Space Telescope Science Institute, which is operated bythe Association of Universities for Research in Astronomy (AURA), Inc.,under NASA contract NAS5-26555.
| An extensive library of 2500 10 500 Å synthetic spectra
We present a complete library of synthetic spectra based on Kurucz'scodes that covers the 2500 10 500 Å wavelength range at resolvingpowers R{P}=20 000, 11 500 (≡GAIA), 8500 (≡RAVE),2000 (≡SLOAN) and uniform dispersions of 1 and 10 Å/pix. Thelibrary maps the whole HR diagram, exploring 51 288 combinations ofatmospheric parameters spanning the ranges: 3500 ≤ T_eff ≤ 47500 K, 0.0≤ log g ≤ 5.0 , -2.5 ≤ [M/H] ≤ 0.5, [α/Fe] = 0.0,+0.4, ξ = 1, 2, 4 km s-1, 0 ≤ V_rot≤ 500 km s-1. The spectra are available both as absolutefluxes as well as continuum normalized. Performance tests andspectroscopic applications of the library are discussed, includingautomatic classification of data from spectroscopic surveys like RAVE,SLOAN, GAIA. The entire library of synthetic spectra is accessible viathe web.
| The distance to the Pleiades. Main sequence fitting in the near infrared
Hipparcos parallax measurements of stars in the Pleiades notoriouslyresult in a cluster distance of 118 pc, which is approximately 10%shorter than the ``classical'' result obtained from earlier mainsequence (MS) fitting studies. In an earlier paper we developed a purelyempirical MS-fitting method in an attempt to address this problem. Thiswork produced conflicting results for the Pleiades between the (B-V) and(V-I) colour indices, indicating that the cluster's photometricmetallicity is substantially lower than its generally acceptedspectroscopic metallicity of [Fe/H]=-0.03. We were able to reconcile thediscrepancy by assuming [Fe/H]=-0.4, the appropriate metallicityindicated from (B-V)/(V-I) colour-colour plots, and the distance moduliobtained from the two colour indices were in agreement with theHipparcos result, within the 1σ errors. With the release of the2MASS All Sky Catalogue, we now apply our MS-fitting method to thePleiades using the infrared colours in addition to the optical bands, inorder to test the plausibility of our earlier result. Using the fullfield dwarf sample and fitting in the V/(V-K) and K/(J-K) colour planes,we find that assuming a substantially subsolar metallicity does notproduce distances in agreement with the (B-V) and (V-I) results. Howeverthe infrared plus (V-I) distances are in mutual agreement when adoptingthe spectroscopic metallicity. By considering only the field dwarfs withMV≤ 6, i.e. brighter than the magnitude where thePleiades (B-V) colours start to be anomalous (Stauffer et al. 2003, AJ,126, 833), the infrared and optical colour indices all yield consistentdistances using the spectroscopic [Fe/H]. The concordant distances thusobtained from the V/(B-V), V/(V-I), V/(V-K) and K/(J-K) planes yield amean of 133.8 ±3 pc, in excellent agreement with both thepre-Hipparcos MS-fitting results, and the most recent determinationsfrom other methods. We conclude that there are two distinct, andunrelated, issues affecting the Pleiades: 1) the Hipparcos parallax isin error by 10%, as previously claimed; 2) the (B-V) colours of thelower MS are anomalous, and we caution against using the (B-V) index forMS-fitting to the Pleiades and similarly young open clusters.
| Eclipsing binaries as standard candles. HD 23642 and the distance to the Pleiades
We present a reanalysis of the light curves of HD 23642, a detachedeclipsing binary star in the Pleiades open cluster, with emphasis on adetailed error analysis. We compare the masses and radii of the twostars to predictions of stellar evolutionary models and find that themetal and helium abundances of the Pleiades are approximately solar. Wepresent a new method for finding distances to eclipsing binaries, ofspectral types A to M, using the empirical calibrations of effectivetemperature versus surface brightness given by Kervella et al. (2004,A&A, 426, 297). We use the calibration for K-filter surfacebrightness to determine a distance of 139.1 ± 3.5 pc to HD 23642and the Pleiades. This distance is in excellent agreement with distancesfound from the use of theoretical and empirical bolometric corrections.We show that the determination of distance, both from the use of surfacebrightness relations and from the use of bolometric corrections, is moreaccurate and precise at infrared wavelengths than at opticalwavelengths. The distance to HD 23642 is consistent with that derivedfrom photometric methods and Hubble Space Telescope parallaxes, but isinconsistent with the distance measured using Hipparcos parallaxes of HD23642 and of other Pleiades stars.
| A purely geometric distance to the binary star Atlas, a member of the Pleiades
We present radial velocity and new interferometric measurements of thedouble star Atlas, which permit, with the addition of publishedinterferometric data, to precisely derive the orbital parameters of thebinary system and the masses of the components. The derived semi-majoraxis, compared with its measured angular size, allows to determine adistance to Atlas of 132± 4 pc in a purely geometrical way. Underthe assumption that the location of Atlas is representative of theaverage distance of the cluster, we confirm the distance value generallyobtained through main sequence fitting, in contradiction with the earlyHipparcos result (118.3± 3.5 pc).Based on observations made with the ELODIE echelle spectrograph mountedon the 1.93-m telescope at the Observatoire de Haute-Provence (CNRS),with the FEROS echelle spectrograph mounted on the 2.2-m telescope atESO-La Silla Observatory (programme No. 072.D-0235B), with the CORALIEechelle spectrograph mounted on the 1.2-m Euler Swiss telescope atESO-La Silla Observatory, with the Naval Prototype OpticalInterferometer (US Naval Observatory) and with the Mark III stellarinterferometer at Mt Wilson.
| The distance to the Pleiades from orbital solution of the double-lined eclipsing binary HD 23642
Combining precise B, V photometry and radial velocities, we have beenable to derive a firm orbital solution and accurate physical parametersfor the newly discovered eclipsing binary HD 23642 in the Pleiades opencluster. The resulting distance to the binary and therefore to thecluster is 132 ± 2 pc. This closely confirms the distance modulusobtained by classical main sequence fitting methods (m - M= 5.60 or 132pc), moving cluster techniques and the astrometric orbit of Atlas. Thisis the first time the distance to a member of the Pleiades is derived byorbital solution of a double-lined eclipsing binary, and it is intendedto contribute to the ongoing discussion about the discordant Hipparcosdistance to the cluster.The photometric and spectroscopic observations discussed in this paperare electronically available from the web pagehttp://ulisse.pd.astro.it/HD_23642/Based in part on spectra collected with the ELODIE spectrograph at the1.93 m telescope of the Observatoire de Haute Provence (OHP), France.
| XMM-Newton and the Pleiades - I. Bright coronal sources and the X-ray emission from intermediate-type stars
We present results of X-ray spectral and timing analyses of solar-like(spectral types F5-K8) and intermediate-type (B4-F4) Pleiads observed ina 40-ks XMM-Newton EPIC exposure, probing X-ray luminosities(LX) up to a factor 10 fainter than previous studies usingthe ROSAT PSPC. All eight solar-like members have`quasi-steady'LX>~ 1029erg s-1,consistent with the known rotation-activity relation and four exhibitflares. Using a hydrodynamic modelling technique, we derive loophalf-lengths for the two strongest flares, on H II 1032 and H II 1100.Near the beginning of its flare, the light curve of H II 1100 shows afeature with a profile suggestive of a total occultation of the flaringloop. Eclipse by a substellar companion in a close orbit is possible butwould seem an extraordinarily fortuitous event; absorption by afast-moving cloud of cool material requires NH at least twoorders of magnitude greater than any solar or stellar prominence. Anoccultation may have been mimicked by the coincidence of two flares,though the first, with its decay time being shorter than its rise timeand suggestive of , would be unusual.Spectral modelling of the quasi-steady emission shows a rising trend incoronal temperature from F and slowly rotating G stars to K stars tofast-rotating G stars, and a preference for low coronal metallicity.These features are consistent with those of nearby solar-like stars,although none of the three stars showing `saturated' emission bears thesignificant component at 2 keV seen in the saturated coronae of AB Dorand 47 Cas. Of five intermediate-type stars, two are undetected(LX < 4 × 1027erg s-1) andthree show X-ray emission with a spectrum and LX consistentwith origin from an active solar-like companion.
| Times of Minima of Eclipsing Binary Stars
Not Available
| Discovery of a Bright Eclipsing Binary in the Pleiades Cluster
The bright star HD 23642 in the Pleiades cluster has been found to be aneclipsing binary with shallow eclipses. The discovery is based on animproved spectroscopic period and the Hipparcos Epoch Photometry.
| Interstellar Matter near the Pleiades. VI. Evidence for an Interstellar Three-Body Encounter
This paper seeks a comprehensive interpretation of new data on Na Iabsorption toward stars in and near the Pleiades, together with existingvisible and infrared data on the distribution of dust and with radiodata on H I and CO in the cluster vicinity. The use of dust and gasmorphology to constrain tangential motions in connection with themeasured radial velocities yields estimates for the space motion of gasnear the Pleiades. Much of the kinematic complexity in the interstellarabsorption toward the Pleiades, including the presence of stronglyblueshifted components that arise in shocked gas, finds explanation inthe interaction between the cluster and foreground gas withVr(LSR)~7 km s-1 associated with the Taurus dustclouds. Taurus gas, however, cannot readily account for an absorptioncomponent having Vr(LSR)~10 km s-1 with a wide,but not continuous distribution and 21 cm emission from gas in thecluster having Vr(LSR)~0 km s-1 associated witheast-west dust filaments. Successive hypotheses for the origin of theseadditional features include Taurus gas at a higher velocity than thepervasive foreground component, additional gas at a radial velocityintermediate between that of the Taurus component and the cluster, and acloud having Vr(LSR)~10 km s-1 approaching thePleiades from the west. A satisfactory account of the full complexity ofthe interstellar medium near the Pleiades requires the last feature andthe Taurus gas, both interacting with the Pleiades and also with eachother.
| Observations of Star-Forming Regions with the Midcourse Space Experiment
We have imaged seven nearby star-forming regions, the Rosette Nebula,the Orion Nebula, W3, the Pleiades, G300.2-16.8, S263, and G159.6-18.5,with the Spatial Infrared Imaging Telescope on the Midcourse SpaceExperiment (MSX) satellite at 18" resolution at 8.3, 12.1, 14.7, and21.3 μm. The large angular scale of the regions imaged (~7.2-50deg2) makes these data unique in terms of the combination ofsize and resolution. In addition to the star-forming regions, twocirrus-free fields (MSXBG 160 and MSXBG 161) and a field near the southGalactic pole (MSXBG 239) were also imaged. Point sources have beenextracted from each region, resulting in the identification over 500 newsources (i.e., no identified counterparts at other wavelengths), as wellas over 1300 with prior identifications. The extended emission from thestar-forming regions is described, and prominent structures areidentified, particularly in W3 and Orion. The Rosette Nebula isdiscussed in detail. The bulk of the mid-infrared emission is consistentwith that of photon-dominated regions, including the elephant trunkcomplex. The central clump, however, and a line of site toward thenorthern edge of the cavity show significantly redder colors than therest of the Rosette complex.
| 3D mapping of the dense interstellar gas around the Local Bubble
We present intermediate results from a long-term program of mapping theneutral absorption characteristics of the local interstellar medium,motivated by the availability of accurate and consistent parallaxes fromthe Hipparcos satellite. Equivalent widths of the interstellar NaID-line doublet at 5890 Å are presented for the lines-of-sighttowards some 311 new target stars lying within ~ 350 pc of the Sun.Using these data, together with NaI absorption measurements towards afurther ~ 240 nearby targets published in the literature (for many ofthem, in the directions of molecular clouds), and the ~ 450lines-of-sight already presented by (Sfeir et al. \cite{sfeir99}), weshow 3D absorption maps of the local distribution of neutral gas towards1005 sight-lines with Hipparcos distances as viewed from a variety ofdifferent galactic projections.The data are synthesized by means of two complementary methods, (i) bymapping of iso-equivalent width contours, and (ii) by densitydistribution calculation from the inversion of column-densities, amethod devised by Vergely et al. (\cite{vergely01}). Our present dataconfirms the view that the local cavity is deficient in cold and neutralinterstellar gas. The closest dense and cold gas ``wall'', in the firstquadrant, is at ~ 55-60 pc. There are a few isolated clouds at closerdistance, if the detected absorption is not produced by circumstellarmaterial.The maps reveal narrow or wide ``interstellar tunnels'' which connectthe Local Bubble to surrounding cavities, as predicted by the model ofCox & Smith (1974). In particular, one of these tunnels, defined bystars at 300 to 600 pc from the Sun showing negligible sodiumabsorption, connects the well known CMa void (Gry et al. \cite{gry85}),which is part of the Local Bubble, with the supershell GSH 238+00+09(Heiles \cite{heiles98}). High latitude lines-of-sight with the smallestabsorption are found in two ``chimneys'', whose directions areperpendicular to the Gould belt plane. The maps show that the LocalBubble is ``squeezed'' by surrounding shells in a complicated patternand suggest that its pressure is smaller than in those expandingregions.We discuss the locations of several HI and molecular clouds. Usingcomparisons between NaI and HI or CO velocities, in some cases we areable to improve the constraints on their distances. According to thevelocity criteria, MBM 33-37, MBM 16-18, UT 3-7, and MBM 54-55 arecloser than ~ 100 pc, and MBM 40 is closer than 80 pc. Dense HI cloudsare seen at less than 90 pc and 85 pc in the directions of the MBM 12and MBM 41-43 clouds respectively, but the molecular clouds themselvesmay be far beyond. The above closest molecular clouds are located at theneutral boundary of the Bubble. Only one translucent cloud, G192-67, isclearly embedded within the LB and well isolated.These maps of the distribution of local neutral interstellar NaI gas arealso briefly compared with the distribution of both interstellar dustand neutral HI gas within 300 pc.Tables 1 and 2 are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp:cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/411/447
| 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.
| Multiplicity among chemically peculiar stars. II. Cool magnetic Ap stars
We present new orbits for sixteen Ap spectroscopic binaries, four ofwhich might in fact be Am stars, and give their orbital elements. Fourof them are SB2 systems: HD 5550, HD 22128, HD 56495 and HD 98088. Thetwelve other stars are: HD 9996, HD 12288, HD 40711, HD 54908, HD 65339,HD 73709, HD 105680, HD 138426, HD 184471, HD 188854, HD 200405 and HD216533. Rough estimates of the individual masses of the components of HD65339 (53 Cam) are given, combining our radial velocities with theresults of speckle interferometry and with Hipparcos parallaxes.Considering the mass functions of 74 spectroscopic binaries from thiswork and from the literature, we conclude that the distribution of themass ratio is the same for cool Ap stars and for normal G dwarfs.Therefore, the only differences between binaries with normal stars andthose hosting an Ap star lie in the period distribution: except for thecase of HD 200405, all orbital periods are longer than (or equal to) 3days. A consequence of this peculiar distribution is a deficit of nulleccentricities. There is no indication that the secondary has a specialnature, like e.g. a white dwarf. Based on observations collected at theObservatoire de Haute-Provence (CNRS), France.Tables 1 to 3 are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/394/151Appendix B is only available in electronic form athttp://www.edpsciences.org
| 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
| Interstellar Matter Near the Pleiades. V. Observations of NA I toward 36 Stars
This paper reports high-resolution, moderate to high signal-to-noiseratio observations of 23 certain Pleiades members, four possiblemembers, and nine nonmembers in the Na I D lines, as well asobservations of 12 of the stars in the Na I ultraviolet doublet. Inspite of the relative proximity of the stars to the sun (even most ofthe nonmembers lie within 200 pc), the line profiles exhibit remarkablecomplexity, with up to five absorption components and equally remarkablestar-to-star variation. The velocity range, 2-20 km s-1,conforms well to the range expected for gas deflected by the passage ofthe cluster. The paper includes a careful discussion of uncertainties inthe data, the most important conclusions of which are that the velocityscatter is consistent with that expected from random errors in thewavelength calibration and that systematic errors probably are <~0.1km s-1. Appendices detail the choice of stellar data and theprocedure adopted for removing telluric absorption lines. Analysisfollows in a separate paper.
| 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
| Search for X-ray flares in the Pleiades using SoHO LASCO C3 images.
Not Available
| On the Variability of A0-A2 Luminosity Class III-V Stars
We study the Hipparcos Satellite photometry of A0-A2 luminosity classIII-V stars. Most are not particularly variable. A few stars for whichfurther study is desirable are identified.
| 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.
| Post-Hipparcos Galactic distance scales.
Not Available
| Do the physical properties of Ap binaries depend on their orbital elements?
We reveal sufficient evidence that the physical characteristics of Apstars are related to binarity. The Ap star peculiarity [represented bythe Δ(V1-G) value and magnetic field strength] diminishes witheccentricity, and it may also increase with orbital period(Porb). This pattern, however, does not hold for largeorbital periods. A striking gap that occurs in the orbital perioddistribution of Ap binaries at 160-600d might well mark a discontinuityin the above-mentioned behaviour. There is also an interestingindication that the Ap star eccentricities are relatively lower thanthose of corresponding B9-A2 normal binaries for Porb>10d.All this gives serious support to the pioneering idea of Abt &Snowden concerning a possible interplay between the magnetism of Apstars and their binarity. Nevertheless, we argue instead in favour ofanother mechanism, namely that it is binarity that affects magnetism andnot the opposite, and suggest the presence of a newmagnetohydrodynamical mechanism induced by the stellar companion andstretching to surprisingly large Porb.
| Radial velocities. Measurements of 2800 B2-F5 stars for HIPPARCOS
Radial velocities have been determined for a sample of 2930 B2-F5 stars,95% observed by the Hipparcos satellite in the north hemisphere and 80%without reliable radial velocity up to now. Observations were obtainedat the Observatoire de Haute Provence with a dispersion of 80Ä,mm(-1) with the aim of studying stellar and galactic dynamics.Radial velocities have been measured by correlation with templates ofthe same spectral class. The mean obtained precision is 3.0 km s(-1)with three observations. A new MK spectral classification is estimatedfor all stars. Based on observations made at the Haute ProvenceObservatory, France and on data from The Hipparcos Catalogue, ESA.Tables 4, 5 and 6 are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr or viahttp://cdsweb.u-strasbg.fr/Abstract.htm
| 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
| Determination of highly precise proper motions for 32 HIPPARCOS stars from photographic plates.
Not Available
| Atmospheric chemical composition of Am stars in the Pleiades
Based on new spectroscopic observational data (high-resolution CCDspectrograms obtained with the coude spectrograph of the 2.6-mtelescope), we perform a model-atmosphere analysis of six A stars in thePleiades cluster which are classified as metallic-line stars. Wedetermine the atmospheric parameters of these stars and estimate theirmasses, radii, and luminosities from a comparison with evolutionarycalculations. Acomparison of the observed and synthetic spectra hasyielded the atmospheric abundances of O, Si, Ca, Fe, and Ba in thestars. Using the currently available oscillator strengths, we determinethe iron abundance in the atmospheres of the stars by themodel-atmosphere method from published measurements of line equivalentwidths. These results and the results of our previous abundance analysisof Coma stars lend support to the conclusion that the differences in theatmospheric abundances of chemically peculiar stars in the same clusterare real.
| The Problem of HIPPARCOS Distances to Open Clusters. I. Constraints from Multicolor Main-Sequence Fitting
Parallax data from the Hipparcos mission allow the direct distance toopen clusters to be compared with the distance inferred frommain-sequence (MS) fitting. There are surprising differences between thetwo distance measurements, indicating either the need for changes in thecluster compositions or reddening, underlying problems with thetechnique of MS fitting, or systematic errors in the Hipparcosparallaxes at the 1 mas level. We examine the different possibilities,focusing on MS fitting in both metallicity-sensitive B-V andmetallicity-insensitive V-I for five well-studied systems (the Hyades,Pleiades, alpha Per, Praesepe, and Coma Ber). The Hipparcos distances tothe Hyades and alpha Per are within 1 sigma of the MS-fitting distancein B-V and V-I, while the Hipparcos distances to Coma Ber and thePleiades are in disagreement with the MS-fitting distance at more thanthe 3 sigma level. There are two Hipparcos measurements of the distanceto Praesepe; one is in good agreement with the MS-fitting distance andthe other disagrees at the 2 sigma level. The distance estimates fromthe different colors are in conflict with one another for Coma but inagreement for the Pleiades. Changes in the relative cluster metalabundances, age related effects, helium, and reddening are shown to beunlikely to explain the puzzling behavior of the Pleiades. We presentevidence for spatially dependent systematic errors at the 1 mas level inthe parallaxes of Pleiades stars. The implications of this result arediscussed.
| Correlation of the HIPPARCOS and Allegheny Observatory Parallax Catalogs
No significant difference is found between the systems of the Hipparcosand Allegheny Observatory MAP parallax catalogs. The correlation of theparallaxes of 63 stars common to both programs is 0.9995 +/- 0.0001,with an average standard deviation of the difference of 0.0023". Whilethere is no indication of systematic difference in the two programs, ourstudy suggests that the formal errors in one or both catalogs aresomewhat underestimated.
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