Ýçindekiler
Görüntüler
Resim Yükleyin
DSS Images Other Images
Ýlgili Makaleler
The lithium-rotation correlation for WTTS in Taurus–Auriga
Surface lithium abundance and rotation velocity can serve as powerfuland mutually complementary diagnostics of interior structure of stars.So far, the processes responsible for the lithium depletion duringpre-main sequence evolution are still poorly understood. We investigatewhether a correlation exists between equivalent widths of Li (EW(Li))and rotation period (Prot) for weak-line T Tauri stars(WTTSs). We find that rapidly rotating stars have lower EW(Li) and thefast burning of Li begins at the phase when star’s Protevolves towards 3 days among 0.9Mȯ to1.4Mȯ WTTSs in Taurus–Auriga. Our results supportthe conclusion by Piau and Turch-Chiéze about a model for lithiumdepletion with age of the star and by Bouvier et al. in relation torotation evolution. The turn over of the curve for the correlationbetween EW(Li) and Prot is at the phase of zero-age mainsequence (ZAMS). The EW(Li) decreases with decreasing Protbefore the star reaches the ZAMS, while it decreases with increasingProt (decreasing rotation velocity) for young low-mass mainsequence stars. This result could be explained as an age effect of Lidepletion and the rapid rotation does not inhibit Li destruction amonglow-mass PMS stars.
| Effective temperature scale and bolometric corrections from 2MASS photometry
We present a method to determine effective temperatures, angularsemi-diameters and bolometric corrections for population I and II FGKtype stars based on V and 2MASS IR photometry. Accurate calibration isaccomplished by using a sample of solar analogues, whose averagetemperature is assumed to be equal to the solar effective temperature of5777 K. By taking into account all possible sources of error we estimateassociated uncertainties to better than 1% in effective temperature andin the range 1.0-2.5% in angular semi-diameter for unreddened stars.Comparison of our new temperatures with other determinations extractedfrom the literature indicates, in general, remarkably good agreement.These results suggest that the effective temperaure scale of FGK starsis currently established with an accuracy better than 0.5%-1%. Theapplication of the method to a sample of 10 999 dwarfs in the Hipparcoscatalogue allows us to define temperature and bolometric correction (Kband) calibrations as a function of (V-K), [m/H] and log g. Bolometriccorrections in the V and K bands as a function of T_eff, [m/H] and log gare also given. We provide effective temperatures, angularsemi-diameters, radii and bolometric corrections in the V and K bandsfor the 10 999 FGK stars in our sample with the correspondinguncertainties.
| A Catalog of Temperatures and Red Cousins Photometry for the Hyades
Using Hyades photometry published by Mendoza and other authors,Pinsonneault et al. have recently concluded that Cousins V-I photometrypublished by Taylor & Joner is not on the Cousins system. Extensivetests of the Taylor-Joner photometry and other pertinent results aretherefore performed in this paper. It is found that in part, thePinsonneault et al. conclusion rests on (1) a systematic error inMendoza's (R-I)J photometry and (2) a small error in anapproximate Johnson-to-Cousins transformation published by Bessell. Forthe Taylor-Joner values of (V-R)C, it is found that there arepossible (though not definite) differences of several mmag with otherresults. However, the Taylor-Joner values of (R-I)C data aresupported at the 1 mmag level. Using the (R-I)C data andother published results, an (R-I)C catalog is assembled for146 Hyades stars with spectral types earlier than about K5. For singlestars with multiple contributing data, the rms errors of the catalogentries are less than 4.4 mmag. Temperatures on the Di Benedettoangular-diameter scale are also given in the catalog and are used tohelp update published analyses of high-dispersion values of [Fe/H] forthe Hyades. The best current mean Hyades value of [Fe/H] is found to be+0.103+/-0.008 dex and is essentially unchanged from its previous value.In addition to these numerical results, recommendations are made aboutimproving attitudes and practices that are pertinent to issues likethose raised by Pinsonneault et al.
| Astrometric orbits of SB^9 stars
Hipparcos Intermediate Astrometric Data (IAD) have been used to deriveastrometric orbital elements for spectroscopic binaries from the newlyreleased Ninth Catalogue of Spectroscopic Binary Orbits(SB^9). This endeavour is justified by the fact that (i) theastrometric orbital motion is often difficult to detect without theprior knowledge of the spectroscopic orbital elements, and (ii) suchknowledge was not available at the time of the construction of theHipparcos Catalogue for the spectroscopic binaries which were recentlyadded to the SB^9 catalogue. Among the 1374 binaries fromSB^9 which have an HIP entry (excluding binaries with visualcompanions, or DMSA/C in the Double and Multiple Stars Annex), 282 havedetectable orbital astrometric motion (at the 5% significance level).Among those, only 70 have astrometric orbital elements that are reliablydetermined (according to specific statistical tests), and for the firsttime for 20 systems. This represents a 8.5% increase of the number ofastrometric systems with known orbital elements (The Double and MultipleSystems Annex contains 235 of those DMSA/O systems). The detection ofthe astrometric orbital motion when the Hipparcos IAD are supplementedby the spectroscopic orbital elements is close to 100% for binaries withonly one visible component, provided that the period is in the 50-1000 drange and the parallax is >5 mas. This result is an interestingtestbed to guide the choice of algorithms and statistical tests to beused in the search for astrometric binaries during the forthcoming ESAGaia mission. Finally, orbital inclinations provided by the presentanalysis have been used to derive several astrophysical quantities. Forinstance, 29 among the 70 systems with reliable astrometric orbitalelements involve main sequence stars for which the companion mass couldbe derived. Some interesting conclusions may be drawn from this new setof stellar masses, like the enigmatic nature of the companion to theHyades F dwarf HIP 20935. This system has a mass ratio of 0.98 but thecompanion remains elusive.
| Spectral Classification of Stars in A Supplement to the Bright Star Catalogue
MK spectral types are given for about 584 stars in A Supplement to theBright Star Catalogue. These are compared with Hipparcos parallaxes tocheck the reliability of those classifications. The estimated errors are+/-1.2 subtypes, and 10% of the luminosity classes may be wrong.
| Beryllium Abundances in F and G Dwarfs in Praesepe and Other Young Clusters from Keck HIRES Observations
The study of both Be and Li gives useful clues about stellar internalstructure. Of particular interest is the study of these light elementsin open clusters, which have a known age and metallicity. In this paperwe present a study of Be abundances in 10 F-type stars in Praesepe and acomprehensive discussion about Be abundances in other open clusters:Hyades, Pleiades, α Per, Coma, and UMa. We have made observationsof the doublet of Be II around 3130 Å in Praesepe stars, using theKeck I telescope and the High Resolution Echelle Spectrometer (HIRES).Beryllium abundances were derived from the spectra using the spectrumsynthesis method. We find four stars with definite Be depletion in thetemperature range of the Li dip like we found in our previous clusterstudies, notably for the Hyades and Coma clusters. Putting all theclusters together, we confirm the existence of a Be dip in a narrowtemperature range for F stars. Beryllium depletion in this dip is lesspronounced than Li depletion. For the cooler stars there is little or noBe depletion, even though there are large depletions of Li. For starsthat have little or no Li depletion, A(Li)>=3.0, the ratio Li/Be is75+/-4.6, compared to the meteoritic ratio of 77.6. For stars coolerthan ~5900 K there appears to be little or no Be depletion, and the meanA(Be) is 1.30+/-0.02. For these cooler stars within a given clusterthere is no evidence for intrinsic star-to-star differences in A(Be),with the possible exception of the cool Pleiades stars. In thetemperature range of the Li-Be dip, a strong correlation exists betweenLi and Be, consistent with the theory of rotationally induced mixing.Moreover, the slopes of the Li versus Be correlations are differentdepending on the temperature range. For the full sample of 42 starsbetween 5900 and 6650 K the slope is 0.43+/-0.05 [where A(Li) is theabscissa]. The slope is 0.48+/-0.08 for 6300K
| The Geneva-Copenhagen survey of the Solar neighbourhood. Ages, metallicities, and kinematic properties of 14 000 F and G dwarfs
We present and discuss new determinations of metallicity, rotation, age,kinematics, and Galactic orbits for a complete, magnitude-limited, andkinematically unbiased sample of 16 682 nearby F and G dwarf stars. Our63 000 new, accurate radial-velocity observations for nearly 13 500stars allow identification of most of the binary stars in the sampleand, together with published uvbyβ photometry, Hipparcosparallaxes, Tycho-2 proper motions, and a few earlier radial velocities,complete the kinematic information for 14 139 stars. These high-qualityvelocity data are supplemented by effective temperatures andmetallicities newly derived from recent and/or revised calibrations. Theremaining stars either lack Hipparcos data or have fast rotation. Amajor effort has been devoted to the determination of new isochrone agesfor all stars for which this is possible. Particular attention has beengiven to a realistic treatment of statistical biases and errorestimates, as standard techniques tend to underestimate these effectsand introduce spurious features in the age distributions. Our ages agreewell with those by Edvardsson et al. (\cite{edv93}), despite severalastrophysical and computational improvements since then. We demonstrate,however, how strong observational and theoretical biases cause thedistribution of the observed ages to be very different from that of thetrue age distribution of the sample. Among the many basic relations ofthe Galactic disk that can be reinvestigated from the data presentedhere, we revisit the metallicity distribution of the G dwarfs and theage-metallicity, age-velocity, and metallicity-velocity relations of theSolar neighbourhood. Our first results confirm the lack of metal-poor Gdwarfs relative to closed-box model predictions (the ``G dwarfproblem''), the existence of radial metallicity gradients in the disk,the small change in mean metallicity of the thin disk since itsformation and the substantial scatter in metallicity at all ages, andthe continuing kinematic heating of the thin disk with an efficiencyconsistent with that expected for a combination of spiral arms and giantmolecular clouds. Distinct features in the distribution of the Vcomponent of the space motion are extended in age and metallicity,corresponding to the effects of stochastic spiral waves rather thanclassical moving groups, and may complicate the identification ofthick-disk stars from kinematic criteria. More advanced analyses of thisrich material will require careful simulations of the selection criteriafor the sample and the distribution of observational errors.Based on observations made with the Danish 1.5-m telescope at ESO, LaSilla, Chile, and with the Swiss 1-m telescope at Observatoire deHaute-Provence, France.Complete Tables 1 and 2 are only available in electronic form at the CDSvia anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/418/989
| What Is Happening at Spectral Type F5 in Hyades F Stars?
Aiming at a better understanding of the mechanisms heating thechromospheres, transition regions, and coronae of cool stars, we studyultraviolet, low-resolution Hubble Space Telescope/Space TelescopeImaging Spectrograph spectra of Hyades main-sequence F stars. We studythe B-V dependence(s) of the chromospheric and transition layer emissionline fluxes and their dependences on rotational velocities. We find thatthe transition layer emission line fluxes and also those of strongchromospheric lines decrease steeply between B-V=0.42 and 0.45, i.e., atspectral type F5, for which the rotational velocities also decreasesteeply. The magnitude of the line-flux decrease increases for lines ofions with increasing degree of ionization. This shows that the line-fluxdecrease is not due to a change in the surface filling factor but ratherdue to a change of the relative importance of different heatingmechanisms. For early F stars with B-V<0.42 we find for thetransition layer emission lines increasing fluxes for increasing vsini,indicating magnetohydrodynamic heating. The vsini dependence isstrongest for the high-ionization lines. On the other hand, the lowchromospheric lines show no dependence on vsini, indicating acousticshock heating for these layers. This also contributes to the heating ofthe transition layers. The Mg II and Ca II lines show decreasing fluxesfor increasing vsini, as long as vsini is less than ~40 kms-1. The coronal X-ray emission also decreases for increasingvsini, except for vsini larger than ~100 km s-1. We have atpresent no explanation for this behavior. For late F stars thechromospheric lines show vsini dependences similar to those observed forearly F stars, again indicating acoustic heating for these layers. Wewere unable to determine the vsini dependence of the transition layerlines because of too few single star targets. The decrease of emissionline fluxes at the spectral type F5, with steeply decreasing vsini,indicates, however, a decreasing contribution of magnetohydrodynamicheating for the late F stars. The X-ray emission for the late F starsincreases for increasing vsini, indicating magnetohydrodynamic heatingfor the coronae of the late F stars, different from the early F stars.Based on observations with the NASA/ESA Hubble Space Telescope obtainedat the Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy, Incorporated,under NASA contract NAS5-26555.
| Beryllium in the Hyades F and G Dwarfs from Keck HIRES Spectra
Although there are extensive observations of Li in field stars of alltypes and in both open and (recently) globular cluster stars, there arerelatively few observations of Be. Because Be is not destroyed as easilyas Li, the abundances of Li and Be together can tell us more about theinternal physical processes in stars than either element can alone. Wehave obtained high-resolution (45,000) and high signal-to-noise ratio(typically 90 per pixel) spectra of the Be II resonance lines in 34Hyades F and G dwarfs with the Keck I telescope and HIRES. In additionwe took a spectrum of the daytime sky to use as a surrogate for thesolar spectrum so we could determine the value for Be in the Sun,analyzed in the same manner as that for the stars. We have adoptedstellar temperatures and some Li abundances for these stars from theliterature. For most of the F dwarfs we have rederived Li abundances.The Be abundances have been derived with the spectrum synthesis method.We find that Be is depleted, but detected, in the Li gap in the F starsreaching down to values of A(Be)=0.60 dex, or a factor of nearly 7 belowthe meteoritic Be abundance (a factor of 3.5 below the solar value ofChmielewski et al.). There is little or no depletion of Be in starscooler than 6000 K, in spite of the large depletions (0.5-2.5 dex) inLi. The mean value of A(Be) for the 10 coolest stars is 1.33+/-0.06, notfar from the meteoritic value of 1.42. The pattern in the Beabundances-a Be dip and undepleted Be in the cool stars-is well matchedby the predictions of slow mixing due to stellar rotation. We haveinterpolated the calculations of Deliyannis and Pinsonneault for Bedepletion due to rotational mixing to the age of the Hyades; we findexcellent agreement of the predictions with the observed Be abundancesbut less good agreement with the observed Li abundances. Some of ourHyades stars have photometrically determined rotation periods, but thereis no relation between Be and rotation period. (Generally, the lowermass stars have less Li and longer periods, which may indicate greaterspin-down and thus more Li depletion relative to Be.) The Li and Beabundances are correlated for stars in the temperature range of5850-6680 K, similar to results from earlier work on Li and Be in F andG field stars. This indicates that the depletions are not justcorrelated-the only claim that can be made for the field stars-but areprobably occurring together during main-sequence evolution. The Hyades Gdwarfs have more Be than the Sun; their initial Be may have been largeror they may not be old enough to have depleted much Be. For those Hyadesstars that seem to have little or no depletion of Li or Be, the Li/Beratio is found to be 75+/-30 the meteoritic ratio Li/Be is 78. TheHyades ratio is a representative value for the initial ratio in thematerial out of which the Hyades cluster was formed.
| HIPPARCOS age-metallicity relation of the solar neighbourhood disc stars
We derive age-metallicity relations (AMRs) and orbital parameters forthe 1658 solar neighbourhood stars to which accurate distances aremeasured by the HIPPARCOS satellite. The sample stars comprise 1382 thindisc stars, 229 thick disc stars, and 47 halo stars according to theirorbital parameters. We find a considerable scatter for thin disc AMRalong the one-zone Galactic chemical evolution (GCE) model. Orbits andmetallicities of thin disc stars show now clear relation each other. Thescatter along the AMR exists even if the stars with the same orbits areselected. We examine simple extension of one-zone GCE models whichaccount for inhomogeneity in the effective yield and inhomogeneous starformation rate in the Galaxy. Both extensions of the one-zone GCE modelcannot account for the scatter in age - [Fe/H] - [Ca/Fe] relationsimultaneously. We conclude, therefore, that the scatter along the thindisc AMR is an essential feature in the formation and evolution of theGalaxy. The AMR for thick disc stars shows that the star formationterminated 8 Gyr ago in the thick disc. As already reported by Grattonet al. (\cite{Gratton_et.al.2000}) and Prochaska et al.(\cite{Prochaska_et.al.2000}), thick disc stars are more Ca-rich thanthin disc stars with the same [Fe/H]. We find that thick disc stars showa vertical abundance gradient. These three facts, the AMR, verticalgradient, and [Ca/Fe]-[Fe/H] relation, support monolithic collapseand/or accretion of satellite dwarf galaxies as likely thick discformation scenarios. Tables 2 and 3 are only available in electronicform at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5)or via http:/ /cdsweb.u-strasbg.fr/ cgi-bin/qcat?J/ A+A/394/927
| 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
| Mg II Emission Lines of Hyades F Stars
With the Hubble Space Telescope (HST) we have observed Hyades F stars,using the Space Telescope Imaging Spectrograph (STIS), in order to getmore information about the heating mechanism(s) for the chromospheresand transition layers and their dependence on rotation and age. In thispaper we study the Mg II lines at 2800 Å. We include earlierobservations with the International Ultraviolet Explorer (IUE)satellite. The Mg II emission lines become observable for B-V>0.3.The emission line fluxes increase steeply until B-V~0.40. For singlestars there is a steep decrease in flux between B-V=0.41 and B-V=0.44,similar to the behavior of the Ca II emission line cores. For larger B-Vthe Mg II emission line fluxes again increase, but much more slowly thanfor the Ca II lines. Generally, the low point of the emission is reachedbetween B-V=0.43 and B-V=0.45, i.e., similar to the Ca II emissioncores. For the Hyades F stars there appears to be a difference betweenthe emissions for single stars and those for binaries. We find that forHyades stars with surface line fluxes larger than 106 ergscm-2 s-1 the emission line fluxes decrease withincreasing vsini. For smaller fluxes they may increase with increasingvsini. We have only three stars that perhaps show this. We study theflux ratios of the Mg II k and h lines at 2795.7 and 2802.5 Å inorder to determine where the lines fall on the curve of growth. For theearliest F stars studied here the ratio is close to 2, as expected foroptically thin lines. Generally, it seems that the optical depths in theline centers are less than 10. There remain problems in understandingthe size of the line widths. We discuss the interpretation of theWilson-Bappu effect. For the Hyades F stars there is a strong dependenceof the line width on the effective temperature. Based on observationswith the NASA/ESA Hubble Space Telescope obtained at the Space TelescopeScience Institute, which is operated by the Association of Universitiesfor Research in Astronomy. Incorporated, under NASA contract NAS5-26555.
| Speckle Interferometry of New and Problem Hipparcos Binaries. II. Observations Obtained in 1998-1999 from McDonald Observatory
The Hipparcos satellite made measurements of over 9734 known doublestars, 3406 new double stars, and 11,687 unresolved but possible doublestars. The high angular resolution afforded by speckle interferometrymakes it an efficient means to confirm these systems from the ground,which were first discovered from space. Because of its coverage of adifferent region of angular separation-magnitude difference(ρ-Δm) space, speckle interferometry also holds promise toascertain the duplicity of the unresolved Hipparcos ``problem'' stars.Presented are observations of 116 new Hipparcos double stars and 469Hipparcos ``problem stars,'' as well as 238 measures of other doublestars and 246 other high-quality nondetections. Included in these areobservations of double stars listed in the Tycho-2 Catalogue andpossible grid stars for the Space Interferometry Mission.
| The Physical Basis of Luminosity Classification in the Late A-, F-, and Early G-Type Stars. II. Basic Parameters of Program Stars and the Role of Microturbulence
Paper I of this series presented precise MK spectral types for 372 lateA-, F-, and early G-type stars with the aim of understanding the natureof luminosity classification on the MK spectral classification systemfor this range of spectral types. In this paper, a multidimensionaldownhill simplex technique is introduced to determine the basicparameters of the program stars from fits of synthetic spectra andfluxes with observed spectra and fluxes from Strömgren uvbyphotometry. This exercise yields useful calibrations of the MK spectralclassification system but, most importantly, gives insight into thephysical nature of luminosity classification on the MK spectralclassification system. In particular, we find that in this range ofspectral types, microturbulence appears to be at least as important asgravity in determining the MK luminosity type.
| The Physical Basis of Luminosity Classification in the Late A-, F-, and Early G-Type Stars. I. Precise Spectral Types for 372 Stars
This is the first in a series of two papers that address the problem ofthe physical nature of luminosity classification in the late A-, F-, andearly G-type stars. In this paper, we present precise spectralclassifications of 372 stars on the MK system. For those stars in theset with Strömgren uvbyβ photometry, we derive reddenings andpresent a calibration of MK temperature types in terms of the intrinsicStrömgren (b-y)0 index. We also examine the relationshipbetween the luminosity class and the Strömgren c1 index,which measures the Balmer jump. The second paper will address thederivation of the physical parameters of these stars, and therelationships between these physical parameters and the luminosityclass. Stars classified in this paper include one new λ Bootisstar and 10 of the F- and G-type dwarfs with recently discoveredplanets.
| A Hipparcos study of the Hyades open cluster. Improved colour-absolute magnitude and Hertzsprung-Russell diagrams
Hipparcos parallaxes fix distances to individual stars in the Hyadescluster with an accuracy of ~ 6 percent. We use the Hipparcos propermotions, which have a larger relative precision than the trigonometricparallaxes, to derive ~ 3 times more precise distance estimates, byassuming that all members share the same space motion. An investigationof the available kinematic data confirms that the Hyades velocity fielddoes not contain significant structure in the form of rotation and/orshear, but is fully consistent with a common space motion plus a(one-dimensional) internal velocity dispersion of ~ 0.30 kms-1. The improved parallaxes as a set are statisticallyconsistent with the Hipparcos parallaxes. The maximum expectedsystematic error in the proper motion-based parallaxes for stars in theouter regions of the cluster (i.e., beyond ~ 2 tidal radii ~ 20 pc) isla 0.30 mas. The new parallaxes confirm that the Hipparcos measurementsare correlated on small angular scales, consistent with the limitsspecified in the Hipparcos Catalogue, though with significantly smaller``amplitudes'' than claimed by Narayanan & Gould. We use the Tycho-2long time-baseline astrometric catalogue to derive a set of independentproper motion-based parallaxes for the Hipparcos members. The newparallaxes provide a uniquely sharp view of the three-dimensionalstructure of the Hyades. The colour-absolute magnitude diagram of thecluster based on the new parallaxes shows a well-defined main sequencewith two ``gaps''/``turn-offs''. These features provide the first directobservational support of Böhm-Vitense's prediction that (the onsetof) surface convection in stars significantly affects their (B-V)colours. We present and discuss the theoretical Hertzsprung-Russelldiagram (log L versus log T_eff) for an objectively defined set of 88high-fidelity members of the cluster as well as the delta Scuti startheta 2 Tau, the giants delta 1, theta1, epsilon , and gamma Tau, and the white dwarfs V471 Tau andHD 27483 (all of which are also members). The precision with which thenew parallaxes place individual Hyades in the Hertzsprung-Russelldiagram is limited by (systematic) uncertainties related to thetransformations from observed colours and absolute magnitudes toeffective temperatures and luminosities. The new parallaxes providestringent constraints on the calibration of such transformations whencombined with detailed theoretical stellar evolutionary modelling,tailored to the chemical composition and age of the Hyades, over thelarge stellar mass range of the cluster probed by Hipparcos.
| Kinematics versus X-ray luminosity segregation in the Hyades
State of the art Tycho-2 proper motions of bona fide Hyades members areused in combination with the ROSAT X-ray survey of this open cluster inStern et al. (1995) to study the internal velocity dispersion versusX-ray luminosity by the classical convergent point method. It is shownin this paper for the Hyades members with B-V below 0.7 mag that starsbrighter than 1.3x 1029 erg s-1 (0.1-1.8 keV) havemuch more coherent motions than those with smaller X-ray luminositiesand those not detected by ROSAT at all. In fact, the proper motions ofthe X-ray luminous members are in agreement with a null internaldispersion of velocities for this subpopulation, whereas theintermediate bright X-ray stars exhibit a dispersion at 320 ms-1. The non-emitters, despite their being more massive andoptically brighter, have a velocity dispersion of about 440 ms-1. Probably related to this fact, the strong X-ray emittersare more concentrated within the tidal radius (10 pc) of the clustercentre than the non-emitters. Nearly half of strong X-ray emitters withcoherent motions are visual or orbiting pairs with typical separations 5to 50 AU. This find implies that a group of intermediate mass (1.0 to1.5 solar masses) stars in the Hyades is significantly younger than therest and their system is not yet dynamically relaxed.
| Speckle Interferometry of New and Problem HIPPARCOS Binaries
The ESA Hipparcos satellite made measurements of over 12,000 doublestars and discovered 3406 new systems. In addition to these, 4706entries in the Hipparcos Catalogue correspond to double star solutionsthat did not provide the classical parameters of separation and positionangle (rho,theta) but were the so-called problem stars, flagged ``G,''``O,'' ``V,'' or ``X'' (field H59 of the main catalog). An additionalsubset of 6981 entries were treated as single objects but classified byHipparcos as ``suspected nonsingle'' (flag ``S'' in field H61), thusyielding a total of 11,687 ``problem stars.'' Of the many ground-basedtechniques for the study of double stars, probably the one with thegreatest potential for exploration of these new and problem Hipparcosbinaries is speckle interferometry. Results are presented from aninspection of 848 new and problem Hipparcos binaries, using botharchival and new speckle observations obtained with the USNO and CHARAspeckle cameras.
| Color indices of the Sun and Hyades stars in the WBVR system
Using an original setup at high altitudes, we measured the color indicesof the Sun in the WBVR photometric system relative to standard stars: (W- B)_solar = -0.05, (B - V)_solar = +0.67, and (V - R)_solar = +0.53. Wepresent the WBVR photometry for the Hyades members selected by vanBueren by their space velocities. The solar position is shown intwo-color diagrams relative to Hyades stars and bright G2 V stars. Acomparison of our results with Kurucz's models reveals a discrepancybetween the metallicities of the models and the Hyades members.
| The Age Range of Hyades Stars
On the basis of canonical models, the age of Hyades supercluster stars,whether in the Hyades and Praesepe clusters or the noncluster field,ranges from (5-6) x 10^8 to 10^9 yr. The difference between the parallaxderived from the supercluster motion and that obtained from Hipparcosobservations has a dispersion only twice that of the mean dispersion ofthe individual Hipparcos values. The supercluster appears not to containred giants on the first ascent of the red giant branch, but onlyasymptotic giant branch (``clump'') stars. The masses obtained forindividual components of binary stars in the supercluster show adispersion of less than 10% when compared with model predictions.
| The Multiplicity of the Hyades and Its Implications for Binary Star Formation and Evolution
A 2.2 μm speckle imaging survey of 167 bright (K < 8.5 mag) Hyadesmembers reveals a total of 33 binaries with separations spanning 0.044"to 1.34" and magnitude differences as large as 5.5 mag. Of thesebinaries, 9 are new detections and an additional 20 are now spatiallyresolved spectroscopic binaries, providing a sample from which dynamicalmasses and distances can be obtained. The closest three systems,marginally resolved at Palomar Observatory, were reobserved with the 10m Keck Telescope in order to determine accurate binary star parameters.Combining the results of this survey with previous radial velocity,optical speckle, and direct-imaging Hyades surveys, the detectedmultiplicity of the sample is 98 singles, 59 binaries, and 10 triples. Astatistical analysis of this sample investigates a variety of multiplestar formation and evolution theories. Over the binary separation range0.1"-1.07" (5-50 AU), the sensitivity to companion stars is relativelyuniform, with = 4 mag, equivalent to a mass ratio = 0.23. Accounting for the inability to detect high fluxratio binaries results in an implied companion star fraction (CSF) of0.30 +/- 0.06 in this separation range. The Hyades CSF is intermediatebetween the values derived from observations of T Tauri stars (CSF_TTS =0.40 +/- 0.08) and solar neighborhood G dwarfs (CSF_SN = 0.14 +/- 0.03).This result allows for an evolution of the CSF from an initially highvalue for the pre-main sequence to that found for main-sequence stars.Within the Hyades, the CSF and the mass ratio distribution provideobservational tests of binary formation mechanisms. The CSF isindependent of the radial distance from the cluster center and theprimary star mass. The distribution of mass ratios is best fitted by apower law q^-1.3+/-0.3 and shows no dependence on the primary mass,binary separation, or radial distance from the cluster center. Overall,the Hyades data are consistent with scale-free fragmentation, butinconsistent with capture and disk-assisted capture in small clusters.Without testable predictions, scale-dependent fragmentation and diskfragmentation cannot be assessed with the Hyades data.
| The Hyades: distance, structure, dynamics, and age
{We use absolute trigonometric parallaxes from the Hipparcos Catalogueto determine individual distances to members of the Hyades cluster, fromwhich the 3-dimensional structure of the cluster can be derived.Inertially-referenced proper motions are used to rediscuss distancedeterminations based on convergent-point analyses. A combination ofparallaxes and proper motions from Hipparcos, and radial velocities fromground-based observations, are used to determine the position andvelocity components of candidate members with respect to the clustercentre, providing new information on cluster membership: 13 newcandidate members within 20 pc of the cluster centre have beenidentified. Farther from the cluster centre there is a gradual mergingbetween certain cluster members and field stars, both spatially andkinematically. Within the cluster, the kinematical structure is fullyconsistent with parallel space motion of the component stars with aninternal velocity dispersion of about 0.3 km s(-1) . The spatialstructure and mass segregation are consistent with N-body simulationresults, without the need to invoke expansion, contraction, rotation, orother significant perturbations of the cluster. The quality of theindividual distance determinations permits the cluster zero-age mainsequence to be accurately modelled. The helium abundance for the clusteris determined to be Y =3D 0.26+/-0.02 which, combined with isochronemodelling including convective overshooting, yields a cluster age of625+/-50 Myr. The distance to the observed centre of mass (a conceptmeaningful only in the restricted context of the cluster memberscontained in the Hipparcos Catalogue) is 46.34+/-0.27 pc, correspondingto a distance modulus m-M=3D3.33+/-0.01 mag for the objects within 10 pcof the cluster centre (roughly corresponding to the tidal radius). Thisdistance modulus is close to, but significantly better determined than,that derived from recent high-precision radial velocity studies,somewhat larger than that indicated by recent ground-based trigonometricparallax determinations, and smaller than those found from recentstudies of the cluster convergent point. These discrepancies areinvestigated and explained. } Based on observations made with the ESAHipparcos astrometry satellite. Table~2 is also available in electronicform at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5)or via http://cdsweb.u-strasbg.fr/Abstract.html
| A New and Comprehensive Determination of the Distance to Member Stars of the Hyades
This paper critiques the methods used in the past for estimating thedistances to the stars that make up the Hyades moving cluster, anddevelops one which is new and comprehensive. We develop a formalism forthe application of stochastic restrictions to the adjustment parameters(velocity components and distances) and test it on constructed clustermodels. We perform model calculations with fictitious model starclusters in which the `true' values of the input parameters (ie.positions, distances and velocity components) are therefore known. Thecomparison of the distances recovered by different methods with the trueinput distances shows our approach to be very superior to thetraditional ones that estimate distances by the method of streamparallaxes without restricting the adjustment parameters. Applying ouralgorithm to the Hyades, we assume a dispersion of 0.25 kps in eachcoordinate about a common value for the velocity components of thecluster members and a dispersion of 5pc about the distance to thecluster's centre for the distances to the stars in the cluster. Underthese stochastic restrictions, we analyse the known estimates (ie.measurements) of trigonometric parallaxes, sets of high-precision propermotion pairs and high-precision radial velocities subject to theabove-stated stochastic constraints, and as a result estimate thecluster's centre to be at a distance of 45.8+/-1.25 (standard error) pc.
| Loop modeling of X-ray coronae: solar-type stars in young clusters and associations
Not Available
| Spectrophotometry of 237 Stars in 7 Open Clusters
Spectrophotometry is presented for 237 stars in 7 nearby open clusters:Hyades, Pleiades, Alpha Persei, Praesepe, Coma Berenices, IC 4665, andM39. The observations were taken by Lee McDonald and David Bursteinusing the Wampler single-channel scanner on the Crossley 0.9m telescopeat Lick Observatory from July 1973 through December 1974. Sixteenbandpasses spanning the spectral range 3500 Angstroms to 7780 Angstromswere observed for each star, with bandwidths 32Angstroms, 48 Angstromsor 64 Angstroms. Data are standardized to the Hayes-Latham system tomutual accuracy of 0.016 mag per passband. The accuracy of thespectrophotometry is assessed in three ways on a star-by-star basis.First, comparisons are made with previously published spectrophotometryfor 19 stars observed in common. Second, (B-V) colors and uvby colorsare compared for 236 stars and 221 stars, respectively. Finally,comparsions are made for 200 main sequence stars to the spectralsynthesis models of Kurucz, fixing log g = 4.0 and [Fe/H] = 0.0, andonly varying effective temperature. The accuracy of tests using uvbycolors and the Kurucz models are shown to track each other closely,yielding an accuracy estimate (1 sigma ) of 0.01 mag for the 13 colorsformed from bandpasses longward of the Balmer jump, and 0.02 mag for the3 colors formed from the three bandpasses below the Balmer jump. Incontrast, larger scatter is found relative to the previously publishedspectrophotometry of Bohm-Vitense & Johnson (16 stars in common) andGunn & Stryker (3 stars). We also show that the scatter in the fitsof the spectrophotometric colors and the uvby filter colors is areasonable way to identify the observations of which specific stars areaccurate to 1 sigma , 2 sigma , .... As such, the residuals from boththe filter color fits and the Kurucz model fits are tabulated for eachstar where it was possible to make a comparison, so users of these datacan choose stars according to the accuracy of the data that isappropriate to their needs. The very good agreement between the modelsand these data verifies the accuracy of these data, and also verifiesthe usefulness of the Kurucz models to define spectrophotometry forstars in this temperature range (>5000 K). These data define accuratespectrophotometry of bright, open cluster stars that can be used as asecondary flux calibration for CCD-based spectrophotometric surveys.
| 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
| The role of convection on the UVBY colours of A, F, and G stars
We discuss the effects of convection on the theoretical uvby colours ofA, F, and G stars. The standard mixing-length theory atlas9 models ofKurucz (1993), with and without approximate overshooting, are comparedto models using the turbulent convection theory proposed by Canuto &Mazzitelli (1991, 1992) and implemented by Kupka (1996a). Comparisonwith fundamental T_eff and log g stars reveals that the Canuto &Mazzitelli models give results that are generally superior to standardmixing-length theory (MLT) without convective overshooting. MLT modelswith overshooting are found to be clearly discrepant. This is supportedby comparisons of non-fundamental stars, with T_eff obtained from theInfrared Flux Method and log g from stellar evolutionary models for opencluster stars. The Canuto & Mazzitelli theory gives values of(b-y)_0 and c_0 that are in best overall agreement with observations.Investigations of the m_0 index reveal that all of the treatments ofconvection presented here give values that are significantly discrepantfor models with T_eff < 6000 K. It is unclear as to whether this isdue to problems with the treatment of convection, missing opacity, orsome other reason. None of the models give totally satisfactory m_0indices for hotter stars, but the Canuto & Mazzitelli models are inclosest overall agreement above 7000 K. Grids of uvby colours, based onthe CM treatment of convection, are presented. These grids represent animprovement over the colours obtained from models using themixing-length theory. The agreement with fundamental stars enables thecolours to be used directly without the need for semi-empiricaladjustments that were necessary with the earlier colour grids. Table~5is only available at the CDS via anonymous ftp 130.79.128.5 or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| ROSAT coronal temperatures of young late type stars.
We analyze ROSAT X-ray spectra of a sample of young late type stars byfitting them with a continuous emission measure model with power-lawtemperature dependence up to a maximum temperature. This model givessuccessful fits to all spectra of our sample. While most of the spectracan also fitted equally successful with the frequently used twotemperature model, we argue and provide evidence that the continuoustemperature distribution model probably describes the coronaltemperature structure in a more meaningful way. From our fitting resultswe find a very good correlation between the X-ray surface flux and themaximum coronal temperature with the functional formF_X_{prop.to}T_max_^(2.2+/-0.2)^ . This correlation extends over severalorders of magnitude and is valid for the very young T Tauri stars aswell as for the much older young main sequence stars in our sample.Since the X-ray surface flux is a good measure of the stellar activityand shows a clear dependence on the stellar rotational velocity, thecorrelation means that the coronal temperature is related to the X-rayactivity and ultimately determined by stellar rotation. Finally, weinterpret our results with a simple analytic loop model and find thatthe coronae of the active young stars are probably dominated by loopswith generally higher pressure as compared to typical solar activeregion loops.
| X-Ray-emitting T Tauri Stars in the L1551 Cloud
Low-mass pre-main-sequence stars in the nearby Lynds 1551 star-formingcloud are studied with the ROSAT and ASCA X-ray satellites. An 8 ksROSAT image reveals 38 sources including seven well-known T Tauri stars,two likely new weak-lined T Tauri stars, five potential new weak-lined TTauri stars, one young B9 star, and the remaining sources are unrelatedto the cloud or poorly identified. A 40 ks ASCA image of the clouddetects seven of the ROSAT sources. Spectral fitting of the brighterX-ray-emitting stars suggests the emission is produced in either amulti-temperature plasma, with temperatures near 0.2 and 1 keV, or asingle-temperature plasma with low metal abundances. XZ Tau, a youngclassical T Tauri star, is much stronger in ASCA than ROSATobservations, showing a harder (1.5-2.0 keV) component. Timing analysisreveals all but one of the T Tauri stars are variable on timescalesranging from 1 hr to 1 year. A powerful flare, emitting 3 x 10^34^ ergswithin a 40 minute rise and fall, was observed by ASCA on the weak-linedT Tauri star V826 Tau. The event was preceded and followed by constantquiescent X-ray emission. The extreme classical T Tauri star XZ Tau wasalso caught during both high and low states, varying by a factor of 15between the ASCA and ROSAT observations. Neither of the luminousinfrared embedded protostars L1551 IRS 5 or L1551NE were detected byROSAT or ASCA.
| New weak-line T Tauri stars in Taurus-Auriga.
On the basis of the ROSAT All-Sky-Survey, a study of the Taurus-Aurigastar forming region has been performed in order to search for hithertoundiscovered TTauri stars. Our study covers an area of about 280 squaredegrees, located between 4^h^ and 5^h^ in right ascension and between15deg and 34deg in declination. Identification of ROSAT All-Sky Surveysources in this area by means of optical spectroscopy revealed 2 newclassical T Tauri stars (CTTS) and 66 new weak-line-T Tauri stars (WTTS)with Wlambda_(Hα)<=10A. Additional pointed ROSATobservations led to the identification of 6 more WTTS and 2 CTTS, givinga total of 76 new T Tauri stars. The large area of our study, ascompared with previous works, allows us to study the spatialdistribution of WTTS in this star forming region. We find the WTTS ofour survey to be distributed over the whole region investigated. Thereis a noticeable decline of the surface density from south to northwithin our study area, but the spatial distribution extends mostprobably beyond our study region. No clustering towards the populationof TTauri stars known prior to ROSAT in Taurus-Auriga could be observed.We suggest that the WTTS found in our study might in part be somewhatolder than the previously known TTauri stars in Taurus-Auriga, and thattheir broad spatial distribution is due to the typical velocitydispersion of a few km/s measured for Taurus TTauri stars, in which casefor some of our WTTS an age on the order of 10^7^years would be requiredfor reaching the observed distances from the Taurus dark clouds. Weestimate a WTTS/CTTS ratio of about 6 within our study area, butconclude that because of the different spatial distribution of WTTS andCTTS this ratio will be most probably significantly larger for a moreextended area.
|
Yeni bir Makale Öner
Ýlgili Baðlantýlar
Yeni Bir Baðlantý Öner
sonraki gruplarýn üyesi:
|
Gözlemler ve gökölçümü verileri
| Takýmyýldýz: | Boga |
| Sað Açýklýk: | 04h29m20.55s |
| Yükselim: | +17°32'41.8" |
| Görünürdeki Parlaklýk: | 7.022 |
| Uzaklýk: | 43.011 parsek |
| özdevim Sað Açýklýk: | 102.8 |
| özdevim Yükselim: | -30.6 |
| B-T magnitude: | 7.618 |
| V-T magnitude: | 7.072 |
Kataloglar ve belirtme:
|
INHABITED SKY
