Contenuti
Immagini
Carica la tua immagine
DSS Images Other Images
Articoli relazionati
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.
| Chemical Homogeneity in the Hyades
We present an abundance analysis of the heavy elements Zr, Ba, La, Ce,and Nd for Hyades F-K dwarfs based on high-resolution, highsignal-to-noise ratio spectra from Keck HIRES. The derived abundancesshow the stellar members to be highly uniform, although some elementsshow a small residual trend with temperature. The rms scatter for eachelement for the cluster members is as follows: Zr=0.055, Ba=0.049,Ce=0.025, La=0.025, and Nd=0.032 dex. This is consistent with themeasurement errors and implies that there is little or no intrinsicscatter among the Hyades members. Several stars thought to be nonmembersof the cluster based on their kinematics are found to deviate from thecluster mean abundances by about 2 σ. Establishing chemicalhomogeneity in open clusters is the primary requirement for theviability of chemically tagging Galactic disk stars to common formationsites in order to unravel the dissipative history of early diskformation.
| 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.
| Fine structure of the chromospheric activity in Solar-type stars - The Hα line
A calibration of Hα as both a chromospheric diagnostic and an ageindicator is presented, complementing the works previously done on thissubject (\cite{herbig}; \cite{luca1}). The chromospheric diagnostic wasbuilt with a statistically significant sample, covering nine years ofobservations, and including 175 solar neighborhood stars. Regarding theage indicator, the presence of stars for which very accurate ages aredetermined, such as those belonging to clusters and kinematic groups,lends confidence to our analysis. We also investigate the possibilitythat stars of the same age might have gone through different tracks ofchromospheric decay, identifying - within the same age range - effectsof metallicity and mass. These parameters, however, as well as age, seemto be significant only for dwarf stars, losing their meaning when weanalyze stars in the subgiant branch. This result suggests that, inthese evolved stars, the emission mechanism cannot bemagnetohydrodynamical in nature, in agreement with recent models (Fawzyet al. 2002c, and references therein). The Sun is found to be a typicalstar in its Hα chromospheric flux, for its age, mass andmetallicity. As a byproduct of this work, we developed an automaticmethod to determine temperatures from the wings of Hα, which meansthe suppression of the error inherent to the visual procedure used inthe literature.Based on observations collected at Observatório do Pico dos Dias,operated by the Laboratório Nacional de Astrofísica, CNPq,Brazil.Table 5 is only available in electronic form at thehttp://www.edpsciences.org
| Magnesium Isotope Ratios in Hyades Stars
Using classical model atmospheres and an LTE analysis, Mg isotope ratios24Mg:25Mg:26Mg are measured in 32Hyades dwarfs covering effective temperatures4000K<=Teff<=5000K. We find no significant trend in anyisotope ratio versus Teff, and the mean isotope ratio is inexcellent agreement with the solar value. We determine stellarparameters and Fe abundances for 56 Hyades dwarfs covering4000K<=Teff<=6200K. For stars warmer than 4700 K, wederive a cluster mean value of [Fe/H]=0.16+/-0.02 (σ=0.1), in goodagreement with previous studies. For stars cooler than 4700 K, we findthat the abundance of Fe from ionized lines exceeds the abundance of Fefrom neutral lines. At 4700 K,[Fe/H]II-[Fe/H]I~=0.3dex, while at 4000 K[Fe/H]II-[Fe/H]I~=1.2dex. This discrepancy betweenthe Fe abundance from neutral and ionized lines likely reflectsinadequacies in the model atmospheres and the presence of non-LTE orother effects. Despite the inability of the models to reproduce theionization equilibrium for Fe, the Mg isotope ratios appear immune tothese problems and remain a powerful tool for studying Galactic chemicalevolution.Data presented here were obtained at the W. M. Keck Observatory, whichis operated as a scientific partnership among the California Instituteof Technology, the University of California, and the NationalAeronautics and Space Administration. The Observatory was made possibleby the generous financial support of the W. M. Keck Foundation.
| Searching for Planets in the Hyades. V. Limits on Planet Detection in the Presence of Stellar Activity
We present the results of a radial velocity survey of a sample of Hyadesstars and discuss the effects of stellar activity on radial velocitymeasurements. The level of radial velocity scatter due to rotationalmodulation of stellar surface features for the Hyades is in agreementwith the 1997 predictions of Saar & Donahue-the maximum radialvelocity rms of up to ~50 m s-1, with an average rms of ~16 ms-1. In this sample of 94 stars we find one new binary, twostars with linear trends indicative of binary companions, and noclose-in giant planets. We discuss the limits on extrasolar planetdetection in the Hyades and the constraints imposed on radial velocitysurveys of young stars.Some of the data presented herein were obtained at the W. M. KeckObservatory, which is operated as a scientific partnership among theCalifornia Institute of Technology, the University of California, andthe National Aeronautics and Space Administration (NASA). TheObservatory was made possible by the generous financial support of theW. M. Keck Foundation. The authors wish to recognize and acknowledge thevery significant cultural role and reverence that the summit of MaunaKea has always had within the indigenous Hawaiian community. We are mostfortunate to have the opportunity to conduct observations from thismountain. Additional data were obtained with the Hobby-Eberly Telescope,which is operated by McDonald Observatory on behalf of the University ofTexas at Austin, the Pennsylvania State University, Stanford University,Ludwig-Maximilians-Universität München, andGeorg-August-Universität Göttingen.
| The age-activity-rotation relationship in solar-type stars
We present Ca II K line chromospheric fluxes measured fromhigh-resolution spectra in 35 G dwarf stars of 5 open clusters todetermine the age-activity-rotation relationship from the young Hyadesand Praesepe (0.6 Gyr) to the old M 67 (4.5 Gyr) through the twointermediate age clusters IC 4651 and NGC 3680 (1.7 Gyr). The fullamplitude of the activity index within a cluster is slightly above 60 %for all clusters but one, NGC 3680, in which only two stars wereobserved. As a comparison, the same Solar Ca II index varies by 40% during a solar cycle. Four of our clusters (Hyades and Praesepe, IC4651 and NGC 3680) are pairs of twins as far as age is concerned; theHyades have the same chromospheric-activity level as Praesepe, at oddswith early claims based on X-ray observations. Both stars in NGC 3680are indistinguishable, as far as chromospheric activity is concerned,from those in the coeval IC 4651. This is a validation of the existenceof an age-activity relationship. On the other hand, the two intermediateage clusters have the same activity level as the much older M 67 and theSun. Our data therefore shows that a dramatic decrease in chromosphericactivity takes place in solar stars between the Hyades and the IC 4651age, of about 1 Gyr. Afterwards, activity remains virtually constant formore than 3 Gyr. We have also measured v sin i for all of our stars andthe average rotational velocity shows the same trend as thechromospheric-activity index. We briefly investigate the impact of thisresult on the age determinations of field G dwarfs in the solarneighborhood; the two main conclusions are that a consistent group of``young'' stars (i.e. as active as Hyades stars) is present, and that itis virtually impossible to give accurate chromospheric ages for starsolder than 2 Gyr. The observed abrupt decline in activity explainsvery well the Vaughan-Preston gap.Observations collected at the ESO VLT. Some data presented herein wereobtained at the W. M. Keck Observatory, which is operated as ascientific partnership among the California Institute of Technology, theUniversity of California, and the National Aeronautics and SpaceAdministration. The Observatory was made possible by the generousfinancial support of the W. M. Keck Foundation.
| 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
| Searching for Planets in the Hyades. IV. Differential Abundance Analysis of Hyades Dwarfs
We present a differential abundance analysis of Hyades F-K dwarfs insearch for evidence of stellar enrichment from accretedhydrogen-deficient disk material. Metallicities and relative abundanceratios of several species have been determined. We derive a cluster mean[Fe/H]=0.13+/-0.01. Two stars show abundances ~0.2 dex larger than thecluster mean. In addition, one star, which was added by a recent studyas a cluster member, shows significantly lower abundances than thecluster mean. These three stars have questionable membershipcharacteristics. The remaining stars in the survey have an rms of 0.04dex in the differential [Fe/H] values. The Hyades cluster members haveapparently not been significantly chemically enriched. The abundanceratios of Si, Ti, Na, Mg, Ca, and Zn with respect to Fe are in theirsolar proportions.Some data presented herein were obtained at the W. M. Keck Observatory,which is operated as a scientific partnership among the CaliforniaInstitute of Technology, the University of California, and the NationalAeronautics and Space Administration. The Observatory was made possibleby the generous financial support of the W. M. Keck Foundation.
| Dependence of coronal X-ray emission on spot-induced brightness variations in cool main sequence stars
The maximum amplitude (Amax) of spot-induced brightnessvariations from long-term V-band photometry and the ratioLX/Lbol between X-ray and bolometric luminositiesare suitable indicators of the level of magnetic activity in thephotosphere and in the corona of late-type stars, respectively. By usingthese activity indicators we investigate the dependence of coronal X-rayemission on the level of photospheric starspot activity in a homogeneoussample of low mass main sequence field and cluster stars of differentages (IC 2602, IC 4665,IC 2391, alpha Persei,Pleiades and Hyades). First, theactivity-rotation connection at the photospheric level is re-analysed,as well as its dependence on spectral type and age. The upper envelopeof Amax increases monotonically with decreasing rotationalperiod (P) and Rossby number (R0) showing a break around 1.1d that separates two rotation regimes where the starspot activity showsdifferent behaviours. The Amax-P andAmax-R0 relations are fitted with linear,exponential and power laws to look for the function which bestrepresents the trend of the data. The highest values of Amaxare found among K-type stars and at the ages of alphaPersei and Pleiades. We also analyse theactivity-rotation connection at the coronal level as well as itsdependence on spectral type. The level of X-ray emission increases withincreasing rotation rate up to a saturation level. The rotational periodat which saturation occurs is colour-dependent and increases withadvancing spectral type. Also the LX/Lbol-P andLX/Lbol-R0 relations are fitted withlinear, exponential and power laws to look for the best fittingfunction. Among the fastest rotating stars (P<=0.3 d) there isevidence of super-saturation. Also the highest values ofLXLbol are found among K-type stars. Finally, thephotospheric-coronal activity connection is investigated by using forthe first time the largest ever sample of light curve amplitudes asindicators of the magnetic filling factor. The activity parametersLX/Lbol and Amax are found to becorrelated with each other, thus confirming the dependence of coronalactivity on photospheric magnetic fields. More precisely, theLX/Lbol-Amax distribution shows thepresence of an upper envelope, which is constant at theLX/Lbol =~ -3.0 saturation level, and of a lowerenvelope. The best fit to the lower envelope is given by a power lawwith steepness decreasing from F-G to M spectral types. However, it isconsidered a tentative result, since the fit reduced chi-squares arelarge. Such spectral-type dependence may be related to a colourdependence of Amax on the total starspot filling factor, aswell as to the coronal emission being possibly more sensitive tostarspot activity variations in F- and G-type than in M-type stars. TheLX/Lbol-Amax mean values for eachcluster in our sample decrease monotonically with increasing age,showing that the levels of photospheric and coronal activity evolve intime according to a single power law till the Sun's age.Tables of the photometric and X-ray data sets are only available inelectronic form at the 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/410/671
| Statistical cataloging of archival data for luminosity class IV-V stars. II. The epoch 2001 [Fe/H] catalog
This paper describes the derivation of an updated statistical catalog ofmetallicities. The stars for which those metallicities apply are ofspectral types F, G, and K, and are on or near the main sequence. Theinput data for the catalog are values of [Fe/H] published before 2002February and derived from lines of weak and moderate strength. Theanalyses used to derive the data have been based on one-dimensional LTEmodel atmospheres. Initial adjustments which are applied to the datainclude corrections to a uniform temperature scale which is given in acompanion paper (see Taylor \cite{t02}). After correction, the data aresubjected to a statistical analysis. For each of 941 stars considered,the results of that analysis include a mean value of [Fe/H], an rmserror, an associated number of degrees of freedom, and one or moreidentification numbers for source papers. The catalog of these resultssupersedes an earlier version given by Taylor (\cite{t94b}).Catalog is only available in electronic form at the CDS via anonymousftp cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/398/731
| Statistical cataloging of archival data for luminosity class IV-V stars. I. The epoch 2001 temperature catalog
This paper is one of a pair in which temperatures and metallicitycatalogs for class IV-V stars are considered. The temperature catalogdescribed here is derived from a calibration based on stellar angulardiameters. If published calibrations of this kind are compared by usingcolor-index transformations, temperature-dependent differences among thecalibrations are commonly found. However, such differences are minimizedif attention is restricted to calibrations based on Johnson V-K. Acalibration of this sort from Di Benedetto (\cite{dib98}) is thereforetested and adopted. That calibration is then applied to spectroscopicand photometric data, with the latter predominating. Cousins R-Iphotometry receives special attention because of its high precision andlow metallicity sensitivity. Testing of temperatures derived from thecalibration suggests that their accuracy and precision are satisfactory,though further testing will be warranted as new results appear. Thesetemperatures appear in the catalog as values of theta equiv5040/T(effective). Most of these entries are accompanied by measured orderived values of Cousins R-I. Entries are given for 951 stars.Catalog is only available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/398/721
| The stellar activity-rotation relationship revisited: Dependence of saturated and non-saturated X-ray emission regimes on stellar mass for late-type dwarfs
We present the results of a new study on the relationship betweencoronal X-ray emission and stellar rotation in late-type main-sequencestars. We have selected a sample of 259 dwarfs in the B-V range 0.5-2.0,including 110 field stars and 149 members of the Pleiades, Hyades, alphaPersei, IC 2602 and IC 2391 open clusters. All the stars have beenobserved with ROSAT, and most of them have photometrically-measuredrotation periods available. Our results confirm that two emissionregimes exist, one in which the rotation period is a good predictor ofthe total X-ray luminosity, and the other in which a constant saturatedX-ray to bolometric luminosity ratio is attained; we present aquantitative estimate of the critical rotation periods below which starsof different masses (or spectral types) enter the saturated regime. Inthis work we have also empirically derived a characteristic time scale,taue , which we have used to investigate the relationshipbetween the X-ray emission level and an X-ray-based Rossby numberRe = Prot/taue: we show that ourempirical time scale taue resembles the theoreticalconvective turnover time for 0.4 <~ M/Msun <~ 1.2, butit also has the same functional dependence on B-V asLbol-1/2 in the color range 0.5 <~ B-V <~1.5. Our results imply that - for non-saturated coronae - theLx - Prot relation is equivalent to theLx/Lbol vs. Re relation. Tables 1 and 2are only available in electronic form at \ http://www.edpsciences.org
| 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.
| Searching for Planets in the Hyades. II. Some Implications of Stellar Magnetic Activity
The Hyades constitute a homogeneous sample of stars ideal forinvestigating the dependence of planet formation on the mass of thecentral star. Because of their youth, Hyades members are much morechromospherically active than stars traditionally surveyed for planetsusing high-precision radial velocity techniques. Therefore, we haveconducted a detailed investigation of whether magnetic activity of ourHyades target stars will interfere with our ability to make preciseradial velocity (vrad) searches for substellar companions. Wemeasure chromospheric activity (which we take as a proxy for magneticactivity) by computing the equivalent of the R'HKactivity index (which is corrected for photospheric contributions) fromthe Ca II K line. The value of is notconstant in the Hyades: we confirm that it decreases with increasingtemperature in the F stars and also find it decreases for stars coolerthan mid K. We examine correlations between simultaneously measuredR'HK and radial velocities using both a classicalstatistical test and a Bayesian odds ratio test. We find that there is asignificant correlation between R'HK and theradial velocity in only five of the 82 stars in this sample. Thus,simple R'HK-vrad correlations willgenerally not be effective in correcting the measured vradvalues for the effects of magnetic activity in the Hyades. We argue thatthis implies long-timescale activity variations (of order a few years;i.e., magnetic cycles or growth and decay of plage regions) will notsignificantly hinder our search for planets in the Hyades if the starsare closely monitored for chromospheric activity. The trends in theradial velocity scatter (σ'v) with, vsini, and Prot for ourstars is generally consistent with those found in field stars in theLick planet search data, with the notable exception of a shallowerdependence of σ'v on for F stars. Data presented hereinwere obtained at the W.M. Keck Observatory, which is operated as ascientific partnership among the California Institute of Technology, theUniversity of California, and the National Aeronautics and SpaceAdministration. The Observatory was made possible by the generousfinancial support of the W.M. Keck Foundation.
| Searching for Planets in the Hyades. I. The Keck Radial Velocity Survey
We describe a high-precision radial velocity search for Jovian-masscompanions to main-sequence stars in the Hyades. The Hyades provides anextremely well controlled sample of stars of the same age, the samemetallicity, and a common birth and early dynamical environment. Thissample allows us to explore the dependence of the process of planetformation on only a single independent variable: the stellar mass. Inthis paper, we describe the survey and summarize results for the first 5years. Data presented herein were obtained at the W.M. Keck Observatory,which is operated as a scientific partnership among the CaliforniaInstitute of Technology, the University of California, and the NationalAeronautics and Space Administration. The Observatory was made possibleby the generous financial support of the W.M. Keck Foundation.
| 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.
| 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.
| The ``rotation-activity connection'': Its extension to photospheric activity diagnostics
In this paper we present the results of a different approach in thestudy of the so-called rotation-activity connection, which is a wellestablished correlation between rotation and magnetic activity atchromospheric and outer atmospheric levels. The present study concernsthe photospheric level and was carried out by using V-band photometriclight curve amplitudes as indicators of starspot coverage and ofmagnetic activity. A high degree of correlation between the envelope ofmaximum V-band light curve amplitudes and the rotation period is foundfor the active star members of young open clusters (IC 2602, IC 2391,Alpha Persei, Pleiades and Hyades), as well as for active field stars.This correlation shows a different behaviour in two different rotationperiod ranges. Moreover, some evidence of a possible activity``saturation'' is found among the most rapidly rotating stars of thesample. Additional correlations between photospheric and other magneticactivity indicators in the chromosphere, transition region and coronaare also investigated. The results presented here can be considered asan extension of the well established rotation-activity connection validfrom the corona, transition region and chromosphere, down to thephotosphere.
| 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.
| Photometric Abundance Calibration of delta Scuti Stars Using HK Photometry
The hk index has been used as a metallicity indicator for RR Lyraevariable stars. It is now being applied to the shorter period deltaScuti variables. Employing spectroscopic abundances of stars withpublished hk values and photometric indices calculated from stellaratmosphere models, a three-dimensional interpolation is used todetermine [Fe/H] from intrinsic b-y, c_1, and hk values. The resulting[Fe/H], log g, and T_eff values for 10 delta Scuti stars are presented.
| 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.
| The Abundance of CN. Calcium and Heavy Elements in High Velocity Stars.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1997AJ....114..825E&db_key=AST
|
Sottometti un nuovo articolo
Link relazionati
Sottometti un nuovo link
Membro dei seguenti gruppi:
|
Osservazione e dati astrometrici
| Costellazione: | Toro |
| Ascensione retta: | 04h24m28.33s |
| Declinazione: | +16°53'10.2" |
| Magnitudine apparente: | 7.794 |
| Distanza: | 48.239 parsec |
| Moto proprio RA: | 113.5 |
| Moto proprio Dec: | -23 |
| B-T magnitude: | 8.494 |
| V-T magnitude: | 7.852 |
Cataloghi e designazioni:
|
