Contenuti
Immagini
Carica la tua immagine
DSS Images Other Images
Articoli relazionati
Modelling the components of binaries in the Hyades: the dependence of the mixing-length parameter on stellar mass
We present our findings based on a detailed analysis of the binaries ofthe Hyades, in which the masses of the components are well known. We fitthe models of the components of a binary system to observations so as togive the observed total V and B-V of that system and the observed slopeof the main sequence in the corresponding parts. According to ourfindings, there is a very definite relationship between themixing-length parameter and the stellar mass. The fitting formula forthis relationship can be given as α= 9.19(M/Msolar-0.74)0.053- 6.65, which is valid for stellar masses greaterthan 0.77Msolar. While no strict information is gathered forthe chemical composition of the cluster, as a result of degeneracy inthe colour-magnitude diagram, by adopting Z= 0.033 and using models forthe components of 70 Tau and θ2 Tau we find thehydrogen abundance to be X= 0.676 and the age to be 670 Myr. If weassume that Z= 0.024, then X= 0.718 and the age is 720 Myr. Our findingsconcerning the mixing-length parameter are valid for both sets of thesolution. For both components of the active binary system V818 Tau, thedifferences between radii of the models with Z= 0.024 and the observedradii are only about 4 per cent. More generally, the effectivetemperatures of the models of low-mass stars in the binary systemsstudied are in good agreement with those determined by spectroscopicmethods.
| Abundances of Baade's Window Giants from Keck HIRES Spectra. I. Stellar Parameters and [Fe/H] Values
We present the first results of a new abundance survey of the Milky Waybulge based on Keck HIRES spectra of 27 K giants in the Baade's Window(l=1deg, b=-4deg) field. The spectral data used inthis study are of much higher resolution and signal-to-noise ratio thanprevious optical studies of Galactic bulge stars. The [Fe/H] values ofour stars, which range between -1.29 and +0.51, were used to recalibratelarge low-resolution surveys of bulge stars. Our best value for the mean[Fe/H] of the bulge is -0.10+/-0.04. This mean value is similar to themean metallicity of the local disk and indicates that there cannot be astrong metallicity gradient inside the solar circle. The metallicitydistribution of stars confirms that the bulge does not suffer from theso-called G dwarf problem. This paper also details the new abundancetechniques necessary to analyze very metal-rich K giants, including anew Fe line list and regions of low blanketing for continuumidentification.Based on data obtained at the W. M. Keck Observatory, which is operatedas a scientific partnership among the California Institute ofTechnology, the University of California, and NASA and was made possibleby the generous financial support of the W. M. Keck Foundation.
| Hyades Oxygen Abundances from the λ6300 [O I] Line: The Giant-Dwarf Oxygen Discrepancy Revisited1,
We present the results of our abundance analysis of Fe, Ni, and O inhigh signal-to-noise ratio, high-resolution Very Large Telescope UVESand McDonald 2dcoudé spectra of nine dwarfs and three giants inthe Hyades open cluster. The difference in Fe abundances derived from FeII and Fe I lines ([Fe II/H]-[Fe I/H]) and Ni I abundances derived frommoderately high-excitation (χ~4.20 eV) lines is found to increasewith decreasing Teff for the dwarfs. Both of these findingsare in concordance with previous results ofoverexcitation/overionization in cool young dwarfs. Oxygen abundancesare derived from the [O I] λ6300 line, with careful attentiongiven to the Ni I blend. The dwarf O abundances are in star-to-staragreement within uncertainties, but the abundances of the three coolestdwarfs (4573 K<=Teff<=4834 K) evince an increase withdecreasing Teff. Possible causes for the apparent trend areconsidered, including the effects of overdissociation of O-containingmolecules. O abundances are derived from the near-UV OH λ3167line in high-quality Keck HIRES spectra, and no such effects are found;indeed, the OH-based abundances show an increase with decreasingTeff, leaving the nature and reality of the cool dwarf [OI]-based O trend uncertain. The mean relative O abundance of the sixwarmest dwarfs (5075 K<=Teff<=5978 K) is[O/H]=+0.14+/-0.02, and we find a mean abundance of [O/H]=+0.08+/-0.02for the giants. Thus, our updated analysis of the [O I] λ6300line does not confirm the Hyades giant-dwarf oxygen discrepancyinitially reported by King & Hiltgen, suggesting that thediscrepancy was a consequence of analysis-related systematic errors. LTEoxygen abundances from the near-IR, high-excitation O I triplet are alsoderived for the giants, and the resulting abundances are approximately0.28 dex higher than those derived from the [O I] line, in agreementwith non-LTE predictions. Non-LTE corrections from the literature areapplied to the giant triplet abundances; the resulting mean abundance is[O/H]=+0.17+/-0.02, in decent concordance with the giant and dwarf [O I]abundances. Finally, Hyades giant and dwarf O abundances derived fromthe [O I] λ6300 line and high-excitation triplet, as well asdwarf O abundances derived from the near-UV OH λ3167 line, arecompared, and a mean cluster O abundance of [O/H]=+0.12+/-0.02 isachieved, which represents the best estimate of the Hyades O abundance.This paper includes data taken with the Harlan J. Smith 2.7 m and theOtto Struve 2.1 m telescopes at the McDonald Observatory of theUniversity of Texas at Austin.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 and theNational Aeronautics and Space Administration. The Observatory was madepossible by the generous financial support of the W. M. Keck Foundation.
| Elemental abundances in the atmosphere of clump giants
Aims.The aim of this paper is to provide the fundamental parameters andabundances for a large sample of local clump giants with a highaccuracy. This study is a part of a big project, in which the verticaldistribution of the stars in the Galactic disc and the chemical anddynamical evolution of the Galaxy are being investigated. Methods:.The selection of clump stars for the sample group was made applying acolour-absolute magnitude window to nearby Hipparcos stars. Theeffective temperatures were estimated by the line depth ratio method.The surface gravities (log {g}) were determined by two methods (thefirst one was the method based on the ionization balance of iron and thesecond one was the method based on fitting of the wings of the Ca I6162.17 Å line). The abundances of carbon and nitrogen wereobtained from the molecular synthetic spectrum, and the Mg and Naabundances were derived using the non-LTE approximation. The "classical"models of stellar evolution without atomic diffusion androtation-induced mixing were employed. Results: .The atmosphericparameters ({T_eff}, log {g}, [Fe/H], {Vt}) and Li, C, N, O,Na, Mg, Si, Ca, and Ni abundances in 177 clump giants of the Galacticdisc were determined. The underabundance of carbon, overabundance ofnitrogen, and "normal" abundance of oxygen were detected. A small sodiumoverabundance was found. A possibility of a selection of the clumpgiants based on their chemical composition and the evolutionary trackswas explored. Conclusions: .The theoretical predictions based onthe classical stellar evolution models are in good agreement with theobserved surface variations of the carbon and nitrogen just after thefirst dredge-up episode. The giants show the same behaviour of thedependencies of O, Mg, Ca, and Si (α-elements) and Ni (iron-peakelement) abundances vs. [Fe/H] as dwarfs do. This allows us to use suchabundance ratios to study the chemical and dynamical evolution of theGalaxy.
| 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.
| Statistical Constraints for Astrometric Binaries with Nonlinear Motion
Useful constraints on the orbits and mass ratios of astrometric binariesin the Hipparcos catalog are derived from the measured proper motiondifferences of Hipparcos and Tycho-2 (Δμ), accelerations ofproper motions (μ˙), and second derivatives of proper motions(μ̈). It is shown how, in some cases, statistical bounds can beestimated for the masses of the secondary components. Two catalogs ofastrometric binaries are generated, one of binaries with significantproper motion differences and the other of binaries with significantaccelerations of their proper motions. Mathematical relations betweenthe astrometric observables Δμ, μ˙, and μ̈ andthe orbital elements are derived in the appendices. We find a remarkabledifference between the distribution of spectral types of stars withlarge accelerations but small proper motion differences and that ofstars with large proper motion differences but insignificantaccelerations. The spectral type distribution for the former sample ofbinaries is the same as the general distribution of all stars in theHipparcos catalog, whereas the latter sample is clearly dominated bysolar-type stars, with an obvious dearth of blue stars. We point outthat the latter set includes mostly binaries with long periods (longerthan about 6 yr).
| CHARM2: An updated Catalog of High Angular Resolution Measurements
We present an update of the Catalog of High Angular ResolutionMeasurements (CHARM, Richichi & Percheron \cite{CHARM}, A&A,386, 492), which includes results available until July 2004. CHARM2 is acompilation of direct measurements by high angular resolution methods,as well as indirect estimates of stellar diameters. Its main goal is toprovide a reference list of sources which can be used for calibrationand verification observations with long-baseline optical and near-IRinterferometers. Single and binary stars are included, as are complexobjects from circumstellar shells to extragalactic sources. The presentupdate provides an increase of almost a factor of two over the previousedition. Additionally, it includes several corrections and improvements,as well as a cross-check with the valuable public release observationsof the ESO Very Large Telescope Interferometer (VLTI). A total of 8231entries for 3238 unique sources are now present in CHARM2. Thisrepresents an increase of a factor of 3.4 and 2.0, respectively, overthe contents of the previous version of CHARM.The catalog is only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/431/773
| Seismic constraints on open clusters
The aim of this theoretical and modelling paper is to derive knowledgeon the global and structural parameters of low-mass stars usingasteroseismology and taking advantage of the stellar collective behaviorwithin open clusters. We build stellar models and compute the seismicsignal expected from main sequence objects in the 0.8-1.6Mȯ range. We first evaluate apparent magnitudes andoscillations-induced luminosity fluctuations expected in the Hyades, thePleiades and the α Persei clusters. The closest cluster presents afeasible challenge to observational asteroseismology in the present andnear future. The remainder of the work therefore focuses on the Hyades.We combine seismological and classical computations to address threequestions: what can be inferred about 1) mass; 2) composition; and 3)extension of outer convection zones of solar analogs in the Hyades. Thefirst issue relies on the strong sensitivity of the large separation tomass. We show that seismic constraints provide masses to a precisionlevel (0.05 Mȯ) that is competitive with the actualmass estimations from binary systems. Then large separations (Δν) and second differences (δ2 ν) are used torespectively constrain metal and helium fractions in the Hyades. Whenplotted for several masses, the relation of effective temperature(Teff) vs. large separation (Δ ν) is found to bestrongly dependent on the metal content. Besides this the seconddifference main modulation is related to the second ionization ofhelium. An accuracy in the helium mass fraction of 0.02 to 0.01 can beachieved provided mass and age are accurately known, which is the casefor a few Hyades binary systems. The second difference modulations arealso partly due to the discontinuity in stellar stratification at theconvective envelope/radiative core transition. They permit directinsight in the stellar structure. We compute acoustic radii of theconvective bases for different values of the mixing length theoryparameter αMLT in convection modelling, i.e. differentconvective efficiency in the superadiabatic layers. For a giveneffective temperature we show that the acoustic radius changes withconvection efficiency. This suggests that seismology can provideconstraints on the extension of outer convection and also more generallyon the direct approaches of convection and dynamical phenomena beingcurrently developed.
| The coronae of bright late-type stars observed with EPIC and RGS
X-ray bright late-type stars have been selected as targets of XMM-Newtonobservations, with the aim to study in detail the plasma thermaldistributions and chemical abundances of their coronae, and thecharacteristics of their X-ray emission variability. Bothhigh-resolution spectra with RGS and high signal-to-noisemedium-resolution EPIC spectra have been employed to this aim. I presentsome representative cases of such studies.
| XMM-Newton EPIC observations of stellar clusters and star forming regions
We report on observations of open clusters (OCs) and star formingregions (SFRs) obtained with the EPIC camera as part of the MissionScientist Guaranteed Time on XMM-Newton. These observations provide apowerful tool to investigate the evolution of coronal activity inlate-type convective stars and its dependence on magnetic fieldgeneration by dynamo processes. We discuss the motivations for thisprogram and present some results for the SFRs sigma Orionis (2-5Myr) and Taurus-Auriga (1-10 Myr) as well as for the OCs IC 2602(30 Myr), alpha Persei (50 Myr), Praesepe (600 Myr) andthe Hyades (600 Myr). We discuss imaging and spectral data providedby the EPIC MOS and PN detectors focussing on the determination of thecluster X-ray luminosity function and of the temperature structure,chemical abundances and time variability of cluster stars.Based on observations collected with the ESA mission XMM-Newton as partof the Mission Scientist (R. Pallavicini) Guaranteed Time
| The Indo-US Library of Coudé Feed Stellar Spectra
We have obtained spectra for 1273 stars using the 0.9 m coudéfeed telescope at Kitt Peak National Observatory. This telescope feedsthe coudé spectrograph of the 2.1 m telescope. The spectra havebeen obtained with the no. 5 camera of the coudé spectrograph anda Loral 3K×1K CCD. Two gratings have been used to provide spectralcoverage from 3460 to 9464 Å, at a resolution of ~1 Å FWHMand at an original dispersion of 0.44 Å pixel-1. For885 stars we have complete spectra over the entire 3460 to 9464 Åwavelength region (neglecting small gaps of less than 50 Å), andpartial spectral coverage for the remaining stars. The 1273 stars havebeen selected to provide broad coverage of the atmospheric parametersTeff, logg, and [Fe/H], as well as spectral type. The goal ofthe project is to provide a comprehensive library of stellar spectra foruse in the automated classification of stellar and galaxy spectra and ingalaxy population synthesis. In this paper we discuss thecharacteristics of the spectral library, viz., details of theobservations, data reduction procedures, and selection of stars. We alsopresent a few illustrations of the quality and information available inthe spectra. The first version of the complete spectral library is nowpublicly available from the National Optical Astronomy Observatory(NOAO) via ftp and http.
| Synthetic Lick Indices and Detection of α-enhanced Stars. II. F, G, and K Stars in the -1.0 < [Fe/H] < +0.50 Range
We present an analysis of 402 F, G, and K solar neighborhood stars, withaccurate estimates of [Fe/H] in the range -1.0 to +0.5 dex, aimed at thedetection of α-enhanced stars and at the investigation of theirkinematical properties. The analysis is based on the comparison of 571sets of spectral indices in the Lick/IDS system, coming from fourdifferent observational data sets, with synthetic indices computed withsolar-scaled abundances and with α-element enhancement. We useselected combinations of indices to single out α-enhanced starswithout requiring previous knowledge of their main atmosphericparameters. By applying this approach to the total data set, we obtain alist of 60 bona fide α-enhanced stars and of 146 stars withsolar-scaled abundances. The properties of the detected α-enhancedand solar-scaled abundance stars with respect to their [Fe/H] values andkinematics are presented. A clear kinematic distinction betweensolar-scaled and α-enhanced stars was found, although a one-to-onecorrespondence to ``thin disk'' and ``thick disk'' components cannot besupported with the present data.
| Empirically Constrained Color-Temperature Relations. I. BV(RI)C
This investigation presents a set of transformations to Johnson B-V,Cousins V-R, and Cousins V-I, as well as bolometric corrections to V,for [Fe/H]=-3, -2, -1, -0.5, 0.0, and +0.3 and, in each case, values oflogg from -0.5 to 5.0 for 3000 K<=Teff<=5500 K and from2.0 to 5.0 for 6000 K<=Teff<=40,000 K. Thesetransformations employ the predictions from Kurucz model atmospheres athigh temperatures (Teff>=8000 K) and from MARCS modelatmospheres at intermediate temperatures (from 7000 K down to atemperature in the range 4000 K<=Teff<=5500 K,depending on [Fe/H], where adjustments to satisfy observationalconstraints become necessary). Thus, theoretical color-Teffrelations are used exclusively down to a minimum temperature that iscooler than the temperatures of turnoff stars in open and globular starclusters. To better represent the color transformations obeyed by coolstars (down to 3000 K), corrections to the synthetic transformationshave been determined from a careful consideration of observations for afew globular clusters (M92, M68, and 47 Tucanae), the color-magnitudediagrams (CMDs) of several open clusters (M67, the Pleiades, the Hyades,and NGC 6791), the CMDs and mass-luminosity diagram for solarneighborhood stars having good distance measurements from Hipparcos,empirical (B-V)-Teff and (V-K)-Teff relations, andcolor-color diagrams for field giants. The semiempirical colortransformations that have been produced as a result of our analysis arealso compared with several others that have been published in recentyears: some of the deficiencies of the latter are revealed.
| Catalogue of averaged stellar effective magnetic fields. I. Chemically peculiar A and B type stars
This paper presents the catalogue and the method of determination ofaveraged quadratic effective magnetic fields < B_e > for 596 mainsequence and giant stars. The catalogue is based on measurements of thestellar effective (or mean longitudinal) magnetic field strengths B_e,which were compiled from the existing literature.We analysed the properties of 352 chemically peculiar A and B stars inthe catalogue, including Am, ApSi, He-weak, He-rich, HgMn, ApSrCrEu, andall ApSr type stars. We have found that the number distribution of allchemically peculiar (CP) stars vs. averaged magnetic field strength isdescribed by a decreasing exponential function. Relations of this typehold also for stars of all the analysed subclasses of chemicalpeculiarity. The exponential form of the above distribution function canbreak down below about 100 G, the latter value representingapproximately the resolution of our analysis for A type stars.Table A.1 and its references are only available in electronic form atthe CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/407/631 and Tables 3 to 9are only available in electronic form at http://www.edpsciences.org
| Line Absorption as a Metallicity Index for Giant Stars
The fraction of light removed from a star's spectrum by the spectrallines, the line absorption, is shown to be a precise empirical indicatorof metallicity. We measured the line absorption in 89 class III giantstars in a 42.5 Å window between 6219.0 and 6261.5 Å andthen calibrated these values against published metallicities. We showthat the line absorption can be measured precisely enough to improve themetallicity precision about fivefold over the original calibrationmetallicities, reaching a precision of 0.01 dex in favorable cases.
| Capella: Separating the Giants
Images from the Faint Object Camera (FOC) on the Hubble Space Telescope(HST) are used to spatially separate the two giants of Capella (αAurigae; HD 34029) for the first time at ultraviolet wavelengths. Theimages were obtained with broadband filters that isolate the wavelengthregions 2500-3000 Å and 1300-1500 Å. The cool G8 giant isfound to be weaker than the hot G1 giant by factors of around 4 and 17,respectively, in these bands. The latter factor is largely due to themuch stronger G1 continuum at short wavelengths. No evidence is foundfor material lying between the two stars in the images. In addition, theobjective prisms of the FOC were used to obtain low-resolution spectrafrom 1200 to 3000 Å, allowing individual emission lines from eachstar to be spatially separated. Cool-to-hot star ratios for the emissionlines H I Lyα, O I λ1305, Si II λ1816, C IIλ1335, He II λ1640, and Si IV λ1393 are presented,showing that the cool giant is weaker than the hot giant by factors of5-10 in these lines. The O I emission is only a factor of 2.5 weaker inthe cool giant, most probably resulting from fluorescence in theextended atmosphere of the cool giant. The line ratios are compared withvalues derived from International Ultraviolet Explorer and HST/GoddardHigh Resolution Spectrograph spectra, which could separate the starsspectrally but not spatially. Reasonable agreement is found although theFOC ratios generally imply lower contributions from the cool giant.Based on observations with the NASA/ESA Hubble Space Telescope obtainedat the Space Telescope Science Institute, which is operated by AURA,Inc., under NASA contract NAS 5-26555.
| Measuring starspots on magnetically active stars with the VLTI
We present feasibility studies to directly image stellar surfacefeatures, which are caused by magnetic activity, with the Very LargeTelescope Interferometer (VLTI). We concentrate on late typemagnetically active stars, for which the distribution of starspots onthe surface has been inferred from photometric and spectroscopic imaginganalysis. The study of the surface spot evolution during consecutiverotation cycles will allow first direct measurements (apart from theSun) of differential rotation which is the central ingredient ofmagnetic dynamo processes. The VLTI will provide baselines of up to 200m, and two scientific instruments for interferometric studies at near-and mid-infrared wavelengths. Imaging capabilities will be made possibleby closure-phase techniques. We conclude that a realistically modeledcool surface spot can be detected on stars with angular diametersexceeding ~ 2 mas using the VLTI with the first generation instrumentAMBER. The spot parameters can then be derived with reasonable accuracy.We discuss that the lack of knowledge of magnetically active stars ofthe required angular size, especially in the southern hemisphere, is acurrent limitation for VLTI observations of these surface features.
| CHARM: A Catalog of High Angular Resolution Measurements
The Catalog of High Angular Resolution Measurements (CHARM) includesmost of the measurements obtained by the techniques of lunaroccultations and long-baseline interferometry at visual and infraredwavelengths, which have appeared in the literature or have otherwisebeen made public until mid-2001. A total of 2432 measurements of 1625sources are included, along with extensive auxiliary information. Inparticular, visual and infrared photometry is included for almost allthe sources. This has been partly extracted from currently availablecatalogs, and partly obtained specifically for CHARM. The main aim is toprovide a compilation of sources which could be used as calibrators orfor science verification purposes by the new generation of largeground-based facilities such as the ESO Very Large Interferometer andthe Keck Interferometer. The Catalog is available in electronic form atthe CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/386/492, and from theauthors on CD-Rom.
| 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
| Lick Spectral Indices for Super-Metal-rich Stars
We present Lick spectral indices for a complete sample of 139 candidatesuper-metal-rich stars of different luminosity classes (MK type from Ito V). For 91 of these stars we were able to identify, in anaccompanying paper, the fundamental atmosphere parameters. This confirmsthat at least 2/3 of the sample consists of stars with [Fe/H] in excessof +0.1 dex. Optical indices for both observations and fiducialsynthetic spectra have been calibrated to the Lick system according toWorthey et al. and include the Fe I indices of Fe5015, Fe5270, andFe5335 and the Mg I and MgH indices of Mg2 and Mg b at 5180Å. The internal accuracy of the observations is found to beσ(Fe5015)=+/-0.32 Å, σ(Fe5270)=+/-0.19 Å,σ(Fe5335)=+/-0.22 Å, σ(Mg2)=+/-0.004 mag,and σ(Mg b)=+/-0.19 Å. This is about a factor of 2 betterthan the corresponding theoretical indices from the synthetic spectra,the latter being a consequence of the intrinsic limitations in the inputphysics, as discussed by Chavez et al. By comparing models andobservations, we find no evidence for nonstandard Mg versus Fe relativeabundance, so [Mg/Fe]=0, on the average, for our sample. Both theWorthey et al. and Buzzoni et al. fitting functions are found tosuitably match the data and can therefore confidently be extended forpopulation synthesis application also to supersolar metallicity regimes.A somewhat different behavior of the two fitting sets appears, however,beyond the temperature constraints of our stellar sample. Its impact onthe theoretical output is discussed, as far as the integratedMg2 index is derived from synthesis models of stellaraggregates. A two-index plot, such as Mg2 versus Fe5270, isfound to provide a simple and powerful tool for probing distinctiveproperties of single stars and stellar aggregates as a whole. The majoradvantage, over a classical CM diagram, is that it is both reddeningfree and distance independent. Based on observations collected at theInstituto Nacional de Astrofísica, Optica y Electrónica(INAOE) ``G. Haro'' Observatory, Cananea (Mexico).
| Line-Depth Ratios: Temperature Indices for Giant Stars
Ratios of the depths of appropriately chosen spectral lines are shown tobe excellent indicators of stellar temperatures for giant stars in theG3 to K3 spectral type range. We calibrate five line-depth ratiosagainst B-V and R-I color indices and then translate these intotemperatures. Our goal is to set up line-depth ratios to (1) accuratelymonitor any temperature variations of a few degrees or less that mayoccur during magnetic cycles or oscillations and (2) rank giantsprecisely on a temperature coordinate. This is not an absolutecalibration of stellar temperatures. We show how giant spectra can bemisleading because of the complex dependences of spectral lines onmetallicity and absolute magnitude as well as temperature, and it isessential to make corrections to accommodate these complications. Thefive line-depth ratios we use yield precision for monitoring, i.e.,detecting temperature variations, of 4 K from a single exposure. Rankinggiants by temperature can be done with errors of ~25 K but could beimproved with better determinations of the metallicity andabsolute-magnitude corrections.
| Absolute spectrophotometry of late-type stars.
Not Available
| The ASCA Medium Sensitivity Survey (the GIS Catalog Project): Source Catalog
We present the first X-ray source catalog of the ASCA Medium SensitivitySurvey (AMSS, or the GIS catalog project), constructed from data atGalactic latitudes b>10deg observed between 1993 May and 1996December. The catalog utilizes 368 combined fields and contains 1343sources with the detection significance above 5 σ either in thesurvey bands of 0.7-7 keV, 2-10 keV, or 0.7-2 keV, including targetsources. For each source, the ASCA source name, position, a 90% errorradius, count rates in the three bands, detection significances, fluxes,and a hardness ratio are provided. With extensive simulations, wecarefully evaluate the data quality of the catalog. Results fromcross-correlation with other existing catalogs are briefly summarized.
| 3 Ms in the Life of β Ceti: Sustained Flare Activity on a Clump Giant Detected by the Extreme Ultraviolet Explorer
A 34 day Extreme Ultraviolet Explorer (EUVE) pointing on the ``clump''giant β Ceti (HD 4128; K0 III) recorded a series of strikingcoronal flare events, reminiscent of a singular outburst seen previouslyfrom μ Velorum (HD 93497; G6 III + dF). The recent flaring episodecontrasts with a more placid behavior in a 6 day EUVE observation ofβ Cet 6 years earlier. The average 70-180 Å Deep Survey countrate in the new observation is twice as high, and the 75-150 Åspectrum displays a distinct hardening. The discovery of sustained flareactivity on β Cet raises the possibility that such episodes aremore common than suspected among the core helium-burning giants andsharpens the puzzle of the survival of magnetic activity beyond heliumflash.
| On the Wilson-Bappu relationship in the Mg II k line
An investigation is carried out on the Wilson-Bappu effect in the Mg Iik line at 2796.34 Å. The work is based on a selection of 230 starsobserved by both the IUE and HIPPARCOS satellites, covering a wide rangeof spectral types (F to M) and absolute visual magnitudes (-5.4<=MV <=9.0). A semi-automatic procedure is used to measurethe line widths, which applies also in the presence of strong centralabsorption reversal. The Wilson-Bappu relationship here provided isconsidered to represent an improvement over previous recent results forthe considerably larger data sample used, as well as for a properconsideration of the measurement errors. No evidence has been found fora possible dependence of the WB effect on stellar metallicity andeffective temperature.
| Lithium in the intermediate age cluster NGC 3680: Following Li evolution along the C-M diagram
We present an analysis of high resolution spectroscopic observations (R~ 30 000, S/N=60-150) of 24 members of the intermediate age ( ~ 1.5 Gyr)open cluster NGC 3680, covering all regions of the clustercolour-magnitude (C-M) diagram where cluster members are known to exist.These observations represent in many aspects challenges to ourunderstanding of stellar interior and mixing. Four main sequence G starshave, within the errors, the same Li abundance, 0.3 dex lower thansimilar stars in the ~ 1 Gyr younger Hyades but comparable with thoseobserved in the coeval cluster IC 4651. The clustershows a clear Li-dip located around the turn-off; two stars on the upperpart of the turn-off are out of the dip and reach solar systemmeteoritic Li abundances. Just above the turn-off, in a very small rangeof magnitudes ( ~ 0.2 in V), a factor of ~ 5 Li depletion occurs. Thissudden decrease explains puzzling results recently obtained on fieldsubgiants but it is not at all reproduced by standard (e.g. no rotation,no diffusion) models, whereas it is in somewhat better agreement withthe predictions of recent models which include rotational mixing andatomic diffusion. Out of the six cluster giants, one is probably abinary; of the remaining five single cluster members, three have a Liabundance log n(Li) ~ 1.1 while two have Li abundances from a factor 6to more than a factor 30 lower than the other three. The star with nodetected Li is the coolest and most luminous object in the sample and ismost likely an AGB star; the other has instead a similar magnitude andeffective temperature as the three more Li rich giants. The reasons forthis difference in Li abundance among otherwise similar stars can beascribed either to differential depletion during main-sequence orpost-main sequence evolution, possibly induced by rotation, or todifferences in the evolutionary status of these evolved stars. Bycomparing our results with those found for clusters of similar age andfor field stars, we find that none of the possible scenarios gives afully satisfactory explanation if the present population of NGC 3680giants reflect the expected ratio of clump vs. first-ascent RGB stars.If the more abundant Li-rich giants in NGC 3680 are indeed clump giants,their relatively high Li content requires that Li is produced, orbrought to the surface, between the tip of the RGB and the clump, whichis not consistent with observations of the similar age cluster NGC 752,where the more abundant, presumably clump giants have low Li abundances.Finally, we have used our spectra to determine the metallicity of thecluster giants, finding [Fe/H]=-0.17+/-0.12. This value is in very goodagreement with that derived from spectral indexes analysis, butsubstantially lower than the value inferred from Strömgrenphotometry. Based on observations collected at ESO, la Silla, and at theVLT.
| The helium content and age of the Hyades:. Constraints from five binary systems and Hipparcos parallaxes
We compare the accurate empirical mass-luminosity (M-L) relation basedon five Hyades binary systems to predictions of stellar modelscalculated with various input parameters (helium, metallicity and age)or physics (mixing-length ratio, model atmosphere, equation of state andmicroscopic diffusion). Models based on a helium content Y ~ 0.28inferred from the Delta Y/Delta Z enrichment law are more than 3sigmabeyond the observations, suggesting that the Hyades initial heliumabundance is lower than expected from its supersolar metallicity. Withthe photometric metallicity ([Fe/H] = 0.144+/- 0.013 dex, Grenon\cite{gre00}) we derive Y=0.255+/-0.009. Because of the (Y, [Fe/H])degeneracy in the M-L plane, the uncertainty grows to Delta Y=0.013 ifthe metallicity from spectroscopy is adopted ([Fe/H] = 0.14+/-0.05 dex,Cayrel de Strobel et al. \cite{cayG97}). We use these results to discussthe Hertzsprung-Russell (HR) diagram of the Hyades, in the(MV, B-V) plane, based on the very precise Hipparcosdynamical parallaxes. Present models fit the tight observed sequencevery well except at low temperatures. We show that the HR diagram doesnot bring further constraints on the helium abundance or metallicity ofthe cluster. In the low mass region of the HR diagram sensitive to themixing-length parameter (alpha MLT), the slope of the mainsequence (MS) suggests that alpha MLT could decrease from asolar (or even supersolar) value at higher mass to subsolar values atlow mass, which is also supported by the modeling of the vB22 M-Lrelation. We find that the discrepancy at low temperatures ((B-V)>≈ 1.2) remains, even if an improved equation of state or bettermodel atmospheres are used. Finally, we discuss the positions of thestars at turn-off in the light of their observed rotation rates and wededuce that the maxiμm age of the Hyades predicted by the presentmodels is ~650 Myr.
| Catalogue of Apparent Diameters and Absolute Radii of Stars (CADARS) - Third edition - Comments and statistics
The Catalogue, available at the Centre de Données Stellaires deStrasbourg, consists of 13 573 records concerning the results obtainedfrom different methods for 7778 stars, reported in the literature. Thefollowing data are listed for each star: identifications, apparentmagnitude, spectral type, apparent diameter in arcsec, absolute radiusin solar units, method of determination, reference, remarks. Commentsand statistics obtained from CADARS are given. The Catalogue isavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcar?J/A+A/367/521
| Research Note Hipparcos photometry: The least variable stars
The data known as the Hipparcos Photometry obtained with the Hipparcossatellite have been investigated to find those stars which are leastvariable. Such stars are excellent candidates to serve as standards forphotometric systems. Their spectral types suggest in which parts of theHR diagrams stars are most constant. In some cases these values stronglyindicate that previous ground based studies claiming photometricvariability are incorrect or that the level of stellar activity haschanged. Table 2 is only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/367/297
| 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.
|
Sottometti un nuovo articolo
Link relazionati
Sottometti un nuovo link
Membro dei seguenti gruppi:
|
Osservazione e dati astrometrici
| Costellazione: | Toro |
| Ascensione retta: | 04h28m34.50s |
| Declinazione: | +15°57'44.0" |
| Magnitudine apparente: | 3.84 |
| Distanza: | 48.403 parsec |
| Moto proprio RA: | 107.4 |
| Moto proprio Dec: | -24.3 |
| B-T magnitude: | 5.039 |
| V-T magnitude: | 3.936 |
Cataloghi e designazioni:
|
