Contents
Images
Upload your image
DSS Images Other Images
Related articles
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.
| Shapes of Spectral Line Bisectors for Cool Stars
The shape of the line bisector for the prototype spectral line Fe Iλ6253 was measured for an array of 54 stars on the cool half ofthe HR diagram. These bisectors are given in tables along with theirerrors. The classic C shape is shown by only a rather restricted rangein effective temperature and luminosity. The detailed change in bisectorshape with effective temperature and luminosity is documented moreprecisely than in previous work. The most blueward point on the bisectorchanges its height systematically with luminosity and can be used as aluminosity or gravity discriminant. The wide range of bisector shapescontains significant information about the velocity fields in theatmospheres of these stars, but extracting that information may requireextensive modeling.
| Analysis of the Na, Mg, Al, and Si Abundances in the Atmospheres of Red Giants of Different Spectral Subgroups
We analyze the Na, Mg, Al, and Si abundances in the atmospheres of morethan 40 stars, includingred giants of different spectral subgroups(normal red giants, mild and classical barium stars) and severalsupergiants. All these elements exhibit abundance excesses, with theoverabundance increasing with the star’s luminosity. Thedependence of the overabundances for each of these elements on theluminosity (or log g) is the same for all the spectral subgroups,testifying to a common origin: they are all products of hydrogen burningin the NeNa and MgAl cycles that have been dredged up from the stellarinteriors to the outer atmospheric layers by convection that graduallydevelops during the star’s evolution from the main sequence to thered-giant stage. The sodium abundances derived for several stars arelower than for other stars with similar atmospheric parameters. The agesand kinematic characteristics of these two groups of stars suggest thatthey probably belong to different stellar generations.
| 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.
| Survey for Transiting Extrasolar Planets in Stellar Systems. II. Spectrophotometry and Metallicities of Open Clusters
We present metallicity estimates for seven open clusters based onspectrophotometric indices from moderate-resolution spectroscopy.Observations of field giants of known metallicity provide a correlationbetween the spectroscopic indices and the metallicity of open clustergiants. We use χ2 analysis to fit the relation ofspectrophotometric indices to metallicity in field giants. The resultingfunction allows an estimate of the target-cluster giants' metallicitieswith an error in the method of +/-0.08 dex. We derive the followingmetallicities for the seven open clusters: NGC 1245, [M/H]=-0.14+/-0.04NGC 2099, [M/H]=+0.05+/-0.05 NGC 2324, [M/H]=-0.06+/-0.04 NGC 2539,[M/H]=-0.04+/-0.03 NGC 2682 (M67), [M/H]=-0.05+/-0.02 NGC 6705,[M/H]=+0.14+/-0.08 NGC 6819, [M/H]=-0.07+/-0.12. These metallicityestimates will be useful in planning future extrasolar planet transitsearches, since planets may form more readily in metal-richenvironments.
| 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
| Heavy element abundances in AGB stars . The Hyades giants
This research summarises our methodology for obtaining equivalent widthsand abundances in giant stars. As an initial study, two of the fourgiants from the Hyades cluster have been analysed for heavy elementabundances, specifically s-process elements. This paper displays theresults obtained for HD28305 and HD27371, with most heavy elementabundances proving to be the same in both stars and similar to thoseobtained in previous studies. The extension of these techniques to M, Sand C stars is also discussed.
| Determination of fundamental characteristics for stars of the F, G, and K spectral types. The surface gravities and metallicity parameters.
Not Available
| 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.
| Precise radial velocity measurements of G and K giants. Multiple systems and variability trend along the Red Giant Branch
We present the results of our radial velocity (RV) measurements of G andK giants, concentrating on the presence of multiple systems in oursample. Eighty-three giants have been observed for 2.5 years with thefiber-fed echelle spectrograph FEROS at the 1.52 m ESO telescope in LaSilla, Chile. Seventy-seven stars (93%) of the targets have beenanalyzed for RV variability using simultaneous Th-Ar calibration and across-correlation technique. We estimate the long-term precision of ourmeasurement as better than 25 m s-1. Projected rotationalvelocities have been measured for most stars of the sample. Within ourtime-base only 21 stars (or 27%) show variability below 2\sigma, whilethe others show RV variability with amplitudes up to several kms-1. The large amplitude (several km s-1) andshape (high eccentricity) of the RV variations for 11 of the programstars are consistent with stellar companions, and possibly brown dwarfcompanions for two of the program stars. In those systems for which afull orbit could be derived, the companions have minimum masses from0.6 M\sun down to 0.1 M\sun. To thesemultiple systems we add the two candidates of giant planets alreadydiscovered in the sample. This analysis shows that multiple systemscontribute substantially to the long-term RV variability of giant stars,with about 20% of the sample being composed of multiple systems despitescreening our sample for known binary stars. After removing binaries,the range of RV variability in the whole sample clearly decreases, butthe remaining stars retain a statistical trend of RV variability withluminosity: luminous cool giants with B-V≥1.2 show RV variationswith \sigma_{/lineRV} > 60 m s-1, while giants with B-V< 1.2 including those in the clump region exhibit less variability orthey are constant within our accuracy. The same trend is observed withrespect to absolute visual magnitudes: brighter stars show a largerdegree of variability and, when plotted in the RV variability vs.magnitude diagram a trend of increasing RV scatter with luminosity isseen. The amplitude of RV variability does not increase dramatically, aspredicted, for instance, by simple scaling laws. At least two luminousand cooler stars of the sample show a correlation between RV andchromospheric activity and bisector asymmetry, indicating that in thesetwo objects RV variability is likely induced by the presence of(chromospheric) surface structures.Based on observations collected at the 1.52 m-ESO telescope at the LaSilla Observatory from Oct 1999 to Feb. 2002 under ESO programs and theESO-Observatório Nacional, Brazil, agreement and in part onobservations collected on the Alfred Jensch 2 m telescope of theThüringer Landessternwarte Tautenburg.
| 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.
| Empirical calibration of the near-infrared Ca II triplet - III. Fitting functions
Using a near-infrared stellar library of 706 stars with a wide coverageof atmospheric parameters, we study the behaviour of the CaII tripletstrength in terms of effective temperature, surface gravity andmetallicity. Empirical fitting functions for recently definedline-strength indices, namely CaT*, CaT and PaT, are provided. Thesefunctions can be easily implemented into stellar population models toprovide accurate predictions for integrated CaII strengths. We alsopresent a thorough study of the various error sources and their relationto the residuals of the derived fitting functions. Finally, the derivedfunctional forms and the behaviour of the predicted CaII are comparedwith those of previous works in the field.
| 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.
| X-ray Emission from the Clump Giant Gamma Tau
We report the analysis of the XMM-Newton spectrum of the Hyades clumpgiant Gamma Tau. This observation of one of the late-type Hyades giantshas implications for understanding the formation of late-type stellarcoronae as a function of the evolutionary state of the star. The RGSspectrum was obtained 13 March 2001 for a total exposure of 58.2 ks;however, background from solar flare activity compromises nearly halfthe data. The spectrum shows strong lines of Fe XVII and XVIII, O VIIand VIII, and Ne IX and X. Other weaker lines are also apparent. Wepresent the emission measure distribution and elemental abundances, anddiscuss the comparison with other clump giant spectra. This project wasfunded by NASA NAG-9986.
| The extreme ultraviolet excess emission in five clusters of galaxies revisited
Evidence for excess extreme ultraviolet (EUV) emission over a tail ofX-ray gas bremsstrahlung emission has been building up recently, but insome cases remains controversial, mostly due to the moderate quality ofthe EUV data. In order to improve the signal to noise ratio in the EUV,we have performed the wavelet analysis and image reconstructions forfive clusters of galaxies observed both at EUV and X-ray energies withthe EUVE and ROSAT satellites respectively. The profiles of the EUV andX-ray reconstructed images all differ at a very large confidence leveland an EUV excess over a thermal bremsstrahlung tail is detected in allfive clusters (Abell 1795, Abell 2199, Abell 4059, Coma and Virgo) up tolarge radii. These results, coupled with recent XMM-Newton observations,suggest that the EUV excess is probably non thermal in origin.
| Determination of accurate stellar radial-velocity measures
Wavelength measurements in stellar spectra cannot readily be interpretedas true stellar motion on the sub-km s-1 accuracy level dueto the presence of many other effects, such as gravitational redshiftand stellar convection, which also produce line shifts. Following arecommendation by the IAU, the result of an accurate spectroscopicradial-velocity observation should therefore be given as the``barycentric radial-velocity measure'', i.e. the absolute spectralshift as measured by an observer at zero gravitational potential locatedat the solar-system barycentre. Standard procedures for reducingaccurate radial-velocity observations should be reviewed to take intoaccount this recommendation. We describe a procedure to determineaccurate barycentric radial-velocity measures of bright stars, based ondigital cross-correlation of spectra obtained with the ELODIEspectrometer (Observatoire de Haute-Provence) with a synthetic templateof Fe I lines. The absolute zero point of the radial-velocity measuresis linked to the wavelength scale of the Kurucz (1984) Solar Flux Atlasvia ELODIE observations of the Moon. Results are given for the Sun and42 stars, most of them members of the Hyades and Ursa Major clusters.The median internal standard error is 27 m s-1. The externalerror is estimated at around 120 m s-1, mainly reflecting theuncertainty in the wavelength scale of the Solar Flux Atlas. For the Sunwe find a radial-velocity measure of +257+/- 11 m s-1referring to the full-disk spectrum of the selected Fe I lines. Based onobservations made at Observatoire de Haute-Provence
| 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
| A Comparative Analysis of Chemical Abundances in the Atmospheres of Red Giants of Different Age Groups
We analyze previously published chemical abundances in the atmospheresof red giants. Excess abundances are observed not only for Na, but alsofor Al and Si, with the overabundances increasing with the stars’luminosity. The observed anomalies provide evidence that, in addition tothe CNO hydrogen-burning cycle, the Mg-Al and Ne-Na cycles operate inthe interiors of main-sequence stars; their products are brought to thestellar atmospheres by convection after the transition to the red-giantphase. The abundance anomalies for s-process elements, also observed inthe atmospheres of field stars, testify to the presence of a substantialnumber of neutrons. The s-process abundance anomalies are absent fromgiants of the young Hyades cluster.
| 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.
| Accurate Vsin i measurements in M 67: The angular momentum evolution of 1.2 Msun stars
By using FEROS spectrograph commissioning observations, we build acalibration of the FEROS cross-correlation function (CCF) to determineaccurate projected rotational velocities Vsin i for slow rotating F-Kdwarf and giant stars. We apply this calibration to a sample of 28 mainsequence, turnoff and giant stars belonging to the old open cluster M67. We find that the stars behave in a very regular manner, depending ontheir position in the Color-Magnitude (C-M) diagram. Early main sequenceG stars have a rotational velocity two times larger than the Sun, andthey show a possible trend with (B-V) color, in that redder colorscorrespond to lower Vsin i. The stars at the turnoff are the fastestrotators, with Vsin i between 6.3 and 7.6 km s-1, while starsjust above the turn-off are already significantly slower , with valuesbetween 4.6 and 4.9 km s-1. Along the Red Giant Branch (RGB),rotation decreases smoothly and for stars above (B-V)ga1 , only upperlimits can be found, including for 4 clump stars. Analyzing the angularmomentum history of 1.2 Msun stars with the help oftheoretical evolutionary tracks, we see that these stars probably obeydifferent angular momentum evolution laws on the main sequence and alongthe RGB: while on the main sequence some extra braking is required inaddition to angular momentum conservation, along the RGB the data arewell represented by the IOmega =C law. Finally, comparing the Vsin i ofthe M 67 turnoff stars with their main sequence progenitors in theyounger open clusters NGC 3680 and Hyades we find that the youngerclusters show substantially higher rotation rates. This indicates that1.2 Msun stars do experience main sequence braking. Thiscould be relevant also for the interpretation of the nature of the``Lithium gap''. Based on observations collected at ESO, La Silla.
| 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.
|
Submit a new article
Related links
Submit a new link
Member of following groups:
|
Observation and Astrometry data
| Constellation: | Stier |
| Right ascension: | 04h19m47.60s |
| Declination: | +15°37'39.0" |
| Apparent magnitude: | 3.65 |
| Distance: | 47.237 parsecs |
| Proper motion RA: | 116 |
| Proper motion Dec: | -22.7 |
| B-T magnitude: | 4.885 |
| V-T magnitude: | 3.75 |
Catalogs and designations:
|
