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Ýlgili Makaleler
The chemical composition of R-stars
The preliminary results of the chemical analysis of a sample of 22galactic R-stars with measured parallaxes are presented. We have derivedthe C/O and 12C/13C ratios, the averagemetallicity ([M/H]) and the Li abundances. We find that most of thestars have 12C/13C below 20 with nosignificant difference between cool and hot R-stars. The C/O ratioranges between 1 to 2. We obtain a clear separation in the Liabundance between hot and cool R stars, with a mean value of log epsilon(Li) = +0.90 and log epsilon (Li) =-0.50, respectively. Cool stars areof near solar metallicity whereas hot stars show a larger spread,-0.5< [Fe/H] < +0.1. Concerning the possibility of s-elementenhancements, we obtain [Rb/M] > 0 in seven hot stars and in three ofthem (R-hot) the analysis of the 5924 {Å} TcI line might becompatible with Tc detection. However, the detection of other s-elementenhancements is necessary to elucidate the evolutionary stage of thesecarbon stars of which origin is still unknown.
| Polarimetry of Li-rich giants
Context: .Protoplanetary nebulae typically present non-sphericalenvelopes. The origin of such geometry is still controversial. There areindications that it may be carried over from an earlier phase of stellarevolution, such as the AGB phase. But how early in the star's evolutiondoes the non-spherical envelope appear? Aims.Li-rich giants show dustycircumstellar envelopes that can help answer that question. We study asample of fourteen Li-rich giants using optical polarimetry in order todetect non-spherical envelopes around them. Methods.We used the IAGPOLimaging polarimeter to obtain optical linear polarization measurementsin {V} band. Foreground polarization was estimated using the field starsin each CCD frame.Results.After foreground polarization was removed,seven objects presented low intrinsic polarization (0.19-0.34)% and two(V859 Aql and GCSS 557) showedhigh intrinsic polarization values (0.87-1.16)%. This intrinsicpolarization suggests that Li-rich giants present a non-sphericaldistribution of circumstellar dust. The intrinsic polarization level isprobably related to the viewing angle of the envelope, with higherlevels indicating objects viewed closer to edge-on. The correlation ofthe observed polarization with optical color excess gives additionalsupport to the circumstellar origin of the intrinsic polarization inLi-rich giants. The intrinsic polarization correlates even better withthe IRAS 25 μ m far infrared emission. Analysis of spectral energydistributions for the sample show dust temperatures for the envelopesthat tend to be between 190 and 260 K. We suggest that dust scatteringis indeed responsible for the optical intrinsic polarization in Li-richgiants.Conclusions.Our findings indicate that non-spherical envelopesmay appear as early as the red giant phase of stellar evolution.
| Three Li-rich K Giants: IRAS 12327-6523, 13539-4153, and 17596-3952
We report on spectroscopic analyses of three K giants previouslysuggested to be Li-rich: IRAS 12327-6523, 13539-4153, and 17596-3952.High-resolution optical spectra and the LTE model atmospheres are usedto derive the stellar parameters (Teff, logg, [Fe/H]),elemental abundances, and the isotopic ratio12C/13C. IRAS 13539-4153 shows an extremely highLi abundance of logɛ(Li)~4.2, a value 10 times greater than thepresent Li abundance in the local interstellar medium. This is the thirdhighest Li abundance yet reported for a K giant. IRAS 12327-6523 shows aLi abundance of logɛ(Li)~1.4. IRAS 17596-3952 is a rapidlyrotating (Vsini~35 km s-1) K giant with logɛ(Li)~2.2.Infrared photometry shows the presence of an IR excess, suggesting massloss. A comparison is made between these three stars and previouslyrecognized Li-rich giants.
| On the nature of lithium-rich giant stars. Constraints from beryllium abundances
We have derived beryllium abundances for 7 Li-rich giant (A(Li) >1.5) stars and 10 other Li-normal giants with the aim of investigatingthe origin of the lithium in the Li-rich giants. In particular, we testthe predictions of the engulfment scenario proposed by Siess & Livio(1999, MNRAS, 308, 1133), where the engulfment of a brown dwarf or oneor more giant planets would lead to simultaneous enrichment of7Li and 9Be. We show that regardless of theirnature, none of the stars studied in this paper were found to havedetectable beryllium. Using simple dilution arguments we show thatengulfment of an external object as the sole source of Li enrichment isruled out by the Li and Be abundance data. The present results favor theidea that Li has been produced in the interior of the stars by aCameron-Fowler process and brought up to the surface by an extra mixingmechanism.
| Local kinematics of K and M giants from CORAVEL/Hipparcos/Tycho-2 data. Revisiting the concept of superclusters
The availability of the Hipparcos Catalogue has triggered many kinematicand dynamical studies of the solar neighbourhood. Nevertheless, thosestudies generally lacked the third component of the space velocities,i.e., the radial velocities. This work presents the kinematic analysisof 5952 K and 739 M giants in the solar neighbourhood which includes forthe first time radial velocity data from a large survey performed withthe CORAVEL spectrovelocimeter. It also uses proper motions from theTycho-2 catalogue, which are expected to be more accurate than theHipparcos ones. An important by-product of this study is the observedfraction of only 5.7% of spectroscopic binaries among M giants ascompared to 13.7% for K giants. After excluding the binaries for whichno center-of-mass velocity could be estimated, 5311 K and 719 M giantsremain in the final sample. The UV-plane constructed from these datafor the stars with precise parallaxes (σπ/π≤20%) reveals a rich small-scale structure, with several clumpscorresponding to the Hercules stream, the Sirius moving group, and theHyades and Pleiades superclusters. A maximum-likelihood method, based ona Bayesian approach, has been applied to the data, in order to make fulluse of all the available stars (not only those with precise parallaxes)and to derive the kinematic properties of these subgroups. Isochrones inthe Hertzsprung-Russell diagram reveal a very wide range of ages forstars belonging to these groups. These groups are most probably relatedto the dynamical perturbation by transient spiral waves (as recentlymodelled by De Simone et al. \cite{Simone2004}) rather than to clusterremnants. A possible explanation for the presence of younggroup/clusters in the same area of the UV-plane is that they have beenput there by the spiral wave associated with their formation, while thekinematics of the older stars of our sample has also been disturbed bythe same wave. The emerging picture is thus one of dynamical streamspervading the solar neighbourhood and travelling in the Galaxy withsimilar space velocities. The term dynamical stream is more appropriatethan the traditional term supercluster since it involves stars ofdifferent ages, not born at the same place nor at the same time. Theposition of those streams in the UV-plane is responsible for the vertexdeviation of 16.2o ± 5.6o for the wholesample. Our study suggests that the vertex deviation for youngerpopulations could have the same dynamical origin. The underlyingvelocity ellipsoid, extracted by the maximum-likelihood method afterremoval of the streams, is not centered on the value commonly acceptedfor the radial antisolar motion: it is centered on < U > =-2.78±1.07 km s-1. However, the full data set(including the various streams) does yield the usual value for theradial solar motion, when properly accounting for the biases inherent tothis kind of analysis (namely, < U > = -10.25±0.15 kms-1). This discrepancy clearly raises the essential questionof how to derive the solar motion in the presence of dynamicalperturbations altering the kinematics of the solar neighbourhood: doesthere exist in the solar neighbourhood a subset of stars having no netradial motion which can be used as a reference against which to measurethe solar motion?Based on observations performed at the Swiss 1m-telescope at OHP,France, and on data from the ESA Hipparcos astrometry satellite.Full Table \ref{taba1} is 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/430/165}
| 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.
| Hipparcos red stars in the HpV_T2 and V I_C systems
For Hipparcos M, S, and C spectral type stars, we provide calibratedinstantaneous (epoch) Cousins V - I color indices using newly derivedHpV_T2 photometry. Three new sets of ground-based Cousins V I data havebeen obtained for more than 170 carbon and red M giants. These datasetsin combination with the published sources of V I photometry served toobtain the calibration curves linking Hipparcos/Tycho Hp-V_T2 with theCousins V - I index. In total, 321 carbon stars and 4464 M- and S-typestars have new V - I indices. The standard error of the mean V - I isabout 0.1 mag or better down to Hp~9 although it deteriorates rapidly atfainter magnitudes. These V - I indices can be used to verify thepublished Hipparcos V - I color indices. Thus, we have identified ahandful of new cases where, instead of the real target, a random fieldstar has been observed. A considerable fraction of the DMSA/C and DMSA/Vsolutions for red stars appear not to be warranted. Most likely suchspurious solutions may originate from usage of a heavily biased color inthe astrometric processing.Based on observations from the Hipparcos astrometric satellite operatedby the European Space Agency (ESA 1997).}\fnmsep\thanks{Table 7 is onlyavailable 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/qcat?J/A+A/397/997
| Spectroscopic Study of IRAS 19285+0517 (PDS 100): A Rapidly Rotating Li-Rich K Giant
We report on photometry and high-resolution spectroscopy for IRAS19285+0517. The spectral energy distribution based on visible andnear-infrared photometry and far-infrared fluxes shows that the star issurrounded by dust at a temperature of Td~250 K. Spectralline analysis shows that the star is a K giant with a projectedrotational velocity of vsini=9+/-2 km s-1. We determined theatmospheric parameters: Teff=4500 K, logg=2.5,ξt=1.5 km s-1, and [Fe/H]=0.14 dex. The LTEabundance analysis shows that the star is Li-rich[logɛ(Li)=2.5+/-0.15], but with essentially normal C, N, O, andmetal abundances. Spectral synthesis of molecular CN lines yields thecarbon isotopic ratio 12C/13C=9+/-3, a signatureof post-main-sequence evolution and dredge-up on the red giant branch(RGB). Analysis of the Li resonance line at 6707 Å for different6Li/7Li ratios shows that the Li profile can bebest fitted with a predicted profile for pure 7Li.Far-infrared excess, high Li abundance, and rapid rotation suggest thata planet has been swallowed or, perhaps, that an instability in the RGBouter layers triggered a sudden enrichment of Li and caused mass loss.
| Reprocessing the Hipparcos data for evolved giant stars II. Absolute magnitudes for the R-type carbon stars
The Hipparcos Intermediate Astrometric Data for carbon stars have beenreprocessed using an algorithm which provides an objective criterion forrejecting anomalous data points and constrains the parallax to bepositive. New parallax solutions have been derived for 317 cool carbonstars, mostly of types R and N. In this paper we discuss the results forthe R stars. The most important result is that the early R stars (i.e.,R0 - R3) have absolute magnitudes and V-K colors locating them among redclump giants in the Hertzsprung-Russell diagram. The average absolutemagnitude MK for early R-type stars (with V - K < 4) hasbeen derived from a Monte-Carlo simulation implicitly incorporating allpossible biases. It appears that the simulated magnitude distributionfor a population with a true Gaussian distribution of mean MK= -2.0 and intrinsic standard deviation 1.0 mag provides a satisfactorymatch to the observed distribution. These values are consistent with theaverage absolute magnitude MK = -1.6 for clump red giants inthe solar neighborhood (Alves 2000). Further, early R-type stars arenon-variable, and their infrared photometric properties show that theyare not undergoing mass loss, properties similar to those of the redclump giants. Stars with subtypes R4 - R9 tend to be cooler and havesimilar luminosity to the N-type carbon stars, as confirmed by theirposition in the (J-H, H-K) color-color diagram. The sample of earlyR-type stars selected from the Hipparcos Catalogue appears to beapproximately complete to magnitude K0 ~ 7, translating intoa completeness distance of 600 pc if all R stars had MK= -2(400 pc if MK= -1). With about 30 early R-type stars in thatvolume, they comprise about 0.04% (0.14% for MK= -1) of thered clump stars in the solar neighborhood. Identification with the redclump locates these stars at the helium core burning stage of stellarevolution, while the N stars are on the asymptotic giant branch, wherehelium shell burning occurs. The present analysis suggests that for asmall fraction of the helium core burning stars (far lower than thefraction of helium shell-burning stars), carbon produced in the interioris mixed to the atmosphere in sufficient quantities to form a carbonstar. Based on observations from the Hipparcos astrometric satelliteoperated by the European Space Agency (ESA 1997).
| Two K Giants with Supermeteoritic Lithium Abundances: HDE 233517 and HD 9746
Two unusual Li-rich K giants, HDE 233517 and HD 9746, have been studied.Optical spectroscopy and photometry have been obtained to determine thefundamental parameters of HDE 233517, a single K2 III with an extremelylarge infrared excess. The spectra yield Teff=4475 K,logg=2.25, [Fe/H]=-0.37, vsini=17.6 km s-1, and a non-LTElogɛ(7Li)=4.22. Photometric observations reveallow-amplitude light variability with a period of 47.9 days. Combinedwith other parameters, this results in a minimum radius of 16.7Rsolar and minimum distance of 617 pc. Comparison of spectraobtained in 1994 and 1996 show profile variations in Hα and the NaD lines indicative of changing mass loss. Optical spectra of HD 9746, achromospherically active giant, were analyzed. The Teff=4400K and revised Hipparcos-based gravity of logg=2.30 lead to a non-LTElogɛ(7Li)=3.75. The Li abundances in both stars aresupermeteoritic. By the inclusion and exclusion of 6Li in thesyntheses, we show that consistent 7Li abundances areobtained only when 6Li is absent in the synthetic fit. Thisprovides evidence for fresh 7Li production and excludes bothpreservation of primordial Li and planetary accretion as viablescenarios for the formation of Li-rich giants. Both stars lie in closeproximity to the red giant luminosity bump supporting the hypothesisthat 7Li production is caused by the same mixing mechanismthat later results in CN processing and lowers the12C/13C ratio to nonstandard values.
| A Survey for Enhanced Lithium in 261 Globular Cluster Giants
A survey of 261 giants in the globular clusters M3, M13, M15, and M92carried out with the Hydra multiobject spectrograph on the WIYN 3.5 mtelescope has identified no stars with Li I 6707 Å resonance linesstronger than 20 mÅ. The limiting equivalent widths of the lithiumline provide upper limits of logɛ(Li)<+0.0 atTeff=4500 K and logɛ(Li)<+1.0 at Teff=5000K. The absence of lithium-rich stars in the sample suggests that thepercentage of giants exhibiting enhanced lithium abundances in globularclusters is low.
| Rotation and lithium in single giant stars
In the present work, we study the link between rotation and lithiumabundance in giant stars of luminosity class III, on the basis of alarge sample of 309 single stars of spectral type F, G and K. We havefound a trend for a link between the discontinuity in rotation at thespectral type G0III and the behavior of lithium abundances around thesame spectral type. The present work also shows that giant starspresenting the highest lithium contents, typically stars earlier thanG0III, are those with the highest rotation rates, pointing for adependence of lithium content on rotation, as observed for otherluminosity classes. Giant stars later than G0III present, as a rule, thelowest rotation rates and lithium contents. A large spread of about fivemagnitudes in lithium abundance is observed for the slow rotators.Finally, single giant stars with masses 1.5 < M/Msun<=2.5 show a clearest trend for a correlation between rotational velocityand lithium abundance. Based on observations collected at theObservatoire de Haute -- Provence (France) and at the European SouthernObservatory, La Silla (Chile). Table 2 is only available electronicallywith the On-Line publication athttp://link.springer.de/link/service/00230/
| The Nature of the lithium rich giants. Mixing episodes on the RGB and early-AGB
We present a critical analysis of the nature of the so-called Li-richRGB stars. For a majority of the stars, we have used Hipparcosparallaxes to determine masses and evolutionary states by comparingtheir position on the Hertzsprung-Russell diagram with theoreticalevolutionary tracks. Among the twenty Li-rich giants whose location onthe HR diagram we were able to determine precisely, five appear to beLi-rich because they have not completed the standard first dredge-updilution, and three have abundances compatible with the maximum allowedby standard dilution. Thus, these should be re-classified as Li-normal.For the remaining stars, the high Li abundance must be a result of freshsynthesis of this fragile element. We identify two distinct episodes ofLi production which occur in advanced evolutionary phases depending uponthe mass of the star. Low-mass RGB stars, which later undergo the heliumflash, produce Li at the phase referred to as the bump in the luminosityfunction. At this evolutionary phase, the outwardly-moving hydrogenshell burns through the mean molecular weight discontinuity created bythe first dredge-up. Any extra-mixing process can now easily connect the3He-rich envelope material to the outer regions of thehydrogen-burning shell, enabling Li production by the Cameron &Fowler (1971) process. While very high Li abundances are then reached,this Li-rich phase is extremely short lived because once the mixingextends deep enough to lower the carbon isotopic ratio below thestandard dilution value, the freshly synthesized Li is quicklydestroyed. In intermediate-mass stars, the mean molecular weightgradient due to the first dredge-up is not erased until after the starhas begun to burn helium in its core. The Li-rich phase in these starsoccurs when the convective envelope deepens at the base of the AGB,permitting extra-mixing to play an effective role. Li production ceaseswhen a strong mean molecular weight gradient is built up between thedeepening convective envelope and the shell of nuclear burning thatsurrounds the inert CO core. This episode is also very short lived.Low-mass stars may undergo additional mixing at this phase. The compileddata provide constraints on the time scales for extra mixing and someinsight on processes suggested in the literature. However, our resultsdo not suggest any specific trigger mechanism. Since the Li-rich phasesare extremely short, enrichment of the Li content of the ISM as a resultof these episodes is negligible.
| LTE and non-LTE atmospheric lithium abundances for the K giant HD 9746
Not Available
| An Extremely Lithium-rich Bright Red Giant in the Globular Cluster M3
We have serendipitously discovered an extremely lithium-rich star on thered giant branch of the globular cluster M3 (NGC 5272). An echellespectrum obtained with the Keck I High-Resolution Echelle Spectrographreveals a Li I lambda6707 resonance doublet of 520 mÅ equivalentwidth, and our analysis places the star among the most Li-rich giantsknown: logɛ(Li)~=+3.0. We determine the elemental abundances ofthis star, IV-101, and three other cluster members of similar luminosityand color and conclude that IV-101 has abundance ratios typical ofgiants in M3 and M13 that have undergone significant mixing. We discussmechanisms by which a low-mass star may be so enriched in Li, focusingon the mixing of material processed by the hydrogen-burning shell justbelow the convective envelope. While such enrichment could conceivablyhappen only rarely, it may in fact regularly occur during giant-branchevolution but be rarely detected because of rapid subsequent Lidepletion. Based on observations obtained with the Keck I Telescope ofthe W. M. Keck Observatory, which is operated by the CaliforniaAssociation for Research in Astronomy (CARA), Inc., on behalf of theUniversity of California and the California Institute of Technology.
| A catalog of rotational and radial velocities for evolved stars
Rotational and radial velocities have been measured for about 2000evolved stars of luminosity classes IV, III, II and Ib covering thespectral region F, G and K. The survey was carried out with the CORAVELspectrometer. The precision for the radial velocities is better than0.30 km s-1, whereas for the rotational velocity measurementsthe uncertainties are typically 1.0 km s-1 for subgiants andgiants and 2.0 km s-1 for class II giants and Ib supergiants.These data will add constraints to studies of the rotational behaviourof evolved stars as well as solid informations concerning the presenceof external rotational brakes, tidal interactions in evolved binarysystems and on the link between rotation, chemical abundance and stellaractivity. In this paper we present the rotational velocity v sin i andthe mean radial velocity for the stars of luminosity classes IV, III andII. Based on observations collected at the Haute--Provence Observatory,Saint--Michel, France and at the European Southern Observatory, LaSilla, Chile. Table \ref{tab5} also available in electronic form at CDSvia anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| Catalogs of temperatures and [Fe/H] averages for evolved G and K stars
A catalog of mean values of [Fe/H] for evolved G and K stars isdescribed. The zero point for the catalog entries has been establishedby using differential analyses. Literature sources for those entries areincluded in the catalog. The mean values are given with rms errors andnumbers of degrees of freedom, and a simple example of the use of thesestatistical data is given. For a number of the stars with entries in thecatalog, temperatures have been determined. A separate catalogcontaining those data is briefly described. Catalog only available atthe CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| Late-type giants with infrared excess. I. Lithium abundances
de la Reza et al. (1997) suggested that all K giants become Li-rich fora short time. During this period the giants are associated with anexpanding thin circumstellar shell supposedly triggered by an abruptinternal mixing mechanism resulting in the surface Li enrichment. Inorder to test this hypothesis twenty nine late-type giants withfar-infrared excess from the list of Zuckerman et al. (1995) wereobserved in the Li-region to study the connection between thecircumstellar shells and Li abundance. Eight giants have been found tohave log epsilon (Li) > 1.0. In the remaining giants the Li abundanceis found to be much lower. HD 219025 is found to be a rapidly rotating(projected rotational velocity of 23 +/-3 km s(-1) ), dusty and Li-rich(log epsilon (Li) = 3.0+/-0.2) K giant. Absolute magnitude derived fromthe Hipparcos parallax reveals that it is a giant and not apre-main-sequence star. The evolutionary status of HD 219025 seems to besimilar to that of HDE 233517 which is also a rapidly rotating, dustyand Li-rich K giant. The Hipparcos parallaxes of all the well studiedLi-rich K giants show that most of them are brighter than the ``clump"giants. Their position in the H-R diagram indicates that they have gonethrough mixing and the initial abundance of Li is not preserved. Thereseems to be no correlations between Li abundances, rotational velocitiesand carbon isotope ratios. The only satisfactory explanation for theoverabundance of lithium in these giants is the creation of Li by theextra deep mixing and the associated ``cool bottom processing". Based onobservations obtained at the European Southern Observatory, La Silla,Chile, and at the Observatoire de Haute Provence, France.
| Statistical equilibrium of lithium in the atmospheres of late-type stars: lithium-rich G-K giants.
Not Available
| On a Rapid Lithium Enrichment and Depletion of K Giant Stars
A model scenario has recently been introduced by de la Reza andcolleagues to explain the presence of very strong Li lines in thespectra of some low-mass K giant stars. In this scenario all ordinary,Li-poor, K giants become Li rich during a short time (~105 yr) whencompared to the red giant phase of 5 x 107 yr. In this "Li period," alarge number of the stars are associated with an expanding thincircumstellar shell supposedly triggered by an abrupt internal mixingmechanism resulting in a surface new 7Li enrichment. This Letterpresents nearly 40 Li-rich K giants known up to now. The distribution ofthese Li-rich giants, along with 41 other observed K giants that haveshells but are not Li rich, in a color-color IRAS diagram confirms thisscenario, which indicates, also as a new result, that a rapid Lidepletion takes place on a timescale of between ~103 and 105 yr. Thismodel explains the problem of the presence of K giants with far-infraredexcesses presented by Zuckerman and colleagues. Other present and futuretests of this scenario are briefly discussed.
| A catalogue of [Fe/H] determinations: 1996 edition
A fifth Edition of the Catalogue of [Fe/H] determinations is presentedherewith. It contains 5946 determinations for 3247 stars, including 751stars in 84 associations, clusters or galaxies. The literature iscomplete up to December 1995. The 700 bibliographical referencescorrespond to [Fe/H] determinations obtained from high resolutionspectroscopic observations and detailed analyses, most of them carriedout with the help of model-atmospheres. The Catalogue is made up ofthree formatted files: File 1: field stars, File 2: stars in galacticassociations and clusters, and stars in SMC, LMC, M33, File 3: numberedlist of bibliographical references The three files are only available inelectronic form at the Centre de Donnees Stellaires in Strasbourg, viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5), or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| An Atlas of the infrared spectral region. II. The late-type stars (G - M)
This Atlas illustrates the behavior of late type stars (F, G, K and M)in the near infrared 8400-8800 Angstrom region with a resolution ofabout 2 degrees. Seventeen figures illustrate the spectral sequence andluminosity classes V, III, Ib and Ia. Four figures illustrate peculiarspectra, namely those of Am stars, composites, weak metal stars and Sand C type objects. The complete Atlas is also available as FITS filesfrom the CDS de Strasbourg and other data centers.
| Barium stars, galactic populations and evolution.
In this paper HIPPARCOS astrometric and kinematical data together withradial velocities from other sources are used to calibrate bothluminosity and kinematics parameters of Ba stars and to classify them.We confirm the results of our previous paper (where we used data fromthe HIPPARCOS Input Catalogue), and show that Ba stars are aninhomogeneous group. Five distinct classes have been found i.e. somehalo stars and four groups belonging to disk population: roughlysuper-giants, two groups of giants (one on the giant branch, the otherat the clump location) and dwarfs, with a few subgiants mixed with them.The confirmed or suspected duplicity, the variability and the range ofknown orbital periods found in each group give coherent resultssupporting the scenario for Ba stars that are not too highly massivebinary stars in any evolutionary stages but that all were previouslyenriched with Ba from a more evolved companion. The presence in thesample of a certain number of ``false'' Ba stars is confirmed. Theestimates of age and mass are compatible with models for stars with astrong Ba anomaly. The mild Ba stars with an estimated mass higher than3Msun_ may be either stars Ba enriched by themselves or``true'' Ba stars, which imposes new constraints on models.
| IUE observations of Beryllium II lines in Lithium-rich giant stars.
IUE spectra of the Beryllium II region are presented for twoLithium-rich giant stars, HD 9746 and HD 112127. In either spectrumthere is no sign of significant Beryllium features at the BeII lines3130.416Å or 3131.064Å, which seems to indicate that Be wasdepleted in these two Lithium-rich giants. This suggests that someadditional Lithium was produced in their stellar interiors during theevolution along the giant branch.
| Absolute magnitudes and kinematics of barium stars.
The absolute magnitude of barium stars has been obtained fromkinematical data using a new algorithm based on the maximum-likelihoodprinciple. The method allows to separate a sample into groupscharacterized by different mean absolute magnitudes, kinematics andz-scale heights. It also takes into account, simultaneously, thecensorship in the sample and the errors on the observables. The methodhas been applied to a sample of 318 barium stars. Four groups have beendetected. Three of them show a kinematical behaviour corresponding todisk population stars. The fourth group contains stars with halokinematics. The luminosities of the disk population groups spread alarge range. The intrinsically brightest one (M_v_=-1.5mag,σ_M_=0.5mag) seems to be an inhomogeneous group containing bariumbinaries as well as AGB single stars. The most numerous group (about 150stars) has a mean absolute magnitude corresponding to stars in the redgiant branch (M_v_=0.9mag, σ_M_=0.8mag). The third group containsbarium dwarfs, the obtained mean absolute magnitude is characteristic ofstars on the main sequence or on the subgiant branch (M_v_=3.3mag,σ_M_=0.5mag). The obtained mean luminosities as well as thekinematical results are compatible with an evolutionary link betweenbarium dwarfs and classical barium giants. The highly luminous group isnot linked with these last two groups. More high-resolutionspectroscopic data will be necessary in order to better discriminatebetween barium and non-barium stars.
| A Moderate-Resolution Spectral Atlas of Carbon Stars: R, J, N, CH, and Barium Stars
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1996ApJS..105..419B&db_key=AST
| Supermetallicity at the Quarter-Century Mark: A Conservative Statistician's Review of the Evidence
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1996ApJS..102..105T&db_key=AST
| Lithium Enrichment--Mass-Loss Connection in K Giant Stars
Based on observed far-infrared properties of K giant stars, we propose ascenario linking the high Li abundances of some of these stars to theevolution of circumstellar shells. In this model, every K giant withmasses between 1.0 and 2.5 Mȯ become Li rich during the red giantbranch stage, and the internal mechanism responsible for the Lienrichment will initiate a prompt mass-loss event. The evolutionarypaths of the detached shells are compatible with observations for lowexpansion velocities of the order of 2 km s-1 and mass loss of (2--5) x10-8 Mȯ yr-1. This modest mass loss is, however, 2 orders ofmagnitude larger than those of normal, Li-poor K giants. A "Li time" ofthe order of 80,000 yr or somewhat larger is possible. This Li phase is,nevertheless, not related to the 12C/13C ratio which appears to evolve,for these low-mass stars, over a much longer time. This model requires arapid internal process of Li enrichment and depletion. New argumentsappearing in the literature concerning this internal process arediscussed. Reference is also made to the importance of these stars tothe Galactic Li evolution.
| Spectra of late type stars from 4800 to 9000A.
We present optical spectra of 21 stars from 4800 to 8920A, coveringessentially the late spectral types, G, K, M and the luminosity classesI and III. Half of the stars are super metal rich (SMR) ones. Thespectra were obtained at a resolution of 1.25A using the Aureliespectrograph, equipped with a linear array CCD-like detector, attachedto the OHP 1.52m telescope. Also presented are the spectra of 7 stars,covering the region 5000-9783A at a resolution of 8.5A, observed at theCFHT with the Herzberg spectrograph. The spectral types are F, G, K, Mand the luminosity classes III and V. Five stars are SMR. These spectrahave been obtained with the aim of extending existing libraries used forpopulation synthesis purposes. The inclusion of SMR stars in a stellarlibrary dedicated to the study of stellar populations in the centralpart of galaxies is crucial as abundance gradients have been observed inthe optical range.
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| Takýmyýldýz: | Berenisin Saci |
| Sað Açýklýk: | 12h53m55.75s |
| Yükselim: | +26°46'48.0" |
| Görünürdeki Parlaklýk: | 6.895 |
| Uzaklýk: | 123.609 parsek |
| özdevim Sað Açýklýk: | 2.7 |
| özdevim Yükselim: | -3.7 |
| B-T magnitude: | 8.54 |
| V-T magnitude: | 7.031 |
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