Contenidos
Imágenes
Subir su imagen
DSS Images Other Images
Artículos relacionados
The ACS Virgo Cluster Survey. VII. Resolving the Connection between Globular Clusters and Ultracompact Dwarf Galaxies
NO>1Based on observations with the NASA/ESA Hubble Space Telescopeobtained at the Space Telescope Science Institute, which is operated bythe association of Universities for Research in Astronomy, Inc., underNASA contract NAS 5-26555.
| The relation between X-ray activity and rotation in intermediate-mass G giants
I study the relation between X-ray activity and rotation amongintermediate-mass single G giants. The results show evidence that thequiescent coronal activity of these stars, as measured by their X-raysurface flux, increases linearly with the angular rotation velocity andwith the inverse of the Rossby number. Even the most rapidly rotating Ggiants do not reach the canonical log(L_X/Lbol) ≈ -3saturation level. The effect of rapid rotation on these stars couldresult mainly in an increased coverage of their surface with magneticclose loop structures. The empirical activity-rotation relationshipaccounts for the occurrence of a maximum of magnetic activity in theatmosphere of intermediate-mass stars as they evolve off themain-sequence near the bottom of the red giant branch. Remarkably, therelation between X-ray to bolometric luminosity ratio and the Rossbynumber or rotation period for G giants differs from the power lawdependence with an index of about -2 that is observed for main-sequencestars. Possible implications for the dynamo generation of magneticfields on giants are discussed.
| 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}
| 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.
| Rotationally Supported Virgo Cluster Dwarf Elliptical Galaxies: Stripped Dwarf Irregular Galaxies?
New observations of 16 dwarf elliptical galaxies (dEs) in the VirgoCluster indicate that at least seven dEs have significant velocitygradients along their optical major axis, with typical rotationamplitudes of 20-30 km s-1. Of the remaining nine galaxies inthis sample, six have velocity gradients of less than 20 kms-1 kpc-1, while the other three observations hadtoo low a signal-to-noise ratio to determine an accurate velocitygradient. Typical velocity dispersions for these galaxies are ~44+/-5 kms-1, indicating that rotation can be a significant componentof the stellar dynamics of Virgo dEs. When corrected for the limitedspatial extent of the spectral data, the rotation amplitudes of therotating dEs are comparable to those of similar-brightness dwarfirregular galaxies (dIs). Evidence of a relationship between therotation amplitude and galaxy luminosity is found and, in fact, agreeswell with the Tully-Fisher relation. The similarity in the scalingrelations of dIs and dEs implies that it is unlikely that dEs evolvefrom significantly more luminous galaxies. These observations reaffirmthe possibility that some cluster dEs may be formed when the neutralgaseous medium is stripped from dIs in the cluster environment. Wehypothesize that several different mechanisms are involved in thecreation of the overall population of dEs and that stripping ofinfalling dIs may be the dominant process in the creation of dEs inclusters like Virgo.
| Estimation of anisoplanatism in adaptive optics by generalized SCIDAR profiling
We present the results of contemporaneous seeing measurements using amulti-object Shack-Hartmann wavefront sensor, SCIDAR optical turbulenceprofiling, and a DIMM seeing monitor at the William Herschel Telescopein La Palma. The data are used to determine the accuracy with whichangular anisoplanatism can be predicted from generalized SCIDARturbulence profile measurements. Theoretical predictions of the angularcorrelation of the Zernike aberration coefficients, based on the SCIDARprofiles, agree with direct measurements from the wavefront sensor towithin 7 per cent rms. Estimates of the total seeing (integrated opticalturbulence strength) from the methods agree to 10 per cent rms. Weconclude that SCIDAR represents a reliable means for calibration of thespatially and temporally variable point-spread function for imaging withadaptive optics in astronomy.
| The Wilson-Bappu effect: A tool to determine stellar distances
Wilson & Bappu (\cite{orig}) have shown the existence of aremarkable correlation between the width of the emission in the core ofthe K line of Ca II and the absolute visual magnitude of late-typestars.Here we present a new calibration of the Wilson-Bappu effect based on asample of 119 nearby stars. We use, for the first time, widthmeasurements based on high resolution and high signal to noise ratio CCDspectra and absolute visual magnitudes from the Hipparcos database.Our primary goal is to investigate the possibility of using theWilson-Bappu effect to determine accurate distances to single stars andgroups.The result of our calibration fitting of the Wilson-Bappu relationshipis MV=33.2-18.0 log W0, and the determinationseems free of systematic effects. The root mean square error of thefitting is 0.6 mag. This error is mostly accounted for by measurementerrors and intrinsic variability of W0, but in addition apossible dependence on the metallicity is found, which becomes clearlynoticeable for metallicities below [Fe/H] ~ -0.4. This detection ispossible because in our sample [Fe/H] ranges from -1.5 to 0.4.The Wilson-Bappu effect can be used confidently for all metallicitiesnot lower than ~ -0.4, including the LMC. While it does not provideaccurate distances to single stars, it is a useful tool to determineaccurate distances to clusters and aggregates, where a sufficient numberof stars can be observed.We apply the Wilson-Bappu effect to published data of the open cluster M67; the retrieved distance modulus is of 9.65 mag, in very goodagreement with the best distance estimations for this cluster, based onmain sequence fitting.Observations collected at ESO, La Silla.
| Rapidly Rotating Lithium-rich K Giants: The New Case of the Giant PDS 365
PDS 365 is a newly detected, rapidly rotating (vsini=20 kms-1), single, low-mass giant star that with HD 233517 and HD219025 forms a remarkable ensemble of single K giants with the uniqueproperties of rapid rotation, very strong Li lines, an asymmetricalHα profile, and a large far-infrared excess. Their vsini valuesare between 18 and 23 km s-1, and their LTE Li abundances,logɛ(Li), are between 2.9 and 3.9. Detailed analysis of PDS 365reveals it to be a ~1 Msolar giant with a value of12C/13C approximately equal to 12. A clearrelation between high rotational velocities and very high Li abundancesfor K giant stars is found only when asymmetrical Hα profiles andlarge far-infrared excesses are present. If we consider single K giants,we find that among rapid (vsini>=8 km s-1) rotators, avery large proportion (~50%) are Li-rich giants. This proportion is incontrast with a very low proportion (~2%) of Li-rich stars among themuch more common slowly rotating K giants. This striking difference isdiscussed in terms of proposed mechanisms for Li enrichment.
| 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).
| On the Accuracy of GAIA Radial Velocities
We have obtained 782 real spectra and used them as inputs for 6700automatic cross-correlation runs to investigate the GAIA potential interms of radial velocity accuracy. We have explored the dispersions0.25, 0.5, 1 and 2 Å/pix over the 8490--8740 Å GAIA range.We have investigated late-F to early-M stars (constituting the vastmajority of GAIA targets), slowly rotating ( = 4 km/s), of solar metallicity (<[Fe/H]> = --0.07) and notbinary. The results are accurately described by the simple law: {logsigma = 0.6 (log S/N)2 -- 2.4 log S/N + 1.75 log D + 3, wheresigma is the cross-correlation standard error (in km/s) and D is thespectral dispersion (in Å/pix). The spectral dispersion has turnedout to be the dominant factor, with S/N being less important and thespectral mis-match being a weak player at the lowest S/N. Our resultsshow that mission-averaged radial velocities of faint GAIA targets (V ~15 mag) can match the ~ 0.5 km/s accuracy of tangential motions,provided the observations are performed at a dispersion not less than0.5 Å/pix.
| Activity Study in 3 Intermediate Mass Evolved Stars
A spectroscopic CaII K&H and Hα study was carriedout for 3 single fast rotating evolved stars - HD145001, HD185958 andHD121107. Photometric UBV observations were also done. Data for theirX-ray behavior were analysed too. One of the stars appear to bechromospherically active. One of the rest two fast rotating giants seemsto be active from time to time, and the last one was found to beinactive, nevertheless its fast rotation. The situation of the stars onthe H-R diagram was found and their evolutionary status was determinedby using also literature data for their chemical abundances. Thepossible reasons for activity in these evolved single stars werediscussed too.
| The proper motions of fundamental stars. I. 1535 stars from the Basic FK5
A direct combination of the positions given in the HIPPARCOS cataloguewith astrometric ground-based catalogues having epochs later than 1939allows us to obtain new proper motions for the 1535 stars of the BasicFK5. The results are presented as the catalogue Proper Motions ofFundamental Stars (PMFS), Part I. The median precision of the propermotions is 0.5 mas/year for mu alpha cos delta and 0.7mas/year for mu delta . The non-linear motions of thephotocentres of a few hundred astrometric binaries are separated intotheir linear and elliptic motions. Since the PMFS proper motions do notinclude the information given by the proper motions from othercatalogues (HIPPARCOS, FK5, FK6, etc.) this catalogue can be used as anindependent source of the proper motions of the fundamental stars.Catalogue (Table 3) is only available at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strastg.fr/cgi-bin/qcat?J/A+A/365/222
| Dating Ptolemy's star catalogue through proper motions : the Hipparchan epoch.
Not Available
| 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/
| Library of Medium-Resolution Fiber Optic Echelle Spectra of F, G, K, and M Field Dwarfs to Giant Stars
We present a library of Penn State Fiber Optic Echelle (FOE)observations of a sample of field stars with spectral types F to M andluminosity classes V to I. The spectral coverage is from 3800 to 10000Å with a nominal resolving power of 12,000. These spectra includemany of the spectral lines most widely used as optical and near-infraredindicators of chromospheric activity such as the Balmer lines (Hαto Hepsilon), Ca II H & K, the Mg I b triplet, Na I D_1, D_2, He ID_3, and Ca II IRT lines. There are also a large number of photosphericlines, which can also be affected by chromospheric activity, andtemperature-sensitive photospheric features such as TiO bands. Thespectra have been compiled with the goal of providing a set of standardsobserved at medium resolution. We have extensively used such data forthe study of active chromosphere stars by applying a spectralsubtraction technique. However, the data set presented here can also beutilized in a wide variety of ways ranging from radial velocitytemplates to study of variable stars and stellar population synthesis.This library can also be used for spectral classification purposes anddetermination of atmospheric parameters (T_eff, logg, [Fe/H]). A digitalversion of all the fully reduced spectra is available via ftp and theWorld Wide Web (WWW) in FITS format.
| 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
| Evolution of X-ray activity and rotation on G-K giants
The recent availability of stellar parallaxes provided by the Hipparcosstar catalogue (ESA 1997) enables an accurate determination of thepositions of single field giants in a theoretical H-R diagram and areliable estimate of their masses. The present study combines these newastrometric data with previously published X-ray fluxes and rotationalvelocities. The results confirm the existence of a sharp decrease ofX-ray emission at spectral type K1 for 2.5 M_sun < M < 5 M_sungiants. The study shows that the rotational velocity of these starsreaches a minimum at the same location in the H-R diagram. However, notight relationship between X-ray luminosities and projected equatorialvelocities was found among the sample stars. I suggest that theseresults could reflect the importance of differential rotation indetermining the level of coronal emission among >= 2.5Msun G and K giants. The restoration of rigid rotation at thebottom of the red giant branch could prevent the maintenance of largescale magnetic fields, thus explaining the sharp decrease of coronalX-ray emission at spectral type K1.
| Circumstellar emission and flares on FK Comae Berenices. Analysis and modelling of Balmer and He I D3 line variations
We present results on spectroscopic observations of the fast-rotatingactive giant FK Comae, obtained mainly with theESA-MUSICOS spectrograph at the Isaac Newton Telescope (INT) in 1996 and1997 and also with the Aurélie spectrograph at the Observatoirede Haute Provence (OHP) in 1997. The profiles analysed are those of theBalmer, Hα \ and Hβ , and He I D3 (lambda 5876 Angstroms)lines. We analyse the Balmer line variability and phase behaviour. Weconfirm these lines as highly variable, with excess emission thatoriginates from extended structures and exhibits clear signs ofrotational modulation. We have described the line profiles for twodistinct states of activity, using different modelling approaches.Similar techniques were applied to the He I D3 spectra. A large flareevent lasting several days was detected in both Balmer lines and in theHe I D3 line. The energy released during this flare in Hα \ is ofthe order of 10(37) erg, making it the largest Hα \ flare reportedon a cool star. Our results confirm the extreme complexity of thecircumstellar environment of FK Comae. The activity level of this staris quite variable demanding different approaches to the line profileanalysis. Based on observations obtained at the Isaac Newton Telescopewith the ESA-MUSICOS spectrograph and at the Observatoire de HauteProvence with the Aurélie spectrograph.
| The ROSAT all-sky survey catalogue of optically bright late-type giants and supergiants
We present X-ray data for all late-type (A, F, G, K, M) giants andsupergiants (luminosity classes I to III-IV) listed in the Bright StarCatalogue that have been detected in the ROSAT all-sky survey.Altogether, our catalogue contains 450 entries of X-ray emitting evolvedlate-type stars, which corresponds to an average detection rate of about11.7 percent. The selection of the sample stars, the data analysis, thecriteria for an accepted match between star and X-ray source, and thedetermination of X-ray fluxes are described. Catalogue only available atCDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| The Tokyo PMC catalog 90-93: Catalog of positions of 6649 stars observed in 1990 through 1993 with Tokyo photoelectric meridian circle
The sixth annual catalog of the Tokyo Photoelectric Meridian Circle(PMC) is presented for 6649 stars which were observed at least two timesin January 1990 through March 1993. The mean positions of the starsobserved are given in the catalog at the corresponding mean epochs ofobservations of individual stars. The coordinates of the catalog arebased on the FK5 system, and referred to the equinox and equator ofJ2000.0. The mean local deviations of the observed positions from theFK5 catalog positions are constructed for the basic FK5 stars to comparewith those of the Tokyo PMC Catalog 89 and preliminary Hipparcos resultsof H30.
| OH 1.563 micron Absorption from Starspots on Active Stars
We present results from a study of starspots on active stars using apair of vibrational-rotational absorption lines of the OH molecule near1.563mu m. We detect excess OH absorption due to dark, cool starspots onthe RS CVn binaries II Pegasi, V1762 Cygni, and lambda Andromedae. Thisis the first detection of OH absorption from spots on stars other thanthe Sun. We have measured absorption equivalent widths of these OH lines(which are blended at the resolution of our observations) in inactivegiant and dwarf stars of spectral types G, K, and M. We find that thetotal equivalent width of the line pair increases approximately linearlyas effective temperature decreases from 5000 K to 3000 K. This greatlyextends the temperature range over which starspots can be detectedthrough molecular absorption features. We measure starspot fillingfactors by fitting the spectra of active stars with linear combinationsof comparison star spectra representing the spot and non-spot regions ofthe star. Fitting only one spectral feature, we cannot deriveindependent constraints on starspot area and temperature. Assuming spottemperatures based on previous analyses, we find (for one epoch) spotfilling factors between 35% and 48% for II Peg, 22% and 26% for lambdaAnd, and 27% and 32% for V1762 Cyg.
| 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
| Core velocity dispersions for 25 Galactic and 10 old Magellanic globular clusters.
We present, for 25 Galactic and 10 old Magellanic globular clusters,projected velocity dispersion (σ_p_) measurements obtained byapplying a cross-correlation technique to integrated-light spectra. Inorder to understand and estimate the statistical errors of thesemeasurements due to small numbers of bright stars dominating theintegrated light, we provide an extensive discussion based on detailednumerical simulations. These errors are smaller if the integration areais larger and/or the cluster concentration higher. The simulations showthat measurements are reliable when the integrated light within theintegration area is brighter than a given magnitude. The statisticalerrors on the σ_p_ measurements of Magellanic globular clustersare small because of a physically large integration area, whereas theycan be important for measurements carried out over small central areasin Galactic clusters. The present observational results are used tooutline a few characteristics of the globular cluster fundamental plane.In this respect, the old Magellanic globular clusters appear similar tothe Galactic clusters.
| Giants with infrared excess.
We have correlated optical and infrared catalogs in order to extract alarge sample of luminosity class III stars with known infrared fluxdensities. For a non-negligible fraction of G and K giants, afar-infrared excess emission was found, starting beyond 25μm. Anexplanation in terms of present-day mass loss thus becomes unlikely,since the dust should then be warmer and the excess emission less far inthe infrared. We believe that the far-infrared excesses of theseobjects, most likely first-ascent giants, are related to the Vegaphenomenon. The dusty disks around these stars, gradually cooled downduring their main-sequence phase, could be reheated once the star leavesthe main sequence and enters the luminous post-main-sequence phase. Thefairly large sample we constructed enables us to derive an estimationfor the occurrence of excesses. This fraction of G or K giants withfar-infrared excess appears to be distinctly smaller than amongmain-sequence stars. Since the higher radiation field of giants couldlead to a larger evaporation rate of the circumstellar debris, this factdoes not conflict with our hypothesis.
| Correlating Coronal Temperature and Gravitational Potential: A Test of the Nonthermal Boundary Hypothesis
It has been suggested that a nonthermal boundary condition can accountfor the existence of a coronal temperature inversion in stars. Thenonthermal hypothesis makes falsifiable predictions of the maximumcoronal temperature obtainable. This maximum temperature corresponds tothe depth of the potential well at the boundary, which is effectivelyR*, and so varies as M*/R*. Here, we compare this prediction withone-temperature fits to single stars in Einstein data. We find that thecoronal temperatures in dwarfs are consistent with a nonthermal boundarycondition, but the nonthermal hypothesis cannot readily account for thecoronal temperatures of class III giants. However, interpreting theemission from the non-compliant giants in alternate terms suggests thata dwarf companion may be the source of the X-rays. More generally,gravitational potential is found to be a useful quantity to orderingcoronal observations: (1) the "dividing line" near K4, beyond whichgiant star coronae are not observed, corresponds to a rather precipitousfactor of 10 drop in the strength of the stellar gravitationalpotential; and (2) M dwarfs and A stars of class III--V, whose coronaehave been somewhat of a puzzle, all have relatively deep gravitationalpotentials.
| New Extreme Ultraviolet-Selected Active Binaries from the ROSAT Wide Field Camera all Sky Survey
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1995MNRAS.276..397J&db_key=AST
| The kinematics of lithium-rich, active late-type stars: evidence for a low-mass Local Association
An EUV-selected sample of late-type stars has been observed for lithiumabundances and radial velocity radiations. It is found that essentiallyall of the single stars have lithium abundances comparable to or greaterthan late-type stars in the Pleiades. These observations have confirmedthe hypothesis that a high proportion of the most coronally active,late-type stars are Li-rich, because they are young, have not yet spundown, and consequently exhibit enhanced dynamo activity. From these dataand other activity-selected samples in the literature, a Li-rich sampleof single stars has been chosen, which, by comparison with open clusterLi abundances and rotation rates, is substantially younger than 300 Myr.The kinematics of this sample has been investigated, and from the radialvelocities a one-dimensional dispersion of only 5.7 km s^-1 is found,about a centroid space motion extremely similar to that of a group ofyoung open clusters and nearby B stars, known as the Local Association.Consideration of the space motions confirms that, if this low-mass LocalAssociation can be represented by a three-dimensional velocitydispersion of about 10 km s^-1, then the majority of the Li-rich starsare members. The origin of the Local Association is still obscure, but anumber of suggestions are put forward, including ejection of stars fromnearby open clusters, the disruption of pre-existing clusters, or thedissolution of the unbound remnants from one or more star formationregions.
| H-alpha measurements for cool giants
The H-alpha line in a cool star is usually an indication of theconditions in its chromosphere. I have collected H-alpha spectra of manynorthern G-M stars, which show how the strength and shape of the H-alphaline change with spectral type. These observations detect surprisinglittle variation in absoption-line depth (Rc approximately0.23 +/- 0.08), linewidth (FWHD approximately 1.44 +/- 0.22 A), orequivalent width (EW approximately 1.12 +/- 0.17 A) among G5-M5 IIIgiants. Lines in the more luminous stars tend to be broader and strongerby 30%-40% than in the Class III giants, while the H-alpha absorptiontends to weaken among the cooler M giants. Velocities of H-alpha andnearby photospheric lines are the same to within 1.4 +/- 4.4 km/s forthe whole group. To interpret these observations, I have calculatedH-alpha profiles, Ly-alpha strengths, and (C II) strengths for a seriesof model chromospheres representing a cool giant star like alpha Tau.Results are sensitive to the mass of the chromosphere, to chromospherictemperature, to clumping of the gas, and to the assumed physics of lineformation. The ubiquitous nature of H-alpha in cool giants and the greatdepth of observed lines argue that chromospheres of giants cover theirstellar disks uniformly and are homogeneous on a large scale. This isquite different from conditions on a small scale: To obtain a highenough electron density with the theoretical models, both to explain theexitation of hydrogen and possibly also to give the observed C IImultiplet ratios, the gas is probably clumped. The 6540-6580 A spectraof 240 stars are plotted in an Appendix, which identifies the date ofobservation and marks positions of strong telluric lines on eachspectrum. I assess the effects of telluric lines and estimates that thestrength of scattered light is approximately 5% of the continuum inthese spectra. I give the measurements of H-alpha as well as equivalentwidths of two prominent photospheric lines, Fe I lambda 6546 and Ca Ilambda 6572, which strengthen with advancing spectral type.
|
Enviar un nuevo artículo
Enlaces relacionados
- - No se han encontrado enlaces -
En viar un nuevo enlace
Miembro de los siguientes grupos:
|
Datos observacionales y astrométricos
| Constelación: | Hércules |
| Ascensión Recta: | 16h08m04.50s |
| Declinación: | +17°02'49.0" |
| Magnitud Aparente: | 5 |
| Distancia: | 119.048 parsecs |
| Movimiento Propio en Ascensión Recta: | -34 |
| Movimiento Propio en Declinación: | -5.6 |
| B-T magnitude: | 6.165 |
| V-T magnitude: | 5.097 |
Catálogos y designaciones:
|
INHABITED SKY
