תוכן
תמונות
הוסף תמונה שלך
DSS Images Other Images
מאמרים קשורים
The Vertical Stellar Kinematics in Face-On Barred Galaxies: Estimating the Ages of Bars
In order to perform a detailed study of the stellar kinematics in thevertical axis of bars, we obtained high signal-to-noise spectra alongthe major and minor axes of the bars in a sample of 14 face-on galaxiesand used them to determine the line-of-sight stellar velocitydistribution, parameterized as a Gauss-Hermite series. With these data,we developed a diagnostic tool that allows one to distinguish betweenrecently formed and evolved bars, as well as to estimate their ages,assuming that bars form in vertically thin disks that are recognizableby low values for the vertical velocity dispersion σz.Through N-body realizations of bar unstable disk galaxies we were alsoable to check the timescales involved in the processes that give bars animportant vertical structure. We show that σz inevolved bars is roughly 100 km s-1, which translates to aheight scale of about 1.4 kpc, giving support to scenarios in whichbulges form through disk material. Furthermore, the bars in ournumerical simulations have values for σz generallysmaller than 50 km s-1, even after evolving for 2 Gyr,suggesting that a slow process is responsible for making bars asvertically thick as we observe. We verify theoretically that theSpitzer-Schwarzschild mechanism is quantitatively able to explain theseobservations if we assume that giant molecular clouds are twice asconcentrated along the bar as in the rest of the disk.
| 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
| 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
Synthetic Lick indices computed with solar scaled abundances and withα-element enhancement are presented and compared with predictionsfrom both theoretical computations (Tripicco & Bell; Thomas,Maraston, & Bender; Barbuy et al.) and empirical fitting functions(de Freitas Pacheco). We propose selected combinations of indicescapable of singling out α-enhanced stars without requiringprevious knowledge of their main atmospheric parameters. By applyingthis approach to the 460 stars in the Worthey et al. catalog, wedetected a list of 82 candidate α-enhanced stars. The confirmationof α-enhancement was obtained by searching the literature forindividual element abundance determinations from high-resolutionspectroscopy for a subsample of 34 stars. Preliminary discussion of theproperties of the detected α-enhanced stars with respect to their[Fe/H] values and kinematics is presented.
| The Integrated Spectrum of M67 and the Spectroscopic Age of M32
We construct an integrated spectrum of the intermediate-age,solar-metallicity Galactic cluster M67, from individual spectroscopicobservations of bona fide cluster members. The spectrum so obtained isused as a template to test our stellar population synthesis models, inan age and metallicity regime where such models remain largely untested.As a result, we demonstrate that our models predict a spectroscopic ageof 3.5+/-0.5 Gyr for M67, which is the same age we obtain from fittingisochrones to the color-magnitude diagram of the cluster. Fullconsistency is reached when using either Hβ, Hγ, or Hδas the age indicator. We also check if the models, when applied to thecluster integrated spectrum, predict elemental abundances in agreementwith the known detailed abundance pattern of the cluster. The modelsalso pass the latter test, by predicting the abundances of iron,magnesium, carbon, and nitrogen in agreement with detailed abundanceanalyses of cluster stars to within 0.1 dex. Encouraged by the highdegree of consistency of our models, we apply them to the study of theintegrated spectrum of the central 3" of the compact elliptical galaxyM32. The resulting luminosity-weighted age of the galaxy ranges between2 and 3.5 Gyr, depending on the age indicator adopted. According to ourmodels, the center of M32 seems to have a supersolar iron abundance,ranging between [Fe/H]~+0.1 and +0.3, depending on the spectral indexadopted. The light element magnesium seems to be underabundant in thecenter of M32 relative to iron by about ~0.1-0.2 dex, whereas the dataare consistent with nearly solar carbon and nitrogen abundances relativeto iron. We find that single-age, single-metallicity stellar populationmodels with solar-scaled abundance patterns cannot fit all the Balmerand metal lines in the integrated spectrum of M32. In particular, thereis a systematic trend in the sense that bluer absorption lines indicatea younger age and a higher metallicity. This slight inconsistency can bedue either to (unaccounted for) abundance ratio effects on blue iron andBalmer line indices or to a spread of the ages of the stellarpopulations in M32. Current stellar population models cannot break thisdegeneracy at the level of accuracy required to address this problem.
| Interferometric observations of the supergiant stars α Orionis and α Herculis with FLUOR at IOTA
We report the observations in the K band of the red supergiant starα Orionis and of the bright giant star α Herculis with theFLUOR beamcombiner at the IOTA interferometer. The high quality of thedata allows us to estimate limb-darkening and derive precise diametersin the K band which combined with bolometric fluxes yield effectivetemperatures. In the case of Betelgeuse, data collected at high spatialfrequency although sparse are compatible with circular symmetry andthere is no clear evidence for departure from circular symmetry. We havecombined the K band data with interferometric measurements in the L bandand at 11.15 μm. The full set of data can be explained if a 2055 Klayer with optical depths τK=0.060±0.003,τL=0.026±0.002 and τ11.15 μm=2.33±0.23 is added 0.33 R* above the photosphereproviding a first consistent view of the star in this range ofwavelengths. This layer provides a consistent explanation for at leastthree otherwise puzzling observations: the wavelength variation ofapparent diameter, the dramatic difference in limb darkening between thetwo supergiant stars, and the previously noted reduced effectivetemperature of supergiants with respect to giants of the same spectraltype. Each of these may be simply understood as an artifact due to notaccounting for the presence of the upper layer in the data analysis.This consistent picture can be considered strong support for thepresence of a sphere of warm water vapor, proposed by \cite{tsuji2000}when interpreting the spectra of strong molecular lines.Based on observations collected at the IOTA interferometer, WhippleObservatory, Mount Hopkins, Arizona.
| Very Low Mass Stars and Brown Dwarfs in Taurus-Auriga
We present high-resolution optical spectra obtained with the HIRESspectrograph on the W. M. Keck I Telescope of seven low-mass T Tauristars (LMTTs) and brown dwarfs in Taurus-Auriga. The observed Li I 6708Å absorption, low surface gravity signatures, and radialvelocities confirm that all are members of the Taurus star-formingregion; no new spectroscopic binaries are identified. Four of the seventargets observed appear to be T Tauri brown dwarfs. Of particularinterest is the previously classified ``continuum T Tauri star'' GM Tau,which has a spectral type of M6.5 and possibly a substellar mass. Thesespectra, in combination with previous high-resolution spectra of LMTTs,are used to understand the formation and early evolution of objects inTaurus-Auriga with masses near and below the stellar/substellarboundary. None of the LMTTs in Taurus are rapidly rotating (vsini<30km s-1), unlike low-mass objects in Orion. Many of the slowlyrotating, nonaccreting stars and brown dwarfs exhibit prominent Hαemission (equivalent widths of 3-36 Å), indicative of activechromospheres. We demonstrate empirically that the full width at 10% ofthe Hα emission profile peak is a more practical and possibly moreaccurate indicator of accretion than either the equivalent width ofHα or optical veiling: 10% widths >270 km s-1 areclassical T Tauri stars (i.e., accreting), independent of stellarspectral type. Although LMTTs can have accretion rates comparable tothat of more typical, higher mass T Tauri stars (e.g., K7-M0 spectraltypes), the average mass accretion rate appears to decrease withdecreasing mass. A functional form of M~M is consistent with theavailable data, but the dependence is difficult to establish because ofboth selection biases in observed samples and the decreasing frequencyof active accretion disks at low masses (M<0.2 Msolar).The diminished frequency of accretion disks for LMTTs, in conjunctionwith their lower, on average, mass accretion rates, implies that theyare formed with less massive disks than higher mass T Tauri stars. Theradial velocities, circumstellar properties, and known binaries do notsupport the suggestion that many of the lowest mass members of Taurushave been ejected from higher stellar density regions within the cloud.Instead, LMTTs appear to have formed and are evolving in the same way ashigher mass T Tauri stars, but with smaller disks and shorter disklifetimes.
| Wing Near-Infrared, TiO-Band, and V-Band Photometry of Chromospherically Active Star λ Andromedae
As a pilot program, Wing near-IR, TiO-band, and V-band photometry isbeing conducted of the RS Canum Venaticorum type, chromosphericallyactive, G8 IV-III star λ Andromedae. The objective is toinvestigate a possible relationship between variation of the ~54 dayrotationally starspot modulated visual light curve and TiO absorptionstrength. The TiO (γ,0,0) absorption band strength at λ=719nm is very sensitive to temperature for cool stars and manifests itselfin cooler starspot regions (T<=4000 K). TiO photometry has anadvantage over conventional photometry in that it provides unambiguousmeasures of the fractional cool starspot coverage. In addition, as thestars rotate, the variation in the TiO index yields information aboutthe longitudinal distribution of the starspots. Importantly, combiningthe TiO photometry with the V-band and near-IR light curves allows thediscrimination of white-light faculae (=hot spot) and cool starspotcontributions. Initial results of this study indicate that the observedV-band and near-IR continua light variations found for λ Andprimarily arise from bright spot (plage) features rather than darkstarspots as is usually assumed. This is in contrast to current theoriesthat the visual light variation is solely due to dark spots. Modelsusing both bright and dark spot features have been developed and arebeing used to fit the light and TiO-index curves. The models account forcool/hot spot characteristics such as projected filling factor andtemperature. The long-term variation of V light and TiO index have beeninvestigated to search for any activity cycles.
| Technetium and the third dredge up in AGB stars. I. Field stars
We searched for Tc in a sample of long period variables selected bystellar luminosity derived from Hipparcos parallaxes. Tc, as an unstables-process element, is a good indicator for the evolutionary status ofstars on the asymptotic giant branch (AGB). In this paper we study theoccurrence of Tc as a function of luminosity to provide constraints onthe minimum luminosity for the third dredge up as estimated from recentstellar evolution models.A large number of AGB stars above the estimated theoretical limit forthe third dredge up are found not to show Tc. We confirm previousfindings that only a small fraction of the semiregular variables show Tclines in their spectra. Contrary to earlier results by Little et al.(\cite{llmb87}) we find also a significant number of Miras without Tc.The presence and absence of Tc is discussed in relation to the massdistribution of AGB stars. We find that a large fraction of the stars ofour sample must have current masses of less than 1.5 Msun .Combining our findings with stellar evolution scenarios we conclude thatthe fraction of time a star is observed as a SRV or a Mira is dependenton its mass.Partly based on observations collected at the European SouthernObservatory, Paranal, Chile (ESO-Programme 65.L-0317(A)).
| High resolution spectroscopy over lambda lambda 8500-8750 Å for GAIA. IV. Extending the cool MK stars sample
A library of high resolution spectra of MK standard and reference stars,observed in support to the GAIA mission, is presented. The aim of thispaper is to integrate the MK mapping of Paper I of this series as wellas to consider stars over a wider range of metallicities. Radialvelocities are measured for all the target stars.The spectra are available in electronic form (ASCII format) at CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/406/995 and from the webpage http://ulisse.pd.astro.it/MoreMK/, where further bibliographicalinformation for the target stars is given.
| 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
| Do red giant stars pulsate in high overtones?
About 30 photometrically variable red giant stars have periods less than10d, as determined by the compilers of the Hipparcos Catalogue fromHipparcos photometric measurements. These periods, when combined withestimates of the radii and masses of these stars, and with pulsationtheory, imply that these stars are pulsating in very high overtones. Wepresent several pieces of evidence which suggest that the periods may bespurious, as a result of the particular aliasing properties of theHipparcos photometry. We conclude that the evidence for high-overtonepulsation in red giant stars is equivocal.
| 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.
| Periods of 25 Pulsating Red Giants
We report periods and amplitudes of 25 pulsating red giants: T Cet, EGAnd, AK Hya, TV UMa, GK Com, SW Vir, FH Vir, EV Vir, tau-4 Ser, AZ Dra,V973 Cyg, V1070 Cyg, and also HR 211, 284, 648, 2286, 2999, 3521, 4267,4483, 5331, 5352, 6543, 6815, and 7009, determined from AmericanAssociation of Variable Stars (AAVSO) photoelectric photometry over upto 5800 days.
| Hydroxyl 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 near1.563 μm. We detect excess OH absorption due to dark, cool starspotson several active stars of the RS CVn and BY Dra classes. Our resultsfor the single-lined spectroscopic binaries II Pegasi, V1762 Cygni, andλ Andromedae augment those from a previous study that used a lesssensitive detector. In this study, we were able for the first time touse molecular absorption features to measure starspot properties ondouble-lined spectroscopic binaries. Measuring the equivalent widths ofthese OH lines in inactive giant and dwarf stars of spectral types G, K,and M, we find that the total equivalent width of the line pairincreases approximately linearly as effective temperature decreases from5000 to 3000 K. We measure starspot filling factors by fitting thespectra of active stars with linear combinations of comparison starspectra representing the spot and nonspot regions of the star.
| Long period variable stars: galactic populations and infrared luminosity calibrations
In this paper HIPPARCOS astrometric and kinematic data are used tocalibrate both infrared luminosities and kinematical parameters of LongPeriod Variable stars (LPVs). Individual absolute K and IRAS 12 and 25luminosities of 800 LPVs are determined and made available in electronicform. The estimated mean kinematics is analyzed in terms of galacticpopulations. LPVs are found to belong to galactic populations rangingfrom the thin disk to the extended disk. An age range and a lower limitof the initial mass is given for stars of each population. A differenceof 1.3 mag in K for the upper limit of the Asymptotic Giant Branch isfound between the disk and old disk galactic populations, confirming itsdependence on the mass in the main sequence. LPVs with a thin envelopeare distinguished using the estimated mean IRAS luminosities. The levelof attraction (in the classification sense) of each group for the usualclassifying parameters of LPVs (variability and spectral types) isexamined. Table only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/374/968 or via ASTRIDdatabase (http://astrid.graal.univ-montp2.fr).
| Catalogue of Apparent Diameters and Absolute Radii of Stars (CADARS) - Third edition - Comments and statistics
The Catalogue, available at the Centre de Données Stellaires deStrasbourg, consists of 13 573 records concerning the results obtainedfrom different methods for 7778 stars, reported in the literature. Thefollowing data are listed for each star: identifications, apparentmagnitude, spectral type, apparent diameter in arcsec, absolute radiusin solar units, method of determination, reference, remarks. Commentsand statistics obtained from CADARS are given. The Catalogue isavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcar?J/A+A/367/521
| Rapidly oscillating M giant stars?
The Hipparcos mission discovered a few dozen M giant stars with periodsP shorter than 10d. Similar stars may be found in other large data basesof new variables (e.g., OGLE). The three possible sources of themagnitude variations - pulsation, starspots and ellipsoidal deformation- are discussed in general terms. The parallaxes and V-I colour indicesare used to calculate radii and temperatures for all M giant variableswith P<100d. Masses are estimated from the positions of the stars ina Hertzsprung-Russell (HR) diagram, using evolutionary tracks. Usingthese data, it is shown that starspots can be ruled out as a variabilitymechanism in almost all cases, and ellipsoidal variations in about halfof the stars. Pulsation in very high-overtone modes appears to be theonly viable explanation for the stars with P<10d. Many of the starsmay be multiperiodic. IRAS data are used to deduce information aboutreddening and circumstellar dust. The apparently low level of mass-loss,as well as the kinematics and the spatial distribution of the stars,indicates that they are from a relatively young (i.e., thin disc) giantstar population.
| A library of 0.5 to 2.5 mu m spectra of luminous cool stars
We present a library of 0.5-2.5 mu m digital spectra of cool, mostlyvariable, giant and supergiant stars which should be useful for spectralsynthesis applications, as well as for comparison with theoretical modelatmosphere calculations. The library includes carbon stars, GalacticBulge and Magellanic Cloud stars, and OH/IR stars. The stars observedcover a range of metallicities and masses. Most variable objects wereobserved several times, at different phases of their pulsation cycle. Wediscuss the effects of various fundamental parameters and of variabilityon the spectra. Based on period-luminosity relations and stellarevolutionary tracks, we tentatively assign initial masses to eachobject.
| Magnitudes absolues des étoiles standards MK des types G à M à partir des parallaxes Hipparcos. The absolute magnitudes of the G to M type MK standards from the Hipparcos parallaxes
We analyse a sample of about 500 MK standards of cool spectral types (Gto M) for to compare the visual absolute magnitudes obtained from bothHipparcos data and Schmidt-Kaler calibrations. Our purpose is tovalidate our spectroscopic work \cite[(Ginestet et al. 1997, 1999)]{G97}on stars with composite spectra with the help of Hipparcos data.Contrary to what is claimed in other papers, the absolute magnitudedomain devoted to the giant stars does not overlap the domain of dwarfs.We find that the discrepancies between absolute magnitudes fromHipparcos data and absolute magnitudes deduced from Schmidt-Kalercalibrations increase with the relative error sigma (pi )/pi on theparallaxes. So, for sigma (pi )/pi <= 0.05 only 3% of the starspresent a discrepancy of one luminosity class, while this percentagereaches 54% for 0.25 < sigma (pi )/pi <= 0.50. Curiously, theluminosity of the giants seems to increase with the distance of thestars, whereas the supergiants of the sample appear underluminous atleast for d < 600 pc! We point out a list of 14 MK standards whoseluminosity classes may be erroneous and need a new spectralclassification, in the near infrared. The case of composite-spectrumbinaries is also discussed. Most of these are too distant for accurateparallaxes even with Hipparcos: only sixteen stars have sigma (pi )/pi<= 0.10; for these, we give new spectral classifications in agreementwith both our classifications in the near infrared of the coolcomponents and Hipparcos data. Finally, for stars having high-precisionparallaxes (sigma (pi )/pi <= 5%) there is no serious problem forSchmidt-Kaler calibrations whith respect to Hipparcos data. The datacorresponding to parallaxes of lower precisions should be used withcaution and only for statistical analyses.
| The CaII-M_v Correlation (Wilson-Bappu Effect) Calibrated by HIPPARCOS Parallaxes
We have used Hipparcos parallaxes to derive absolute visual magnitudesof G, K, and M stars with Ca II emission line widths previously measuredby O. C. Wilson. A linear relationship similar to the one derivedoriginally by Wilson & Bappu and improved by Lutz & Kelker wasfound from M_v=+7 to -2. For stars brighter than M_v=-2 a substantialnumber of stars show Ca II emission lines that are broader than expectedfrom the linear fit. Most of those stars are bright giants andsupergiants of type G. In appendices we show some sample Ca II profilesand identify emission lines of Fe II as well as the Hepsilon line insome stars.
| Photometric modelling of starspots - I. A Barnes-Evans-like surface brightness-colour relation using (Ic-K)
In the first part of this work, the empirical correlation of stellarsurface brightness FV with (Ic-K) broad-bandcolour is investigated by using a sample of stars cooler than the Sun. Abilinear correlation is found to represent well the brightness of G, Kand M giant stars. The change in slope occurs at (Ic-K)~2.1or at about the transition from K to M spectral types. The samerelationship is also investigated for dwarf stars and found to bedistinctly different from that of the giants. The dwarf star correlationdiffers by an average of -0.4 in (Ic-K) or by a maximum inFV of ~-0.1, positioning it below that of the giants, withboth trends tending towards convergence for the hotter stars in oursample. The flux distribution derived from theFV-(Ic-K) relationship for the giant stars,together with that derived from an FV-(V-K) relationship andthe blackbody flux distribution, is then utilized to compute syntheticlight V and colour (V-R)c, (V-I)c and (V-K) curvesof cool spotted stars. We investigate the effects on the amplitudes ofthe curves by using these FV-colour relations and by assumingthe effective gravity of the spots to be lower than the gravity of theunspotted photosphere. We find that the amplitudes produced by using theFV-(Ic-K) relationship are larger than thoseproduced by the other two brightness correlations, meaning smallerand/or warmer spots.
| 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.
| Speckle Interferometry of New and Problem HIPPARCOS Binaries
The ESA Hipparcos satellite made measurements of over 12,000 doublestars and discovered 3406 new systems. In addition to these, 4706entries in the Hipparcos Catalogue correspond to double star solutionsthat did not provide the classical parameters of separation and positionangle (rho,theta) but were the so-called problem stars, flagged ``G,''``O,'' ``V,'' or ``X'' (field H59 of the main catalog). An additionalsubset of 6981 entries were treated as single objects but classified byHipparcos as ``suspected nonsingle'' (flag ``S'' in field H61), thusyielding a total of 11,687 ``problem stars.'' Of the many ground-basedtechniques for the study of double stars, probably the one with thegreatest potential for exploration of these new and problem Hipparcosbinaries is speckle interferometry. Results are presented from aninspection of 848 new and problem Hipparcos binaries, using botharchival and new speckle observations obtained with the USNO and CHARAspeckle cameras.
| Empirical calibration of the lambda 4000 Å break
Empirical fitting functions, describing the behaviour of the lambda 4000Ä break, D4000, in terms of effective temperature,metallicity and surface gravity, are presented. For this purpose, thebreak has been measured in 392 stars from the Lick/IDS Library. We havefollowed a very detailed error treatment in the reduction and fittingprocedures, allowing for a reliable estimation of the breakuncertainties. This calibration can be easily incorporated into stellarpopulation models to provide accurate predictions of the break amplitudefor, relatively old, composite systems. Table 1 is only available inelectronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.128.5) or via http://cdsweb.u-strasbg.fr/Abstract.html
| Spectral classification of the cool giants in symbiotic systems
We derive the spectral types of the cool giants in about 100 symbioticsystems. Our classification is mainly based on near IR spectra in orderto avoid the contamination of the spectrum by the nebula and the hotcomponent in the visual region. The accuracy of our spectral types isapproximately one spectral subclass, similar to previous near IRclassification work, and much better than visual spectral typeestimates. Strong, intrinsic spectral type variations (>2 spectralsubtypes) are only seen in systems containing pulsating mira variables.We present a catalogue of spectral types for cool giants in symbioticsystems which also includes determinations taken from the literature.The catalogue gives spectral types for the cool giants in about 170systems which is nearly the full set of confirmed symbiotics. Based onour classifications we discuss the distribution of spectral types of thecool giants in galactic symbiotic binaries. We find that the spectraltypes cluster strongly between M3 and M6, with a peak at M5. Thedistribution of systems with a mira variable component peaks even later,at spectral types M6 and M7. This is a strong bias towards late spectraltypes when compared to red giants in the solar neighbourhood. Also thefrequency of mira variables is much larger among symbiotic giants. Thispredominance of very late M-giants in symbiotic systems seems toindicate that large mass loss is a key ingredient for triggeringsymbiotic activity on a white dwarf companion. Further we find forsymbiotic systems a strong correlation between the spectral type of thecool giant and the orbital period. In particular we find a tightrelation for the minimum orbital period for symbiotic systems with redgiants of a given spectral type. This limiting line in the spectral type- orbital period diagram seems to be equivalent with the relationR<=l_1/2, where R is the radius of the red giant and l_1 the distancefrom the center of the giant to the inner Lagrangian point L_1. Thiscorrelation possibly discloses that symbiotic stars are - with probablyonly one exception in our sample - well detached binary systems. Basedon observations obtained with the 1.52~m and 3.6~m telescopes of theEuropean Southern Observatory (ESO), the 1.93~m telescope of theObservatoire de Haute-Provence (OHP), the 2.3~m telescope of theAustralian National University (ANU) at Siding Spring, and the WilliamHerschel Telescope (WHT) at La Palma. This research has made use of theAFOEV database, operated at CDS, France.
| Pulsation Modes in Small-Amplitude Red Variable Stars
We have investigated the pulsation modes of small-amplitude redvariables with well-determined periods and accurate Hipparcos parallaxesby comparing observational and theoretical Q-values. In our sample of 13stars, one appears to be pulsating in the third overtone, nine in thefirst or second overtone, and three in the fundamental or firstovertone. This result is consistent with the predictions of recentpulsation models by Xiong et al.
| Stellar radii of M giants
We determine the stellar radii of the M giant stars in the Hipparcoscatalogue that have a parallax measured to better than 20% accuracy.This is done with the help of a relation between a visual surfacebrightness parameter and the Cousins (V - I) colour index, which wecalibrate with M giants with published angular diameters.The radii of(non-Mira) M giants increase from a median value of 50 R_Sun at spectraltype M0 III to 170 R_Sun at M7/8 III. Typical intermediate giant radiiare 65 R_Sun for M1/M2, 90 R_Sun for M3, 100 R_Sun for M4, 120 R_Sun forM5 and 150 R_Sun for M6. There is a large intrinsic spread for a givenspectral type. This variance in stellar radius increases with latertypes but in relative terms, it remains constant.We determineluminosities and, from evolutionary tracks, stellar masses for oursample stars. The M giants in the solar neighbourhood have masses in therange 0.8-4 M_Sun. For a given spectral type, there is a close relationbetween stellar radius and stellar mass. We also find a linear relationbetween the mass and radius of non-variable M giants. With increasingamplitude of variability we have larger stellar radii for a given mass.
| Measurements of Starspot Parameters on Active Stars using Molecular Bands in Echelle Spectra
We present results from a study of starspot areas (f_S) and temperatures(T_S), primarily on active, single-lined spectroscopic binaries,determined using molecular absorption bands. Expanding upon our previousstudies, we have analyzed multiorder echelle spectra of eight systems tosimultaneously measure several different molecular bands andchromospheric emission lines. We determined starspot parameters byfitting the molecular bands of interest, using spectra of inactive G andK stars as proxies for the nonspotted photosphere of the active stars,and using spectra of M stars as proxies for the spots. At least twobands with different T_eff sensitivities are required. We found thatfitting bands other than the TiO 7055 and 8860 Å features does notgreatly extend the temperature range or sensitivity of our technique.The 8860 Å band is particularly important because of its sharplydifferent temperature sensitivity. We did not find any substantialdepartures from f_S or T_S that we have measured previously based onsingle-order spectra. We refined our derived spot parameters usingcontemporaneous photometry where available. We found that using M giantsas spot proxies for subgiant active stars often underestimates f_Sneeded to fit the photometry; this is presumably due to the increase instrength of the TiO bands with decreasing gravity. We also investigatedcorrelations between f_S and chromospheric emission, and we developed asimple method to measure nonspot temperature (T_Q) solely from ourechelle spectra.
| Spectroscopic Evidence for Nonuniform Starspot Properties on II Pegasi
We present spectroscopic evidence for multiple spot temperatures on theRS CVn star II Pegasi (HD 224085). We model the strengths of the 7055and 8860 A TiO absorption bands in the spectrum of II Peg using weightedsums of inactive comparison spectra: a K star to represent thenonspotted photosphere and an M star to represent the spots. The bestfit yields independent measurements of the starspot filling factor (fS ) and mean spot temperature ( ) averagedover the visible hemisphere of the star. During three-fourths of arotation of II Peg in late 1996, we measure a constantfS~55%+/-5% . However, varies from3350+/-60 to 3550+/-70 K. We compute for twosimple models: (1) a star with two distinct spot temperatures, and (2) astar with different umbral/penumbral area ratios. The changing correlates with emission strengths of H alpha andthe Ca II infrared triplet in the sense that cooler accompanies weaker emission. We explore possibleimplications of these results for the physical properties of the spotson II Peg and for stellar surface structure in general.
|
הכנס מאמר חדש
לינקים קשורים
הכנס לינק חדש
משמש של הקבוצה הבאה
|
תצפית ומידע אסטרומטרי
| קבוצת-כוכבים: | אריה |
| התרוממות ימנית: | 10h56m01.50s |
| סירוב: | +06°11'07.0" |
| גודל גלוי: | 5.81 |
| מרחק: | 99.701 פארסק |
| תנועה נכונה: | -21 |
| תנועה נכונה: | -5.4 |
| B-T magnitude: | 7.577 |
| V-T magnitude: | 6.116 |
קטלוגים וכינוים:
|
