Contents
Images
Upload your image
DSS Images Other Images
Related articles
Lithium abundances for early F stars: new observational constraints for the Li dilution
Aims.To investigate any correlation between Li abundances and rotationalvelocities among F-G evolved stars, we study a large sample of early Fstars from the Bright Star Catalogue (BSC), most of them classified inthe literature as giant stars.Methods.Physical parameters and Liabundances are estimated for each star, often for the first time, bycomparing observed and synthetic spectra. We analyse the position of thestars in the H-R Diagram based on Hipparcos data using stellarevolutionary tracks and we discuss their Li abundances and projectedrotational velocities.Results.Observed stars are mostly on theturnoff, with masses between 1.5 and 2.0 Mȯ. The starswith measured A(Li) abundance show high Li content, most of them withabundance near the cosmic value. The A(Li) versus V sin i diagram showsthe same trend as reported in previous studies: fast rotators (V sinigse 30 km s-1) are also stars with high Li content, whereasslow rotators present a wide range of values of A(Li), ranging from nodetected Li to the cosmic value.
| Rotation- and temperature-dependence of stellar latitudinal differential rotation
More than 600 high resolution spectra of stars with spectral type F andlater were obtained in order to search for signatures of differentialrotation in line profiles. In 147 stars the rotation law could bemeasured, with 28 of them found to be differentially rotating.Comparison to rotation laws in stars of spectral type A reveals thatdifferential rotation sets in at the convection boundary in theHR-diagram; no star that is significantly hotter than the convectionboundary exhibits the signatures of differential rotation. Four lateA-/early F-type stars close to the convection boundary and at v sin{i}≈ 100 km s-1 show extraordinarily strong absolute shear atshort rotation periods around one day. It is suggested that this is dueto their small convection zone depth and that it is connected to anarrow range in surface velocity; the four stars are very similar inTeff and v sin{i}. Detection frequencies of differentialrotation α = ΔΩ/Ω > 0 were analyzed in starswith varying temperature and rotation velocity. Measurable differentialrotation is more frequent in late-type stars and slow rotators. Thestrength of absolute shear, ΔΩ, and differential rotationα are examined as functions of the stellar effective temperatureand rotation period. The highest values of ΔΩ are found atrotation periods between two and three days. In slower rotators, thestrongest absolute shear at a given rotation rateΔΩmax is given approximately byΔΩmax ∝ P-1, i.e.,αmax ≈ const. In faster rotators, bothαmax and ΔΩmax diminish lessrapidly. A comparison with differential rotation measurements in starsof later spectral type shows that F-stars exhibit stronger shear thancooler stars do and the upper boundary in absolute shear ΔΩwith temperature is consistent with the temperature-scaling law found inDoppler Imaging measurements.
| 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
| Rotation and Lithium Surface Abundances, Revisited
For giants in the Hertzsprung gap, the relations betweenTeff, vsini, and lithium surface abundances arereinvestigated and compared with the relations found for Hyadesmain-sequence F stars. For the Hyades main-sequence F stars, the vsinidecrease steeply around Teff~6450 K. At the same temperaturethe lithium surface abundances show a narrow, deep dip. For most giantsthere is also a steep decrease of vsini for Teff around 6450K. At this temperature the lithium surface abundances of the giants alsodecrease steeply and remain low for Teff<6400 K. Thechanges in rotation and Li surface abundances occur over a temperatureinterval of less than 300 K, which for a 2 Msolar giantcorresponds to an age interval of about 106 yr. Thesimultaneous steep decreases of rotation velocities and Li surfaceabundances indicate that for the giants these changes are due to thesame cause, which we suggest to be deep mixing. It then seems ratherlikely that for the Hyades main-sequence F5 V stars the decrease ofrotation and Li surface abundance is also caused by deep mixing. Wesuggest that in both cases the changes are related to the merging of thehydrogen and helium convection zones.
| The Geneva-Copenhagen survey of the Solar neighbourhood. Ages, metallicities, and kinematic properties of 14 000 F and G dwarfs
We present and discuss new determinations of metallicity, rotation, age,kinematics, and Galactic orbits for a complete, magnitude-limited, andkinematically unbiased sample of 16 682 nearby F and G dwarf stars. Our63 000 new, accurate radial-velocity observations for nearly 13 500stars allow identification of most of the binary stars in the sampleand, together with published uvbyβ photometry, Hipparcosparallaxes, Tycho-2 proper motions, and a few earlier radial velocities,complete the kinematic information for 14 139 stars. These high-qualityvelocity data are supplemented by effective temperatures andmetallicities newly derived from recent and/or revised calibrations. Theremaining stars either lack Hipparcos data or have fast rotation. Amajor effort has been devoted to the determination of new isochrone agesfor all stars for which this is possible. Particular attention has beengiven to a realistic treatment of statistical biases and errorestimates, as standard techniques tend to underestimate these effectsand introduce spurious features in the age distributions. Our ages agreewell with those by Edvardsson et al. (\cite{edv93}), despite severalastrophysical and computational improvements since then. We demonstrate,however, how strong observational and theoretical biases cause thedistribution of the observed ages to be very different from that of thetrue age distribution of the sample. Among the many basic relations ofthe Galactic disk that can be reinvestigated from the data presentedhere, we revisit the metallicity distribution of the G dwarfs and theage-metallicity, age-velocity, and metallicity-velocity relations of theSolar neighbourhood. Our first results confirm the lack of metal-poor Gdwarfs relative to closed-box model predictions (the ``G dwarfproblem''), the existence of radial metallicity gradients in the disk,the small change in mean metallicity of the thin disk since itsformation and the substantial scatter in metallicity at all ages, andthe continuing kinematic heating of the thin disk with an efficiencyconsistent with that expected for a combination of spiral arms and giantmolecular clouds. Distinct features in the distribution of the Vcomponent of the space motion are extended in age and metallicity,corresponding to the effects of stochastic spiral waves rather thanclassical moving groups, and may complicate the identification ofthick-disk stars from kinematic criteria. More advanced analyses of thisrich material will require careful simulations of the selection criteriafor the sample and the distribution of observational errors.Based on observations made with the Danish 1.5-m telescope at ESO, LaSilla, Chile, and with the Swiss 1-m telescope at Observatoire deHaute-Provence, France.Complete Tables 1 and 2 are 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/418/989
| High-Precision Near-Infrared Photometry of a Large Sample of Bright Stars Visible from the Northern Hemisphere
We present the results of 8 yr of infrared photometric monitoring of alarge sample of stars visible from Teide Observatory (Tenerife, CanaryIslands). The final archive is made up of 10,949 photometric measuresthrough a standard InSb single-channel photometer system, principally inJHK, although some stars have measures in L'. The core of this list ofstars is the standard-star list developed for the Carlos SánchezTelescope. A total of 298 stars have been observed on at least twooccasions on a system carefully linked to the zero point defined byVega. We present high-precision photometry for these stars. The medianuncertainty in magnitude for stars with a minimum of four observationsand thus reliable statistics ranges from 0.0038 mag in J to 0.0033 magin K. Many of these stars are faint enough to be observable with arraydetectors (42 are K>8) and thus to permit a linkage of the bright andfaint infrared photometric systems. We also present photometry of anadditional 25 stars for which the original measures are no longeravailable, plus photometry in L' and/or M of 36 stars from the mainlist. We calculate the mean infrared colors of main-sequence stars fromA0 V to K5 V and show that the locus of the H-K color is linearlycorrelated with J-H. The rms dispersion in the correlation between J-Hand H-K is 0.0073 mag. We use the relationship to interpolate colors forall subclasses from A0 V to K5 V. We find that K and M main-sequence andgiant stars can be separated on the color-color diagram withhigh-precision near-infrared photometry and thus that photometry canallow us to identify potential mistakes in luminosity classclassification.
| Differential rotation in rapidly rotating F-stars
We obtained high quality spectra of 135 stars of spectral types F andlater and derived ``overall'' broadening functions in selectedwavelength regions utilizing a Least Squares Deconvolution (LSD)procedure. Precision values of the projected rotational velocity v \siniwere derived from the first zero of the Fourier transformed profiles andthe shapes of the profiles were analyzed for effects of differentialrotation. The broadening profiles of 70 stars rotating faster than v\sini = 45 km s-1 show no indications of multiplicity nor ofspottedness. In those profiles we used the ratio of the first two zerosof the Fourier transform q_2/q_1 to search for deviations from rigidrotation. In the vast majority the profiles were found to be consistentwith rigid rotation. Five stars were found to have flat profilesprobably due to cool polar caps, in three stars cuspy profiles werefound. Two out of those three cases may be due to extremely rapidrotation seen pole on, only in one case (v \sini = 52 km s-1)is solar-like differential rotation the most plausible explanation forthe observed profile. These results indicate that the strength ofdifferential rotation diminishes in stars rotating as rapidly as v \sini>~ 50 km s-1.Table A.1 is only available at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.125.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/412/813Based on observations collected at the European Southern Observatory, LaSilla, 69.D-0015(B).
| Lithium and rotation in F and G dwarfs and subgiants
Lithium abundances have been determined in 127 F and G Pop I stars basedon new measurements of the equivalent width of the lambda 6707 ÅLi I line from their high resolution CCD spectra. Distances and absolutemagnitudes of these stars have been obtained from the HipparcosCatalogue and their masses and ages derived, enabling us to investigatethe behaviour of lithium as a function of these parameters. Based ontheir location on the HR diagram superposed on theoretical evolutionarytracks, the sample of the stars has been chosen to ensure that they havemore or less completed their Li depletion on the main sequence. A largespread in the Li abundances is found at any given effective temperatureespecially in the already spun down late F and early G stars. Thisspread persists even if the ``Li-dip'' stars that have evolved from themain sequence temperature interval 6500-6800 K are excluded. Stars inthe mass range up to 2 M/Msun when divided into threemetallicity groups show a linear correlation between Li abundance andmass, albeit with a large dispersion around it which is not fullyaccounted for by age either. The large depletions and the observedspread in Li are in contrast to the predictions of the standard stellarmodel calculations and suggest that they are aided by non-standardprocesses depending upon variables besides mass, age and metallicity.The present study was undertaken to examine, in particular, the effectsof rotation on the depletion of Li. No one-to-one correlation is foundbetween the Li abundance and the present projected rotational velocity.Instead the observed abundances seem to be dictated by the rotationalhistory of the star. However, it is noted that even this interpretationis subject to the inherent limitation in the measurement of the observedLi EQW for large rotational velocities.Table 1 is only available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/409/251
| 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.
| 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
| Sixth Catalogue of Fundamental Stars (FK6). Part III. Additional fundamental stars with direct solutions
The FK6 is a suitable combination of the results of the HIPPARCOSastrometry satellite with ground-based data, measured over a longinterval of time and summarized mainly in the FK5. Part III of the FK6(abbreviated FK6(III)) contains additional fundamental stars with directsolutions. Such direct solutions are appropriate for single stars or forobjects which can be treated like single stars. Part III of the FK6contains in total 3272 stars. Their ground-based data stem from thebright extension of the FK5 (735 stars), from the catalogue of remainingSup stars (RSup, 732 stars), and from the faint extension of the FK5(1805 stars). From the 3272 stars in Part III, we have selected 1928objects as "astrometrically excellent stars", since their instantaneousproper motions and their mean (time-averaged) ones do not differsignificantly. Hence most of the astrometrically excellent stars arewell-behaving "single-star candidates" with good astrometric data. Thesestars are most suited for high-precision astrometry. On the other hand,354 of the stars in Part III are Δμ binaries in the sense ofWielen et al. (1999). Many of them are newly discovered probablebinaries with no other hitherto known indication of binarity. The FK6gives, besides the classical "single-star mode" solutions (SI mode),other solutions which take into account the fact that hidden astrometricbinaries among "apparently single-stars" introduce sizable "cosmicerrors" into the quasi-instantaneously measured HIPPARCOS proper motionsand positions. The FK6 gives, in addition to the SI mode, the "long-termprediction (LTP) mode" and the "short-term prediction (STP) mode". TheseLTP and STP modes are on average the most precise solutions forapparently single stars, depending on the epoch difference with respectto the HIPPARCOS epoch of about 1991. The typical mean error of anFK6(III) proper motion in the single-star mode is 0.59 mas/year. This isa factor of 1.34 better than the typical HIPPARCOS errors for thesestars of 0.79 mas/year. In the long-term prediction mode, in whichcosmic errors are taken into account, the FK6(III) proper motions have atypical mean error of 0.93 mas/year, which is by a factor of about 2better than the corresponding error for the HIPPARCOS values of 1.83mas/year (cosmic errors included).
| Testing convection theories using Balmer line profiles of A, F, and G stars
We consider the effects of convection on the Balmer line profiles({H_α} and {H_β}) of A, F, and G stars. The standardmixing-length theory (MLT) atlas9 models of Kurucz (1993), with andwithout overshooting, are compared to atlas9 models based on theturbulent convection theory proposed by Canuto & Mazzitelli (1991,1992) and implemented by Kupka (1996), and the improved version of thismodel proposed by Canuto et al. (1996) also implemented by Kupka. TheBalmer line profiles are a useful tool in investigating convectionbecause they are very sensitive to the parameters of convection used inthe stellar atmosphere codes. The {H_α} and {H_β} lines areformed at different depths in the atmosphere. The {H_α} line isformed just above the convection zone. The {H_β} line, however, ispartially formed inside the convection zone. We have calculated the{T_eff} of observed stars by fitting Balmer line profiles to syntheticspectra and compared this to: (i) the {T_eff} of the fundamental stars;(ii) the {T_eff} of stars determined by the Infra-Red Flux Method and(iii) the {T_eff} determined by Geneva photometry for the stars in theHyades cluster. We find that the results from the {H_α} and{H_β} lines are different, as expected, due to the differing levelsof formation. The tests are inconclusive between three of the fourmodels; MLT with no overshooting, CM and CGM models, which all giveresults in reasonable agreement with fundamental values. The resultsindicate that for the MLT theory with no overshooting it is necessary toset the mixing length parameter alpha equal to 0.5 for stars with {T_eff<= 6000} K or {T_eff >= 7000} K. However for stars with {6000}K{<= T_eff <= 7000} K the required value for the parameter is{alpha >= 1.25}. Models with overshooting are found to be clearlydiscrepant, consistent with the results with uvby photometry by Smalley& Kupka (1997). Based on observations made at the Observatorio delRoque de los Muchachos using the Richardson-Brealey Spectrograph on the1.0m Jacobs Kapteyn Telescope.
| On the rotation-activity correlation for active binary stars
We present an investigation of rotation-activity correlations usingInternational Ultraviolet Explorer (IUE) SWP measurements of the Civemission line at 1550Angstroms for 72 active binary systems. We use astandard stellar evolution code to derive non-empirical Rossby numbers,R_0, for each star in our sample and compare the resulting Civrotation-activity correlation to that found for empirically derivedvalues of the Rossby number and that based on rotation alone. For dwarfstars our values of R_0 do not differ greatly from empirical ones and wefind a corresponding lack of improvement in correlation. Only a marginalimprovement in correlation is found for evolved components in oursample. We discuss possible additional factors, other than rotation orconvection, that may influence the activity levels in active binaries.Our observational data imply, in contrast to the theoretical predictionsof convective motions, that activity is only weakly related to mass inevolved stars. We conclude that current dynamo theory is limited in itsapplication to the study of active stars because of the uncertainty inthe angular velocity-depth profile in stellar interiors and the unknowneffects of binarity and surface gravity.
| Determination of the temperatures of selected ISO flux calibration stars using the Infrared Flux Method
Effective temperatures for 420 stars with spectral types between A0 andK3, and luminosity classes between II and V, selected for a fluxcalibration of the Infrared Space Observatory, ISO, have been determinedusing the Infrared Flux Method (IRFM). The determinations are based onnarrow and wide band photometric data obtained for this purpose, andtake into account previously published narrow-band measures oftemperature. Regression coefficients are given for relations between thedetermined temperatures and the photometric parameters (B2-V1), (b-y)and (B-V), corrected for interstellar extinction through use ofHipparcos parallaxes. A correction for the effect of metallicity on thedetermination of integrated flux is proposed. The importance of aknowledge of metallicity in the representation of derived temperaturesfor Class V, IV and III stars by empirical functions is discussed andformulae given. An estimate is given for the probable error of eachtemperature determination. Based on data from the ESA HipparcosAstrometry Satellite.
| Are metallic A-F giants evolved AM stars? Rotation and rate of binaries among giant F stars
We test the hypothesis of Berthet (1992) {be91} which foresees that Amstars become giant metallic A and F stars (defined by an enhanced valueof the blanketing parameter Delta m_2 of the Geneva photometry) whenthey evolve. If this hypothesis is right, Am and metallic A-FIII starsneed to have the same rate of binaries and a similar distribution ofvsin i. From our new spectroscopic data and from vsin i and radialvelocities in the literature, we show that it is not the case. Themetallic giant stars are often fast rotators with vsin i larger than 100kms(-1) , while the maximum rotational velocity for Am stars is about100 kms(-1) . The rate of tight binaries with periods less than 1000days is less than 30% among metallic giants, which is incompatible withthe value of 75% for Am stars - [Abt & Levy 1985] {ab85}).Therefore, the simplest way to explain the existence of giant metallic Fstars is to suggest that all normal A and early F stars might go througha short ``metallic" phase when they are finishing their life on the mainsequence. Besides, it is shown that only giant stars with spectral typecomprised between F0 and F6 may have a really enhanced Delta m_2 value,while all A-type giants seem to be normal. Based on observationscollected at Observatoire de Haute Provence (OHP), France.
| Towards a fundamental calibration of stellar parameters of A, F, G, K dwarfs and giants
I report on the implementation of the empirical surface brightnesstechnique using the near-infrared Johnson broadband { (V-K)} colour assuitable sampling observable aimed at providing accurate effectivetemperatures of 537 dwarfs and giants of A-F-G-K spectral-type selectedfor a flux calibration of the Infrared Space Observatory (ISO). Thesurface brightness-colour correlation is carefully calibrated using aset of high-precision angular diameters measured by moderninterferometry techniques. The stellar sizes predicted by thiscorrelation are then combined with the bolometric flux measurementsavailable for a subset of 327 ISO standard stars in order to determineone-dimensional { (T, V-K)} temperature scales of dwarfs and giants. Theresulting very tight relationships show an intrinsic scatter induced byobservational photometry and bolometric flux measurements well below thetarget accuracy of +/- 1 % required for temperature determinations ofthe ISO standards. Major improvements related to the actual directcalibration are the high-precision broadband { K} magnitudes obtainedfor this purpose and the use of Hipparcos parallaxes for dereddeningphotometric data. The temperature scale of F-G-K dwarfs shows thesmallest random errors closely consistent with those affecting theobservational photometry alone, indicating a negligible contributionfrom the component due to the bolometric flux measurements despite thewide range in metallicity for these stars. A more detailed analysisusing a subset of selected dwarfs with large metallicity gradientsstrongly supports the actual bolometric fluxes as being practicallyunaffected by the metallicity of field stars, in contrast with recentresults claiming somewhat significant effects. The temperature scale ofF-G-K giants is affected by random errors much larger than those ofdwarfs, indicating that most of the relevant component of the scattercomes from the bolometric flux measurements. Since the giants have smallmetallicities, only gravity effects become likely responsible for theincreased level of scatter. The empirical stellar temperatures withsmall model-dependent corrections are compared with the semiempiricaldata by the Infrared Flux Method (IRFM) using the large sample of 327comparison stars. One major achievement is that all empirical andsemiempirical temperature estimates of F-G-K giants and dwarfs are foundto be closely consistent between each other to within +/- 1 %. However,there is also evidence for somewhat significant differential effects.These include an average systematic shift of (2.33 +/- 0.13) % affectingthe A-type stars, the semiempirical estimates being too low by thisamount, and an additional component of scatter as significant as +/- 1 %affecting all the comparison stars. The systematic effect confirms theresults from other investigations and indicates that previousdiscrepancies in applying the IRFM to A-type stars are not yet removedby using new LTE line-blanketed model atmospheres along with the updatedabsolute flux calibration, whereas the additional random component isfound to disappear in a broadband version of the IRFM using an infraredreference flux derived from wide rather than narrow band photometricdata. Table 1 and 2 are only available in the electronic form of thispaper
| Mesures de vitesses radiales. VIII. Accompagnement AU sol DU programme d'observation DU satellite HIPPARCOS
We publish 1879 radial velocities of stars distributed in 105 fields of4^{\circ} \times 4^{\circ}. We continue the PPO series \cite[(Fehrenbachet al. 1987;]{Feh87} \cite[Duflot et al. 1990, 1992 and 1995),]{Du90}using the Fehrenbach objective prism method. Table 1 only available inelectronic form at CDS via to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| The role of convection on the UVBY colours of A, F, and G stars
We discuss the effects of convection on the theoretical uvby colours ofA, F, and G stars. The standard mixing-length theory atlas9 models ofKurucz (1993), with and without approximate overshooting, are comparedto models using the turbulent convection theory proposed by Canuto &Mazzitelli (1991, 1992) and implemented by Kupka (1996a). Comparisonwith fundamental T_eff and log g stars reveals that the Canuto &Mazzitelli models give results that are generally superior to standardmixing-length theory (MLT) without convective overshooting. MLT modelswith overshooting are found to be clearly discrepant. This is supportedby comparisons of non-fundamental stars, with T_eff obtained from theInfrared Flux Method and log g from stellar evolutionary models for opencluster stars. The Canuto & Mazzitelli theory gives values of(b-y)_0 and c_0 that are in best overall agreement with observations.Investigations of the m_0 index reveal that all of the treatments ofconvection presented here give values that are significantly discrepantfor models with T_eff < 6000 K. It is unclear as to whether this isdue to problems with the treatment of convection, missing opacity, orsome other reason. None of the models give totally satisfactory m_0indices for hotter stars, but the Canuto & Mazzitelli models are inclosest overall agreement above 7000 K. Grids of uvby colours, based onthe CM treatment of convection, are presented. These grids represent animprovement over the colours obtained from models using themixing-length theory. The agreement with fundamental stars enables thecolours to be used directly without the need for semi-empiricaladjustments that were necessary with the earlier colour grids. Table~5is only available at the CDS via anonymous ftp 130.79.128.5 or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| Notes on the convection in the ATLAS9 model atmospheres.
The mixing-length theory for the convection, as it is used in the ATLAS9code (Kurucz, 1993a), is summarized and discussed. We investigated theeffect of the modification called ``approximate overshooting'' on themodel structure of the Sun and of stars with T_eff_ included between4000K and 8500K, logg included between 2.5 and 4.5, and metallicities[M/H]=0.0 and [M/H]=-3.0. We found that the Kurucz solar model (SUNK94)with the ``overshooting'' option switched on reproduces moreobservations than that without ``overshooting''. In theHgamma_ and Hbeta_ regions no solar model is ableto reproduce the level of the true continuum deduced fromhigh-resolution observations absolutely calibrated. At 486 nm thecomputed continuum is about 6.6% higher than that inferred from theobserved spectrum. We found that the largest effect of the ``approximateovershooting'' on the model structure occurs for models withT_eff_>6250K and it decreases with decreasing gravity. Thedifferences in (b-y), (B-V), and (V-K) indices computed from models withthe ``overshooting'' option switched on and off, correspond to T_eff_differences which may amount up to 180K, 100K, 60K respectively. Thedifferences in T_eff_ from Balmer profiles may amount up to 340K andthey occur also for T_eff_<6250K down to about 5000K. The c_1_ indexyields gravity differences {DELTA}logg as a function of logg which, foreach T_eff_, grow to a maximum value. The maximum {DELTA}logg decreaseswith increasing temperatures and ranges, for solar metallicity, from 0.7dex at logg=0.5 and T_eff_=5500K to 0.2dex at logg=4.5 and T_eff_=8000K.This behaviour does not change for [M/H]=-3.0. Comparisons with theobservations indicate that model parameters derived with differentmethods are more consistent when the ``overshooting'' option is switchedoff (NOVER models), except for the Sun. In particular for Procyon,T_eff_ and logg from NOVER models are closer to the parameters derivedfrom model independent methods than are T_eff_ and logg derived from theKurucz (1995) grids. However, no model is able to explain the wholeobserved spectrum of either the Sun or Procyon with a unique T_eff_,regardless of whether the ``overshooting'' option is switched on or off.Independently of the convection option, the largest differences inT_eff_ derived with different methods are of the order of 200K forProcyon and 150K for the Sun.
| Transformations from Theoretical Hertzsprung-Russell Diagrams to Color-Magnitude Diagrams: Effective Temperatures, B-V Colors, and Bolometric Corrections
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1996ApJ...469..355F&db_key=AST
| Vitesses radiales. Catalogue WEB: Wilson Evans Batten. Subtittle: Radial velocities: The Wilson-Evans-Batten catalogue.
We give a common version of the two catalogues of Mean Radial Velocitiesby Wilson (1963) and Evans (1978) to which we have added the catalogueof spectroscopic binary systems (Batten et al. 1989). For each star,when possible, we give: 1) an acronym to enter SIMBAD (Set ofIdentifications Measurements and Bibliography for Astronomical Data) ofthe CDS (Centre de Donnees Astronomiques de Strasbourg). 2) the numberHIC of the HIPPARCOS catalogue (Turon 1992). 3) the CCDM number(Catalogue des Composantes des etoiles Doubles et Multiples) byDommanget & Nys (1994). For the cluster stars, a precise study hasbeen done, on the identificator numbers. Numerous remarks point out theproblems we have had to deal with.
| The lithium content and other properties of F2-G5 giants in the Hertzsprung Gap
As stars of 2-5 solar mass evolve across the Hertzsprung Gap they shouldfirst deplete their surface lithium by convective dilution and then,when convection penetrates deeper, begin to bring CN processed materialto their surfaces. To investigate this process we have observed 52giants, 25 of which have known C/N ratios, for their Li abundances.After eliminating four stars that may actually be dwarfs and includingthe two components of Capella analyzed by Pilachowski and Sowell we havecompared our (Li/Fe) ratios with models of Swenson. For stars showing vsin i greater than 50 km/s we find (Li/Fe) to be unaffected by mixingfor B - V less than 0.7 as predicted. For stars cooler than B - V = 0.7both v sin i and (Li/Fe) drop to smaller values. For the sharp linedstars (v sin i less than 50 km/s) we find a drop in Li between B - V =0.45 and 0.60 which cannot be understood in terms of dilution byconvection. Various possible causes of such an early depletion ordilution of surface Li are discussed including diffusion at the base ofthe convection zone, mass loss possibly enhanced by pulsation, andmagnetic activity as in the magnetic A and B type stars. The models ofRicher & Michaud (1993) with diffusion point toward a satisfactorysolution. A few giants with low v sin i values stand out with muchhigher than expected (Li/Fe) values despite their cool effectivetemperatures. We do not understand why those stars have not depletedtheir lithium as have most giants of similar color. The correlation of(N/C) with (Li/Fe) follows expectations in so far as almost all starswith enhanced (N/C) have depleted their Li as well.
| Stellar effective temperatures and angular diameters determined by the infrared flux method (IRFM): Revisions using improved Kurucz LTE stellar atmospheres
Infrared flux method (IRFM) determinations of stellar effectivetemperatures and angular diameters are revised using new Kurucz localthermodynamic equilibrium (LTE) line-blanketed model atmospheres, whichmore accurately predict the emergent stellar radiation flux than modelsused previously. An improved method for deriving integrated stellarfluxes is described, together with polynomial coefficients forevaluating them from V and V-K. Tables were given for making smallcorrections appropiate to changes in log g, metallicity, fluxcalibration in J, K and L and interstellar extinction, to avoid the needfor additional tabular material. The determined temperatures areexpressed in terms of V and V-K, giving a mean absolute deviation of0.53%. A comparison of the derived angular diameters for three starswith determinations using Michelson interferometer shows an averageagreement to better than 1%.
| Improved Mean Positions and Proper Motions for the 995 FK4 Sup Stars not Included in the FK5 Extension
Not Available
| A catalog of stellar Lyman-alpha fluxes
We present a catalog of stellar Ly-alpha emission fluxes, based on newand archival images obtained with the IUE spacecraft. The catalogincludes 227 stars with detectable Ly-alpha emission fluxes, and upperlimits on the Ly-alpha emission flux for another 48 stars. Multiple fluxmeasurements are given for 52 stars. We present a model for correctingthe observed Ly-alpha flux for attenuation by the local interstellarmedium, and we apply this model to derive intrinsic Ly-alpha fluxes for149 catalog stars which are located in low H I column density directionsof the local interstellar medium. In our catalog, there are 14 late-Aand early-F stars at B-V = 0.29 or less that show detectable emission atLy-alpha. We find a linear correlation between the intrinsic Ly-alphaflux and C II 1335 A flux for stars with B-V greater than 0.60, but theA and F stars deviate from this relation in the sense that theirLy-alpha flux is too low. We also find a good correlation betweenLy-alpha strength and coronal X-ray emission. This correlation holdsover most of the H-R diagram, even for the F stars, where an X-raydeficit has previously been found relative to the transition regionlines of C II and C IV.
| Stromgren Four-Colour UVBY Photometry of G5-TYPE Hd-Stars Brighter than MV=8.6
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1993A&AS..102...89O&db_key=AST
| Corrections to the right ascension to be applied to the apparent places of 1217 stars given in "The Chinese Astronomical Almanach" for the year 1984 to 1992.
Not Available
| Spectral synthesis in the ultraviolet. IV - A library of mean stellar groups
A library of mean UV stellar energy distributions is derived from IUEspectrophotometry of 218 stars. The spectra cover 1230-3200 A with aspectral resolution of about 6 A. They have been corrected forinterstellar extinction and converted to a common flux and wavelengthscale. Individual stars were combined into standard groups according totheir continuum colors, observed UV spectral morphology, MK luminosityclass, and metal abundance. The library consists of 56 groups: 21dwarf(V), 8 subgiant(IV), 16 giant(III), and supergiant(I + II) groups,covering O3-M4 spectral types. A metal-poor sequence is included,containing four dwarf and two giant groups, as is a metal-enhancedsequence with a single dwarf, subgiant, and giant group. Spectralindices characterizing the continuum and several strong absorptionfeatures are examined as temperature, luminosity, and abundancediagnostics. The library is intended to serve as a basis forinterpreting the composite UV spectra of a wide variety of stellarsystems, e.g., elliptical galaxies, starburst systems, and high-redshiftgalaxies.
| Carbon and nitrogen abundances determined from transition layer lines
The possibility of determining relative carbon, nitrogen, and siliconabundances from the emission-line fluxes in the lower transition layersbetween stellar chromospheres and coronae is explored. Observations formain-sequence and luminosity class IV stars with presumably solarelement abundances show that for the lower transition layers Em =BT-gamma. For a given carbon abundance the constants gammaand B in this relation can be determined from the C II and C IVemission-line fluxes. From the N V and S IV lines, the abundances ofthese elements relative to carbon can be determined from their surfaceemission-line fluxes. Ratios of N/C abundances determined in this wayfor some giants and supergiants agree within the limits of errors withthose determined from molecular bands. For giants, an increase in theratio of N/C at B-V of about 0.8 is found, as expected theoretically.
| Rotation and transition layer emission in cool giants
Gray (1981, 1982) found that field giants with T(eff) less than about5500 K experience a steep decrease in rotational velocities coupled witha decrease in transition layer emission. This decrease may beattributable to fast magnetic braking or to redistribution of angularmomentum for rapidly increasing depths of the convection zones if theserotate with depth independent specific angular momentum. Additionalarguments in favor of the latter interpretation are presented. Theincrease of N/C abundances due to deep mixing occurs at the same pointas the decrease in v sin i. On the other hand, the ratios of the C IV toC II emission line fluxes decrease at this point indicating smallercontributions of MHD wave heating. The X-ray fluxes decrease at nearlythe same T(eff). Thus, no observations are found which would indicatelarger magnetic activity which could lead to fast magnetic braking.Theory predicts a rapid increase in the convection zone depth at theT(eff) where the decrease in v sin i is observed. This can explain theobserved phenomena.
|
Submit a new article
Related links
Submit a new link
Member of following groups:
|
Observation and Astrometry data
| Constellation: | Δίδυμοι |
| Right ascension: | 07h39m09.90s |
| Declination: | +34°35'03.0" |
| Apparent magnitude: | 4.9 |
| Distance: | 48.497 parsecs |
| Proper motion RA: | -34.5 |
| Proper motion Dec: | -114.8 |
| B-T magnitude: | 5.37 |
| V-T magnitude: | 4.928 |
Catalogs and designations:
|
