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51 Tau




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The Puzzle of the Metallic Line Stars
In the puzzle of the metallic line (Am) stars, there still seem to bemissing pieces. While the ``normal'' A stars have elemental abundancesclose to solar, the classical Am stars show stronger absorption linesfor most heavy elements in their spectra. Elements with ionizationpotentials that nearly agree with those of hydrogen or helium havereduced abundances. The Ca II and Sc II lines are especially weak. TheAm stars have no ultraviolet emission lines. They are binaries that,with very few exceptions, have rotational velocities vsini lower than100 km s-1. Of the main-sequence A stars, 20% to 30% are Amstars. Here we rediscuss previous suggestions that tried to explain thepeculiar line strengths in the Am star spectra. In particular, wecompare the well-studied properties of Hyades A and Am stars in order toidentify reasons that can or cannot explain the differences. We findthat accretion of interstellar material by A stars with distortedmagnetic fields, which are weaker than those in peculiar A (Ap) stars,has the best chance of explaining the main characteristics of thepeculiar heavy-element abundances in Am star photospheres.Charge-exchange reactions also seem to be important.

Modelling the components of binaries in the Hyades: the dependence of the mixing-length parameter on stellar mass
We present our findings based on a detailed analysis of the binaries ofthe Hyades, in which the masses of the components are well known. We fitthe models of the components of a binary system to observations so as togive the observed total V and B-V of that system and the observed slopeof the main sequence in the corresponding parts. According to ourfindings, there is a very definite relationship between themixing-length parameter and the stellar mass. The fitting formula forthis relationship can be given as α= 9.19(M/Msolar-0.74)0.053- 6.65, which is valid for stellar masses greaterthan 0.77Msolar. While no strict information is gathered forthe chemical composition of the cluster, as a result of degeneracy inthe colour-magnitude diagram, by adopting Z= 0.033 and using models forthe components of 70 Tau and θ2 Tau we find thehydrogen abundance to be X= 0.676 and the age to be 670 Myr. If weassume that Z= 0.024, then X= 0.718 and the age is 720 Myr. Our findingsconcerning the mixing-length parameter are valid for both sets of thesolution. For both components of the active binary system V818 Tau, thedifferences between radii of the models with Z= 0.024 and the observedradii are only about 4 per cent. More generally, the effectivetemperatures of the models of low-mass stars in the binary systemsstudied are in good agreement with those determined by spectroscopicmethods.

Adaptive Optics Photometry and Astrometry of Binary Stars
We present astrometric and photometric measurements of 39 binary starsmade with the adaptive optics system on the 3.6 m AdvancedElectro-Optical System (AEOS) telescope, taken from 2002 November to2003 March. The binaries have separations ranging from 0.08" to 5.11"and differential magnitudes ranging from 0.096 to 7.9. Also, we includea list of observations of 23 known binaries that we were unable toresolve. In the process of these measurements, we discovered three newcompanions to two previously known binary stars. We also discuss theeffects of scintillation and anisoplanatism on measurements of binarystar photometry in adaptive optics images. Suggestions on how tominimize these effects are then given.Based on observations made at the Maui Space Surveillance Systemoperated by Detachment 15 of the US Air Force Research Laboratory'sDirected Energy Directorate.

Astrometric orbits of SB^9 stars
Hipparcos Intermediate Astrometric Data (IAD) have been used to deriveastrometric orbital elements for spectroscopic binaries from the newlyreleased Ninth Catalogue of Spectroscopic Binary Orbits(SB^9). This endeavour is justified by the fact that (i) theastrometric orbital motion is often difficult to detect without theprior knowledge of the spectroscopic orbital elements, and (ii) suchknowledge was not available at the time of the construction of theHipparcos Catalogue for the spectroscopic binaries which were recentlyadded to the SB^9 catalogue. Among the 1374 binaries fromSB^9 which have an HIP entry (excluding binaries with visualcompanions, or DMSA/C in the Double and Multiple Stars Annex), 282 havedetectable orbital astrometric motion (at the 5% significance level).Among those, only 70 have astrometric orbital elements that are reliablydetermined (according to specific statistical tests), and for the firsttime for 20 systems. This represents a 8.5% increase of the number ofastrometric systems with known orbital elements (The Double and MultipleSystems Annex contains 235 of those DMSA/O systems). The detection ofthe astrometric orbital motion when the Hipparcos IAD are supplementedby the spectroscopic orbital elements is close to 100% for binaries withonly one visible component, provided that the period is in the 50-1000 drange and the parallax is >5 mas. This result is an interestingtestbed to guide the choice of algorithms and statistical tests to beused in the search for astrometric binaries during the forthcoming ESAGaia mission. Finally, orbital inclinations provided by the presentanalysis have been used to derive several astrophysical quantities. Forinstance, 29 among the 70 systems with reliable astrometric orbitalelements involve main sequence stars for which the companion mass couldbe derived. Some interesting conclusions may be drawn from this new setof stellar masses, like the enigmatic nature of the companion to theHyades F dwarf HIP 20935. This system has a mass ratio of 0.98 but thecompanion remains elusive.

Differential photometry of speckle-interferometric binary and multiple stars
A method for differential photometry of speckle-interferometric binaryand multiple stars is presented. Both the accuracy and sources ofsystematic errors of the method are analysed. The photometric accuracyranges between 0.02 m and 0.20 m, depending on the atmospheric seeing,the brightness and the separation of the system components. A comparisonbetween our magnitude differences and those of other authors ispresented.

Seismic constraints on open clusters
The aim of this theoretical and modelling paper is to derive knowledgeon the global and structural parameters of low-mass stars usingasteroseismology and taking advantage of the stellar collective behaviorwithin open clusters. We build stellar models and compute the seismicsignal expected from main sequence objects in the 0.8-1.6Mȯ range. We first evaluate apparent magnitudes andoscillations-induced luminosity fluctuations expected in the Hyades, thePleiades and the α Persei clusters. The closest cluster presents afeasible challenge to observational asteroseismology in the present andnear future. The remainder of the work therefore focuses on the Hyades.We combine seismological and classical computations to address threequestions: what can be inferred about 1) mass; 2) composition; and 3)extension of outer convection zones of solar analogs in the Hyades. Thefirst issue relies on the strong sensitivity of the large separation tomass. We show that seismic constraints provide masses to a precisionlevel (0.05 Mȯ) that is competitive with the actualmass estimations from binary systems. Then large separations (Δν) and second differences (δ2 ν) are used torespectively constrain metal and helium fractions in the Hyades. Whenplotted for several masses, the relation of effective temperature(Teff) vs. large separation (Δ ν) is found to bestrongly dependent on the metal content. Besides this the seconddifference main modulation is related to the second ionization ofhelium. An accuracy in the helium mass fraction of 0.02 to 0.01 can beachieved provided mass and age are accurately known, which is the casefor a few Hyades binary systems. The second difference modulations arealso partly due to the discontinuity in stellar stratification at theconvective envelope/radiative core transition. They permit directinsight in the stellar structure. We compute acoustic radii of theconvective bases for different values of the mixing length theoryparameter αMLT in convection modelling, i.e. differentconvective efficiency in the superadiabatic layers. For a giveneffective temperature we show that the acoustic radius changes withconvection efficiency. This suggests that seismology can provideconstraints on the extension of outer convection and also more generallyon the direct approaches of convection and dynamical phenomena beingcurrently developed.

An Assessment of Dynamical Mass Constraints on Pre-Main-Sequence Evolutionary Tracks
We have assembled a database of stars having both masses determined frommeasured orbital dynamics and sufficient spectral and photometricinformation for their placement on a theoretical H-R diagram. Our sampleconsists of 115 low-mass (M<2.0 Msolar) stars, 27pre-main-sequence and 88 main-sequence. We use a variety of availablepre-main-sequence evolutionary calculations to test the consistency ofpredicted stellar masses with dynamically determined masses. Despitesubstantial improvements in model physics over the past decade, largesystematic discrepancies still exist between empirical and theoreticallyderived masses. For main-sequence stars, all models considered predictmasses consistent with dynamical values above 1.2 Msolar andsome models predict consistent masses at solar or slightly lower masses,but no models predict consistent masses below 0.5 Msolar,with all models systematically underpredicting such low masses by5%-20%. The failure at low masses stems from the poor match of mostmodels to the empirical main sequence below temperatures of 3800 K, atwhich molecules become the dominant source of opacity and convection isthe dominant mode of energy transport. For the pre-main-sequence samplewe find similar trends. There is generally good agreement betweenpredicted and dynamical masses above 1.2 Msolar for allmodels. Below 1.2 Msolar and down to 0.3 Msolar(the lowest mass testable), most evolutionary models systematicallyunderpredict the dynamically determined masses by 10%-30%, on average,with the Lyon group models predicting marginally consistent masses inthe mean, although with large scatter. Over all mass ranges, theusefulness of dynamical mass constraints for pre-main-sequence stars isin many cases limited by the random errors caused by poorly determinedluminosities and especially temperatures of young stars. Adopting awarmer-than-dwarf temperature scale would help reconcile the systematicpre-main-sequence offset at the lowest masses, but the case for this isnot compelling, given the similar warm offset at older ages between mostsets of tracks and the empirical main sequence. Over all age ranges, thesystematic discrepancies between track-predicted and dynamicallydetermined masses appear to be dominated by inaccuracies in thetreatment of convection and in the adopted opacities.

Empirically Constrained Color-Temperature Relations. I. BV(RI)C
This investigation presents a set of transformations to Johnson B-V,Cousins V-R, and Cousins V-I, as well as bolometric corrections to V,for [Fe/H]=-3, -2, -1, -0.5, 0.0, and +0.3 and, in each case, values oflogg from -0.5 to 5.0 for 3000 K<=Teff<=5500 K and from2.0 to 5.0 for 6000 K<=Teff<=40,000 K. Thesetransformations employ the predictions from Kurucz model atmospheres athigh temperatures (Teff>=8000 K) and from MARCS modelatmospheres at intermediate temperatures (from 7000 K down to atemperature in the range 4000 K<=Teff<=5500 K,depending on [Fe/H], where adjustments to satisfy observationalconstraints become necessary). Thus, theoretical color-Teffrelations are used exclusively down to a minimum temperature that iscooler than the temperatures of turnoff stars in open and globular starclusters. To better represent the color transformations obeyed by coolstars (down to 3000 K), corrections to the synthetic transformationshave been determined from a careful consideration of observations for afew globular clusters (M92, M68, and 47 Tucanae), the color-magnitudediagrams (CMDs) of several open clusters (M67, the Pleiades, the Hyades,and NGC 6791), the CMDs and mass-luminosity diagram for solarneighborhood stars having good distance measurements from Hipparcos,empirical (B-V)-Teff and (V-K)-Teff relations, andcolor-color diagrams for field giants. The semiempirical colortransformations that have been produced as a result of our analysis arealso compared with several others that have been published in recentyears: some of the deficiencies of the latter are revealed.

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 ( 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).

What Is Happening at Spectral Type F5 in Hyades F Stars?
Aiming at a better understanding of the mechanisms heating thechromospheres, transition regions, and coronae of cool stars, we studyultraviolet, low-resolution Hubble Space Telescope/Space TelescopeImaging Spectrograph spectra of Hyades main-sequence F stars. We studythe B-V dependence(s) of the chromospheric and transition layer emissionline fluxes and their dependences on rotational velocities. We find thatthe transition layer emission line fluxes and also those of strongchromospheric lines decrease steeply between B-V=0.42 and 0.45, i.e., atspectral type F5, for which the rotational velocities also decreasesteeply. The magnitude of the line-flux decrease increases for lines ofions with increasing degree of ionization. This shows that the line-fluxdecrease is not due to a change in the surface filling factor but ratherdue to a change of the relative importance of different heatingmechanisms. For early F stars with B-V<0.42 we find for thetransition layer emission lines increasing fluxes for increasing vsini,indicating magnetohydrodynamic heating. The vsini dependence isstrongest for the high-ionization lines. On the other hand, the lowchromospheric lines show no dependence on vsini, indicating acousticshock heating for these layers. This also contributes to the heating ofthe transition layers. The Mg II and Ca II lines show decreasing fluxesfor increasing vsini, as long as vsini is less than ~40 kms-1. The coronal X-ray emission also decreases for increasingvsini, except for vsini larger than ~100 km s-1. We have atpresent no explanation for this behavior. For late F stars thechromospheric lines show vsini dependences similar to those observed forearly F stars, again indicating acoustic heating for these layers. Wewere unable to determine the vsini dependence of the transition layerlines because of too few single star targets. The decrease of emissionline fluxes at the spectral type F5, with steeply decreasing vsini,indicates, however, a decreasing contribution of magnetohydrodynamicheating for the late F stars. The X-ray emission for the late F starsincreases for increasing vsini, indicating magnetohydrodynamic heatingfor the coronae of the late F stars, different from the early F stars.Based on observations with the NASA/ESA Hubble Space Telescope obtainedat the Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy, Incorporated,under NASA contract NAS5-26555.

Rotational velocities of A-type stars in the northern hemisphere. II. Measurement of v sin i
This work is the second part of the set of measurements of v sin i forA-type stars, begun by Royer et al. (\cite{Ror_02a}). Spectra of 249 B8to F2-type stars brighter than V=7 have been collected at Observatoirede Haute-Provence (OHP). Fourier transforms of several line profiles inthe range 4200-4600 Å are used to derive v sin i from thefrequency of the first zero. Statistical analysis of the sampleindicates that measurement error mainly depends on v sin i and thisrelative error of the rotational velocity is found to be about 5% onaverage. The systematic shift with respect to standard values fromSlettebak et al. (\cite{Slk_75}), previously found in the first paper,is here confirmed. Comparisons with data from the literature agree withour findings: v sin i values from Slettebak et al. are underestimatedand the relation between both scales follows a linear law ensuremath vsin inew = 1.03 v sin iold+7.7. Finally, thesedata are combined with those from the previous paper (Royer et al.\cite{Ror_02a}), together with the catalogue of Abt & Morrell(\cite{AbtMol95}). The resulting sample includes some 2150 stars withhomogenized rotational velocities. Based on observations made atObservatoire de Haute Provence (CNRS), France. Tables \ref{results} and\ref{merging} are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr ( or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/393/897

Speckle interferometry of nearby multiple stars
We present the results of diffraction-limited optical speckleinterferometry and infrared bispectrum speckle interferometry of 111double and 10 triple systems performed in 1998-1999 with the 6-mtelescope of the Special Astrophysical Observatory in Zelenchuk. Theobservations concentrated on nearby close binaries discovered during theHipparcos mission. Many nearby fast-orbiting low-mass binaries knownbefore Hipparcos were also included in the program. New companions werefirst resolved in 4 systems: HIP 5245, ADS 3179, Kui 99, and ADS 16138.In addition to accurate relative positions, magnitude differences weremeasured for most of the pairs. We combined our results with theHipparcos parallaxes to derive absolute magnitudes and spectral typesfor 63 binaries and 4 triples. Preliminary orbital elements and themass-sum are derived for HIP 689, and improved orbits are presented forHIP 16602 (CHR 117) and HIP 21280 (CHR 17). Based on data collected atthe Special Astrophysical Observatory, Russia Table 1 is only, and Table2 also, available in electronic form at the CDS via anonymons ftp tocdsarc.u-strasbg.fr ( or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/385/87

Astrometric radial velocities. III. Hipparcos measurements of nearby star clusters and associations
Radial motions of stars in nearby moving clusters are determined fromaccurate proper motions and trigonometric parallaxes, without any use ofspectroscopy. Assuming that cluster members share the same velocityvector (apart from a random dispersion), we apply a maximum-likelihoodmethod on astrometric data from Hipparcos to compute radial and spacevelocities (and their dispersions) in the Ursa Major, Hyades, ComaBerenices, Pleiades, and Praesepe clusters, and for theScorpius-Centaurus, alpha Persei, and ``HIP 98321'' associations. Theradial motion of the Hyades cluster is determined to within 0.4 kms-1 (standard error), and that of its individual stars towithin 0.6 km s-1. For other clusters, Hipparcos data yieldastrometric radial velocities with typical accuracies of a few kms-1. A comparison of these astrometric values withspectroscopic radial velocities in the literature shows a good generalagreement and, in the case of the best-determined Hyades cluster, alsopermits searches for subtle astrophysical differences, such as evidencefor enhanced convective blueshifts of F-dwarf spectra, and decreasedgravitational redshifts in giants. Similar comparisons for the ScorpiusOB2 complex indicate some expansion of its associations, albeit slowerthan expected from their ages. As a by-product from the radial-velocitysolutions, kinematically improved parallaxes for individual stars areobtained, enabling Hertzsprung-Russell diagrams with unprecedentedaccuracy in luminosity. For the Hyades (parallax accuracy 0.3 mas), itsmain sequence resembles a thin line, possibly with wiggles in it.Although this main sequence has underpopulated regions at certaincolours (previously suggested to be ``Böhm-Vitense gaps''), suchare not visible for other clusters, and are probably spurious. Futurespace astrometry missions carry a great potential for absoluteradial-velocity determinations, insensitive to the complexities ofstellar spectra. Based on observations by the ESA Hipparcos satellite.Extended versions of Tables \ref{tab1} and \ref{tab2} are available inelectronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr( or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/381/446

The helium content and age of the Hyades:. Constraints from five binary systems and Hipparcos parallaxes
We compare the accurate empirical mass-luminosity (M-L) relation basedon five Hyades binary systems to predictions of stellar modelscalculated with various input parameters (helium, metallicity and age)or physics (mixing-length ratio, model atmosphere, equation of state andmicroscopic diffusion). Models based on a helium content Y ~ 0.28inferred from the Delta Y/Delta Z enrichment law are more than 3sigmabeyond the observations, suggesting that the Hyades initial heliumabundance is lower than expected from its supersolar metallicity. Withthe photometric metallicity ([Fe/H] = 0.144+/- 0.013 dex, Grenon\cite{gre00}) we derive Y=0.255+/-0.009. Because of the (Y, [Fe/H])degeneracy in the M-L plane, the uncertainty grows to Delta Y=0.013 ifthe metallicity from spectroscopy is adopted ([Fe/H] = 0.14+/-0.05 dex,Cayrel de Strobel et al. \cite{cayG97}). We use these results to discussthe Hertzsprung-Russell (HR) diagram of the Hyades, in the(MV, B-V) plane, based on the very precise Hipparcosdynamical parallaxes. Present models fit the tight observed sequencevery well except at low temperatures. We show that the HR diagram doesnot bring further constraints on the helium abundance or metallicity ofthe cluster. In the low mass region of the HR diagram sensitive to themixing-length parameter (alpha MLT), the slope of the mainsequence (MS) suggests that alpha MLT could decrease from asolar (or even supersolar) value at higher mass to subsolar values atlow mass, which is also supported by the modeling of the vB22 M-Lrelation. We find that the discrepancy at low temperatures ((B-V)>≈ 1.2) remains, even if an improved equation of state or bettermodel atmospheres are used. Finally, we discuss the positions of thestars at turn-off in the light of their observed rotation rates and wededuce that the maxiμm age of the Hyades predicted by the presentmodels is ~650 Myr.

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 ( or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcar?J/A+A/367/521

A Hipparcos study of the Hyades open cluster. Improved colour-absolute magnitude and Hertzsprung-Russell diagrams
Hipparcos parallaxes fix distances to individual stars in the Hyadescluster with an accuracy of ~ 6 percent. We use the Hipparcos propermotions, which have a larger relative precision than the trigonometricparallaxes, to derive ~ 3 times more precise distance estimates, byassuming that all members share the same space motion. An investigationof the available kinematic data confirms that the Hyades velocity fielddoes not contain significant structure in the form of rotation and/orshear, but is fully consistent with a common space motion plus a(one-dimensional) internal velocity dispersion of ~ 0.30 kms-1. The improved parallaxes as a set are statisticallyconsistent with the Hipparcos parallaxes. The maximum expectedsystematic error in the proper motion-based parallaxes for stars in theouter regions of the cluster (i.e., beyond ~ 2 tidal radii ~ 20 pc) isla 0.30 mas. The new parallaxes confirm that the Hipparcos measurementsare correlated on small angular scales, consistent with the limitsspecified in the Hipparcos Catalogue, though with significantly smaller``amplitudes'' than claimed by Narayanan & Gould. We use the Tycho-2long time-baseline astrometric catalogue to derive a set of independentproper motion-based parallaxes for the Hipparcos members. The newparallaxes provide a uniquely sharp view of the three-dimensionalstructure of the Hyades. The colour-absolute magnitude diagram of thecluster based on the new parallaxes shows a well-defined main sequencewith two ``gaps''/``turn-offs''. These features provide the first directobservational support of Böhm-Vitense's prediction that (the onsetof) surface convection in stars significantly affects their (B-V)colours. We present and discuss the theoretical Hertzsprung-Russelldiagram (log L versus log T_eff) for an objectively defined set of 88high-fidelity members of the cluster as well as the delta Scuti startheta 2 Tau, the giants delta 1, theta1, epsilon , and gamma Tau, and the white dwarfs V471 Tau andHD 27483 (all of which are also members). The precision with which thenew parallaxes place individual Hyades in the Hertzsprung-Russelldiagram is limited by (systematic) uncertainties related to thetransformations from observed colours and absolute magnitudes toeffective temperatures and luminosities. The new parallaxes providestringent constraints on the calibration of such transformations whencombined with detailed theoretical stellar evolutionary modelling,tailored to the chemical composition and age of the Hyades, over thelarge stellar mass range of the cluster probed by Hipparcos.

Stellar Structure and Evolution: Deductions from Hipparcos
During the last decade, the understanding of fine features of thestructure and evolution of stars has become possible as a result ofenormous progress made in the acquisition of high-quality observationaland experimental data, and of new developments and refinements in thetheoretical description of stellar plasmas. The confrontation ofhigh-quality observations with sophisticated stellar models has allowedmany aspects of the theory to be validated, and several characteristicsof stars relevant to Galactic evolution and cosmology to be inferred.This paper is a review of the results of recent studies undertaken inthe context of the Hipparcos mission, taking benefit of the high-qualityastrometric data it has provided. Successes are discussed, as well asthe problems that have arisen and suggestions proposed to solve them.Future observational and theoretical developments expected and requiredin the field are also presented.

ICCD Speckle Observations of Binary Stars. XXIII. Measurements during 1982-1997 from Six Telescopes, with 14 New Orbits
We present 2017 observations of 1286 binary stars, observed by means ofspeckle interferometry using six telescopes over a 15 year period from1982 April to 1997 June. These measurements constitute the 23dinstallment in CHARA's speckle program at 2 to 4 m class telescopes andinclude the second major collection of measurements from the MountWilson 100 inch (2.5 m) Hooker Telescope. Orbital elements are alsopresented for 14 systems, seven of which have had no previouslypublished orbital analyses.

Orientation of the Orbital Planes of Visual Binary Systems
The space distribution of orbital poles for 252 visual binaries isanalyzed to check a possible tendency towards parallelism. It isconfirmed that orbital planes do not show any trend to be parallel tothe Galactic plane. No strong evidence is found for a preferentialorientation of the orbital planes for subgroups of binaries with similarperiods and eccentricities. Asymmetry in the distribution of orbitalpoles is seen only for a subgroup of 19 binaries lying closer than 10pc. Small differences in the distribution of orbital poles are alsodetected for subgroups with different location on HR diagram.

uvby photometry of the magnetic CP stars 20 Eridani, HR 1297, 12 Canis Majoris, and HD 134214
Differential Strömgren uvby observations from the Four CollegeAutomated Photoelectric Telescope (FCAPT) are presented for the mCPstars 20 Eri, HR 1297, 12 CMa, and HD 134214. Adelman & Boyce foundsuggestions that the light curves of 20 Eri change between observingseasons. Observations for two additional seasons confirm this behaviorand lead to an improved period of 1.928890 days. For HR 1297 a newperiod of 15.7490 days was derived. The uvby light curves now appear tobe the same for different observing seasons. 12 CMa is found to be aphotometric variable with the 2.18010 day period determined frommagnetic measurements by Bohlender et al. Observations of the rapidlyoscillating Ap star HD 134214 confirm the lack of photometricvariability which is not associated with the rapid oscillations. Tables2, 3, 4 and 5 are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr ( or viahttp://cdsweb.u-strasbg.fr/Abstract.html

Resolved double-lined spectroscopic binaries: A neglected source of hypothesis-free parallaxes and stellar masses
Double-lined spectroscopic binaries, once visually resolved (VB-SB2),provide hypothesis-free orbital parallaxes and masses of bothcomponents. Unlike eclipsing-spectroscopic binaries for which manyaccurate masses are already known (Andersen 1991; Andersen 1997), thenumber of VB-SB2 remains rather small. This paper presents 40 suchsystems for which published visual observations and radial velocitiesallow a simultaneous adjustment of both data sets. The precision of theindividual masses as well as the evolution of that precision withrespect to the published precision is investigated.

A Precision Test of HIPPARCOS Systematics toward the Hyades
We propose a test that can in principle detect any systematic errors inthe Hipparcos parallaxes toward the Hyades cluster at the level of 0.3mas. We show that the statistical parallax algorithm subsumes theclassical moving cluster methods and provides more precise estimates ofthe distance and the first two moments of the velocity distribution ofthe Hyades cluster, namely, its bulk space velocity and the velocitydispersion tensor. To test the Hipparcos parallaxes, we first rescalethe bulk velocity determined from statistical parallax to forceagreement with the distance scale determined from Hipparcos parallaxes.We then predict the parallaxes of Hyades cluster members using thiscommon cluster space velocity and their Hipparcos proper motions. Weshow that the parallaxes determined in this manner (pi_pm) areconsistent at the 1 sigma level with the parallaxes (pi_orb) of threeHyades spectroscopic binary systems with orbital solutions. We find that=0.52+/-0.47 mas, where the error is dominated bythe errors in the orbital parallaxes. A reduction in these errors wouldallow a test of the systematic errors in the Hipparcos parallaxes at the0.3 mas level. If the Hyades distance scale is fixed by Hipparcosparallaxes, then its bulk velocity in equatorial coordinates is(V_x,V_y,V_z)=(-5.70+/-0.20, 45.62+/-0.11, 5.65+/-0.08) kms^-1, itsvelocity dispersion is 320+/-39 m s^-1, and the distance modulus to thecentroid of our sample of 43 cluster members is 3.34+/-0.02 mag.

Binary star speckle measurements during 1992-1997 from the SAO 6-m and 1-m telescopes in Zelenchuk
We present the results of speckle interferometric measurements of binarystars made with the television photon-counting camera at the 6-m BigAzimuthal Telescope (BTA) and 1-m telescope of the Special AstrophysicalObservatory (SAO) between August 1992 and May 1997. The data contain 89observations of 62 star systems on the large telescope and 21 on thesmaller one. For the 6-m aperture 18 systems remained unresolved. Themeasured angular separation ranged from 39 mas, two times above the BTAdiffraction limit, to 1593 mas.

Radial velocities. Measurements of 2800 B2-F5 stars for HIPPARCOS
Radial velocities have been determined for a sample of 2930 B2-F5 stars,95% observed by the Hipparcos satellite in the north hemisphere and 80%without reliable radial velocity up to now. Observations were obtainedat the Observatoire de Haute Provence with a dispersion of 80Ä,mm(-1) with the aim of studying stellar and galactic dynamics.Radial velocities have been measured by correlation with templates ofthe same spectral class. The mean obtained precision is 3.0 km s(-1)with three observations. A new MK spectral classification is estimatedfor all stars. Based on observations made at the Haute ProvenceObservatory, France and on data from The Hipparcos Catalogue, ESA.Tables 4, 5 and 6 are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr or viahttp://cdsweb.u-strasbg.fr/Abstract.htm

Consequences of HIPPARCOS parallaxes for stellar evolutionary models. Three Hyades binaries: V 818 Tauri, 51 Tauri, and theta (2) Tauri
Three binary systems in the Hyades open cluster (51 Tau, V818 Tau, andtheta (2) Tau), with known {metallicity} and good Johnson photometricdata are used to test the validity of three independent sets of stellarevolutionary tracks. A statistical method is described and applied tothe colour-magnitude diagram of the six selected components, giving riseto chi (2) -contours in the metallicity-age plane. The effects of theHipparcos parallaxes on these confidence regions are studied in detailfor these binaries through a comparison with very accurate but olderorbital parallaxes. There are no significant differences in 51 Tau andtheta (2) Tau, and hence the test validates the sets of theoreticaltracks used. However the orbital parallax of V 818 Tau leads tometallicities too large in comparison with the observed range, while theHipparcos parallax give consistent results, at the 1sigma level.Comparisons between the predicted and measured masses are also made andshow a good agreement. However, even if the Padova tracks predictcorrect masses for each component of V 818 Tau, there is no Padova trackthat fits both components simultaneously in the mass-radius diagram.Finally, since the individual masses of these binaries are accuratelyknown, some issues about the influence of the Hipparcos parallaxes ontheir location in the mass-luminosity diagram are also discussed. Basedon data from the ESA Hipparcos satellite}

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.

Visual binary orbits and masses POST HIPPARCOS
The parallaxes from Hipparcos are an important ingredient to derive moreaccurate masses for known orbital binaries, but in order to exploit theparallaxes fully, the orbital elements have to be known to similarprecision. The present work gives improved orbital elements for some 205systems by combining the Hipparcos astrometry with existing ground-basedobservations. The new solutions avoid the linearity constraints andomissions in the Hipparcos Catalog by using the intermediate TransitData which can be combined with ground-based observations in arbitarilycomplex orbital models. The new orbital elements and parallaxes give newmass-sum values together with realistic total error-estimates. To getindividual masses at least for main-sequence systems, the mass-ratioshave been generally estimated from theoretical isochrones and observedmagnitude-differences. For some 25 short-period systems, however, trueastrometric mass-ratios have been determined through the observedorbital curvature in the 3-year Hipparcos observation interval. Thefinal result is an observed `mass-luminosity relation' which falls closeto theoretical expectation, but with `outliers' due to undetectedmultiplicity or to composition- and age-effects in the nonuniformnear-star sample. Based in part on observations collected with the ESAHipparcos astrometry satellite. Tables~ 1, 3, 4 and 6 are also availablein electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr~( or viahttp://cdsweb.u-strasbg.fr/Abstract.html

Diffraction-limited speckle masking interferometry of binary stars with the SAO 6-m telescope.
Not Available

The nature of visual components in 82 multiple systems.
Not Available

UVBY Photometry of the Magnetic Chemically Peculiar Stars HR 1297, 36 Aurigae, and HR 2722 and the Nonmagnetic Chemically Peculiar Stars HR 1576 and alpha CANCRI
Differential Strömgren uvby photometric observations from the FourCollege Automated Photoelectric Telescope are presented for the magneticCP stars HR 1297, 36 Aur, and HR 2722 and the nonmagnetic CP stars HR1576 and alpha Cnc. Both the moderately rotating HR 1576, amercury-manganese star, and alpha Cnc, a metallic-line star, arenonvariable. For HR 1297 we refined Winzer's period to 1.06457 days. Ourperiod of 14.366 days for 36 Aur is an alias of Winzer's period and isin keeping with the sharp-lined nature of this star. We found a moreaccurate period of 2.31523 days for the low-amplitude variable HR 2722.



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距离:54.795 天文距离
B-T magnitude:5.973
V-T magnitude:5.662

适当名称   (Edit)
Flamsteed51 Tau
HD 1989HD 27176
TYCHO-2 2000TYC 1276-1622-1
USNO-A2.0USNO-A2 1050-01210669
BSC 1991HR 1331
HIPHIP 20087

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