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The Distances to Open Clusters from Main-Sequence Fitting. III. Improved Accuracy with Empirically Calibrated Isochrones
We continue our series of papers on open cluster distances with acritical assessment of the accuracy of main-sequence fitting usingisochrones that employ empirical corrections to the color-temperaturerelations. We use four nearby open clusters with multicolor photometryand accurate metallicities and present a new metallicity for Praesepe([Fe/H]=+0.11+/-0.03) from high-resolution spectra. The internalprecision of distance estimates is about a factor of 5 better than thecase without the color calibrations. After taking into account all majorsystematic errors, we obtain distances accurate to about 2%-3% whenthere exists a good metallicity estimate. Metallicities accurate tobetter than 0.1 dex may be obtained from BVICKsphotometry alone. We also derive a helium abundance for the Pleiades ofY=0.279+/-0.015, which is equal within the errors to the Sun's initialhelium abundance and that of the Hyades. Our best estimates of distancesare (m-M)0=6.33+/-0.04, 8.03+/-0.04, and 9.61+/-0.03 toPraesepe, NGC 2516, and M67, respectively. Our Pleiades distance at thespectroscopic metallicity,(m-M)0=5.66+/-0.01(internal)+/-0.05(systematic), is inexcellent agreement with several geometric distance measurements. Wehave made calibrated isochrones for -0.3<=[Fe/H]<=+0.2 availableonline.
| 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.
| 2MASS J05162881+2607387: A New Low-mass Double-lined Eclipsing Binary
We show that the star known as 2MASS J05162881+2607387 (hereafter J0516)is a double-lined eclipsing binary with nearly identical low-masscomponents. The spectroscopic elements derived from 18 spectra obtainedwith the High Resolution Spectrograph on the Hobby-Eberly Telescopeduring the fall of 2005 are K1=88.45+/-0.48 andK2=90.43+/-0.60 km s-1, resulting in a mass ratioof q=K1/K2=0.978+/-0.018 and minimum masses ofM1sin3i=0.775+/-0.016 Msolar andM2sin3i=0.759+/-0.012 Msolar, respectively. Wehave extensive differential photometry of J0516 obtained over severalnights between 2004 January and March (epoch 1) and between 2004 Octoberand 2005 January plus 2006 January (epoch 2) using the 1 m telescope atthe Mount Laguna Observatory. The source was roughly 0.1 mag brighter inall three bandpasses during epoch 1 when compared to epoch 2. Also,phased light curves from epoch 1 show considerable out-of-eclipsevariability, presumably due to bright spots on one or both stars. Incontrast, the phased light curves from epoch 2 show littleout-of-eclipse variability. The light curves from epoch 2 and the radialvelocity curves were analyzed using our ELC code with updated modelatmospheres for low-mass stars. We find the following:M1=0.787+/-0.012 Msolar,R1=0.788+/-0.015 Rsolar,M2=0.770+/-0.009 Msolar, andR2=0.817+/-0.010 Rsolar. The stars in J0516 haveradii that are significantly larger than model predictions for theirmasses, similar to what is seen in a handful of other well-studiedlow-mass double-lined eclipsing binaries. We compiled all recent massand radius determinations from low-mass binaries and determine anempirical mass-radius relation of the formR(Rsolar)=0.0324+0.9343M(Msolar)+0.0374M2(Msolar).Based on observations obtained with the Hobby-Eberly Telescope, which isa joint project of the University of Texas at Austin, the PennsylvaniaState University, Stanford University,Ludwig-Maximilians-Universität München, andGeorg-August-Universität Göttingen.
| First Results from the CHARA Array. IV. The Interferometric Radii of Low-Mass Stars
We have measured the angular diameters of six M dwarfs with the CHARAArray, a long-baseline optical interferometer located at Mount WilsonObservatory. Spectral types range from M1.0 V to M3.0 V and linear radiifrom 0.38 to 0.69 Rsolar. These results are consistent withthe seven other M dwarf radii measurements from optical interferometryand with those for 14 stars in eclipsing binary systems. We compare alldirectly measured M dwarf radii to model predictions and find thatcurrent models underestimate the true stellar radii by up to 15%-20%.The differences are small among the metal-poor stars but becomesignificantly larger with increasing metallicity. This suggests thattheoretical models for low-mass stars may be missing some opacity sourcethat alters the computed stellar radii.
| The Hyades Binary θ2 Tauri: Confronting Evolutionary Models with Optical Interferometry
We determine the masses and magnitude difference of the components ofthe Hyades spectroscopic binary θ2 Tauri. We find thatboth components appear to be less massive and/or brighter than predictedfrom some recent evolutionary models. The rapid rotation and unknownrotational inclination of both components introduce uncertainty in theirluminosities and colors, but not enough to reconcile both of them withthe evolutionary models. We measured the visual orbit with the Mark IIIoptical interferometer and the Navy Prototype Optical Interferometer andcombined it with the Hipparcos proper-motion-based parallax to find atotal system mass ΣM of 4.03+/-0.20 Msolar.We also combined our visual orbit with three recent spectroscopic orbitsto find three spectroscopically based estimates of ΣM andcompared these to the ΣM from the visual orbit and parallax.We chose the spectroscopic orbit that agreed best and used its massratio to estimate individual masses MA,B of 2.15+/-0.12and 1.87+/-0.11 Msolar. From the interferometry, wedetermine Δm=1.13+/-0.05 mag across the 450-850 nm band. Theparallax then implies absolute V magnitudes MA,B of0.48+/-0.05 and 1.61+/-0.06 mag. If the components are rotating nearbreakup velocity and seen nearly pole-on, the true luminosities may beas faint as 1.03 and 2.13 mag; even in that case, however, the secondaryis too blue by ~0.07 mag in B-V.
| Basic physical parameters of a selected sample of evolved stars
We present the detailed spectroscopic analysis of 72 evolved stars,which were previously studied for accurate radial velocity variations.Using one Hyades giant and another well studied star as the referenceabundance, we determine the [Fe/H] for the whole sample. Thesemetallicities, together with the T_eff values and the absolute V-bandmagnitude derived from Hipparcos parallaxes, are used to estimate basicstellar parameters (ages, masses, radii, (B-V)0 and log g)using theoretical isochrones and a Bayesian estimation method. The(B-V)0 values so estimated turn out to be in excellentagreement (to within ~0.05 mag) with the observed (B-V), confirming thereliability of the T_eff-(B-V)0 relation used in theisochrones. On the other hand, the estimated log g values are typically0.2 dex lower than those derived from spectroscopy; this effect has anegligible impact on [Fe/H] determinations. The estimated diametersθ have been compared with limb darkening-corrected ones measuredwith independent methods, finding an agreement better than 0.3 maswithin the 1<θ<10 mas interval (or, alternatively, findingmean differences of just 6%). We derive the age-metallicity relation forthe solar neighborhood; for the first time to our knowledge, such arelation has been derived from observations of field giants rather thanfrom open clusters and field dwarfs and subdwarfs. The age-metallicityrelation is characterized by close-to-solar metallicities for starsyounger than ~4 Gyr, and by a large [Fe/H] spread with a trend towardslower metallicities for higher ages. In disagreement with other studies,we find that the [Fe/H] dispersion of young stars (less than 1 Gyr) iscomparable to the observational errors, indicating that stars in thesolar neighbourhood are formed from interstellar matter of quitehomogeneous chemical composition. The three giants of our sample whichhave been proposed to host planets are not metal rich; this result is atodds with those for main sequence stars. However, two of these starshave masses much larger than a solar mass so we may be sampling adifferent stellar population from most radial velocity searches forextrasolar planets. We also confirm the previous indication that theradial velocity variability tends to increase along the RGB, and inparticular with the stellar radius.
| Tertiary companions to close spectroscopic binaries
We have surveyed a sample of 165 solar-type spectroscopic binaries (SB)with periods from 1 to 30 days for higher-order multiplicity. Asubsample of 62 targets were observed with the NACO adaptive opticssystem and 13 new physical tertiary companions were detected. Anadditional 12 new wide companions (5 still tentative) were found usingthe 2MASS all-sky survey. The binaries belong to 161 stellar systems; ofthese 64 are triple, 11 quadruple and 7 quintuple. After correction forincompleteness, the fraction of SBs with additional companions is foundto be 63% ± 5%. We find that this fraction is a strong functionof the SB period P, reaching 96% for P<3d and dropping to34% for P>12^d. Period distributions of SBs with and withouttertiaries are significantly different, but their mass ratiodistributions are identical. The statistical data on the multiplicity ofclose SBs presented in this paper indicates that the periods and massratios of SBs were established very early, but the periods of SB systemswith triples were further shortened by angular momentum exchange withcompanions.
| A catalogue of eclipsing variables
A new catalogue of 6330 eclipsing variable stars is presented. Thecatalogue was developed from the General Catalogue of Variable Stars(GCVS) and its textual remarks by including recently publishedinformation about classification of 843 systems and making correspondingcorrections of GCVS data. The catalogue1 represents thelargest list of eclipsing binaries classified from observations.
| A Catalog of Temperatures and Red Cousins Photometry for the Hyades
Using Hyades photometry published by Mendoza and other authors,Pinsonneault et al. have recently concluded that Cousins V-I photometrypublished by Taylor & Joner is not on the Cousins system. Extensivetests of the Taylor-Joner photometry and other pertinent results aretherefore performed in this paper. It is found that in part, thePinsonneault et al. conclusion rests on (1) a systematic error inMendoza's (R-I)J photometry and (2) a small error in anapproximate Johnson-to-Cousins transformation published by Bessell. Forthe Taylor-Joner values of (V-R)C, it is found that there arepossible (though not definite) differences of several mmag with otherresults. However, the Taylor-Joner values of (R-I)C data aresupported at the 1 mmag level. Using the (R-I)C data andother published results, an (R-I)C catalog is assembled for146 Hyades stars with spectral types earlier than about K5. For singlestars with multiple contributing data, the rms errors of the catalogentries are less than 4.4 mmag. Temperatures on the Di Benedettoangular-diameter scale are also given in the catalog and are used tohelp update published analyses of high-dispersion values of [Fe/H] forthe Hyades. The best current mean Hyades value of [Fe/H] is found to be+0.103+/-0.008 dex and is essentially unchanged from its previous value.In addition to these numerical results, recommendations are made aboutimproving attitudes and practices that are pertinent to issues likethose raised by Pinsonneault et al.
| Absolute Properties of the Eclipsing Binary Star RW Lacertae
We present 3004 differential observations in the V bandpass measured bya robotic telescope, as well as 36 pairs of radial velocities fromhigh-resolution spectroscopic observations, of the detached, eccentric,EA-type, 10.37 day period, double-lined eclipsing binary star RW Lac.Absolute dimensions of the components are determined with excellentprecision (better than 0.7% in the masses and 0.5% in the radii) for thepurpose of testing various aspects of theoretical modeling. We obtain0.928+/-0.006 Msolar and 1.186+/-0.004 Rsolar forthe hotter, larger, more massive, and more luminous photometric primary(star A) and 0.870+/-0.004 Msolar and 0.964+/-0.004Rsolar for the cooler, smaller, less massive, and lessluminous photometric secondary (star B). A faint, third componentcontributes 2.6% of the light in V but is not detected in ourspectrograms. The effective temperatures and interstellar reddening ofthe stars are accurately determined from UBV and uvbyβ photometryand from analysis of the spectrograms: 5760+/-100 K for the primary and5560+/-150 K for the secondary, corresponding to spectral types of G5and G7, and 0.050 mag for interstellar reddening Eb-y. Theorbits are slightly eccentric, and spectral line widths give observedrotational velocities that are not significantly different fromsynchronous for both components. The components of RW Lac are old,somewhat metal-deficient, low-mass, main-sequence stars with an age ofabout 11 Gyr, according to models.
| Time scales of Li evolution: a homogeneous analysis of open clusters from ZAMS to late-MS
We have performed a new and homogeneous analysis of all the Li dataavailable in the literature for main sequence stars (spectral-types fromlate F to K) in open clusters. In the present paper we focus on adetailed investigation of MS Li depletion and its time scales for starsin the 6350-5500 K effective temperature range. For the first time, wewere able to constrain the age at which non-standard mixing processes,driving MS Li depletion, appear. We have also shown that MS Li depletionis not a continuous process and cannot be simply described by at-α law. We confirm that depletion becomes ineffectivebeyond an age of 1-2 Gyr for the majority of the stars, leading to a Liplateau at old ages. We compared the empirical scenario of Li as afunction of age with the predictions of three non-standard models. Wefound that models including only gravity waves as main mixing processare not able to fit the Li vs. age pattern and thus this kind of mixingcan be excluded as the predominant mechanism responsible for Lidepletion. On the other hand, models including slow mixing induced byrotation and angular momentum loss, and in particular those includingalso diffusive processes not related to rotation, can explain to someextent the empirical evidence. However, none of the currently proposedmodels can fit the plateau at old ages.
| 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.
| Kinematics of chromospherically active binaries and evidence of an orbital period decrease in binary evolution
The kinematics of 237 chromospherically active binaries (CABs) werestudied. The sample is heterogeneous with different orbits andphysically different components from F to M spectral-type main-sequencestars to G and K giants and supergiants. The computed U, V, W spacevelocities indicate that the sample is also heterogeneous in velocityspace. That is, both kinematically younger and older systems exist amongthe non-evolved main sequence and the evolved binaries containing giantsand subgiants. The kinematically young (0.95 Gyr) subsample (N= 95),which is formed according to the kinematical criteria of moving groups,was compared with the rest (N= 142) of the sample (3.86 Gyr) toinvestigate any observational clues of binary evolution. Comparing theorbital period histograms between the younger and older subsamples,evidence was found supporting the finding of Demircan that the CABs losemass (and angular momentum) and evolve towards shorter orbital periods.The evidence of mass loss is noticeable on the histograms of the totalmass (Mh+Mc), which is compared between theyounger (only N= 53 systems available) and older subsamples (only N= 66systems available). The orbital period decrease during binary evolutionis found to be clearly indicated by the kinematical ages of 6.69, 5.19and 3.02 Gyr which were found in the subsamples according to the periodranges of logP<= 0.8, 0.8 < logP<= 1.7 and 1.7 < logP<=3, respectively, among the binaries in the older subsample.
| Eclipsing binaries in open clusters - I. V615 Per and V618 Per in h Persei
We derive absolute dimensions for two early-type main-sequence detachedeclipsing binaries in the young open cluster h Persei (NGC 869). V615Persei has a spectral type of B7 V and a period of 13.7 d. V618 Perseiis A2 V and has a period of 6.4 d. New ephemerides are calculated forboth systems. The masses of the component stars have been derived usinghigh-resolution spectroscopy and are 4.08 +/- 0.06 and 3.18 +/- 0.05Msolar for V615 Per and 2.33 +/- 0.03 and 1.56 +/- 0.02Msolar for V618 Per. The radii have been measured by fittingthe available light curves using EBOP and are 2.29 +/- 0.14 and 1.90 +/-0.09 Rsolar for V615 Per and 1.64 +/- 0.07 and 1.32 +/- 0.07Rsolar for V618 Per. By comparing the observed spectra ofV615 Per with synthetic spectra from model atmospheres we find that theeffective temperatures of the stars are 15000 +/- 500 K for the primaryand 11000 +/- 500 K for the secondary. The equatorial rotationalvelocities of the primary and secondary components of V615 Per are 28+/- 5 and 8 +/- 5 km s-1, respectively. Both components ofV618 Per rotate at 10 +/- 5 km s-1. The equatorial rotationalvelocities for synchronous rotation are about 10 km s-1 forall four stars. The time-scales for orbital circularization for bothsystems, and the time-scale for rotational synchronization of V615 Per,are much greater than the age of h Per. Their negligible eccentricitiesand equatorial rotational velocities therefore support the hypothesisthat they were formed by delayed break-up. We have compared the radii ofthese stars with models by the Granada and the Padova groups for starsof the same masses but different compositions. We conclude that themetallicity of the stars is Z~ 0.01. This appears to be the firstestimate of the bulk metallicity of h Per. Recent photometric studieshave assumed a solar metallicity so their results should be reviewed.
| Testing Blend Scenarios for Extrasolar Transiting Planet Candidates. I. OGLE-TR-33: A False Positive
We report high-resolution spectroscopic follow-up observations of thefaint transiting planet candidate OGLE-TR-33 (V=14.7), located in thedirection of the Galactic center. Small changes in the radial velocityof the star were detected that initially suggested the presence of alarge planet or brown dwarf in orbit. However, further analysis revealedspectral line asymmetries that change in phase with the 1.95 day period,casting doubt on those measurements. These asymmetries make it morelikely that the transit-like events in the light curve are the result ofcontamination from the light of an eclipsing binary along the same lineof sight (referred to as a ``blend''). We performed detailed simulationsin which we generated synthetic light curves resulting from such blendscenarios and fitted them to the measured light curve. Guided by thesefits and the inferred properties of the stars, we uncovered a second setof lines in our spectra that correspond to the primary of the eclipsingbinary and explain the asymmetries. Using all the constraints fromspectroscopy, we were then able to construct a model that satisfies allthe observations and to characterize the three stars based on modelisochrones. OGLE-TR-33 is fully consistent with being a hierarchicaltriple system composed of a slightly evolved F6 star (the brighterobject) near the end of its main-sequence phase and an eclipsing binarywith a K7-M0 star orbiting an F4 star. The application to OGLE-TR-33 ofthe formalism developed to fit light curves of transit candidatesillustrates the power of such simulations for predicting additionalproperties of the blend and for guiding further observations that mayserve to confirm that scenario, thereby ruling out a planet. Tests suchas this can be very important for validating faint candidates.
| 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.
| X-ray astronomy of stellar coronae
X-ray emission from stars in the cool half of the Hertzsprung-Russelldiagram is generally attributed to the presence of a magnetic coronathat contains plasma at temperatures exceeding 1 million K. Coronae areubiquitous among these stars, yet many fundamental mechanisms operatingin their magnetic fields still elude an interpretation through adetailed physical description. Stellar X-ray astronomy is thereforecontributing toward a deeper understanding of the generation of magneticfields in magnetohydrodynamic dynamos, the release of energy in tenuousastrophysical plasmas through various plasma-physical processes, and theinteractions of high-energy radiation with the stellar environment.Stellar X-ray emission also provides important diagnostics to study thestructure and evolution of stellar magnetic fields from the first daysof a protostellar life to the latest stages of stellar evolution amonggiants and supergiants. The discipline of stellar coronal X-rayastronomy has now reached a level of sophistication that makes tests ofadvanced theories in stellar physics possible. This development is basedon the rapidly advancing instrumental possibilities that today allow usto obtain images with sub-arcsecond resolution and spectra withresolving powers exceeding 1000. High-resolution X-ray spectroscopy has,in fact, opened new windows into astrophysical sources, and has played afundamental role in coronal research.
| The shortest period M dwarf eclipsing system BW3 V38. II. Determination of absolute elements
The spectroscopic data for the short-period (0.1984 deg) eclipsingbinary V38, discovered by the OGLE micro-lensing team in Baade's Windowfield BW3, are analyzed. Radial velocity curves are derived frommid-resolution spectra obtained with EMMI-NTT at ESO - La Silla, and asimultaneous solution of the existing light curve by OGLE and of the newradial velocity curves is obtained. The system is formed by almost twinM3e dwarf components that are very close, but not yet in contact. Thespectra of both dwarfs show signatures of the presence of strongchromospheres. Spectroscopy definitely confirms, therefore, what wassuggested on the basis of photometry: BW3 V38 is indeed a unique system,as no other similar binary with M components and in such a tight orbitis known. Within the limits posed by the relatively large errors, dueto the combined effect of system faintness and of the constraints onexposure time, the derived physical parameters seem to agree with therelations obtained from the other few known eclipsing binaries with latetype components (which indicate a discrepancy between the availableevolutionary models and the data at 10% level). A possibleexplanation is the presence of strong magnetic fields and fast rotation(that applies to the BW3 V38 case as well). A simple computation of thesystem secular evolution by angular momentum loss and spin-orbitsynchronization shows that the evolution of a system with M dwarfcomponents is rather slow, and indicates as well a possible reason whysystems similar to BW3 V38 are so rare.Based on observations collected at the European Southern Observatory, LaSilla, Chile.
| 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
| The Distances to Open Clusters from Main-Sequence Fitting. I. New Models and a Comparison with the Properties of the Hyades Eclipsing Binary VB 22
In the first of a new series of papers on open cluster distances, we useupdated stellar evolution models to construct an isochrone appropriatefor the Hyades and compare it with the Hyades eclipsing binary system VB22. We find that the absolute and relative luminosities of the two starsare in good agreement with the model but that the radii do not match thevalues inferred from eclipse data. We present evidence that there is aconsistency problem with the flux ratios and the inferred radii anddiscuss possible theoretical effects that could be responsible for themismatch in the radii. We derive a helium abundance for the Hyades ofY=0.271+/-0.006, which is equal within the errors to the Sun's initialhelium abundance, even though the Hyades is considerably moremetal-rich.
| 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.
| Disentangling discrepancies between stellar evolution theory and sub-solar mass stars. The influence of the mixing length parameter for the UV Psc binary
Serious discrepancies have recently been observed between predictions ofstellar evolution models in the 0.7-1.1 Msun mass range andaccurately measured properties of binary stars with components in thismass range. We study one of these objects, the eclipsing binary UVPiscium, which is particularly interesting because Popper(\cite{popper1997}) derived age estimates for each component thatdiffered by more than a factor of two. In an attempt to solve thissignificant discrepancy (a difference in age of 11 Gyr), we compute alarge grid of stellar evolution models with the CESAM code for eachcomponent. By fixing the masses to their accurately determined values(relative error smaller than 1% for both stars), we consider a widerange of possible metallicities Z (0.01 to 0.05), and helium content Y(0.25 to 0.34) uncorrelated to Z. In addition, the mixing lengthparameter alphaMLT is left as another free parameter. Weobtain a best fit in the Teff-radius diagram for a commonchemical composition (Z, Y) = (0.012, 0.31), but a different MLTparameter alphaMLT a = 0.95 +/-0.12(statistical)+0.30(systematic) and alphaMLT b = 0.65 +/-0.07(stat)+0.10(syst). The apparent age discrepancy found by Popper(\cite{popper1997}) disappears with this solution, the components beingcoeval to within 1%. This suggests that fixing alphaMLT toits solar value ( ~ 1.6), a common hypothesis assumed in most stellarevolutionary models, may not be correct. Secondly, sincealphaMLT is smaller for the less massive component, thissuggests that the alphaMLT parameter may decrease withstellar mass, showing yet another shortcoming of the mixing lengththeory to explain stellar convection. This trend needs furtherconfirmation with other binary stars with accurate data.
| The 0.4-Msun eclipsing binary CU Cancri. Absolute dimensions, comparison with evolutionary models and possible evidence for a circumstellar dust disk
Photometric observations in the R and I bands of the detached M-typedouble-lined eclipsing binary CU Cnc have beenacquired and analysed. The photometric elements obtained from theanalysis of the light curves have been combined with an existingspectroscopic solution to yield high-precision (errors la 2%) absolutedimensions: MA=0.4333+/-0.0017 Msun,MB= 0.3980+/-0.0014 Msun,RA=0.4317+/-0.0052 Rsun, andRB=0.3908+/-0.0094 Rsun. The mean effectivetemperature of the system has been estimated to be Teff=3140+/-150 K by comparing multi-band photometry (optical and infrared)with synthetic colors computed from state-of-the-art model atmospheres.Additionally, we have been able to obtain an estimate for the age ( ~320 Myr) and chemical composition ([Fe/H]~ 0.0) of the binary systemthrough its membership of the Castor moving group. With all theseobservational constraints, we have carried out a critical test of recentstellar models for low-mass stars. The comparison reveals that mostevolutionary models underestimate the radius of the stars by as much as10%, thus confirming the trend observed by Torres & Ribas(\cite{TR02}) for YY Gem and V818Tau. In the mass-absolute magnitude diagram, CU Cnc isobserved to be dimmer than other stars of the same mass and this makesthe comparison with stellar models not so compelling. After ruling out anumber of different scenarios, the apparent faintness of CU Cnc can beexplained if its components are some 10% cooler than similar-mass starsor if there is some source of circumstellar dust absorption. The lattercould be a tantalizing indirect evidence for a coplanar (Vega-like)dusty disk around this relatively young M-type binary.Tables 1 and 2 are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.125.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/398/239}
| First radius measurements of very low mass stars with the VLTI
We present 4 very low mass stars radii measured with the VLTI using the2.2 mu m VINCI test instrument. The observations were carried out duringthe commissioning of the 104-meter-baseline with two 8-meter-telescopes.We measure angular diameters of 0.7-1.5 mas with accuracies of 0.04-0.11mas, and for spectral type ranging from M0V to M5.5V. We determine anempirical mass-radius relation for M dwarfs based on all availableradius measurements. The observed relation agrees well with theoreticalmodels at the present accuracy level, with possible discrepancy around0.5-0.8 Msun that needs to be confirmed. In the near future,dozens of M dwarfs radii will be measured with 0.1-1% accuracy, with theVLTI, thanks to the improvements expected from the near infraredinstrument AMBER. This will bring strong observational constraints onboth atmosphere and interior physics.Based on observations made with the European Southern Observatorytelescopes and obtained from the ESO/ST-ECF Science Archive Facility.
| Absolute Dimensions of the M-Type Eclipsing Binary YY Geminorum (Castor C): A Challenge to Evolutionary Models in the Lower Main Sequence
We present new spectroscopic observations of the detached late-typedouble-lined eclipsing binary YY Geminorum (P=0.814 day), a member ofthe Castor sextuple system and one of the benchmarks for the comparisonbetween observations and stellar evolution theory in the lower mainsequence. In addition, we have reanalyzed existing light curves inseveral passbands using modern techniques that account for theconspicuous presence of spots. This, combined with the spectroscopy, hasyielded a very precise determination of the absolute dimensions of thecomponents, which are virtually identical to each other. We obtain forthe mean mass, radius, and effective temperature the valuesM=0.5992+/-0.0047 Msolar, R=0.6191+/-0.0057Rsolar, and Teff=3820+/-100 K, respectively. Boththe mass and the radius determinations are good to better than 1%, whichin the case of the radius represents a fourfold improvement overprevious results and significantly enhances the value of this quantityfor testing the models. We discuss the importance of systematic effectsin these measurements by comparison with another high-precisiondetermination of the mass by Ségransan and coworkers. Areanalysis of the Hipparcos transit data for Castor AB that accounts forthe relative motion of the pair in its 467 yr period orbit has yieldedan improved parallax for the system of 66.90+/-0.63 mas. With this, wehave estimated the age (~370 Myr) and metal abundance ([Fe/H]~0.0) of YYGem from isochrone fits to Castor A and B under the assumption of acommon origin. This, along with the other physical properties, allow foran unusually stringent test of the models for low-mass stars. We havecompared the observations of YY Gem with a large number of recenttheoretical calculations, and we show that all models underestimate theradius by up to 20% and that most overestimate the effective temperatureby 150 K or more. Both of these trends are confirmed by observations ofanother similar system in the Hyades (V818 Tau). Consequently,theoretical ages for relatively low mass objects such as T Tauri starsderived by placing them on the H-R diagram may be considerably biased.If the radius is used directly as a measure of evolution, ages could beunderestimated by as much as a factor of 10 in this mass regime. In viewof these discrepancies, absolute ages from essentially all currentmodels for the lower main sequence must be viewed with at least somemeasure of skepticism. Finally, we derive a new and very accurateephemeris based on all available times of eclipse, and we lay to restprevious claims of sudden changes in the orbital period of the binary,which we show to be spurious. Some of the observations reported herewere obtained with the Multiple Mirror Telescope, a joint facility ofthe Smithsonian Institution and the University of Arizona.
| Testing Models of Stellar Evolution for Metal-poor Stars: An Interferometric-spectroscopic Orbit for the Binary HD 195987
We report spectroscopic and interferometric observations of themoderately metal-poor double-lined binary system HD 195987, with anorbital period of 57.3 days. By combining our radial-velocity andvisibility measurements, we determine the orbital elements and deriveabsolute masses for the components of MA=0.844+/-0.018Msolar and MB=0.6650+/-0.0079 Msolar,with relative errors of 2% and 1%, respectively. We also determine theorbital parallax, πorb=46.08+/-0.27 mas, corresponding toa distance of 21.70+/-0.13 pc. The parallax and the measured brightnessdifference between the stars in V, H, and K yield the component absolutemagnitudes in those bands. We also estimate the effective temperaturesof the stars as TAeff=5200+/-100 K andTBeff=4200+/-200 K. Together with detailedchemical abundance analyses from the literature giving [Fe/H]~-0.5(corrected for binarity) and [α/Fe]=+0.36, we use these physicalproperties to test current models of stellar evolution for metal-poorstars. Among the four that we considered, we find that no single modelfits all observed properties at the measured composition, although weidentify the assumptions in each one that account for the discrepancy,and we conclude that a model with the proper combination of assumptionsshould be able to reproduce all the radiative properties. Theindications from the isochrone fits and the pattern of enhancement ofthe metals in HD 195987 are consistent with this being a thick diskobject, with an age of 10-12 Gyr.
| 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(130.79.125.8) 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.
| 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.
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Observation and Astrometry data
| Constellation: | Stier |
| Right ascension: | 04h17m38.94s |
| Declination: | +16°56'52.3" |
| Apparent magnitude: | 8.336 |
| Distance: | 46.729 parsecs |
| Proper motion RA: | 112.8 |
| Proper motion Dec: | -24.2 |
| B-T magnitude: | 9.295 |
| V-T magnitude: | 8.416 |
Catalogs and designations:
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