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Modelling eclipsing binaries with pulsating components: Phase dependence of observed pulsation amplitudes
We investigate the phase dependence of the light variations due topulsations, as modulated with light variations due to the eclipses. Thestudy is based, both on a simplified model and on the rigorous lightcurve (LC) synthesis software of Wilson and Devinney. We model AB Cas asa prototype, using both of these approaches. The phase dependence of thedifferential light variations for total systems is also discussed .
| Spectral separation of two pulsating non-single stars.
theta 2 Tau and RS Cha are 2 binary systems also showingdelta Scuti-type pulsations. theta 2 Tau is a Hyades"single-lined" spectroscopic binary with an orbital period of 140.7days. Its secondary component, though of similar temperature, is lessevolved and fainter than the primary and is therefore difficult todetect spectroscopically. RS Cha is a double-lined pre-main-sequenceeclipsing spectroscopic binary orbiting with a short 1.7 days period.Since both systems address several interesting issues related to stellarevolution, metallicism, binarity and pulsation, we explore in thepresent contribution the possibility to study their spectra using theFourier disentangling technique developed by\citet{1995A&AS..114..393H} for extraction of reliable and accuratecomponent properties.
| Recent results on Pre-main sequence delta Scuti stars. .
Intermediate mass Pre-main sequence stars (1.5 Mȯ< M< 5Mȯ) cross the instability strip on their way tothe main sequence. They are therefore expected to be pulsating in asimilar way as the delta Scuti stars. In this contribution we presentthe status of the observational studies of pulsations in these starswith special emphasis on recent results from our group. The prospectsfor future investigations of these objects from the ground and fromspace are discussed.
| RS Cha, a binary system suitable for testing stellar physics modeling during the pre-main sequence phase.
We use accurate spectroscopic data of the pre-main sequence eclipsingbinary system RS Cha to redetermine masses and radii of both componentsand to measure the metallicity of the system. Knowing all fundamentalparameters of both stars we are able to test stellar physics modelingduring the PMS phase. We find no evidence that our models are deficient.Accurate detection of modes and measurements of periods of the recentlydiscovered pulsations in both components are required to constrain moreseverely the physical description of PMS stellar models..
| Computed Hβ indices from ATLAS9 model atmospheres
Aims.Grids of Hβ indices based on updated (new-ODF) ATLAS9 modelatmospheres were computed for solar and scaled solar metallicities[+0.5], [+0.2], [0.0], [ -0.5] , [ -1.0] , [ -1.5] , [ -2.0] , [ -2.5]and for α enhanced compositions [+0.5a], [0.0a], [ -0.5a] , [-1.0a] , [ -1.5a] , [ -2.0a] , [ -2.5a] , and [ -4.0a] . Methods:.Indices for T_eff > 5000 K were computed with the same methods asdescribed by Lester et al. (1986, LGK86) except for a differentnormalization of the computed natural system to the standard system.LGK86 used special ODFs to compute the fluxes. For T_eff ≤ 5000 K wecomputed the fluxes using the synthetic spectrum method. In order toassess the accuracy of the computed indices comparisons were made withthe indices computed by Smalley & Dworetsky (1995, A&A, 293,446, MD95) and with the empirical relations T_eff-Hβ given byAlonso et al. (1996, A&A, 313, 873) for several metallicities.Furthermore, for cool stars, temperatures inferred from the computedindices were compared with those of the fundamental stars listed byMD95. The same kind of comparison was made between gravities for B-typestars. Results: .The temperatures from the computed indices are ingood agreement, within the error limits, with the literature values for4750 K ≤ T_eff ≤ 8000 K, while the gravities agree for T_eff> 9000 K. The computed Hβ indices for the Sun and for Procyonare very close to the observed values. The comparison between theobserved and computed Hβ indices as function of the observedHβ has shown a very small trend which almost completely disappearswhen only stars hotter than 10 000 K are considered. The trend due tothe cool stars is probably related with the low accuracy of thefundamental T_eff which are affected by large errors for most of thestars.
| Asteroseismology of δ Scuti Stars: Problems and Prospects
We briefly outline the state-of-the-art seismology of δ Scutistars from a theoretical point of view: why is it so difficult a task?The recent theoretical advances in the field that these difficultieshave influenced are also discussed.
| Spitzer/IRAC Photometry of the η Chameleontis Association
We present IRAC 3.6, 4.5, 5.8, and 8 μm photometry for the 17 A-, K-,and M-type members of the η Chameleontis association. These datashow infrared excesses toward six of the 15 K and M stars, indicatingthe presence of circumstellar disks around 40% of the stars with massesof 0.1-1 Msolar. The two A stars show no infrared excesses.The excess emission around one of the stars is comparable to the medianexcess for classical T Tauri stars in the Taurus association; theremaining five show comparatively weak excess emission. Taking intoaccount published Hα spectroscopy that shows that five of the sixstars are accreting, we argue that the disks with weak mid-infraredexcesses are disks in which the inner disks have been largely depletedof small grains by grain growth or, in one case, the small grains havesettled to the midplane. This suggests that η Cha has a much higherfraction of disks caught in the act of transitioning into optically thindisks than that measured in younger clusters and associations.
| Constraints on Inner Disk Evolution Timescales: A Disk Census of the η Chamaeleontis Young Cluster
We present new L'-band (3.8 μm) observations of stars inthe nearby (~97 pc) young (~6 Myr) compact cluster around ηChamaeleontis, obtained with the European Southern Observatory's VeryLarge Telescope in Paranal, Chile. Our data, combined withJ,H,Ks photometry from the Two Micron All Sky Survey, revealthat only two of the 12 members surveyed harbor L'-bandexcesses consistent with optically thick inner disks; both are alsolikely accretors. Intriguingly, two other stars with possible evidenceof ongoing accretion, albeit at very low rates, do not show significantinfrared excess: this may imply substantial grain growth and/or partialclearing of the inner disk region, as expected in planet formationscenarios. Our findings suggest that η Cha stars are in an epochwhen disks are rapidly evolving, perhaps due to processes related toplanet building, and provide further constraints on inner disklifetimes.
| Observational Tests and Predictive Stellar Evolution. II. Nonstandard Models
We examine contributions of second-order physical processes to theresults of stellar evolution calculations that are amenable to directobservational testing. In the first paper in the series, we establishedbaseline results using only physics that were common to modern stellarevolution codes. In this paper we establish how much of the discrepancybetween observations and baseline models is due to particular elementsof new physics in the areas of mixing, diffusion, equations of state,and opacities. We then consider the impact of the observationaluncertainties on the maximum predictive accuracy achievable by a stellarevolution code. The Sun is an optimal case because of the precise andabundant observations and the relative simplicity of the underlyingstellar physics. The standard model is capable of matching the structureof the Sun as determined by helioseismology and gross surfaceobservables to better than a percent. Given an initial mass and surfacecomposition within the observational errors, and no current observablesas additional constraints for which the models can be optimized, it isnot possible to predict the Sun's current state to better than ~7%.Convectively induced mixing in radiative regions, terrestriallycalibrated by multidimensional numerical hydrodynamic simulations,dramatically improves the predictions for radii, luminosity, and apsidalmotions of eclipsing binaries while simultaneously maintainingconsistency with observed light element depletion and turnoff ages inyoung clusters. Systematic errors in core size for models of massivebinaries disappear with more complete mixing physics, and acceptablefits are achieved for all of the binaries without calibration of freeparameters. The lack of accurate abundance determinations for binariesis now the main obstacle to improving stellar models using this type oftest.
| Pulsations and metallicity of the pre-main sequence eclipsing spectroscopic binary RS Cha
We present new spectroscopic observations of the pre-main sequenceeclipsing spectroscopic binary RS Cha. A sample of 174 spectra wereobtained with the GIRAFFE spectrograph at the SAAO at 32 000 resolution.The radial velocity curves derived from these spectra were combined withprevious observations spanning a period of about 30 years to correct theephemeris of the system, and the result indicates that the orbitalperiod is not constant. Residuals of the binary radial velocity curvefor both components with amplitudes up to a few km s-1 andperiods on the order of 1 h are clearly seen in our data, which weinterpret as the signatures of delta-Scuti type pulsations. We revisitedthe masses of both components and determined the surface metallicity Zof both components of the RS Cha system by fitting synthetic spectra toobserved spectra in a set of selected spectral regions. The syntheticspectra are calculated with the SYNTH code using stellar atmospheremodels computed with the Kurucz ATLAS 9 code, along with a list of linesobtained from the VALD database. A selection of the best spectra and themost relevant spectral regions allowed us to determine Z = 0.028± 0.005. We also derived new values of v sin i: 64 ± 6 kms-1 and 70 ± 6 km s-1 for the primary andthe secondary star, respectively. Finally, we observationally confirmthat the RS Cha system is a synchronized and circularized system.
| The spectroscopic characteristics of intermediate-aged pre-main-sequence stars: the η Chamaeleontis cluster
We present a study of calibrated low-resolution spectra of the 18 knownprimaries of the ~9-Myr-old η Chamaeleontis (η Cha)pre-main-sequence (PMS) star cluster. Using synthetic broad-band coloursand narrow-band continuum-sensitive, temperature-sensitive andgravity-sensitive indices derived from the spectra, we compare the ηCha stars to standard dwarfs. We find that the VRI colours of the PMSstars are indistinguishable from those of main-sequence stars, but thata B-band excess attaining ~0.2 mag for late-M cluster stars is present,which might be an indicator of gravity, metallicity and/or activitydifferences between the two samples of stars. The narrow-band spectralindices for the η Cha stars possibly indicate higher metallicity andstrongly indicate lower surface gravity than the dwarf indices,consistent with the elevated location of the cluster in theHertzsprung-Russell evolutionary diagram. Using the derived syntheticcolours and indices, we adopt spectral types for the late-type η Chastars. We then produce a table of absolute optical magnitudes andcolours representing the cluster isochrone for comparison with PMSevolutionary models. From our results we also conclude that the ηCha stars are unreddened, consistent with the group being a sample ofolder PMS stars distant from obscuring molecular clouds, except for theA1 member HD 75505 for which we confirm AV= 0.4 mag likelydue to the presence of circumstellar material.
| Population and dynamical state of the η Chamaeleontis sparse young open cluster
We report new results in our continuing study of the unique compact (1pc extent), nearby (d= 97 pc), young (t= 9 Myr) stellar clusterdominated by the B9 star η Chamaeleontis. An optical photometricsurvey spanning 1.3 × 1.3 pc adds two M5-M5.5 weak-lined T Tauri(WTT) stars to the cluster inventory which is likely to be significantlycomplete for primaries with masses M > 0.15 Msolar. Thecluster now consists of 17 primaries and ~=9 secondaries lying within100 au of their primaries. The apparent distribution of 9:7:1single:binary:triple systems shows 2-4 times higher multiplicity than inthe field main-sequence stars, and is comparable to that seen in otherpre-main-sequence populations. The initial mass function (IMF) isconsistent with that of rich young clusters and field stars. Byextending the cluster IMF to lower masses, we predict 10-14 additionallow-mass stars with 0.08 < M < 0.15 Msolar and 10-15brown dwarfs with 0.025 < M < 0.08 Msolar remain to bediscovered. The η Cha cluster extends the established stellardensity and richness relationship for young open clusters. The radialdistribution of stars is consistent with an isothermal sphere, but masssegregation is present with >50 per cent of the stellar mass residingin the inner 6 arcmin (0.17 pc). Considering that the η Cha clusteris sparse, diffuse and young, the cluster may be an ideal laboratory fordistinguishing between mass segregation that is primordial in nature, orarising from dynamical interaction processes.
| The physical properties of normal A stars
Designating a star as of A-type is a result of spectral classification.After separating the peculiar stars from those deemed to be normal usingthe results of a century of stellar astrophysical wisdom, I define thephysical properties of the "normal" stars. The hotter A stars haveatmospheres almost in radiative equilibrium. In the A stars convectivemotions can be found which increase in strength as the temperaturedecreases.
| Spectroscopy of Candidate Members of the η Chamaeleontis and MBM 12 Young Associations
We present an analysis of candidate members of the η Cha and MBM 12Ayoung associations. For an area of 0.7 deg2 toward η Cha,we have performed a search for members of the association by combiningJHKs photometry from the Two Micron All Sky Survey and iphotometry from DENIS with follow-up optical spectroscopy at MagellanObservatory. We report the discovery of three new members with spectraltypes of M5.25-M5.75, corresponding to masses of 0.13-0.08Msolar by theoretical evolutionary models. Two and three ofthese members were found independently by Lyo and coworkers and Song andcoworkers, respectively. Meanwhile, no brown dwarfs were detected inη Cha down to the completeness limit of 0.015 Msolar. ForMBM 12A, we have obtained spectra of three of the remaining candidatemembers that lacked spectroscopy at the end of the survey by Luhman, allof which are found to be field M dwarfs. Ogura and coworkers haverecently presented four ``probable'' members of MBM 12A. However, two ofthese objects were previously classified as field dwarfs by thespectroscopy of Luhman. In this work, we find that the other two objectsare field dwarfs as well.Based on observations performed at Las Campanas Observatory and the MMTObservatory. This publication makes use of data products from 2MASS andDENIS.
| 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.
| Asiago eclipsing binaries program. I. V432 Aurigae
The orbit and physical parameters of the previously unsolved eclipsingbinary V432 Aur, discovered by Hipparcos, have been derived with errorsbetter than 1% from extensive Echelle spectroscopy and B, V photometry.Synthetic spectral analysis of both components has been performed,yielding Teff and log g in close agreement with the orbitalsolution, a metallicity [Z/Zȯ] = -0.6 and rotationalsynchronization for both components. Direct comparison on thetheoretical L, Teff plane with the Padova evolutionary tracksand isochrones for the masses of the two components (1.22 and 1.08Mȯ) provides a perfect match and a 3.75 Gyr age. Themore massive and cooler component is approaching the base of the giantbranch and displays a probable pulsation activity with an amplitude ofΔ V = 0.075 mag and Δrad.vel. = 1.5 km s-1. Witha Teff = 6080 K it falls to the red of the nearby instabilitystrip populated by δ Sct and γ Dor types of pulsatingvariables. Orbital modeling reveals a large and bright surface spot onit. The pulsations activity and the large spot(s) suggest the presenceof macro-turbulent motions in its atmosphere. They reflect in a linebroadening that at cursory inspection could be taken as indication of arotation faster than synchronization, something obviously odd for anold, expanding star.
| Infrared study of the η Chamaeleontis cluster and the longevity of circumstellar discs
We have analysed JHKL observations of the stellar population of the ~9Myr-old η Chamaeleontis cluster. Using infrared (IR) colour-colourand colour-excess diagrams, we find that the fraction of stellar systemswith near-IR excess emission is 0.60 +/- 0.13 (2σ). This resultimplies considerably longer disc lifetimes than found in some recentstudies of other young stellar clusters. For the classical T Tauri (CTT)and weak-lined T Tauri (WTT) star population, we also find a strongcorrelation between the IR excess and Hα emission. The IR excessesof these stars indicate a wide range of star-disc activity: from a CTTstar showing high levels of accretion to CTT-WTT transition objects withevidence for some on-going accretion and WTT stars with weak or absentIR excesses. Of the 15 known cluster members, four stars with IRexcesses Δ(K-L) > 0.4 mag are likely experiencing ongoingaccretion owing to strong or variable optical emission. The resultingaccretion fraction (0.27 +/- 0.13; 2σ) shows that the accretionphase, in addition to the discs themselves, can endure for at least ~10Myr.
| The ɛ Chamaeleontis Young Stellar Group and the Characterization of Sparse Stellar Clusters
We present the outcomes of a Chandra X-Ray Observatory snapshot studyof five nearby Herbig Ae/Be (HAeBe) stars that are kinematically linkedwith the Oph-Sco-Cen association (OSCA). Optical photometric andspectroscopic follow-up was conducted for the HD 104237 field. Theprincipal result is the discovery of a compact group ofpre-main-sequence (PMS) stars associated with HD 104237 and itscodistant, comoving B9 neighbor ɛ Chamaeleontis AB. We name thegroup after the most massive member. The group has five confirmedstellar systems ranging from spectral type B9 to M5, including aremarkably high degree of multiplicity for HD 104237 itself. The HD104237 system is at least a quintet, with four low-mass PMS companionsin nonhierarchical orbits within a projected separation of 1500 AU ofthe HAeBe primary. Two of the low-mass members of the group are activelyaccreting classical T Tauri stars. The Chandra observations alsoincrease the census of companions for two of the other four HAeBe stars,HD 141569 and HD 150193, and identify several additional new members ofthe OSCA. We discuss this work in light of several theoretical issues:the origin of X-rays from HAeBe stars; the uneventful dynamical historyof the high-multiplicity HD 104237 system; and the origin of the ɛCha group and other OSCA outlying groups in the context of turbulentgiant molecular clouds. Together with the similar η Cha cluster, wepaint a portrait of sparse stellar clusters dominated byintermediate-mass stars 5-10 Myr after their formation.
| ECHA J0843.3-7905: Discovery of an `old' classical T Tauri star in the η Chamaeleontis cluster
A limited-area survey of the η Chamaeleontis cluster has identifiedtwo new late-type members. The more significant of these is ECHAJ0843.3-7905 (=IRAS F08450-7854), a slowly rotating (P=12d) M2 classicalT Tauri (CTT) star with a spectrum dominated by Balmer emission. At adistance of 97pc and cluster age of ~9Myr, the star is a nearby rareexample of an `old' CTT star and promises to be a rewarding laboratoryfor the study of disc structure and evolution in pre-main sequence (PMS)stars. The other new member is the M4 weak-lined T Tauri (WTT) star ECHAJ0841.5-7853, which is the lowest mass (M~0.2Msolar) primaryknown in the cluster.
| Studies of Intermediate-Mass Stellar Models Using Eclipsing Binaries
Evolutionary computations for intermediate-mass stars are analyzed usingobserved parameters for eclipsing SB2 binaries and theoreticalparameters based on evolutionary tracks. Modern observations cannot beused to distinguish between models with and without convectiveovershooting for stars in the vicinity of the main sequence.Statistically significant discrepancies between the observed andcomputed stellar parameters are associated with systematic errors inphotometric effective temperatures. After taking into account systematiceffects, the theoretical computations fit the observational datauniformly well throughout the entire mass interval studied. Empiricaland semiempirical (i.e., reduced to the ZAMS and with solar elementalabundances) formulas for the mass-luminosity, mass-effectivetemperature, and mass-radius relations are proposed.
| The Smallest Mass Ratio Young Star Spectroscopic Binaries
Using high-resolution near-infrared spectroscopy with the KeckTelescope, we have detected the radial velocity signatures of the coolsecondary components in four optically identified pre-main-sequence,single-lined spectroscopic binaries. All are weak-lined T Tauri starswith well-defined center-of-mass velocities. The mass ratio for oneyoung binary, NTTS 160905-1859, isM2/M1=0.18+/-0.01, the smallest yet measureddynamically for a pre-main-sequence spectroscopic binary. These newresults demonstrate the power of infrared spectroscopy for the dynamicalidentification of cool secondaries. Visible-light spectroscopy, to date,has not revealed any pre-main-sequence secondary stars with masses<0.5 Msolar, while two of the young systems reported hereare in that range. We compare our targets with a compilation of thepublished young, double-lined spectroscopic binaries and discuss ourunique contribution to this sample.
| Determination of the Ages of Close Binary Stars on the Main Sequence from Evolutionary Model Stars of Claret and Gimenez
A grid of isochrones, covering a wide range of stellar ages from thezero-age main sequence to 10 billion years, is calculated in the presentwork on the basis of the model stars of Claret and Gimenez withallowance for convective overshoot and mass loss by the components. Theages of 88 eclipsing variables on the main sequence from Andersen'scatalog and 100 chromospherically active stars from Strassmeier'scatalog are calculated with a description of the method of optimuminterpolation. Comparisons with age determinations by other authors aregiven and good agreement is established.
| Close Binaries in the η Chamaeleontis Cluster
We have used speckle interferometry and adaptive optics observations tosearch for multiple systems among 13 stars in the η Chamaeleontiscluster. We discovered two previously unknown subarcsecond binaries.Placing the components in infrared color-magnitude diagrams shows thatmost members of η Cha are coeval. Repeated observations of thebinary RECX 1 allow us to determine a preliminary orbit and derive asystem mass of about 2 Msolar.Based on observations obtained at the European Southern Observatory, LaSilla, proposals 56.E-0197, 62.I-0399, 65.I-0350, 65.I-0086, 67.C-0354, and 68.C-0539.
| Absolute Properties of the Main-Sequence Eclipsing Binary Star WW Camelopardalis
We present absolute photometric observations in uvbyβ and 5759differential observations in the V filter (the most complete light curveever obtained) measured by a robotic telescope, as well as radialvelocities from spectroscopic observations of the detached, eccentric,2.3 day, double-lined, eclipsing binary star WW Camelopardalis. Absolutedimensions of the components are determined with high precision (betterthan 1% in the masses and radii) for the purpose of testing variousaspects of theoretical modeling. We obtain 1.920+/-0.013Msolar and 1.911+/-0.016 Rsolar for the primary,and 1.873+/-0.018 Msolar and 1.808+/-0.014 Rsolarfor the secondary. The effective temperatures and interstellar reddeningof the stars are accurately determined from new uvbyβ photometry:8350+/-135 K for the primary and 8240+/-135 K for the secondary,corresponding to a spectral type of A4m for both, and 0.294 mag forEb-y. The metallic-lined character of the stars is revealedby high-resolution spectroscopy and uvbyβ photometry. Spectral linewidths give rotational velocities that are synchronous with the orbitalmotion in a slightly eccentric orbit (e=0.0098). The components of WWCam are main-sequence stars with an age of about 490 Myr according tomodels. Some of the observations reported here were obtained with theMultiple Mirror Telescope, a joint facility of the SmithsonianInstitution and the University of Arizona.
| Detached double-lined eclipsing binaries as critical tests of stellar evolution. Age and metallicity determinations from the HR diagram
Detached, double-lined spectroscopic binaries that are also eclipsingprovide the most accurate determinations of stellar mass, radius,temperature and distance-independent luminosity for each of theirindividual components, and hence constitute a stringent test ofsingle-star stellar evolution theory. We compile a large sample of 60non-interacting, well-detached systems mostly with typical errorssmaller than 2% for mass and radius and smaller than 5% for effectivetemperature, and compare them with the properties predicted by stellarevolutionary tracks from a minimization method. To assess the systematicerrors introduced by a given set of tracks, we compare the resultsobtained using three widely-used independent sets of tracks, computedwith different physical ingredients (the Geneva, Padova and Granadamodels). We also test the hypothesis that the components of thesesystems are coeval and have the same metallicity, and compare thederived ages and metallicities with the ones obtained by fitting asingle isochrone to the system. Overall, there is a good agreement amongthe different determinations, and we provide a comprehensive discussionon the sub-sample of systems which either present problems or haveestimated metallicities. Although within the errors the published trackscan fit most of the systems, a large degeneracy between age andmetallicity remains. The power of the test is thus limited because themetallicities of most of the systems are unknown. The full version ofTable 6 is only available in the electronic form athttp://www.edpsciences.org
| On the anomaly of Balmer line profiles of A-type stars. Fundamental binary systems
In previous work, Gardiner et al. (\cite{GKS99}) found evidence for adiscrepancy between the Teff obtained from Balmer lines withthat from photometry and fundamental values for A-type stars. Aninvestigation into this anomaly is presented using Balmer line profilesof stars in binary system with fundamental values of bothTeff and log g. A revision of the fundamental parameters forbinary systems given by Smalley & Dworetsky (\cite{SD95}) is alsopresented. The Teff obtained by fitting Hα and Hβline profiles is compared to the fundamental values and those obtainedfrom uvby photometry. We find that the discrepancy found by Gardiner etal. (\cite{GKS99}) for stars in the range 7000 K <~ Teff<~ 9000 K is no longer evident. Partly based on DENIS data obtainedat the European Southern Observatory.
| The η Chamaeleontis cluster: photometric study of the ROSAT-detected weak-lined T Tauri stars
We present the results of a photometric study of X-ray-active weak-linedT Tauri (WTT) stars in the η Chamaeleontis star cluster. Multi-epochV-band photometric monitoring during 1999 and 2000 of the 10X-ray-active WTT stars found that all were variable in one or bothyears, with periods ascribed to rotational modulation of starspots.Comparison between the rotational and X-ray properties of these objectsindicates the saturation level, log(LXLbol)~-3,observed in other studies of X-ray-active pre-main-sequence stars,persists in the η Cha stars from the slow- to the fast-rotatorregimes. Cousins VRI photometry of the WTT stars has enabled us toinvestigate further the photometric properties of these stars. The starsappear sufficiently coeval to distinguish near-equal-mass binarieswithin the sample. A new Hertzsprung-Russell diagram for these objectssuggests ages of 4-9Myr for M-type RECX primaries using the tracks ofD'Antona & Mazzitelli.
| Stars with the Largest Hipparcos Photometric Amplitudes
A list of the 2027 stars that have the largest photometric amplitudes inHipparcos Photometry shows that most variable stars are all Miras. Thepercentage of variable types change as a function of amplitude. Thiscompilation should also be of value to photometrists looking forrelatively unstudied, but large amplitude stars.
| Age and Metallicity Estimates for Moderate-Mass Stars in Eclipsing Binaries
We estimate the ages and metallicities for the components of 43 binarysystems using a compilation of accurate observational data on eclipsingbinaries for which lines of both components are visible in theirspectra, together with two independent modern sets of stellar evolutionmodels computed for a wide range of masses and chemical abundances. Theuncertainties of the resulting values are computed, and their stabilityis demonstrated. The ages and metallicity are compared with thosederived in other studies using different methods, as well as withindependent estimates from photometric observations and observations ofclusters. These comparisons con firm the reliability of our ageestimates. The resulting metallicities depend significantly on thechoice of theoretical model. Comparison with independent estimatesfavors the estimates based on the evolutionary tracks of the Genevagroup.
| Observational Tests and Predictive Stellar Evolution
We compare 18 binary systems with precisely determined radii and massesfrom 23 to 1.1 Msolar and stellar evolution models producedwith our newly revised code TYCHO. ``Overshooting'' and rotationalmixing were suppressed in order to establish a baseline for isolatingthese and other hydrodynamic effects. Acceptable coeval fits are foundfor 16 pairs without optimizing for heavy-element or helium abundance.The precision of these tests is limited by the accuracies of theobserved effective temperatures. High-dispersion spectra and detailedatmospheric modeling should give more accurate effective temperaturesand heavy-element abundances. PV Cas, a peculiar early A system, EK CepB, a known post-T Tauri star, and RS Cha, a member of a young OBassociation, are matched by pre-main-sequence models. Predicted massloss agrees with upper limits from IUE for CW Cep A and B. Relativelypoor fits are obtained for binaries having at least one component in themass range 1.7
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| Sað Açýklýk: | 08h43m12.40s |
| Yükselim: | -79°04'11.0" |
| Görünürdeki Parlaklýk: | 6.05 |
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| özdevim Sað Açýklýk: | -29.7 |
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| B-T magnitude: | 6.368 |
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