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Accurate masses and radii of normal stars: modern results and applications
This article presents and discusses a critical compilation of accurate,fundamental determinations of stellar masses and radii. We haveidentified 95 detached binary systems containing 190 stars (94 eclipsingsystems, and ? Centauri) that satisfy our criterion that the massand radius of both stars be known within errors of ±3% accuracyor better. All of them are non-interacting systems, and so the starsshould have evolved as if they were single. This sample more thandoubles that of the earlier similar review by Andersen (Astron AstrophysRev 3:91-126, 1991), extends the mass range at both ends and, forthe first time, includes an extragalactic binary. In every case, we haveexamined the original data and recomputed the stellar parameters with aconsistent set of assumptions and physical constants. To these we addinterstellar reddening, effective temperature, metal abundance,rotational velocity and apsidal motion determinations when available,and we compute a number of other physical parameters, notably luminosityand distance. These accurate physical parameters reveal the effects ofstellar evolution with unprecedented clarity, and we discuss the use ofthe data in observational tests of stellar evolution models in somedetail. Earlier findings of significant structural differences betweenmoderately fast-rotating, mildly active stars and single stars, ascribedto the presence of strong magnetic and spot activity, are confirmedbeyond doubt. We also show how the best data can be used to testprescriptions for the subtle interplay between convection, diffusion,and other non-classical effects in stellar models. The amount andquality of the data also allow us to analyse the tidal evolution of thesystems in considerable depth, testing prescriptions of rotationalsynchronisation and orbital circularisation in greater detail thanpossible before. We show that the formulae for pseudo-synchronisation ofstars in eccentric orbits predict the observed rotations quite well,except for very young and/or widely separated stars. Deviations dooccur, however, especially for stars with convective envelopes. Thesuperior data set finally demonstrates that apsidal motion rates aspredicted from General Relativity plus tidal theory are in goodagreement with the best observational data. No reliable binary dataexist, which challenge General Relativity to any significant extent. Thenew data also enable us to derive empirical calibrations of M and R forsingle (post-) main-sequence stars above {0.6 M_{odot}}. Simple,polynomial functions of T eff, log g and [Fe/H] yield M and Rwithin errors of 6 and 3%, respectively. Excellent agreement is foundwith independent determinations for host stars of transiting extrasolarplanets, and good agreement with determinations of M and R from stellarmodels as constrained by trigonometric parallaxes and spectroscopicvalues of T eff and [Fe/H]. Finally, we list a set of 23interferometric binaries with masses known to be better than 3%, butwithout fundamental radius determinations (except ? Aur). Wediscuss the prospects for improving these and other stellar parametersin the near future.
| Absolute dimensions of solar-type eclipsing binaries. III. EW Orionis: stellar evolutionary models tested by a G0 V system
Context. Recent studies of inactive and active solar-type binariessuggest that chromospheric activity, and its effect on envelopeconvection, is likely to cause significant radius and temperaturediscrepancies. Accurate mass, radius, and abundance determinations fromadditional solar-type binaries exhibiting various levels of activity areneeded for a better insight into the structure and evolution of thesestars. Aims: We aim to determine absolute dimensions andabundances for the G0 V detached eclipsing binary EW Ori, and to performa detailed comparison with results from recent stellar evolutionarymodels. Methods: uvby light curves and {uvby}? standardphotometry were obtained with the Strömgren Automatic Telescope,published radial velocity observations from the CORAVEL spectrometerwere reanalysed, and high-resolution spectra were observed at the FEROSspectrograph; all are/were ESO, La Silla facilities. State-of-the-artmethods were applied for the photometric and spectroscopic analyses. Results: Masses and radii that are precise to 0.9% and 0.5%,respectively, have been established for both components of EW Ori. The1.12 M? secondary component reveals weak Ca ii H and Kemission and is probably mildly active; no signs of activity are seenfor the 1.17 M? primary. We derive an [Fe/H] abundanceof +0.05 ± 0.09 and similar abundances for Si, Ca, Sc, Ti, Cr,and Ni. Yonsai-Yale and Granada solar-scaled evolutionary models for theobserved metal abundance reproduce the components fairly well at an ageof ?2 Gyr. Perfect agreement is, however, obtained at an age of 2.3Gyr for a combination of a) a slight downwards adjustment of theenvelope mixing length parameter for the secondary, as seen for otheractive solar-type stars; and b) a slightly lower helium content thanprescribed by the Y-Z relations adopted for the standard model grids.The orbit is eccentric (e = 0.0758 ± 0.0020), and apsidal motionwith a 62% relativistic contribution has been detected. The apsidalmotion period is U = 16 300 ± 3900 yr, and the inferred meancentral density concentration coefficient, log(k2) = -1.66± 0.30, agrees marginally with model predictions. The measuredrotational velocities, 9.0 ± 0.7 (primary) and 8.8 ± 0.6(secondary) km s-1, are in agreement with both thesynchronous velocities and the theoretically predictedpseudo-synchronous velocities. Finally, the distance (175 ± 7pc), age, and center-of mass velocity (6 km s-1) excludesuggested membership of the open cluster Collinder 70. Conclusions: EW Ori now belongs to the small group of solar-typeeclipsing binaries with well-established astrophysical properties.Based on observations carried out at the Strömgren AutomaticTelescope (SAT) and the 1.5 m telescope (62.L-0284) at ESO, LaSilla.Table 11 is only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/511/A22
| Relativistic apsidal motion in eccentric eclipsing binaries
Context. The study of apsidal motion in detached eclipsing binarysystems is known to be an important source of information about stellarinternal structure as well as the possibility of verifying of GeneralRelativity outside the Solar System. Aims: As part of thelong-term Ond?ejov and Ostrava observational projects, we aim tomeasure precise times of minima for eccentric eclipsing binaries, neededfor the accurate determination of apsidal motion, providing a suitabletest of the effects of General Relativity. Methods: About seventynew times of minimum light recorded with photoelectric or CCDphotometers were obtained for ten eccentric-orbit eclipsing binarieswith significant relativistic apsidal motion. Their O-C diagrams wereanalysed using all reliable timings found in the literature, and new orimproved elements of apsidal motion were obtained. Results: Weconfirm very long periods of apsidal motion for all systems. For BF Draand V1094 Tau, we present the first apsidal-motion solution. Therelativistic effects are dominant, representing up to 100% of the totalobservable apsidal-motion rate in several systems. The theoretical andobserved values of the internal structure constant k 2 werecompared for systems with lower relativistic contribution. Using thelight-time effect solution, we predict a faint third component for V1094Tau orbiting with a short period of about 8 years.Partly based on photoelectric observations secured at the HvarObservatory, Faculty of Geodesy, Zagreb, Croatia, in October 2008.
| Distances to Four Solar Neighborhood Eclipsing Binaries from Absolute Fluxes
Eclipsing binary (EB)-based distances are estimated for four solarneighborhood EBs by means of the Direct Distance Estimation (DDE)algorithm. Results are part of a project to map the solar neighborhoodEBs in three dimensions, independently of parallaxes, and providestatistical comparisons between EB and parallax distances. Apart fromjudgments on adopted temperature and interstellar extinction, DDE'ssimultaneous light-velocity solutions are essentially objective and workas well for semidetached (SD) and overcontact binaries as for detachedsystems. Here, we analyze two detached and two SD binaries, all doublelined. RS Chamaeleontis is a pre-main-sequence (MS), detached EB withweak ? Scuti variations. WW Aurigae is detached and uncomplicated,except for having high metallicity. RZ Cassiopeiae is SD and has veryclear ? Scuti variations and several peculiarities. R CanisMajoris (R CMa) is an apparently simple but historically problematic SDsystem, also with weak ? Scuti variations. Discussions includesolution rules and strategies, weighting, convergence, and third lightproblems. So far there is no indication of systematic band dependenceamong the derived distances, so the adopted band-calibration ratios seemconsistent. Agreement of EB-based and parallax distances is typicallywithin the overlapped uncertainties, with minor exceptions. We alsosuggest an explanation for the long-standing undermassiveness problem ofR CMa's hotter component, in terms of a fortuitous combination of lowmetallicity and evolution slightly beyond the MS.
| Absolute dimensions of solar-type eclipsing binaries. II. V636 Centauri: A 1.05 {M}ȯ primary with an active, cool, oversize 0.85 {M}ȯ secondary
Context: The influence of stellar activity on the fundamental propertiesof stars around and below 1 Mȯ is not well understood.Accurate mass, radius, and abundance determinations from solar-typebinaries exhibiting various levels of activity are needed for a betterinsight into the structure and evolution of these stars. Aims: Weaim to determine absolute dimensions and abundances for the solar-typedetached eclipsing binary V636 Cen, and to perform a detailed comparisonwith results from recent stellar evolutionary models. Methods:uvby light curves and uvbyβ standard photometry were obtained withthe Strömgren Automatic Telescope, radial velocity observationswith the CORAVEL spectrometer, and high-resolution spectra with theFEROS spectrograph, all at ESO, La Silla. State-of-the-art methods wereapplied for the photometric and spectroscopic analyses. Results:Masses and radii that are precise to 0.5% have been established for thecomponents of V636 Cen. The 0.85 Mȯ secondary componentis moderately active with starspots and Ca ii H and K emission, and the1.05 Mȯ primary shows signs of activity as well, but ata much lower level. We derive a [Fe/H] abundance of -0.20 ± 0.08and similar abundances for Si, Ca, Ti, V, Cr, Co, and Ni. Correspondingsolar-scaled stellar models are unable to reproduce V636 Cen, especiallyits secondary component, which is ~10% larger and ~400 K cooler thanpredicted. Models adopting significantly lower mixing-length parametersl/Hp remove these discrepancies, seen also for othersolar-type binary components. For the observed [Fe/H], Claret models forl/Hp = 1.4 (primary) and 1.0 (secondary) reproduce thecomponents of V636 Cen at a common age of 1.35 Gyr. The orbit iseccentric (e = 0.135 ± 0.001), and apsidal motion with a 40%relativistic contribution has been detected. The period is U = 5 270± 335 yr, and the inferred mean central density concentrationcoefficient, log(k_2) = -1.61 ± 0.05, agrees marginally withmodel predictions. The measured rotational velocities, 13.0 ± 0.2(primary) and 11.2 ± 0.5 (secondary) km s-1, are inremarkable agreement with the theoretically predicted pseudo-synchronousvelocities, but are about 15% lower than the periastron values. Conclusions: V636 Cen and 10 other well-studied inactive and activesolar-type binaries suggest that chromospheric activity, and its effecton envelope convection, is likely to cause radius and temperaturediscrepancies, which can be removed by adjusting the model mixing lengthparameters downwards. Noting this, the sample may also lend support totheoretical 2D radiation hydrodynamics studies, which predict a slightdecrease of the mixing length parameter with increasing temperature/massfor inactive main sequence stars. More binaries are, however, needed fora description/calibration in terms of physical parameters and level ofactivity.Based on observations carried out at the Strömgren AutomaticTelescope (SAT), the Danish 1.54 m telescope, and the 1.5 m telescope(62.L-0284) at ESO, La Silla, Chile. Table A.1 is only available inelectronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/502/253
| The Geneva-Copenhagen survey of the solar neighbourhood. III. Improved distances, ages, and kinematics
Context: Ages, chemical compositions, velocity vectors, and Galacticorbits for stars in the solar neighbourhood are fundamental test datafor models of Galactic evolution. The Geneva-Copenhagen Survey of theSolar Neighbourhood (Nordström et al. 2004; GCS), amagnitude-complete, kinematically unbiased sample of 16 682 nearby F andG dwarfs, is the largest available sample with complete data for starswith ages spanning that of the disk. Aims: We aim to improve theaccuracy of the GCS data by implementing the recent revision of theHipparcos parallaxes. Methods: The new parallaxes yield improvedastrometric distances for 12 506 stars in the GCS. We also use theparallaxes to verify the distance calibration for uvby? photometryby Holmberg et al. (2007, A&A, 475, 519; GCS II). We add newselection criteria to exclude evolved cool stars giving unreliableresults and derive distances for 3580 stars with large parallax errorsor not observed by Hipparcos. We also check the GCS II scales of T_effand [Fe/H] and find no need for change. Results: Introducing thenew distances, we recompute MV for 16 086 stars, and U, V, W,and Galactic orbital parameters for the 13 520 stars that also haveradial-velocity measurements. We also recompute stellar ages from thePadova stellar evolution models used in GCS I-II, using the new valuesof M_V, and compare them with ages from the Yale-Yonsei andVictoria-Regina models. Finally, we compare the observed age-velocityrelation in W with three simulated disk heating scenarios to show thepotential of the data. Conclusions: With these revisions, thebasic data for the GCS stars should now be as reliable as is possiblewith existing techniques. Further improvement must await consolidationof the T_eff scale from angular diameters and fluxes, and the Gaiatrigonometric parallaxes. We discuss the conditions for improvingcomputed stellar ages from new input data, and for distinguishingdifferent disk heating scenarios from data sets of the size andprecision of the GCS.Full Table 1 is only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/501/941
| A catalogue of chromospherically active binary stars (third edition)
The catalogue of chromospherically active binaries (CABs) has beenrevised and updated. With 203 new identifications, the number of CABstars is increased to 409. The catalogue is available in electronicformat where each system has a number of lines (suborders) with a uniqueorder number. The columns contain data of limited numbers of selectedcross references, comments to explain peculiarities and the position ofthe binarity in case it belongs to a multiple system, classicalidentifications (RS Canum Venaticorum, BY Draconis), brightness andcolours, photometric and spectroscopic data, a description of emissionfeatures (CaII H and K, Hα, ultraviolet, infrared),X-ray luminosity, radio flux, physical quantities and orbitalinformation, where each basic entry is referenced so users can go to theoriginal sources.
| Four-colour photometry of eclipsing binaries. XLI.. uvby light curves for AD Bootis, HW Canis Majoris, SW Canis Majoris, VZ Hydrae, and WZ Ophiuchi
Context: Accurate mass, radius, and abundance determinations frombinaries provide important information on stellar evolution, fundamentalto central fields in modern astrophysics and cosmology. Aims:Within the long-term Copenhagen Binary Project, we aim to obtainhigh-quality light curves and standard photometry for double-lineddetached eclipsing binaries with late A, F, and G type main-sequencecomponents, needed for the determination of accurate absolute dimensionsand abundances, and for detailed comparisons with results from recentstellar evolutionary models. Methods: Between March 1985 and July2007, we carried out photometric observations of AD Boo, HW CMa, SW CMa,V636 Cen, VZ Hya, and WZ Oph at the Strömgren Automatic Telescopeat ESO, La Silla. Results: We obtained complete {uvby} lightcurves, ephemerides, and standard {uvby}? indices for all sixsystems. For V636 Cen and HW CMa, we present the first modern lightcurves, whereas for AD Boo, SW CMa, VZ Hya, and WZ Oph, they are bothmore accurate and more complete than earlier data. Due to a high orbitaleccentricity (e = 0.50), combined with a low orbital inclination (i =84.7°), only one eclipse, close to periastron, occurs for HW CMa.For the two other eccentric systems, V636 Cen (e = 0.134) and SW CMa (e= 0.316), apsidal motion has been detected with periods of 5270 ±335 and 14 900 ± 3600 years, respectively.Based on observations carried out with the Strömgren AutomaticTelescope (SAT) at ESO, La Silla, Chile. Appendix A is only available inelectronic form at http://www.aanda.org Tables 6-13 are only availablein electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/487/1081
| New absolute magnitude calibrations for detached binaries
Lutz-Kelker bias corrected absolute magnitude calibrations for thedetached binary systems with main-sequence components are presented. Theabsolute magnitudes of the calibrator stars were derived at intrinsiccolours of Johnson-Cousins and 2MASS (Two Micron All Sky Survey)photometric systems. As for the calibrator stars, 44 detached binarieswere selected from the Hipparcos catalogue, which have relative observedparallax errors smaller than 15% (σπ/π≤0.15).The calibration equations which provide the corrected absolute magnitudefor optical and near-infrared pass bands are valid for wide ranges ofcolours and absolute magnitudes: -0.18<(B-V)0<0.91,-1.6
| Four-colour photometry of eclipsing binaries. XL. uvby light curves for the B-type systems DW Carinae, BF Centauri, AC Velorum, and NSV 5783
Aims.In order to increase the limited number of B-stars with accuratelyknown dimensions, and also the number of well studied eclipsing binariesin open clusters, we have undertaken observations and studies of foursouthern double-lined eclipsing B-type binaries; DW Car, BF Cen, AC Vel,and NSV 5783. Methods: .Complete {uvby} light curves were observedbetween January 1982 and April 1991 at the Danish 0.5 m telescope at ESOLa Silla, since 1985 known as the Strömgren Automatic Telescope(SAT). Standard indices for the systems and the comparison stars, aswell as additional minima observations for AC Vel, have been obtainedlater at SAT. For DW Car and AC Vel, high-resolution spectra fordefinitive spectroscopic orbits have also been obtained; they arepresented as part of the detailed analyses of these systems. A fewspectra of NSV 5783 are included in the present paper. Results:.For all four systems, the first modern accurate light curves have beenestablished. DW Car is a detached system consisting of two nearlyidentical components. It is member of the young open cluster Cr228. Adetailed analysis, based on the new light curves and 29 high-resolutionspectra, is published separately. BF Cen is semidetached and is memberof NGC 3766. Modern spectra are needed for a detailed study. AC Vel is adetached system with at least one more star. A full analysis, based onthe new light curves and 18 high-resolution spectra, is publishedseparately. NSV 5783 is discovered to be an eclipsing binary consistingof two well-detached components in an 11-day period eccentric (e = 0.18)orbit. Secondary eclipse is practically total. From the light curves anda few high-resolution spectra, accurate photometric elements andpreliminary absolute dimensions have been determined. The quite similarcomponents have masses of about 5 Mȯ and radii of about3.5 Rȯ, and they seem to have evolved just slightly offthe ZAMS. The measured rotational velocities (≈150 kms-1) are about 6 times those corresponding topseudosynchronization.
| Search for associations containing young stars (SACY). I. Sample and searching method
We report results from a high-resolution optical spectroscopic surveyaimed to search for nearby young associations and young stars amongoptical counterparts of ROSAT All-Sky Survey X-ray sources in theSouthern Hemisphere. We selected 1953 late-type (B-V~≥~0.6),potentially young, optical counterparts out of a total of 9574 1RXSsources for follow-up observations. At least one high-resolutionspectrum was obtained for each of 1511 targets. This paper is the firstin a series presenting the results of the SACY survey. Here we describeour sample and our observations. We describe a convergence method in the(UVW) velocity space to find associations. As an example, we discuss thevalidity of this method in the framework of the β Pic Association.
| 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.
| Updated Elements for Southern Eclipsing Binaries
Analysis of ASAS-3 data for 442 southern eclipsing binaries yielded 131stars with incorrect periods listed in the GCVS catalog
| 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
| Fast-rotating nearby solar-type stars sin i and X-ray luminosities relationships. II. Li abundances, v sin i and X-ray luminosities relationships
We present an analysis of our high-resolution spectroscopic andhigh-precision UBV(RI)_c photometric observations of a sample of 110nearby late-F and G-type stars selected for their large rotationalvelocity. The relationships between Li abundance, X-ray luminosity, andvsin i are investigated. We find that, as expected, the stars in oursample show statistically higher Li abundance and activity level thanfield star samples with similar characteristics, but slower rotation.Surprisingly, however, we also find four rapidly-rotating singlemain-sequence stars with very low Li abundance. For both single andbinary stars we find a large spread of Li abundance for stars withrotation lower than about 18 km s-1. The well-establishedcorrelation between X-ray luminosity and rotation rate is clearlyobserved. All single unevolved solar type stars with vsin i larger than18 km s-1 are strong X-ray emitters and have high Liabundance. Finally, we find also five evolved stars with very low Liabundance that are still rather fast rotators. The results from oursample confirm the presence of young very active stars close to the Sun,in agreement with recent findings from EUV and X-ray surveys, althoughour sample does not show such extreme characteristics as those selectedfrom EUV and X-ray surveys at the current flux limits.Based on data collected at the European Southern Observatory, La Silla,Chile.Tables, Figures and the complete data set are available in electronicform at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5)or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/397/987
| Post-T Tauri Stars in the Nearest OB Association
We present results of a spectroscopic survey of X-ray- andproper-motion-selected samples of late-type stars in the LowerCentaurus-Crux (LCC) and Upper Centaurus-Lupus (UCL) subgroups of thenearest OB association: Scorpius-Centaurus. The primary goals of thesurvey are to determine the star formation history of the OB subgroupsand to assess the frequency of accreting stars in a sample dominated by``post-T Tauri'' pre-main-sequence (PMS) stars. We investigate twosamples: (1) proper-motion candidates from the ACT Catalog and TychoReference Catalog (TRC) with X-ray counterparts in the ROSAT All-SkySurvey (RASS) Bright Source Catalog and (2) G- and K-type stars in theHipparcos catalog found to be candidate members by de Zeeuw et al. Weobtained optical spectra of 130 candidates with the Siding Spring 2.3 mdual-beam spectrograph. PMS stars were identified by (1) strong Liλ6707 absorption, (2) subgiant surface gravities, (3) propermotions consistent with Sco-Cen membership, and (4) H-R diagrampositions consistent with being PMS. We find 93% of the RASS-ACT/TRCstars to be probable PMS members, compared with 73% of the Hipparcoscandidates. We demonstrate that measuring the gravity-sensitive bandratio of Sr II λ4077 to Fe I λ4071 is a valuable means ofdiscriminating PMS and zero-age main-sequence (ZAMS) stars. Usingsecular parallaxes and Hipparcos, Tycho-2, and Two Micron All Sky Surveyphotometry, we construct an H-R diagram. Depending on the choice ofpublished evolutionary tracks, we find the mean ages of the PMSpopulations to range between 17 and 23 Myr for LCC and 15 and 22 Myr forUCL. Taking into account observational errors, it appears that 95% ofthe low-mass star formation in each subgroup must have occurred in lessthan 8 Myr (LCC) and 12 Myr (UCL). Using the Bertelli et al. tracks, wefind main-sequence turnoff ages for Hipparcos B-type members to be16+/-1 Myr for LCC and 17+/-1 Myr for UCL. Contrary to previousfindings, it appears that LCC is coeval with, or slightly older than,UCL. The secular parallaxes of the Sco-Cen PMS stars yield distances of85-215 pc, with 12 of the LCC members lying within 100 pc of the Sun.Only one out of 110 (0.9+2.1-0.8%; 1 σ) PMSsolar-type stars in the sample with ages of 13+/-1 (s.e.)+/-6 (1σ) Myr and masses of 1.3+/-0.2 (1 σ) Msolar showsboth enhanced Hα emission and a K-band excess indicative ofaccretion from a truncated circumstellar disk: the nearby (d~=86 pc)classical T Tauri star PDS 66.
| Fast-rotating nearby solar-type stars, Li abundances and X-ray luminosities. I. Spectral classification, v sin i, Li abundances and X-ray luminosities
We present the results of high-resolution spectroscopic andhigh-precision photometric observations on a sample of 129 late-F andG-type nearby stars selected on the basis of their large rotationalvelocity. Using also data from the Hipparcos satellite, CORAVEL and fromthe ROSAT satellite database, we infer spectral types, compute radialvelocities, v sin i, Li abundances and X-ray luminosities andinvestigate the single or binary nature of the sample stars. Such acareful analysis of our sample shows a large fraction of binaries ( =~62%) and of young single disk stars. In particular, at least 9 stars canbe considered bona-fide PMS or ZAMS objects, and 30 stars are identifiedas SBs for the first time. Information on the presence of Ca II Kemission and on optical variability is given for some of the stars ofthe sample. Based on data collected at the European SouthernObservatory, La Silla, Chile. Tables 1, 3, 4 and 5 and the complete dataset are only available in electronic form at the CDS via anonymous ftpto cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/384/491
| A Multiplicity Survey of Chromospherically Active and Inactive Stars
Surveys of three samples of solar-type stars, segregated bychromospheric emission level, were made to determine their multiplicityfractions and to investigate the evolution of multiplicity with age. Intotal, 245 stars were searched for companions with DeltaV <= 3.0 andseparations of 0.035" to 1.08" using optical speckle interferometry. Byincorporating the visual micrometer survey for duplicity of theLamontHussey Observatory, the angular coverage was extended to 5.0" withno change in the DeltaV limit. This magnitude difference allows massratios of 0.63 and larger to be detected throughout a search region of2-127 AU for the stars observed. The 84 primaries observed in thechromospherically active sample are presumably part of a youngpopulation and are found to have a multiplicity fraction of 17.9% +/-4.6%. The sample of 118 inactive, presumably older, primaries wereselected and observed using identical methods and are found to have amultiplicity fraction of only 8.5% +/- 2.7%. Given the known linkbetween chromospheric activity and age, these results tentatively implya decreasing stellar multiplicity fraction from 1 to 4 Gyr, theapproximate ages of the two samples. Finally, only two of the 14 veryactive primaries observed were found to have a companion meeting thesurvey detection parameters. In this case, many of the systems areeither very young, or close, RS CVn type multiples that are unresolvableusing the techniques employed here.
| The Problem of HIPPARCOS Distances to Open Clusters. II. Constraints from Nearby Field Stars
This paper examines the discrepancy between distances to nearby openclusters as determined by parallaxes from Hipparcos compared totraditional main-sequence fitting. The biggest difference is seen forthe Pleiades, and our hypothesis is that if the Hipparcos distance tothe Pleiades is correct, then similar subluminous zero-age main-sequence(ZAMS) stars should exist elsewhere, including in the immediate solarneighborhood. We examine a color-magnitude diagram of very young andnearby solar-type stars and show that none of them lie below thetraditional ZAMS, despite the fact that the Hipparcos Pleiades parallaxwould place its members 0.3 mag below that ZAMS. We also presentanalyses and observations of solar-type stars that do lie below theZAMS, and we show that they are subluminous because of low metallicityand that they have the kinematics of old stars.
| Metallicity effects on the chromospheric activity-age relation for late-type dwarfs
We show that there is a relationship between the age excess, defined asthe difference between the stellar isochrone and chromospheric ages, andthe metallicity as measured by the index [Fe/H] for late-type dwarfs.The chromospheric age tends to be lower than the isochrone age formetal-poor stars, and the opposite occurs for metal-rich objects. Wesuggest that this could be an effect of neglecting the metallicitydependence of the calibrated chromospheric emission-age relation. Wepropose a correction to account for this dependence. We also investigatethe metallicity distributions of these stars, and show that there aredistinct trends according to the chromospheric activity level. Inactivestars have a metallicity distribution which resembles the metallicitydistribution of solar neighbourhood stars, while active stars appear tobe concentrated in an activity strip on the logR'_HKx[Fe/H] diagram. Weprovide some explanations for these trends, and show that thechromospheric emission-age relation probably has different slopes on thetwo sides of the Vaughan-Preston gap.
| A Survey of Ca II H and K Chromospheric Emission in Southern Solar-Type Stars
More than 800 southern stars within 50 pc have been observed forchromospheric emission in the cores of the Ca II H and K lines. Most ofthe sample targets were chosen to be G dwarfs on the basis of colors andspectral types. The bimodal distribution in stellar activity first notedin a sample of northern stars by Vaughan and Preston in 1980 isconfirmed, and the percentage of active stars, about 30%, is remarkablyconsistent between the northern and southern surveys. This is especiallycompelling given that we have used an entirely different instrumentalsetup and stellar sample than used in the previous study. Comparisons tothe Sun, a relatively inactive star, show that most nearby solar-typestars have a similar activity level, and presumably a similar age. Weidentify two additional subsamples of stars -- a very active group, anda very inactive group. The very active group may be made up of youngstars near the Sun, accounting for only a few percent of the sample, andappears to be less than ~0.1 Gyr old. Included in this high-activitytail of the distribution, however, is a subset of very close binaries ofthe RS CVn or W UMa types. The remaining members of this population maybe undetected close binaries or very young single stars. The veryinactive group of stars, contributting ~5%--10% to the total sample, maybe those caught in a Maunder Minimum type phase. If the observations ofthe survey stars are considered to be a sequence of snapshots of the Sunduring its life, we might expect that the Sun will spend about 10% ofthe remainder of its main sequence life in a Maunder Minimum phase.
| Stroemgren photometry of F- and G-type stars brighter than V = 9.6. I. UVBY photometry
Within the framework of a large photometric observing program, designedto investigate the Galaxy's structure and evolution, Hβ photometryis being made for about 9000 stars. As a by-product, supplementary uvbyphotometry has been made. The results are presented in a cataloguecontaining 6924 uvby observations of 6190 stars, all south ofδ=+38deg. The overall internal rms errors of one observation(transformed to the standard system) of a program star in the interval6.5
| Eclipse Monitoring of Eccentric Binary Systems
Not Available
| Predicting the Secondary Minima Depths and Spectra of the Main Components for the Dm-Type Eclipsing Variable Stars
Not Available
| Program of Parallax Measurements from Space for the Nearest Eclipsing Binaries
Not Available
| The period distribution of unevolved close binary systems
Period distributions have been examined for various spectral types ofabout 600 (eclipsing and spectroscopic) close binaries, which are likelyto be substantially unevolved. The comparison with the previouscorresponding analyses of extensive (but heterogeneous) binary samplesallows a clarification of the extent of the evolutionary andobservational selection effects. Remarkably, this analysis reveals agreat deficiency of short period binaries (with periods corresponding tocase A mass transfer) in the whole spectral range. For the late spectraltypes, this result may be connected with postformation angular momentumloss caused by stellar wind magnetic braking; at least for the late Band A spectral range, a ready interpretation of this finding is thatclose binaries of corresponding periods and spectral types are rarelyformed.
| Statistics of categorized eclipsing binary systems Lightcurve shapes, periods, and spectral types
The statistics of the light curve morphologies, eclipse depths, orbitalperiods, and spectral types of about 1000 eclipsing binary systems areexamined, after attempting to subdivide these binaries into variousbasic evolutionary categories. The applicability of statisticalcriteria, based on light curve morphologies and eclipse depths, for thecategorization of eclipsing binaries has been found more limited thanpreviously believed. In particular, EW-type light curves turn out to begood indicators of contact systems (though not conversely), while EA-and EB-type light curves have little physical significance. Moreover,the study reveals a strong deficit of short-period noncontact systems inthe whole spectral range, together with an underabundance of early-typecontact binaries (compared with the number of late-type contact pairs).Interestingly, the distribution of evolved Algol-type systems isshifted, on average, to periods longer than those of unevolved detachedsystems in the OB and early A spectral range (and to shorter periods inthe F spectral range).
| A catalogue of parameters for eclipsing binaries
Not Available
| Survey of Spectra of Southern Eclipsing Binaries.
Not Available
| Bearbeitung des Lichtwechsels von 10 sudlichen veranderlichen Sternen.
Not Available
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Osservazione e dati astrometrici
| Costellazione: | Centauro |
| Ascensione retta: | 14h16m57.91s |
| Declinazione: | -49°56'42.3" |
| Magnitudine apparente: | 8.708 |
| Distanza: | 65.104 parsec |
| Moto proprio RA: | -23.2 |
| Moto proprio Dec: | -22.1 |
| B-T magnitude: | 9.485 |
| V-T magnitude: | 8.773 |
Cataloghi e designazioni:
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INHABITED SKY
