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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.
| Mass loss and orbital period decrease in detached chromospherically active binaries
The secular evolution of the orbital angular momentum (OAM), thesystemic mass (M=M1+M2) and the orbital period of114 chromospherically active binaries (CABs) were investigated afterdetermining the kinematical ages of the subsamples which were setaccording to OAM bins. OAMs, systemic masses and orbital periods wereshown to be decreasing by the kinematical ages. The first-orderdecreasing rates of OAM, systemic mass and orbital period have beendetermined as per systemic OAM, per systemic mass and per orbitalperiod, respectively, from the kinematical ages. The ratio of d logJ/dlogM= 2.68, which were derived from the kinematics of the presentsample, implies that there must be a mechanism which amplifies theangular momentum loss (AML) times in comparison to isotropic AML ofhypothetical isotropic wind from the components. It has been shown thatsimple isotropic mass loss from the surface of a component or bothcomponents would increase the orbital period.
| Mg II chromospheric radiative loss rates in cool active and quiet stars
The Mg II k emission line is a good indicator of the level ofchromospheric activity in late-type stars. We investigate the dependenceof this activity indicator on fundamental stellar parameters. To thispurpose we use IUE observations of the Mg II k line in 225 late-typestars of luminosity classes I-V, with different levels of chromosphericactivity. We first re-analyse the relation between Mg II k lineluminosity and stellar absolute magnitude, performing linear fits to thepoints. The ratio of Mg II surface flux to total surface flux is foundto be independent of stellar luminosity for evolved stars and toincrease with decreasing luminosity for dwarfs. We also analyse the MgII k line surface flux-metallicity connection. The Mg II k emissionlevel turns out to be not dependent on metallicity. Finally, the Mg II kline surface flux-temperature relation is investigated by treatingseparately, for the first time, a large sample of very active and normalstars. The stellar surface fluxes in the k line of normal stars arefound to be strongly dependent on the temperature and slightly dependenton the gravity, thus confirming the validity of recently proposedmodels. In contrast, data relative to RS CVn binaries and BY Dra stars,which show very strong chromospheric activity, are not justified in theframework of a description based only on acoustic waves and uniformlydistributed magnetic flux tubes so that they require more detailedmodels.
| 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
| 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.
| A study of the Mg II 2796.34 Å emission line in late-type normal and RS CVn stars
We carry out an analysis of the Mg II 2796.34 Å emission line inRS CVn stars and make a comparison with the normal stars studied in aprevious paper (Paper I). The sample of RS CVn stars consists of 34objects with known HIPPARCOS parallaxes and observed at high resolutionwith IUE. We confirm that RS CVn stars tend to possess wider Mg II linesthan normal stars having the same absolute visual magnitude. However, wecould not find any correlation between the logarithmic line width logWdeg and the absolute visual magnitude MV (theWilson-Bappu relationship) for these active stars, contrary to the caseof normal stars addressed in Paper I. On the contrary, we find that astrong correlation exists in the (MV, log LMg II)plane (LMg II is the absolute flux in the line). In thisplane, normal and RS CVn stars are distributed along two nearly parallelstraight lines with RS CVn stars being systematically brighter by ~1dex. Such a diagram provides an interesting tool to discriminate activefrom normal stars. We finally analyse the distribution of RS CVn and ofnormal stars in the (log LMg II, log Wdeg) plane,and find a strong linear correlation for normal stars, which can be usedfor distance determinations.
| New periodic variables from the Hipparcos epoch photometry
Two selection statistics are used to extract new candidate periodicvariables from the epoch photometry of the Hipparcos catalogue. Theprimary selection criterion is a signal-to-noise ratio. The dependenceof this statistic on the number of observations is calibrated usingabout 30000 randomly permuted Hipparcos data sets. A significance levelof 0.1 per cent is used to extract a first batch of candidate variables.The second criterion requires that the optimal frequency be unaffectedif the data are de-trended by low-order polynomials. We find 2675 newcandidate periodic variables, of which the majority (2082) are from theHipparcos`unsolved' variables. Potential problems with theinterpretation of the data (e.g. aliasing) are discussed.
| Long-term monitoring of active stars. IX. Photometry collected in 1993
As a part of an extensive program focused on the global properties andevolution of active stars, high-precision UBV(RI)_c and UBV photometryof 31 selected stars is presented. The UBV(RI)_c observations werecollected at the European Southern Observatory over the 31 December1992-18 January 1993 and the 20 November-3 December 1993 intervals.Additional UBV photometry obtained by the ``Phoenix" and by the CataniaAstrophysical Observatory Automatic Photoelectric Telescopes from 1990to 1993 is also presented for some of the program stars. Significantevolution of the light curves, period variations and evidence forlong-term variability of the global degree of spottedness are found.Some spectral classifications are revised and the inferred photometricparallaxes are compared, whenever possible, with the values measured bythe Hipparcos satellite. These observations are finalized to theconstruction of an extended photometric database, which can giveimportant clues on topics such as the stability of spotted areas,differential rotation, solar-like cycles and the correlation betweeninhomogeneities at different atmospheric levels. Based on data collectedat the European Southern Observatory, La Silla, Chile. Tables and thecomplete data set are also 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/367/910
| On X-Ray Variability in Active Binary Stars
We have compared the X-ray emissions of active binary stars observed atvarious epochs by the Einstein and ROSAT satellites in order toinvestigate the nature of their X-ray variability. The primary aim ofthis work is to determine whether or not active binaries exhibitlong-term variations in X-ray emission, perhaps analogous to theobserved cyclic behavior of solar magnetic activity. We find that, whilethe mean level of emission of the sample remains steady, comparison ofdifferent ROSAT observations of the same stars shows significantvariation on timescales <~2 yr, with an ``effective variability''ΔI/I=0.32+/-0.04, where I and ΔI represent the mean emissionand variation from the mean emission, respectively. A comparison of theROSAT All-Sky Survey and later pointed observations with earlierobservations of the same stars carried out with Einstein yields onlymarginal evidence for a larger variation (ΔI/I=0.38+/-0.04 forEinstein vs. ROSAT All-Sky Survey and 0.46+/-0.05 for Einstein vs. ROSATpointed) at these longer timescales (~10 yr), thus indicating thepossible presence of a long-term component to the variability. Whetheror not this long-term component is due to the presence of cyclicvariability cannot be decided on the basis of existing data. However,assuming that this component is analogous to the observed cyclicvariability of the Sun, we find that the relative magnitude of thecyclic component in the ROSAT passband can, at most, be a factor of 4,i.e., I_cyc/I_min<4. This is to be compared with the correspondingbut significantly higher solar value of ~10-10^2 derived from GOES,Yohkoh, and Solrad data. These results are consistent with thesuggestions of earlier studies that a turbulent or distributive dynamomight be responsible for the observed magnetic activity on the mostactive, rapidly rotating stars.
| Radio star catalogue observed in San Juan (RSSJ95)
Using the data observed in San Juan with the photoelectric AstrolabeMark II of the Beijing Astronomical Observatory from February, 1992 toMarch, 1997, the radio star catalogue in San Juan(RSSJ95) has beencompiled. There are 69 radio stars in this catalogue. The positions ofthe radio stars are for the epoch of observation and the equinox J2000.0and a system close to that of the system FK5. The mean precisions are+/-2.2 ms and +/-0.035'' in right ascensions and declinations,respectively. The magnitudes of stars are from 0.9 to 10.7. Thedeclinations are from -2fdg 5 to -60(deg) . The mean epoch is 1995.1.Finally, the comparison results between the Hipparcos catalogue andRSSJ95 are given.
| The age-mass relation for chromospherically active binaries. III. Lithium depletion in giant components
We present a study of the lithium abundances of a sample of evolvedcomponents of Chromospherically Active Binary Systems. We show that asignificant part of them have lithium excesses, independently of theirmass and evolutionary stage. Therefore, it can be concluded that Liabundance does not depend on age for giant components of CABS. Theseoverabundances appear to be closely related to the stellar rotation, andwe interpret them as a consequence of the transfer of angular momentumfrom the orbit to the rotation as the stars evolve in and off the MainSequence, in a similar way as it happens in the dwarf components of thesame systems and in the Tidally Locked Binaries belonging to the Hyadesand M67. Based on observations collected with the 2.2\,m telescope ofthe German-Spanish Observatorio de Calar Alto (Almeria, Spain), and withthe 2.56\,m Nordic Optical Telescope in the Spanish Observatorio delRoque de los Muchachos of the Instituto de Astrof\'\i sica de Canarias(La Palma, Spain)
| Long-term monitoring of active stars. VIII. UBV(RI)_(c) photometry collected in February 1992
As a part of an extensive program focusing on the global properties andevolution of active stars, high-precision UBV(RI)_c photometry of 31selected stars, collected at the European Southern Observatory over the14-29 February 1992 interval, is presented. Significant evolution of thelight curves, period variations and evidence for long-term variabilityof the global degree of spottedness are found. Some spectralclassifications are revised and the inferred photometric parallaxes arecompared, whenever possible, with the values measured by the Hipparcossatellite. Flare events were detected for the star HD16157 = CC Eri, EXO 055609-3804.4 TY Coland HD 119285 = V851 Cen. Optical variability wasdiscovered for the Pop II binary HD 89499. Theseobservations contribute to the establishment of a time-extendedphotometric database which can give important clues on topics such asthe stability of spotted areas, differential rotation, solar-like cyclesand the correlation between inhomogeneities at different atmosphericlevels. based on data collected at the European Southern Observatory, LaSilla, Chile.
| Further discussion of binary star radio survey data
Statistical information on 8280 individual radio observations of binarystars, predominantly at 8.4 GHz using the Parkes 64 m antenna, ispresented. Three main groups are distinguished: (i) RS CVn stars, (ii)classical Algol binaries (EA2s) and (iii) detached pairs of generallyearly type (ETBs). The RS CVn stars more frequently gave rise todetectable fluxes, while the ETBs, in this data, are a small and ratherheterogeneous class. The Algols' emission appears to increase nearconjunction phases, though we cannot clearly distinguish any specialproperty of the Algols' phase-dependent behaviour which is not alsoshared by the RS CVn binaries. Both these categories' data show abimodal, phase-dependent pattern to the distribution of detections,suggesting both binary types share similar underlying physicalproperties, though there could also be other factors at play. The samplesizes of the Algols and particularly the ETB detections are too smallfor effective, discriminatory statistics, however.
| Long-term monitoring of active stars. VII. UBV(RI)_c photometry collected in March 1991
In the framework of an extensive program focusing on the globalproperties and evolution of active stars, high-precision UBV(RI)_cphotometry of 19 selected stars, collected at the European SouthernObservatory over the 15-31 March 1991 interval is presented. Significantevolution of the light curves, period variations and evidence forlong-term variability of the global degree of spottedness are found.Most of the spectral classifications are discussed. A flare event wasdetected for the star HD 127535 = V 841 Cen. These observationscontribute to the establishment of a time-extended photometric databasewhich can give important clues on topics such as the stability ofspotted areas, differential rotation, solar-like cycles and thecorrelation between inhomogeneities at different atmospheric levelsFigure 1 to 24 are only available on the on-line version of A&A athttp://www.ed-phys.fr}. based on data collected at the European SouthernObservatory, La Silla, Chile. Tables and the complete data set are alsoavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr or via http://cdsweb.u-strasbg.fr/Abstract.html
| San Juan radio star catalogue and comparison with HIPPARCOS catalogue.
Not Available
| The age-mass relation for chromospherically active binaries. II. Lithium depletion in dwarf components.
We present an extensive study of lithium abundances in dwarf componentsof chromospherically active binary stars (CABS). Since most of thesebinaries have known radii, masses and ages, this kind of data isespecially useful for comparisons with theoretical models which try toexplain the Li depletion phenomenon. We show that a significant part ofthese stars have clear Li overabundances with respect to the typicalvalues for stars of the same mass and evolutionary stage. These excessesare evident when comparing our sample of CABS with binary and singlestars belonging to open clusters of different ages, namely Pleiades,Hyades, NGC 752, M 67 and NGC 188, which have ages ranging from 7x10^7^to 10^10^yr. The Li excesses are more conspicuous for masses in therange 0.75-0.95Msun_, indicating that the rate of Lidepletion has been less pronounced in CABS than in single stars. Thisphenomenon is interpreted in the context of transport of angularmomentum from the orbit to the stellar rotation due to tidal effects.This angular momentum transfer would avoid the radial differentialrotation and the associated turbulent mixing of material in the stellarinterior. Other explanations, however, can not be ruled out. This is thecase of transport of material induced by internal gravity waves, whichcould be inhibited due to the presence of strong magnetic fieldsassociated with the effective dynamo in CABS. The confirmed existence ofa relation between Li abundances and the fluxes in CaII H&K linescan also be accommodated within both scenarios.
| EUV Emission from RS Canum Venaticorum binaries.
We performed a study of 104 RS CVn systems in the extreme ultraviolet(EUV) using the all-sky survey data obtained by the Extreme UltravioletExplorer (EUVE). The present sample includes several new RS CVndetections; 11 more than in the published EUVE catalogs, and 8 more thanin the ROSAT Wide Field Camera catalog. The ratio of detections tonon-detections remained constant throughout the sky, implying that ourdetections are not limited by the exposure time but are most likelylimited by absorption from the interstellar medium. A general trend ofincreasing Lex/B (50-180Å) flux with decreasing rotational periodis clear. The dwarf systems exhibit a leveling-off for the fasterrotators. In contrast, the evolved systems exhibit no such effect. Forthe RS CVn systems the losses in the EUV represent a smaller fraction ofthe coronal radiative losses, as compared to active late-type dwarfs.
| Long-term monitoring of active stars. V. UBV(RI)_ c_ photometry collected in Feb.-Mar. 1990.
High-precision UBV(RI)_c_ photometry of 12 selected active stars,collected at the European Southern Observatory (La Silla, Chile) overthe period February - March 1990, is presented. Significant evolution ofthe light curves, period variations and, in most cases, evidence forlong-term variability of the global degree of spottedness are found.Some of the spectral classifications are discussed. This paper is partof a more extensive program focusing on the global properties andevolution of active stars and is aimed at establishing a time-extendedphotometric database which can give important clues on topics such asthe stability of the spotted areas, differential rotation and solar-likecycles.
| The emission of the RS CVn binaries in the IRAS passbands.
In the literature, there is an ambiguity pertaining to the existence ofa far-IR excess in RS CVn systems. In the current paper we undertook astudy of the behaviour of 103 such systems in the IRAS passbands. Wefound 72 acceptable detections in the 12μm band, and 40 in the25μm band (50% more than the IRAS Point Source Catalog). Although ourfindings may be interpreted as indicating towards the existence of anexcess beyond 12μm for some systems, the evidence is not conclusivein all but two cases. These are systems GX Lib and HR 7428, with Capellabeing the only system where the IRAS fluxes in all four bands originatefrom the stellar photosphere. Given the accuracy of the data we did notfind 12μm excess for any system. We argue that the IRAS data alonecannot settle the issue, as their uncertainty is, in many cases, higherthan what the Signal-to-Noise ratio of each observation implies.Furthermore, at the higher wavelength bands the IRAS angular resolutiondrops from 0.5' at 12μm to 2' at 100μm and the background becomesvery complex, so one cannot be certain about the origin of the observedflux. The existence of IR excess in the RS CVn stars is important as faras the evolutionary scenarios for these systems are concerned. Futuremissions such as the Infrared Space Observatory will return moresensitive and accurate measurements and the ambiguity can be removed.The fluxes we quote will be helpful when planning these futureobservations, as we provide more accurate photometry and for a largerselection of sources than the IRAS Point Source Catalog.
| Radio continuum emission from stars: a catalogue update.
An updated version of my catalogue of radio stars is presented. Somestatistics and availability are discussed.
| Are the Rotational Axes Perpendicular to the Orbital Planes in the Binary Systems? II. Synchronous Long-Period RS CVn Stars
Inclinations of the rotational axes, irot, are determined for37 synchronous long-period RS CVn binaries by combining v sin imeasurements with rotational period information. These inclinations arethen compared with the orbital inclinations of the systems,iorb, to test the alignment between the equatorial andorbital planes. Contrary to the asynchronous RS CVn systems (see PaperI), in all 37 cases irot is equal or nearly equal toiorb, and the assumption about coplanarity of theirequatorial and orbital planes is justified.
| Lithium in Rs-Canum Binaries and Related Chromospherically Active Stars - Part Three - Northern Rs-Canum Systems
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1994A&A...283..893R&db_key=AST
| The active dynamo stars: RS CVn, BY Dra, FK Com, Algol, W UMa, and T Tau
Not Available
| Longterm Monitoring of Active Stars - Part Three - UBV Ri/c Photometry of 14 Southern Hemisphere Variables
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1993A&AS..102..655C&db_key=AST
| A catalog of chromospherically active binary stars (second edition)
The catalog contains 206 spectroscopic binary systems with at least onelate-type component that shows Ca II H and K emission in its spectrum.These systems include the classical RS CVn binaries and BY Dra binaries.The catalog summarizes information on the photometric, spectroscopic,orbital, and physical properties of the systems as well as space motionsand positions. Up to 42 'parameters' for each stellar system are listedfollowed by the appropriate reference to direct interested colleagues tothe original papers. A comprehensive selection of further informationfor each star is given in the individual notes. In addition, the catalogcontains a candidate list of 138 stars that have similar characteristicsbut are not definitely known binaries or have not had Ca II H and Kemission observed.
| The ROSAT All-Sky Survey of active binary coronae. I - Quiescent fluxes for the RS Canum Venaticorum systems
One hundred and thirty-six RS CV(n) active binary systems were observedwith the ROSAT Position Sensitive Proportional Counter (PSPC) during theAll-Sky Survey component of the mission. The entire sky was surveyed,which represents the largest sample of RS CV(n) systems observed to dateat any wavelength, including X-rays. X-ray surface fluxes for the RSCV(n) systems are found to lie in the range 10 exp 4 to 10 exp 8 ergs/sqcm seconds. Surface flux as a function of (B - V) color is reported. Adecrease in surface flux with increasing rotation period for the entiresample is observed. The rotation period provides the best stellar ororbital parameter to predict the X-ray surface flux level. The absenceof correlation of F(x) or L(x) with Gamma is noted due to the fact thatthe coronal heating mechanism for these active stars must be magnetic incharacter, and the magnetic field depends on the interaction betweenconvection and differential rotation inside the star. X-ray propertiesof the RS CV(n) systems with 6 cm radio and C IV UV emission systems iscompared.
| Lithium in Rs-Canum Binaries and Related Chromospherically Active Stars - Part Two - Spectrum Synthesis Analysis
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1993A&A...273..194R&db_key=AST
| Kinematics and age of RS Canum Venaticorum and by Draconis stars
Space velocities of 146 chromospherically active binary stars have beencalculated. Containing F-M spectral types on the main sequence togetherwith G and K giants and subgiants, this very heterogeneous sample hasbeen divided into groups in order to segregate stars which have similarkinematics and ages. After many trials for different criteria, thesample was divided into five groups - two groups for giants, two groupsfor main-sequence systems, and one group for main-sequence systems.Kinematics of subgiants implies a stellar age of about 2-3 Gyr. Youngand old groups of giants and main-sequence systems could becharacterized by a kinematical age of about 1 Gyr and more than 5 Gyr,respectively. These ages are estimated approximately according to spacevelocity distributions and dispersions in velocity space in each group.Inferred ages for the groups above agrees with circulation, rotationactivity relation, and stellar evolution theories.
| Lithium in RS CVn binaries and related chromospherically active stars. I - Observational results
The present survey of the Li I 6708 A line in a sample of spectral typeG and K stars with luminosity classes III, IV, and V shows that manyK-type stars in the sample, including a large number of RS CVn binaries,show an anomalously high Li abundance relative to typical inactive starsof the same spectral type. Only a few stars in the sample are likely tobe premain sequence objects of stars which have recently arrived on themain sequence. Mechanisms that could lead to the enhanced Li absorptionin chromospherically active stars are discussed; these encompass largecool spots on the stellar surface, the production of Li in stellarflares by spallation reactions, and the evolution from main-sequenceprogenitors without, or with very shallow, outer convective zones.
| The non-synchronous systems among RS CVn stars
An RS CVn star is considered as nonsynchronous if its orbital perioddiffers by more than 4 percent from its photometric period. Of 88systems examined according to this criterion, 67 are synchronous, 21 arenonsynchronous. For the 21 nonsynchronous systems, it is found thattheir spectral types are most around KO-2 III and that their orbitaleccentricities are larger and their chromospheric activity lower thanthe average level.
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Observation and Astrometry data
| Constellation: | Centaurus |
| Right ascension: | 11h39m22.23s |
| Declination: | -39°23'07.6" |
| Apparent magnitude: | 7.852 |
| Distance: | 121.655 parsecs |
| Proper motion RA: | -2.9 |
| Proper motion Dec: | -53 |
| B-T magnitude: | 8.999 |
| V-T magnitude: | 7.947 |
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