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Membership, binarity, and rotation of red dwarfs in the nearby open cluster Coma Berenices (Melotte 111)
Context: Although several attempts have been made to identify solar-typemembers on the main sequence of the nearby open cluster Coma Berenices(Mel 111), the population of the lower main sequence is still poorlyknown. Aims: We observed 46 new candidates to search for new membersand monitored known spectroscopic-binary members to determine orbitalparameters. Methods: We obtained a total of 903 radial-velocitymeasurements of 69 solar-type stars in the field of Mel 111 with theCORAVEL spectrovelocimeter over 20 years. Results: Among the 35 starsfrom Trumpler's list, 23 are members according to their radialvelocities and photometry. We were able to confirm the membership ofonly 8 stars among the 46 candidates observed. Six double-lined and tensingle-lined spectroscopic binaries were discovered. Six only aremembers and an orbit was determined for them and for 4 double-linednon-members. The binary frequency is 22% (7/32). The cluster mean radialvelocity is +0.01 ± 0.08 km s-1 based on 28 members. Conclusions: The lower main sequence of the Coma Berenices open clusteris still rather poorly populated. The cluster size may be much largerthat usually accepted. Accordingly extensive programmes to determineprecise proper motions, radial velocities and photometry should beundertaken to identify faint cluster members outside the cluster centralarea. If a significant population of faint members cannot be identified,Coma Ber could be a prominent example of dynamical evolution leading tostar evaporation.Based on observations collected at the Haute-Provence Observatory(France). Table 1 is also available, and Full Table 8 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/491/951
| Beryllium Abundances in F and G Dwarfs in the Coma Cluster and the Ursa Major Moving Group from Keck HIRES Observations
The study of both light elements Li and Be in open clusters of knownproperties can reveal the internal structure and the mechanisms ofmixing in main-sequence stars as a function of age and composition. Inprevious work, we have investigated the older Hyades cluster and theyounger Pleiades and α Per clusters. The Coma Berenices clusterand the UMa moving group are intermediate in age between the Hyades andPleiades and provide a good linchpin for the influence of stellar age onlight-element abundances; there are dips in the mid-F stars in both Liand Be in the Hyades but no Be dip and only a minor Li dip in thePleiades. We have made observations of the resonance doublet of Be IInear 3130 Å in 13 Coma and six UMa stars with the Keck I telescopeand the High Resolution Echelle Spectrometer. The Be abundances weredetermined by spectrum synthesis. In the F dwarfs in Coma, there areboth Li and Be deficiencies, indicating that the depletions occur duringthe main-sequence phase of evolution but do not become evident until anage of 200-300 Myr. For both UMa and Coma stars, the Li depletion isgreater than the Be depletion at all temperatures, but there is little,if any, Be depletion in stars with Teff<6000 K. In thefour clusters studied for Be, the mean Be abundance for stars withtemperatures less than 6000 K is logN(Be/H)+12.00=1.27, independent ofage or metal content. For the hotter stars (5850-6680 K), the Li and Beabundances are correlated, indicating that the depletion probably occurssimultaneously; this matches the results for the field stars and theHyades and the predictions of Li and Be depletion by rotationallyinduced mixing.
| Astrometric radial velocities. III. Hipparcos measurements of nearby star clusters and associations
Radial motions of stars in nearby moving clusters are determined fromaccurate proper motions and trigonometric parallaxes, without any use ofspectroscopy. Assuming that cluster members share the same velocityvector (apart from a random dispersion), we apply a maximum-likelihoodmethod on astrometric data from Hipparcos to compute radial and spacevelocities (and their dispersions) in the Ursa Major, Hyades, ComaBerenices, Pleiades, and Praesepe clusters, and for theScorpius-Centaurus, alpha Persei, and ``HIP 98321'' associations. Theradial motion of the Hyades cluster is determined to within 0.4 kms-1 (standard error), and that of its individual stars towithin 0.6 km s-1. For other clusters, Hipparcos data yieldastrometric radial velocities with typical accuracies of a few kms-1. A comparison of these astrometric values withspectroscopic radial velocities in the literature shows a good generalagreement and, in the case of the best-determined Hyades cluster, alsopermits searches for subtle astrophysical differences, such as evidencefor enhanced convective blueshifts of F-dwarf spectra, and decreasedgravitational redshifts in giants. Similar comparisons for the ScorpiusOB2 complex indicate some expansion of its associations, albeit slowerthan expected from their ages. As a by-product from the radial-velocitysolutions, kinematically improved parallaxes for individual stars areobtained, enabling Hertzsprung-Russell diagrams with unprecedentedaccuracy in luminosity. For the Hyades (parallax accuracy 0.3 mas), itsmain sequence resembles a thin line, possibly with wiggles in it.Although this main sequence has underpopulated regions at certaincolours (previously suggested to be ``Böhm-Vitense gaps''), suchare not visible for other clusters, and are probably spurious. Futurespace astrometry missions carry a great potential for absoluteradial-velocity determinations, insensitive to the complexities ofstellar spectra. Based on observations by the ESA Hipparcos satellite.Extended versions of Tables \ref{tab1} and \ref{tab2} are available inelectronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.125.8) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/381/446
| Lithium in the Coma Berenices open cluster
Lithium abundances, radial velocities, and rotational velocities arereported for 20 candidate F, G and K stars of the sparse Coma Berenicesopen cluster. All the stars are proper-motion selected, and our radialvelocities support the membership credentials of at least 12 of thecandidates. Combining our data with that in the literature, we have aclose-to-complete census of Li abundances for late-type stars withMV<5.8. These data show that the Li-depletion pattern inComa Ber is similar, but not identical, to that in the Hyades cluster,which has a similar age but higher metallicity. Several Coma Ber F starshave suffered significantly more Li depletion than their counterparts inthe Hyades. The G and early-K stars of Coma Ber have undergone less Lidepletion than those of the Hyades, but much more than that predicted bystandard evolutionary models featuring only convective mixing. Thisprovides strong evidence for additional mixing and Li depletionoperating in these stars during their first 400 Myr on the mainsequence, amounting to 0.3 dex at 6000 K and rising to 0.8-1.2 dex at5400 K. We find that 4 of the radial-velocity non-members are among asmall number of low-mass stars which were previously reported as part ofan extra-tidal moving group associated with the Coma Ber cluster. As aresult, we now find that the luminosity function of this moving group isindistinguishable from that of the central cluster. It is uncertainwhether there may be a significant number of stars with even lowermasses among the moving group.
| Lithium in the low-mass stars of the Coma Berenices open cluster
I report lithium abundances and radial velocities for 15 late F, G and Kstars in the Coma Berenices open cluster. All the stars are propermotion members, and the radial velocities enable us to confirmmembership for 11 single stars and identify three SB1 systems and oneshort-period SB2 system that are very probable cluster members. Thepattern of Li abundances is similar to that in the Hyades, with someevidence for higher Li abundances in Coma Ber for T_eff<5700 K. Iargue that these data do not support standard models for Li depletioninvolving only convective mixing, because the Coma Ber stars haveconsistently determined lower metallicities and shallower convectionzones than the Hyades and should have suffered a great deal less Lidepletion than observed. Instead, it seems that the Li abundances inthese and other young clusters are primarily ranked according to theirage rather than their metallicity. I propose a scenario where theextreme metallicity dependence of pre-main-sequence Li depletion istempered by a mechanism that severely inhibits convective mixing, wherecontinued main-sequence depletion occurs via non-standard mixingmechanisms, and where any dependence of Li depletion on metallicitytakes at least several hundred Myr to emerge.
| The Coma Berenices star cluster and its moving group
We report on the analysis of astrometric and photometric data fromHipparcos, Tycho and the ACT catalogue in a 1200 sq. degree field aroundthe nearby open star cluster in Coma Berenices and infer thecharacteristic features of this cluster. From a sample of 51 kinematicmembers we derive the cluster's distance, size and spatial structure aswell as its stellar content, mass and age. We find that the clusterconsists of an elliptical core-halo system with major axis parallel tothe direction of galactic motion, and of a moving group of extratidalstars. The latter have tangential distances >= 10 pc from the clustercenter, but have the same distance from the Sun, the same motion and thesame age as the stars in the core and halo. The luminosity function ofthe core-halo system steeply declines beyond absolute magnitude M_V=4.5,but that of the moving group rises towards fainter magnitudes andsuggests the existence of further low-mass members below the currentmagnitude limit. The cloud of extratidal stars witnesses the process ofdissolution of the cluster.
| Member stars of the open cluster Mel 111 in Coma Berenices
Proper motions of AGK3 stars in a 6 x 6 deg field centered on the opencluster Mel 111 in Coma Berenices have been used for selecting starswhich are probable members of the cluster. Available photometric dataand radial velocities have been used to refine the selection. Seventeennew candidates are proposed.
| Space distribution of stars of intermediate spectral type in a North galactic pole region.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1963AJ.....68..194U&db_key=AST
| Investigations on the stars in high galactic latitudes. III. Spectrophotometric analysis of about 3000 stars down to the photographic magnitude 13.5.
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