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Lithium Abundances in Wide Binaries with Solar-Type Twin Components
We present high-resolution spectroscopic observations of the Li Iresonance line in a sample of 62 stars that belong to 31 common propermotion pairs with twin F- or G-type components. Photospheric abundancesof lithium were derived by spectral synthesis analysis. For seven of thepairs, we have measured large lithium abundance differences. Elevenother pairs have components with similar lithium abundances. We cannotdetermine if the remaining 13 pairs have lithium differences because wedid not detect the Li I lines, and hence we can only provide upperlimits to the abundances of both stars. Our results demonstrate thattwin stars do not always share the same lithium abundances. Lithiumdepletion in solar-type stars does not only depend on age, mass, andmetallicity. This result is consistent with the spread in lithiumabundances among solar-type stars in the solar-age open cluster M67. Ourstars are brighter than the M67 members of similar spectral type, makingthem good targets for detailed follow-up studies that could shed lighton the elusive mechanism responsible for the depletion of lithium duringthe main-sequence evolution of the Sun and solar-type stars.
| Precise positions, separations, and position angles of visual binaries
Astrometric observations of 212 large visual binaries are reported. Dataobtained with a GG 495 yellow filter on IIaD photographic plates usingthe Zen astrograph (aperture 20 cm and focal length 2077 mm) of theOsservatorio Astronomico di Brera during 1985-1986 are presented inextensive tables. The potential significance of these measurements forthe catalog being constructed for the Hipparcos space observatory isindicated.
| A catalog of precise reference star positions for the astrometry network of the international comet P/Halley compaign
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1987A&AS...71..525D&db_key=AST
| Common proper motion stars in the AGK 3
A search was made of common-proper-motion (CPM) systems among AGK 3stars. The selection of physical systems was based upon the ratiobetween the angular separation (rho) and the proper motion (mu); the CPMstars found are presented in two tables. Table I lists systems withrho/mu less than 1000 years. It contains 326 entries, and the proportionof optical pairs is estimated to be 1 percent. Table II lists systemswith rho/mu in the range 1000 to 3500 years; it contains 113 systems,but only 60 percent of them are physical. Nevertheless, these systemsoften have separations larger than 10,000 AU and are the mostinteresting for the study of the tail of the distribution function ofthe semimajor axes.
| The rotation of AP stars
Photometric periods of silicon stars belonging to clusters andassociations as well as to the field are discussed in relation withtheories of magnetic braking taking place on the main sequence. It isshown that Si stars do not lose the bulk of their angular momentumduring their main-sequence lifetime, but must have lost it before. Anybraking mechanism which predicts an e-folding time of the order of, orshorter than, the star's lifetime seems ruled out. The theory of purelyhydromagnetic rotational braking is much too efficient to fit theobservations (unless under its earlier form proposed by Kulsrud, 1971),while the accretion theory does not fit them either, although it seemsmore realistic. Preinjection of cosmic rays by means of Ap stars appearsdoubtful.
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