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Radial velocity measurements. IV - Ground-based accompaniment to the HIPPARCOS observation program
The paper presents 396 radial velocities of stars distributed in 19fields of 4 x 4 degrees. The study employs the Fehrenbach objectiveprism method and the same measuring technique used in a previous paper(Fehrenbach et al., 1987).
| The evolution of the lithium abundances of solar-type stars. I - The Hyades and Coma Berenices clusters
High-resolution, high signal-to-noise spectra of the lithium region at6708 A in 28 solar-type stars of the Hyades and Coma Berenices clustersare reported. Given an observational uncertainty of less than about 5 mAin W-lambda (Li), no significant scatter about the mean relation wasseen for most stars. However, there are several stars that haveanomalous abundances. Two of them fall well below the mean relation, andappear to have no distinctive qualities that might account for their lowLi. Two others are close binaries and have significantly greater thanaverage Li. A means by which close binaries might preserve Li issuggested, and Li depletion timescales for stars near the zero-age mainsequence (ZAMS) are estimated by comparing the Hyades to the Pleiades.This comparison indicates that Li depletion for stars near 1 solar massstarts on the ZAMS, not before, and that depletion occurs at a muchslower rate after the age of the Hyades than before.
| Magnetic field structure in the Taurus dark cloud
Optical and infrared polarimetry of sources in the direction of theTaurus cloud are obtained in order to study the magnetic field in thiscloud and its possible role in the cloud's evolution. Most of the starsare background giant stars whose light shines through the cloud and ispolarized by the cloud material. The transverse component of themagnetic field, as delineated by the polarization vectors, is generallyperpendicular to the galactic plane, and the stratified structure of thecloud could be due to the effect of the magnetic field during the earlystages of collapse. Three of the 13 embedded stars are stronglypolarized with position angles nearly perpendicular to those of nearbyfield stars. The polarization of these stars is most likely intrinsic,and the direction of polarization indicates that the materialsurrounding these stars may be magnetic i.e., that the magnetic field isfrozen in this material.
| U, B, V, R, I stellar photometry in the field of the Taurus dark clouds
A program of photoelectric stellar photometry has been carried out inthe field of the Taurus dark clouds with a view to establishing thedistance of the clouds and the absorption law in that region of the sky.It is found that the distance of the dark clouds accompanying the Tau T1and Tau T3 associations is 132 plus or minus 10 pc. Within theabsorption region 0-4 m, the Taurus dark clouds are characterized by anearly normal absorption law (to within 5-10%). In the field of the darkclouds a group of F0-G0 stars close to the main sequence is found whosespace density is three to eight times that in the solar neighborhood.
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