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    Accurate Positions Of 502 Stars in the Region of the Pleiades
    Monthly Notices of the Royal Astronomical Society (1970)
    H. EichhornW. D. GoogeC. F. LukacJ. K. Murphy
    This paper contains the right ascensions and declinations of 502 stars in a region of about 1–0.5 degrees square in the Pleiades cluster, centred on η Tau, and describes the unconventional procedures used to obtain their high relative accuracy which is on the order of 0″.01. Relative proper motions for the reference stars were computed by comparison of the positions given in this paper with those previously determined by König. Means for the reduction of the positions and proper motions to the FK4 system are provided. Other results of significance for astrometric technique are discussed. All plates were exposed with the telescope objectives equipped with coarse diffraction gratings. Analytical expressions for magnitude dependent systematic errors were established by comparing the measured positions of the central images with those of the diffraction spectra, and by utilization of the plate overlap conditions. The pattern in which the plates were taken lent itself to the application of a reduction technique which considers the correlation of plate constants on mutually overlapping plates. The accuracy achieved by the plate overlap method, which was used for the reduction of the measurements, suggests the application of this or another similar technique whenever a high degree of systematic accuracy is required.
    Pleiades
    Astrometry
    Data Reduction
    Square (algebra)
    Photographic plate
    Citations (5)
    Chromospheric activity in the late A- and early F-type stars of open clusters -- II. Pleiades and Alpha Persei
    Monthly Notices of the Royal Astronomical Society (2000)
    B. L. Rachford
    We report observations of the He iλ5876 (D3) line in the late A- and early F-type stars in the Pleiades and Alpha Persei star clusters used to determine chromospheric activity levels. This represents the first sample of young stars in this temperature range with chromospheric activity measurements. We find the same average activity level in the young early F stars as in Hyades-age stars and field stars. In addition, the young star sample shows the same large star-to-star variation in activity as seen in the older stars. Thus, as a whole, chromospheric activity in this photospheric temperature range remains the same over nearly a factor of 100 in stellar age (50 Myr to 3 Gyr), in striking contrast to the behaviour of later-type stars. In the five late A stars we find three certain detections of D3 and one likely detection. This includes the bluest star yet observed with a chromospheric D3 line, Pleiades star HII 1362 at (B–V)0=0.22, making it one of the earliest stars with an observed chromosphere. The late A stars have D3 equivalent widths comparable to the weakest early F stars. However, when comparing D3 measurements in the young late A stars with older late A stars, we find evidence for a slight decrease in activity with age based on the large number of non-detections in the older stars. We find an apparently linear relationship between the activity upper limit and B–V over our entire range of B–V. Extrapolated blueward, this relationship predicts that the chromospheric D3 line would disappear for all stars at B–V≈0.13.
    Pleiades
    H-alpha
    Alpha (finance)
    Citations (5)