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UBV photometry of stars whose positions are accurately known.
Photometric indices V, B-V, and U-B were measured for about 560 stars ofthe AGK3R and NPZT catalogs between BD declinations 11 deg and 23 deg,using the 40-cm Cassegrain telescope of the Kvistaberg Observatory fromApril 1986 to May 1987. The observation procedure and the reductiontechnique were the same as in the earlier papers of this series by Oja(1984, 1985, 1986, and 1987). The mean errors were calculated from theinternal consistency of the data. The accuracy was found to be the sameas for the earlier parts of the survey.
| Chromospheric H-alpha emission in F8-G3 dwarfs, and its connection with the T Tauri stars
A chromospheric H-alpha emission core is measured in 40 stars of typesF8-G3 V on the basis of observations obtained with dispersion 1.6 nm/mm,resolution 74 pm, and S/N 200-350 using a coude spectrograph on the2.05-m reflector and a single-array 1872-channel Reticon in the focalplane of the 1-m camera at Lick Observatory on six nights duringDecember, 1982 - June, 1983. An emission spike of about 100 pm,corresponding to surface flux 1.4 Merg/sq cm sec, is detected in themost active stars by subtracting the spectrum of the reference star BetaCVn. The data are presented in tables and graphs along withcorresponding Ca II H, K data from Mount Wilson and analyzed in detail.It is found that main-sequence H-alpha surface-flux decay can bedescribed by either a power law with exponent -0.4 and rms scatterfactor 1.5 or an exponential relation with e-folding time 3-5 Gyr.Although extrapolation of the power law to very young stars correctlypredicts the H-alpha fluxes of the T Tau stars, important differences(suggesting a different emission mechanism) are shown to make suchextrapolation invalid.
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