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Long-term monitoring of active stars. IV. UBV(RI)_c_ observations obtained at La Silla in December 1989.
High-precision UBV(RI)_c_ photometry of 23 selected acitve stars,collected at the European Southern Observatory (La Silla, Chile) overthe period 9-28 December 1989, is presented. This paper is part of alarger program focusing on the global properties and evolution of activestars and is aimed at establishing a time-extended database which cangive important clues on topics such as the stability of the spottedareas, differential rotation and solar-like cycles. Significantevolution of the wave-like light curves, period variations and, in mostcases, evidence for long-term variability of the global degree ofspottedness are found for the observed stars. Some spectralclassifications are rediscussed and evidence for a newly discoveredvariable star is given.
| Photometric variability in chromospherically active stars. III - The binary stars
Differential UBV photometry of 49 late-type chromospherically activebinary stars has been obtained. A total of 12,353 differential UBVmagnitudes have been acquired and are used to determine seasonalphotometric periods, amplitudes, mean brightnesses, maximum spotamplitudes, 'unspotted' differential magnitudes, and times of minimumlight. Standard Fourier techniques are used to investigate the cause ofthe light variability. Eight stars are found which exhibit lightvariations due not to spot activity but rather to ellipticity and/orreflection effect. Others show combinations of all three effects. Someof the systems also show substantial differences between theirrotational and orbital period.
| Local photometric standards for CaII emission stars
UBV data are given for 108 stars which are suitable local standards for52 stars with strong Ca2 emissions. An additional eight stars wererejected as possible standards because of suspected variability.
| The physical properties of the ZZ Ceti stars and their pulsations
Stromgren colors for the ZZ Ceti variables are used to fix the meanpositions of individual stars in the H-R diagram. The ZZ Ceti stars arefound to be normal white dwarfs occurring in the temperature range fromabout 10,500 K to 13,500 K on the DA cooling sequence. The onset ofvariability among DA white dwarfs is shown to be an evolutionary effectoccurring at the maximum of hydrogen capacity on the DA sequence.Time-resolved Stromgren photometry is used to examine the changes of theeffective temperature, the radius, and the effective surface gravitywith time during a pulsation to evaluate the relative contribution ofthese parameters to the total luminosity variation. The increase ineffective temperature during a pulsation is observed to be sufficient toaccount for the entire change in luminosity. Changes in the radius ofthe star and the effective gravity during a pulsation are shown to besmall. It is suggested that the partial ionization zones associated withthe high opacity provide the driving mechanism for the ZZ Ceti stars.
| Lichtelektrische UBV-Photometrie von Standardsternen und in vier Sternfeldern am Äquator
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Observation and Astrometry data
| Constellation: | Orion |
| Right ascension: | 04h57m04.91s |
| Declination: | +00°13'16.4" |
| Apparent magnitude: | 7.832 |
| Proper motion RA: | 9 |
| Proper motion Dec: | -29.5 |
| B-T magnitude: | 9.164 |
| V-T magnitude: | 7.942 |
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