BY BY Draconis5 stars

- These stars are emission-line dwarfs of dKe-dMe spectral type showing quasiperiodic light changes with periods from a fraction of a day to 12(r and amplitudes from several hundredths to 0".'5 in V. The light variability is caused by axial rotation of a star with a variable degree of nonuniformity of the surface brightness (spots) and chromospheric activity. Some of these stars also show flares similar to those of UV Ceti stars, and in these cases they also belong to the latter type and are simultaneously considered eruptive variables. GCVS

4This model was first suggested by Stibbs in 1950.

5Draco, the Dragon, identified as the beast that guarded the golden apples of the garden of the Hesperides, and was killed by Hercules when he came to fetch the apples as his eleventh labor.

Figure 6.1. Artist's conception of a BY Draconis- type variable showing the large starspols responsible fe, this type of star's variability. Copyright Gerry A Good.

Figure 6.1. Artist's conception of a BY Draconis- type variable showing the large starspols responsible fe, this type of star's variability. Copyright Gerry A Good.

The BY Draconis variables are dKe and dMe stars. Their spectral designation shows that they are late (K-M) dwarfs (prefix "d") that exhibit hydrogen line emissions (suffix "e") in their spectra. The variability in these stars is produced by axial rotation with irregular surface brightness (Figure 6.1). A region of cool spots localized on one hemisphere of the star causes the irregular light variations.

BY Dra variables are one of the many stars showing interesting activity within their chromosphere. A cursory examination of these stars may lead you to believe that they are similar to RS CVn binaries, however, BY Dra variables can be either binary or single. This physical characteristic was used to prove that a binary configuration is not directly responsible for this type of chromospheric activity.

Several of the BY Dra variables also show UV Ceti-type flares and the GCVS recognizes this phenomenon and has added the UV classification to some of these stars.

A few stars classified BY Dra within the GCVS probably should be classified as FK Com stars instead. Though undoubtedly single, spotted, and varying as a result of rotational modulation, they are not the proper spectral type and/or luminosity class for BY Dra variables. F.xamples would be OP And (gKl), V390 Aur (KO III), EK Eri (G8 IV-III), and V491 Per (G8 IV).

The variable star BY Dra, the group's prototype, was discovered in 1966. At that time, star spots were used to explain its variable nature. Another BY Dra star, YY Gem, was discovered in 1926, but was classified as an eclipsing binary. It is nevertheless clearly a BY Dra variable; its spectral type is dMe+dM2e and its variability between the eclipses was correctly identified as a starspot wave. The GCVS, however, emphasizing its eclipses and its flares, classifies it EA+UV.

Observation

Small amplitudes n Mixed periods <2> CCD or PEP

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