MidB Middle B variable stars

- In 1985 a class of "mid-B" variable stars was introduced within the literature. These stars are generally B3-B8 spectral type, luminosity class III-V. They have periods of ld-3 and amplitudes of light variations of a few 0".'01. The color variations are in phase with the light variations, and the color-to-light ratio remains constant despite variability in amplitudes on a cycle-to-cycle and even a year-to-year base, not recognized within the GCVS

In recent decades, astronomers have begun to realize that many, if not all, early-type stars show some kind of intrinsic variability the causes of which are unknown. A number of theoretical studies seem to indicate that the current equilibrium models of massive stars disagree with the observation to some degree. It is likely that our lack of understanding of the causes of the variations of the early-type stars is related to our poor understanding of their precise structure. The study of early-type variable stars is therefore not only of interest within the context of stellar pulsation theory but also when considered within the broader context of the theory of stellar structure.

Numerous observational studies of early-type variables have been carried out recently and the view that emerges is somewhat confusing. Some astronomers distinguish up to nine different classes of variable B stars. The distinctions between various classes seem to be based not only on a comparison of the characteristics of the variations but also on some a priori information. On the other hand, there is a temptation to disregard the accepted boundaries placed between most classes, and to call, for instance, a "fi Cephei star" any B-type star that shows variability on a short time-scale compatible with a radial pulsation mode. A similar kind of reasoning is adopted by some when the descriptive term slow variables was introduced to name the very mixed group of stars that vary on a time-scale significantly longer than the fundamental period of radial pulsation.

These classification problems are partly caused by our inability to define the parameters that are relevant to describe the variability, and so to the very problem of our inability to isolate instability mechanisms that operate in these stars. Probably, it is, however, also true that at least some of the ambiguity in classifying the

Observation

"jAf Bright stars

Small amplitude Mixed periods <®> CCD or PEP

early-type variable stars is due to the diverse observational techniques that are used for detecting and describing these stars.

The distinction between these groups of stars is primarily based on the morphology of their light curves. The physical conditions in the stars of these few classes are different, and it is therefore probable that the variety in the morphologies of their light curves indicates that physically distinct mechanisms are responsible for their variations.

Observation

■4f Bright stars g Small amplitudes fi Short periods <®> CCD or PEP

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