Problems Concerning Pleione

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Abstract
An evolutionary paradox arises in the case of the star Pleione. The implications of this paradox are discussed and several possibilities for overcoming it are investigated. At first sight one would expect a star evolving from the main-sequence to become increasingly stable against rotation. It is shown, however, that increasing central condensation decreases the moment of inertia of the star sufficiently to produce two distinct epochs during which any initial tendency towards rotational instability is rendered more acute. The mass of Pleione must be of order 4⊙ and the radius of order 2 × 10 11 cm. The calculated age depends on the evolutionary interpretation adopted, one interpretation gives an age as low as 4.7 × 10 7 years, another gives an age as high as 1.4 × 10 8 years. Rotational instability causes the star to shed a disk of material, the disk probably being magnetically coupled to the star. The winding of the field, arising through rotation, may well be responsible for the ultimate instability of the disk. The process of disk formation and disruption can be repeated many times, the general order of the time required for disruption being 100 years.

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