Doppler images of the spotted RS CVn star HD 26337: 1984-87

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Abstract
Doppler images of the spot distribution on HD 26337 (EI Eri) for the years 1984–87 are presented. These images were derived using maximum entropy digital image reconstruction on time-resolved spectroscopy of the Ca I 6439-Å line. All the images are characterized by a large spot with several appendages straddling the rotation pole. The image from 1987 marks the appearance of two equatorial spots. The existence of polar spots on RS CVn stars appears to be not only a common phenomenon, but a long-lived one as well. This polar spot underwent significant changes from year to year. Unfortunately, the inadequate time resolution of the Doppler images makes it difficult to understand the morphology of these changes. For one year (1986-87), enough spectral data were available to derive two (albeit with incomplete phase coverage) Doppler images separated by about 3 months. The spot distribution looks remarkably similar in these two images, however, the polar spot in 1987 January appears to have rotated by 22° with respect to the 1986 October image in a manner consistent with its rotation period being less than that of the orbital period. If the equator is tidally locked to the orbital period of this binary system, then the differential rotation in HD 26337 must be at least a factor of 100 less than in the Sun, but in the same sense, i.e. the poles rotate more slowly than the equator. If spots emerge at the equator and migrate poleward to join with the feature there, then the migration rate must be fairly large in order to account for so few equatorial spots seen in the images. The fact that the polar spot undergoes significant changes in its size and shape, in spite of the lack of equatorial spots in previous images, suggests that the polar distribution can change even without the accretion of equatorial spots. Better time resolution of the Doppler images is needed to confirm this.

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