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Astron. Astrophys. 347, 225-234 (1999)

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1. Introduction

The RS CVn binary HD 12545 = XX Tri (K0III, [FORMULA] 24 days) is among the most active stars of the RS CVn class exhibiting strong UV emission-line fluxes up to 400 times the solar values (Bopp et al. 1993), Ca II H & K emission-line intensities 2-3 times that of the nearby continuum (Bidelman 1985, Strassmeier et al. 1990), and strong coronal X-ray emission (Dempsey et al. 1993). But most noteworthy for optical observers is its huge photometric variability with an amplitude of 0:m 60 in V in late 1990 (Nolthenius 1991), and again a record amplitude of 0:m 63 during the observations for this paper in 1997/98. These amplitudes are the largest amplitudes ever recorded for a post-main-sequence star (the absolute record holder is the weak-lined T Tauri star V410 Tau with 0:m 65 in 1994/95; Strassmeier et al. 1997a) and are commonly attributed to rotational modulation of an asymmetrically spotted stellar surface having spots cooler than the photosphere. Such an assumption seems to be appropriate due to the analog of sunspots, despite that the Sun's photometric V-band amplitude is typically only 0:m 001.

Ten years of photometry of HD 12545 from various sources indicated a systematic waxing and waning of the light-curve amplitude reminiscent of a sunspot cycle with a yet unpredictable periodicity (Strassmeier et al. 1997a). Spot modeling of rotationally modulated light and color curves by two independent groups (Strassmeier & Oláh 1992 and Hampton et al. 1996) gave conflicting surface spot distributions. The main reason for these discrepancies is that photometry alone can not constrain a spot's latitude, nor can it reliably recover a surface spot distribution when the inclination of the rotation axis and the level of unspotted brightness are unknown. Just recently, Vogt et al. (1999) presented a comparison of Doppler images and spot distributions from light-curve modeling of the RS CVn binary HR 1099 and showed that simple two-spot model solutions are often misleading and nonunique. Clearly, spot distributions determined from photometry alone are model dependent.

These shortcomings are not present in Doppler imaging, which uses the deformations of rotationally-broadened line profiles to recover the stellar surface temperature distribution (e.g. Rice 1996, and previous papers in this series). Together with simultaneous multi-color photometry, which is used to define the absolute and relative continuum flux, it is possible to obtain accurate physical spot quantities and compare these values with the Sun and other stars.

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© European Southern Observatory (ESO) 1999

Online publication: June 18, 1999