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Astron. Astrophys. 354, 537-550 (2000)

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1. Introduction: Doppler imaging of V711 Tau

Vogt & Penrod (1983) were the first to obtain a Doppler image of V711 Tau (=HR 1099) from data in late 1981 and discovered its prominent polar spot. Because the Sun does not show spots in excess of [FORMULA]40o, this discovery spurred significant interest in these types of stars. V711 Tau is a triple system and consists of a close double-lined spectroscopic pair with a spotted and rapidly rotating K1IV primary and a comparably inactive and slowly rotating G5V secondary in a 2.8-day orbit. The tertiary is a fainter K3V star 6" away. A comparison of the close binary with evolutionary models suggests that mass transfer from the K primary onto the G secondary may begin within 107 years (Fekel 1983). Fekel also summarized the relevant physical properties and we refer to his paper for further systemic details.

Doppler imaging is an indirect image-reconstruction technique that allows us to recover a star's surface temperature distribution from periodic line-profile variations of a series of high-resolution spectra. Detailed reviews of how this technique works, its advantages and shortcomings, and to what class of stars it may be applied can be found in the literature and we refer the reader to the following papers; e.g., Vogt et al. (1987), Rice et al. (1989), Collier Cameron (1992), Piskunov & Rice (1993), Rice (1996), Strassmeier (2000).

V711 Tau is among the brightest spotted stars and therefore attracted many investigators. After the initial discovery by Vogt & Penrod (1983), Vogt (1988) summarized their images for up to 1985. At about the same time Gondoin (1986) obtained a map for 1983.8 by using a technique, originally applied to equivalent-width variations of Ap stars, that is nowadays obsolete. Just recently, Vogt et al. (1999) presented a seminal paper that included 23 Doppler images for the epochs 1981 through 1993 (a preview was presented by Vogt & Hatzes 1996). Donati et al. (1992) independently derived maps for the epochs 1988.9 and 1990.9 and presented the first magnetic maps from Zeeman-Doppler imaging. These maps suggested that the magnetic field lines were emerging radially or poloidally encircling the polar spot. Additionally, a warm feature near the equator seemed to be also the site of significant magnetic fields. However, the field geometry was later revised by Donati (1999) and the most recent magnetic maps show at least one feature in which the field is predominantly azimuthal. It is not yet clear whether this increase in complexity is due to an increase of data quality or is intrinsic to the star. Jankov & Donati (1995) presented further brightness maps for the epochs 1989.9 and 1992.9 and Donati (1999) summarized his data of V711 Tau and presented five annual images from 1991.9 to 1996.9.

All authors, except the early attempt of Gondoin (1986), consistently recovered a polar spot on V711 Tau that seems to have existed throughout the past 15 years. Up to now, almost all maps of V711 Tau were obtained on an annual basis. The short end of the variability time scale was not explored as thoroughly. Nevertheless, Vogt et al. (1999) found evidence that individual spots can come and go rather quickly, in agreement with observations of other spotted stars (see the previous papers in this series). They also found that spots on V711 Tau emerge at low to intermediate latitudes and then migrate up to the pole in a clockwise direction where they possibly merge with the polar spot. Vogt et al. (1999) explained this scenario with differential surface rotation that is opposite to the Sun's, i.e. the polar regions of V711 Tau rotate faster than the equatorial regions. Unfortunately, most of the spots that were used as tracers for the differential rotation either disappeared before they had a chance to migrate and/or the time and phase coverage of the observations was too short and too sparse to follow their complete evolution. Any conclusions concerning differential surface rotation are hampered by the fact mentioned above. Therefore, we decided to observe V711 Tau continuously for two months by using a dedicated portion of telescope time at the National Solar Observatory's McMath-Pierce telescope in combination with our own automatic photometric telescopes in southern Arizona. In this paper, we present the first results from this campaign.

In Sect. 2, we describe the new spectroscopic and photometric data. Sect. 3 presents several details of the data preparation due to the fact that V711 Tau is a double-lined spectroscopic binary and its primary exhibits a mild ellipticity effect. In Sect. 4, we derive and discuss the Doppler images and in Sect. 5, we present our conclusions.

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© European Southern Observatory (ESO) 2000

Online publication: February 9, 2000