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Astron. Astrophys. 336, 587-603 (1998)

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1. Introduction

HD 51066 (V=7:m 0, G8III-II, P=16 days, [FORMULA]15:s3, [FORMULA]=[FORMULA], 2000.0) is one of the few examples of an effectively single star that somehow has maintained its angular momentum through its evolution off the main sequence. It was previously thought to be a single main-sequence star of spectral type K2 (Fleming et al. 1989) but that was revised to K0III by Fekel & Balachandran (1994) from high-resolution red wavelength spectra. Bidelman (1994) found it with strong Ca II H&K emission lines on the Michigan objective-prism survey plates at Kitt Peak which were confirmed by Strassmeier (1994) from high-resolution spectra with the Coudé feed telescope at Kitt Peak. The star also shows strong x-ray emission (Fleming et al. 1989, Stocke et al. 1991) and too large a lithium abundance for a normal late-type giant (Fekel & Balachandran 1994). Its brightness is modulated with a period of approximately 16 days (Henry et al. 1995b) and it has a projected rotational velocity of [FORMULA] km s-1 (Fekel 1997). It has recently received the variable star designation CM Cam (Kazarovets & Samus 1997). Besides being magnetically active and thus an interesting star, its single status raises the question how an evolved star that is not a member in a close binary system, can preserve its angular momentum throughout the expansion phase due to the termination of hydrogen core burning.

There seem to be two roads to an answers. Either the star is not single or the proposed dynamo saturation, thought to account for the high number of ultra-rapidly rotating stars in young open clusters (e.g. Barnes & Sofia 1996), does also apply for post main-sequence stars. The first who concluded that HD 51066 is (apparently) single was Fleming et al. (1989) from optical observations following the Einstein medium sensitivity survey, and was later confirmed by Fekel & Balachandran (1994) and Henry et al. (1995b) from eight radial-velocity observations between 1991 November and 1993 April. Recently, in his list of rotational velocities of late-type stars, Fekel (1997) lists it to be a suspected SB1 on the grounds of annually inconsistent radial velocities.

On the second possible avenue, Solanki et al. (1997) have shown that a decreased angular momentum loss induced by a stellar wind along predominantly polar magnetic field lines of a pre-main-sequence star would have the equivalent effect on the stellar rotation than a field saturation by the dynamo itself and could thus also maintain the star's rapid rotation until the ZAMS is reached. However, it is not clear whether this picture would also apply to post-main-sequence evolution since HD 51066 is at least a class III giant but, if possible and of correct magnitude, this mechanism would predict long-lived polar spots on evolved and rapidly rotating giants. Fortunately, the phenomena of dynamo saturation and fields concentrated in polar spots can not act at the same time and, more or less, exclude each other. It is one of the goals in the present series of papers to find observational evidence for or against polar starspots by means of Doppler imaging. Our observations are described in Sect. 2, Sect. 3summarizes our results from seven years of photometric monitoring, Sect. 4presents 66 radial velocities that show the star to be a long-period spectroscopic binary and then, in Sect. 5, we derive Doppler images from five different spectral-line regions and for four consecutive years along with a determination of the fundamental stellar parameters of the HD 51066 system. Sect. 6compares chromospheric H[FORMULA] line profiles from 1996 and 1997 with the photospheric activity in these years and, finally, Sect. 7summarizes our conclusions.

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© European Southern Observatory (ESO) 1998

Online publication: July 20, 1998