7. Discussion and conclusions
We have presented annual Doppler images of the effectively-single giant HD 51066 = CM Cam from four consecutive years. The 1994 image reveals a weak but significant cool polar-cap like spot, which became even weaker in 1995 and was absent in 1996 and 1997. The simultaneously observed brightening since the discovery of the light variability in 1992 suggests a common cause, namely, the slow decay of the polar spot. The overall spot temperature is only 500 K cooler than the nominal photospheric value of 4950 K, in excellent agreement with independent spot modelling of high-precision V and V-I light curves from 1997/98. Our annual cross-correlation maps contain some evidence for latitude-dependent phase shifts from consecutive (annual) maps but we can not unambiguously interpret them to be due to differential rotation because there is mostly more than one correlation peak per latitude slice. However, the polar feature and the persistent existence of low-latitude inhomogeneity possibly suggests that spot activity on HD 51066 takes place in two belts, a nearly uniformly spotted equatorial belt and at high-latitudes with either one (in 1997 and 1996) or at most two (in 1995 and 1994) isolated features. Since the high-latitude features are generally recovered with cooler temperature than the equatorial features we may suspect them to be areas of higher magnetic flux density. This leads us to speculate that these regions could form bi-polar groups with the numerous spots of lesser field density in the equatorial belt. Such a closed field geometry, or better the absence of an open field geometry, could also explain the anomalously high rotation of HD 51066 by a lack of magnetic braking. However, it would imply that a similarly organized field structure must have already existed during the pre main-sequence stage, otherwise the star would have already considerably slowed down once it had spent a significant part of its lifetime on the main sequence. Today's field strength can be estimated indirectly by assuming that the surface-averaged value is close to, but less than what is required to be in equilibrium with the gas pressure. This would be consistent with the fact that many main-sequence stars show a saturation of activity with rotation (e.g. Vilhu 1984) and that direct field measurements of active dwarf stars indicate for rapid rotators (Saar 1996), where . With the approximate scaling law , and of 2.5 we get G for HD 51066 when we adopt =1500 G.
However, HD 51066 was not a solar-type star when on the main sequence. With the absolute magnitude from Hipparcos and the stellar parameters determined in this paper we can evolve HD 51066 backwards to the main sequence if we assume conservation of mass, angular momentum, and magnetic flux. The values in Table 4 convert then to a spectral type of B7-8, an effective temperature of 12-13,000 K, a radius of (Gray 1992) and nearly the same luminosity as today. Following the analysis of Stepie (1993) for the single, but slowly rotating, active G8 subgiant HR 1362 we may also estimate the rotational and magnetic main-sequence parameters. From the assumption of specific angular-momentum conservation, i.e. , we get days or km s-1 on the main sequence. Assuming the rigid-rotation case instead of specific angular-momentum conservation would even further shorten that period. Furthermore, assuming conservation of magnetic flux we get = kG. While this field strength is typical for magnetic Ap and Bp stars their rotation periods are mostly several days to even months with average 's seldomly exceeding 60 km s-1 (e.g. Wolff 1983). However, there are a few Si stars in the field and in open clusters with significantly higher rotation, e.g. HD 124224 = CU Vir (Borra & Landstreet 1980) with = 150 km s-1 and a rotation period of 0.52 days (e.g. Hiesberger et al. 1995) or ET And, another rapidly-rotating Si-enhanced Ap/Bp star (Piskunov et al. 1994). The sequence of magnetic Ap stars reaches up to B2 (see the review by Landstreet 1992) and would be consistent with our spectral-type estimate for the progenitor. It is thus tempting to conclude that the more massive ( ), rapidly-rotating, active single G-K giants have untypically rapidly-rotating Bp stars as progenitors while, as was concluded by Stepie (1993) for HR 1362, currently slowly-rotating and less massive, active single G-K giants were typical Ap stars. Consequently, we could expect the combined effects of a primordial and a dynamo-generated field within the overshoot region of these 3-4 late-type giants. Whether this has any impact on observable surface magnetic activity like starspots remains to be determined.
An alternative explanation may involve a coalesced binary as the main-sequence progenitor. Such a scenario was originally proposed by Webbink (1976) to explain the rapidly-rotating, single, G-K giants of the FK Comae group and Guinan & Bradstreet (1988) illustrated the evolution of initially detached solar-type binaries into contact systems and eventually into a single A-type star via magnetic braking. The problem in the application of this scenario to HD 51066 is that a typical W UMa system is very old, on average 5-10 Gyr, while its supposedly coalesced product, HD 51066, is a three solar-mass star and still relatively young.
© European Southern Observatory (ESO) 1998
Online publication: July 20, 1998