Doppler Imagery of the Spotted RS Canum Venaticorum Star HR 1099 (V711 Tauri) from 1981 to 1992

Abstract

We present a set of 23 Doppler images of the spotted RS CVn star HR 1099(V711 Tauri;HD 22468) obtained from 1981 to 1992. HR 1099 shows a large,cool polar spot that has persisted for the 11 yr of this study and otherlow-latitude spots that come and So on relatively short (less than 1 yr)timescales and can emerge anywhere on the star. The polar spot hasvariable protuberances that look very similar to the time-variablevertical extensions of the Sun's polar coronal hole. The area of thepolar spot and its extensions shows marginal evidence of being variable,with a period of about 3 yr and an amplitude of about 1\% that isperhaps indicative of a weak cycle, but this is not yet conclusive.Comparison of our Doppler images with previously published ``fewspot{''} model fits to the light curves shows that such simplespot-model solutions, while sometimes in agreement, are often misleadingand nonunique, particularly when the light-curve amplitude is small.Moreover, these spot-model fits do not recover the existence of thepolar spot. The Doppler images show quite good agreement among multipleimages at a given epoch and between different Doppler imaging researchgroups using completely independent data sets and imaging software. Our(cool spots only) Doppler imaging solutions, when properly thresholded,generally well reproduce the published light curves. However, in oneinstance the difficulty of fitting light curves suggests that at leastone hot spot was present on HR 1099 during one observing season.Variations in the mean brightness of the system at the observed 0.05 maglevel seem to correlate with spot area, particularly the polar spot,indicating that the mean light level is a pretty good proxy of spot areaon HR 1099. While the polar spot with variable extensions was alwayspresent, isolated spots also frequently appeared at both mid- and lowlatitudes. On several occasions isolated prominent spots emerged andthen disappeared on or near the equator.The ``migrating photometric wave{''} ion HR 1099 is due not to a simplelongitudinal migration of spots on a differentially rotating star butrather to changes in the spatial distribution of a few spots (some ofwhich move but most of which are fixed in longitude) that emerge andthen disappear. So, at least for HR 1099, the phase drift of thismigrating photometric wave minimum contains very little uniqueinformation about differential rotation or spot migration. While thetracking of individual features involves some uncertainty andspeculation because of our limited time sampling, the tracks of twolong-lived spots suggest that some spots that emerge at low orintermediate latitudes may migrate up to the pole in a clockwise spiral(slower than the orbit), then apparently merge with the polar spot. Ifthese dark spots trace magnetic flux, we speculate that some of themagnetic flux emerging at lower latitudes migrates poleward and mergeswith the polar spot flux. It is not yet clear whether this flux is ofthe same or opposite polarity to the polar spot and thus whether thesepoleward-migrating, low-latitude spots reinforce or cancel the polarspot field. One of the high-latitude spots also appeared to getstretched in longitude as it approached the polar spot, and its overalltrack is quite reminiscent of the annulus of toroidal field found byDonati et al. encircling the polar spot of HR 1099 in 1990.9.In general, the spots appear to be very tightly locked to the orbitalframe of the system, and most disappear before they have had a chance tomigrate significantly; Like solar coronal holes, they show very littleevidence for shear due to differential rotation. A few selected,long-lived features gave longitudinal migration rates of 1 part in 300to 1 part in 3600 of the rotation period, in the sense that intermediateand low latitudes rotate slightly slower than the orbital angularvelocity, while the pole and highest latitudes appear to be synchronizedto the orbit. The implied differential rotation is thus of opposite signand about a factor of 56 less than for the Sun. The rotation rate versuslatitude behavior can be well fitted with a variety of formulae,including the PI launder formula. One of the best fits is provided by arotation period versus latitude that is proportional to the surfacestrength of a centered axisymmetric magnetic dipole field, with the polesynchronized to the orbit and lower latitudes rotating more slowly. Webelieve that these starspots are not measuring photospheric differentialrotation. Instead, like solar coronal holes, their relatively low degreeof shearing and nearly solid body rotation may be enforced by amultikilogauss, axisymmetric, nearly current-free quasi-potential globalmagnetic field. Our Doppler images also agree very closely with theZeeman-Doppler imagery of Donati et al. and support their finding thatregions around the edge of the polar spat and within bright spats showlargely monopolar fields of at least 300-700 G strength, The large,permanent cool polar spots, the very low observable differentialrelation and shearing of starspots, and the evidence of strong,essentially unipolar magnetic fields associated with them leads us tobelieve that NR 1099 and other rapidly rotating RS CVn stars harborquite strong (multikilogauss) axisymmetric global magnetic dipolefields. These fields have historically been largely hidden from view bytheir high degree of rotational symmetry, by being concentrated in thelow surface brightness dark spots, and by these stars' high degree ofrotational line broadening. We propose that the starspots on HR 1099 andother rapidly rotating RS CVn stars are, by analogy with solar coronalholes, large unipolar, magnetic regions that are tightly frozen intomultikilogauss, axisymmetric global dipole fields in these stars. Sincethe large cool polar spots, the signature of these dipoles, are notpresent on more slowly rotating RS CVn stars,we believe that they mustbe dynamo-induced fields rather than remnant fossil fields.