1 Scientific motivation for a Ca II H&K survey

The presence of emission in the core of the Ca II H and K resonance lines is a diagnostic of magnetic activity in the chromospheres of late-type stars. Spatially resolved K-line heliograms and magnetograms amply demonstrate the relation between H&K-emission strength and the surface magnetic field on our Sun (Schrijver [1996]). Furthermore, the fact that we observe generally stronger H&K emission in more rapidly rotating stars is widely known as the rotation-activity relation (e.g. Noyes et al. [1984]) which is heuristically explained by the $\Omega$-effect of the classic $\alpha\Omega$ dynamo (see Stix [1989]). Therefore, rapidly-rotating stars offer laboratories to study the effect of stellar dynamos. The catalog of chromospherically active binary stars (CABS, Strassmeier et al. [1993]) summarized such stars in binaries and proofed to be a valuable data base for further investigations.

It is only the very rapidly-rotating stars where we can also obtain spatially resolved information of their surface temperature distribution, and respectively also of their magnetic surface field, by applying indirect imaging techniques like Doppler imaging (e.g. Rice [1996]). Such rapidly-rotating active stars are relatively rare but can be identified from their Ca II H&K emission with just a single spectrum of low signal-to-noise ratio and moderate resolution. For example, a H&K survey from low-resolution spectra in the southern hemisphere (Henry et al. [1996]) provided the source for the discovery of many rapidly-rotating solar-type stars (Soderblom et al. [1998]). The H&K work of W. Bidelman (e.g. Bidelman [1981]; see also Sect. 2) supplied the target lists for the radial-velocity and photometric survey at SAAO (e.g. Balona [1987]; Lloyd-Evans & Koen [1987]). Another particularly important example of an usolved question in the above context is the angular momentum loss during stellar evolution. Magnetic braking of stellar rotation due to a stellar wind along predominantly equatorial magnetic field lines, like in our Sun, seemed not to have always the power to slow down stars from their initial angular momentum gained during the contraction from the pre-stellar cloud. The many ultra-fast rotators in young open clusters as well as the young field stars AB Dor, LQ Hya, EK Dra etc. are the most cited examples. Moreover, there exists a group of single, rapidly-rotating and evolved stars with strong magnetic activity (Fekel & Balachandran [1993]). This is a paradox since magnetic braking had enough time during the main-sequence stage to halt the rapid rotation, and the radius increase due to the termination of hydrogen-core burning should have resulted in an effectively complete loss of angular momentum. What process maintained these stars angular momentum? Is it the same process suggested for the ultra-rapid cluster rotators, i.e. a saturation of the atmospheric (coronal) volume with magnetic fields so that there is no torque arm for magnetic braking via a stellar wind anymore? Or is it something completely different?

Figure 1: The sky distribution of the sample of stars in this paper. Stars that were found to exhibit Ca II emission are shown as dots, non-emission stars as plusses. The active stars are again subdivided into weak-to-moderate emission (emission intensity, $I_{\rm K}$ = 0.5-2.5 according to Wilson ([1976]) and moderate-to-strong ($I_{\rm K}$ = 3.0-5.0) emission stars

Solanki et al. ([1997]), Strassmeier et al. ([1998]) and Buzasi ([1999]) suggested that magnetic fields concentrated in polar starspots could be the reason for such a lack of angular-momentum loss as described above. Solanki et al. presented numerical simulations that show that the effect would be quantitatively the same as with a dynamo saturation process. The only way to find conclusive observational evidence for or against the polar-spot hypothesis is to Doppler image these stars and search for polar starspots. Since Doppler imaging is an elaborate technique with many restrictions for the stellar sample (rapid rotation, medium inclination, known rotation period, relatively bright star etc.) one needs significantly more stellar candidates as known to date to cover the part in the $\rm H-R$ diagram where stellar activity occurs. It is the primary aim of this survey to provide a larger sample of suitable Doppler-imaging targets. Additional goals are to provide activity-related stellar parameters like absolute Ca II emission-line fluxes, the H$\alpha$ morphology, the lithium abundance, and photometric variations and relate them to absolute stellar parameters based on the distance from the Hipparcos satellite.