Long-term spectroscopic monitoring of the Luminous Blue Variable AG Carinae

We have extensively monitored the Luminous Blue Variable AG Car (HD 94910) spectroscopically. Our data cover the years 1989 to 1999. In this period, t he star underwent almost a full S Dor cycle from visual minimum to maximum and back. Over several seasons, up to four months of almost daily spectra a re available. Our data cover most of the visual spectral range with a high spectral resolution (lambda/Delta lambda approximate to 20 000). This allow s us to investigate the variability in many lines on time scales from days to years. The strongest variability occurs on a time scale of years. Qualit atively, the variations can be understood as changes of the effective tempe rature and radius, which are in phase with the optical light curve. Quantit atively, there are several interesting deviations from this behaviour, howe ver. The Balmer lines show P Cygni profiles and have their maximum strength (both in equivalent width and line flux) after the peak of the optical lig ht curve, at the descending branch of the light curve. The line-width durin g maximum phase is smaller than during minimum, but it has a local maximum close to the peak of the visual light curve. We derive mass-loss rates over the cycle from the H alpha line and find the highest mass loss rates (log (M)over dot/(M-circle dot yr(-1)) approximate to -3.8, about a factor of fi ve higher than in the minimum, where we find log (M)over dot/(M-circle dot yr(-1)) approximate to -4.5) after the visual maximum. Line-splitting is ve ry commonly observed, especially on the rise to maximum and on the descendi ng branch from maximum. The components are very long-lived (years) and are probably unrelated to similar-looking line-splitting events in normal super giants. Small apparent accelerations of the components are observed. The ch ange in radial velocity could be due to successive narrowing of the compone nts, with the absorption disappearing at small expansion velocities first. In general, the line-splitting is more likely the result of missing absorpt ion at intermediate velocities than of excess absorption at the velocities of the components. The HeI lines and other lines which form deep in the atm osphere show the most peculiar variations. The HeI lines show a central abs orption with variable blue- and red-shifted emission components. Due to the variations of the emission components. The HeI lines can change their line profile from a normal P Cyg profile to an inverse P Cyg-profile or double- peak emission. In addition, very broad (+/- 1500 km s(-1)) emission wings a re seen at the strongest HeI lines of AG Car. At some phases, a blue-shifte d absorption is also present. The central absorption of the HeI lines is bl ue-shifted before and red-shifted after maximum. Possibly, we directly see the expansion and contraction of the photosphere. If this explanation is co rrect, the velocity of the continuum-forming layer is not dominated by expa nsion but is only slightly oscillating around the systemic velocity.