Millimeter-VLBI provides an angular resolution of up to a few tens of microarcseconds and allows imaging of compact radio sources, self-absorbed at longer wavelengths, with unsurpassed angular resolution. At 43 GHz the participation of the VLBA and the 30 m-MRT at Pico Veleta (e.g. Krichbaum et al., 1993 a&b), and at 86 GHz the addition of the 100 m-RT at Effelsberg and the 30 m-MRT (Schalinski et al., 1993, and this volume) have improved the imaging capabilities of mm-VLBI observations.
Results: The increased sensitivity of mm-VLBI observations allows the investigation of fainter objects, previously not accessible. As one example we show in Fig.1 the first detection of the compact radio source Sgr A∗ in the Galactic Center with VLBI at 43 GHz in May 1992 (Krichbaum et al., 1993d) and at 86 GHz in April 1993 (Krichbaum et al., 1994). In both observations the size of Sgr A∗ appeared to be larger than its expected scattering size, indicative of intrinsic source structure showing up at mm-wavelengths. Future monitoring with mm-VLBI is necessary to search for (not unexpected) structural variability.
Monitoring of AGN with mm-VLBI reveals in all cases observed in sufficient detail jet curvatures of increasing amplitude towards the self-absorbed VLBI-cores (e.g. in 18034-784: Krichbaum, 1990, OJ 287: Krichbaum et al., 1993c), and sub- or superluminal motion along ‘quasi-helically’ bent trajectories (e.g. 3C 84: Krichbaum et al., 1993b; 3C 273: Krichbaum et al., 1993c), which differ sometimes for adjacent jet components (e.g. 3C 345: Krichbaum & Witzel, 1992, Krichbaum et al., 1992&1993a). In 3C 84, 3C 273 and 3C 345 the apparent velocity of jet components varies systematically along the jet axis, in 4C 39.25 (Alberdi et al., 1993) a moving component decelerates and brightens, all of this suggesting differential Doppler boosting and motion along three-dimensionally curved trajectories. In 3C 345 the complex kinematics of C4 and C5 (Zensus, this volume) has been geometrically modeled by motion along a helical path on the surface of a conical jet (Qian et al., 1992, Steffen et al., 1993, and this volume; see also Camenzind, this volume). As a new example, the oscillations of the inner jet and its velocity variations β
(r) are shown for the BL Lac object 1803+784 in Fig. 2 (see the maps in: Krichbaum et al., 1993b). The frequent occurence of ‘quasi-sinusoidal’ bends in the inner jets of very different classes of AGN (QSO's, BL Lac's, Seyfert's) suggests that this effect is common in a large fraction of AGN and that the underlying jet-physical process may be fundamental for the understanding of the creation of jets.