X iv : a st ro - p h / 98 11 37 4 v 1 2 4 N ov 1 99 8 The circumnuclear disk and ionized gas filaments as remnants of tidally disrupted clouds

The circumnuclear disk and ionized gas filaments as remnants of tidally disrupted clouds Abstract. Sticky particle calculations indicate that a coherent structure , a dispersion ring, forms when a cloud on a low angular momentum orbit passes close to the dynamical center of a potential containing a point mass. The cloud is tidally stretched and differentially wrapped, and dissipation in shocks organizes the gas into a precessing offset elliptical ring which can persist for many rotation periods. The morphology and kinematics of the circumnuclear disk (CND) between 2 and 5 pc and the Northern arm in the inner 1 pc are well-represented by such structures. In the case of the Northern Arm, strong shocks which arise during the formation of the dispersion ring can lead to star formation even in the near tidal field of a massive black hole. 1. The relation between gas flow and periodic orbits To what extent can the gas structures in the inner few parsecs of the Galaxy be understood in terms of gas flow in a gravitational field? I am referring to the well-studied circumnuclear disk (CND) and the ionized gas filaments within the central cavity of the CND (Morris & Serabyn 1996, Mezger et al. 1996). The morphology and kinematics of the ionized gas filaments have been modeled by motion along Keplerian orbits appropriately projected on the plane of the sky (Serabyn et al. 1988, Herbst et al. 1993, Roberts et al. 1996); the success of such models strongly suggests that the motion is primarily orbital and that other possible mechanisms (e.g., stellar winds, magnetic fields, supernova explosions) do not significantly influence the overall, systematic pattern of flow. But the material is, afterall, gaseous, and how well can gas motion be approximated by orbital motion? It is obvious that the hydrodynamic equation of motion written in La-grangian form is the equation of motion of a particle when the pressure gradient and viscous stress terms are omitted. This means that, in a gravitational field, if pressure forces are negligible (including thermal, turbulent, viscous, and magnetic), then gas elements move on orbits. Moreover, for steady state inviscid gas flow in a gravitational field the gas streamlines are non-intersecting periodic orbits. Of course, not all orbits can be streamlines because orbits may loop and there cannot be two values of the fluid velocity at one point. Fig. 1a shows a typical orbit in a gravitational field …