Simple Hydrodynamical Simulations of the Circumnuclear Disk

Robert F. Coker1, M.H. Christopher2, S.R. Stolovy2,3, N.Z. Scoville2

1 Los Alamos National Laboratory
2 Caltech

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The ``circumnuclear disk'' (CND) is a dense, clumpy, asymmetric ring-like feature centered on Sgr A*. The outer edge of the CND is not distinct but extends for more than 7 pc; the distinct inner edge, at a radius of ~1.5 pc, surrounds the ``mini-spiral'' of the HII region, Sgr A West. We present simple 3D hydrodynamical models of the formation and evolution of the CND from multiple self-gravitating in-falling clouds and compare the results with recent observations. We assume the clouds are initially Bonner-Ebert spheres, in equilibrium with a hot confining inter-cloud medium. We include the gravitational potential due to the point-mass of Sgr A* as well as the extended mass distribution of the underlying stellar population. We also include the effects of the ram pressure due to the stellar winds from the central cluster of early-type stars.

A single spherically symmetric cloud cannot reproduce the clumpy morphology of the CND; multiple clouds on diverse trajectories are required so that cloud-cloud collisions can circularize the clouds' orbits while maintaining a clumpy morphology. Collisions also serve to compress the clouds, delaying tidal disruption while potentially hastening gravitational collapse. Low density clumps are disrupted before reaching the inner CND radius, forming short-lived arcs. The outer parts of more massive clumps get tidally stripped, forming long-lived low-density arcs, while their cores undergo gravitational collapse. The fine balance between resisting tidal disruption while also preventing gravitational collapse implies that most if not all clumps are not stable for more than an orbit. Thus, in order for the CND to be a long-lived clumpy object, it must be continually fed by additional in-falling clouds. Clouds which survive to small radii are likely to be the sites of present or future star formation. However, within a few parsecs of Sgr A*, the stellar winds decelerate any in-falling cloud so that the wind-cloud interface becomes Rayleigh-Taylor unstable, potentially disrupting the cloud and inhibiting star formation.