Using Millimeter VLBI to Constrain RIAF Models of Sagittarius A*

The recent detection of Sagittarius A* at λ = 1.3 mm on abaseline from Hawaii to Arizona demonstrates that millimeter wavelengthvery long baseline interferometry (VLBI) can now spatially resolveemission from the innermost accretion flow of the Galactic centerregion. Here, we investigate the ability of future millimeter VLBIarrays to constrain the spin and inclination of the putative black holeand the orientation of the accretion disk major axis within the contextof radiatively inefficient accretion flow (RIAF) models. We examine therange of baseline visibility and closure amplitudes predicted by RIAFmodels to identify critical telescopes for determining the spin,inclination, and disk orientation of the Sgr A* black hole and accretiondisk system. We find that baseline lengths near 3 Gλ have thegreatest power to distinguish amongst RIAF model parameters, and that itwill be important to include new telescopes that will form north-southbaselines with a range of lengths. If an RIAF model describes theemission from Sgr A*, it is likely that the orientation of the accretiondisk can be determined with the addition of a Chilean telescope to thearray. Some likely disk orientations predict detectable fluxes onbaselines between the continental United States and even a single 10-12m dish in Chile. The extra information provided from closure amplitudesby a four-antenna array enhances the ability of VLBI to discriminateamongst model parameters.