Structure of Sagittarius A* at 86 GHZ using VLBI Closure Quantities

At radio wavelengths, images of the compact radio source Sagittarius A*(Sgr A*) in the Galactic center are scatter broadened with aλ² dependence due to an intervening ionized medium. Wepresent VLBI observations of Sgr A* at 86 GHz using a six station array,including the VLBA antennas at Pie Town, Fort Davis, and Los Alamos, the12 m antenna at Kitt Peak, and the millimeter arrays at Hat Creek andOwens Valley. To avoid systematic errors due to imperfect antennacalibration, the data were modeled using interferometric closureinformation. The data are best modeled by a circular Gaussian brightnessdistribution of FWHM 0.18+/-0.02 mas. The data are also shown to beconsistent with an elliptical model corresponding to the scattering of apoint source. The source structure in the north-south direction, whichis less well determined than in the east-west direction because of thelimited north-south u-v coverage of the array, is constrained to be lessthan 0.27 mas by these measurements. These results are consistent withextrapolations of intrinsic structure estimates obtained with VLBI at a7 mm wavelength, assuming the intrinsic size of Sgr A* has a greaterdependence than λ^0.9 with wavelength.