SMALL-SCALE STRUCTURE AND POSITION OF SAGITTARIUS A-ASTERISK FROM VLBI AT 3 MILLIMETER WAVELENGTH

The compact continuum radio source Sgr A associated with the center o f the Galaxy appears unresolved at 86 GHz on a Kitt Peak, Arizona to O wens Valley, California Very Long Baseline Interferometer (VLBI). The lack of structure on this 850 km (2 x 10(8) lambda) baseline sets an u pper limit on the angular size of 0.2 mas. The lack of a detection on the much longer baselines to the Haystack Observatory 37 m sets a lowe r limit of 0.1 mas on the angular size. Both limits are consistent wit h the angular size of 0.16 +/- 0.01 mas expected from the wavelength-s quared dependence of size observed at longer wavelengths produced by d iffractive scattering in the intervening turbulent plasma close to the Galactic center. The upper size limit implies an intrinsic size for t he emitting region of less than 0.13 mas (1 AU) and a brightness tempe rature in excess of 1.4 x 10(10) K. An accurate position of Sgr A was also obtained from the observations.