Model and geometry dependence of radio distance determinations of extragalactic supernovae

Now that supernovae are being detected in the radio, it is possible to use angular size and velocity measurements to obtain the distances to their respective galaxies. The application of measurements to distance determinations requires assumptions, which lead to uncertainties in the derived distances. Other papers in this volume deal with the optical-radio velocity correspondence; here uncertainties involved in extracting angular diameters from VLBI data are discussed. At this point, the weakness of the flux densities of observed radio supernovae combines with the small number of highly sensitive antennas available to limit the information which one can extract using VLBI. This situation should improve as receiver technology advances and the VLBA becomes operational. In regard to spherical symmetry, theoretically, supernovae are extremely energetic, and essentially point explosions. For example, if the pre-supernova wind in Chevalier's model is not spherically symmetric, neither will be the radio emission. In any case, better coverage of the spatial frequencies (viz., more sensitive dishes) uncover asymmetries if they exist. VLBI observations are interpreted by fitting the brightness distribution of a uniform, optically thin sphere to data. The angular diameter of the model sphere is adjusted until the best fit is obtained. If the correct model is not a uniform sphere, the angular diameter obtained is an overestimate, which leads to an underestimate of the distance and an overestimate of H/sub 0/. The ratio of inferred to true angular diameter is shown and therefore the derived value of H/sub 0/, depends on the geometry of the source for several possible models