Model and geometry dependence of radio distance determinations of extragalactic supernovae

Abstract

I hope that the above discussion elucidates the systematic model and geometry dependent uncertainties in using radio supernovae as distance indicators. Of all the models considered, a uniform sphere is the most “conservative” in that it yields the highest value of H _o. Since Chevalier's model currently seems to best satisfy the observational and theoretical constraints, it seems likely that the value of H _o obtained by adopting a uniform spherical geometry is high by 25–34%. However, supernovae may have a few surprises in store for us. The multifrequency VLBI data of Bartel suggest a possible frequency dependence of 0 _inferred even though v ≫ v _m. This should not be possible unless gradients in magnetic field and particle energy exist in the emitting region. For example, the electrons whose radiation peaks at 2.3 GHz could occupy a larger volume than those whose radiation peaks near 5 GHz. It is difficult for this to yield significantly different observed sizes, however, since synchrotron radiation is broadband, which precludes sharp frequency cutoffs within a given volume of emission. Nevertheless, we should remain aware that unexpected complicating effects such as gradients in magnetic field and relativistic particle densities and energies, scattering, coherent emission mechanisms, and inhomogeneous opacity effects could plague our efforts to use radio supernovae as distance indicators.

I thank Norbert Bartel for discussing his results prior to the Workshop. This research was supported in part by the National Science Foundation under grant AST-8315556.