SN 1979C VLBI: 22 Years of Almost Free Expansion

Abstract

VLBI measurements of the size of SN 1979C in M100 (NGC 4321) in the Virgo Cluster, from t = 3.7 yr after the explosion, show an expansion ∝tm, which is, with m = 0.95 ± 0.03, almost consistent with being free for 22 years. The last size measurement, at t = 22 yr, may indicate for the first time a change of the expansion of the supernova and suggests, as an alternative, free expansion ∝tm with m = 1.00 ± 0.05 up to tb ~ 17 yr followed by marginally significant deceleration with m = 0.74 ± 0.17. The possible deceleration could be weaker within the errors for tb 17 yr. With the assumption for the density profile of the circumstellar medium (CSM) of ρCSM ∝ r-s, we derive a model-dependent value of s = 1.95 up to a distance from the progenitor, r = rb, that corresponds to tb ~ 17 yr, which changes to s 1.5 for r > rb. For a kinetic energy of the shocked ejecta and CSM shells of Ekin = 1051 ergs, our results require a mass loss to wind velocity ratio for the progenitor of w/w ~ 1 × 10-5 M☉ yr-1 per w = 10 km s-1, an order of magnitude smaller than estimated from radio light-curve fitting. The swept-up mass at t = 22 yr is then Msw = 0.3 M☉ and the inferred mass of the shocked ejecta Mshock-ej = 2 M☉. Our last observations give an image of a barely resolved source with a first hint on the structure of the supernova, consistent with being circular within 9% and possibly center filled. The expanding shock front method (ESM) of combining the transverse radio expansion velocities with the radial optical velocities gives direct distance estimates to M100, with standard errors of D = 16.5 ± 2.5 to 19.8 ± 3.0 Mpc, depending on whether the supernova has a bright center or is a shell without such a center. These estimates are comparable with those from Cepheid observations (e.g., 16.1 and 15.2 Mpc).