A distorted radio shell in the young supernova SN 1986J

We report here on 5-GHz global very-long-baseline interferometry (VLBI) observations of SN 1986J, 16 yr after its explosion. We obtained a high-resolution image of the supernova, which shows a distorted shell of radio emission, indicative of a deformation of the shock front. The angular size of the shell is similar to4.7 mas, corresponding to a linear size of similar to6.8 x 10(17) cm for a distance of 9.6 Mpc to NGC 891. The average speed of the shell has decreased from similar to7400 km s(-1) in 1988.74 down to about 6300 km s(-1) in 1999.14, indicative of a mild deceleration in the expansion of SN 1986J. Assuming a standard density profile for the progenitor wind (rho(cs) proportional to r(-s), s = 2), the swept-up mass by the shock front is similar to2.2 M.. This large swept-up mass, coupled with the mild deceleration suffered by the supernova, suggests that the mass of the hydrogen-rich envelope ejected at explosion was greater than or similar to12 M.. Thus, the supernova progenitor must have kept intact most of its hydrogen-rich envelope by the time of explosion, which favours a single, massive star progenitor scenario. We find a flux density for SN 1986J of similar to7.2 mJy at the observing frequency of 5 GHz, which results in a radio luminosity of similar to1.4 x 10(37) erg s(-1) for the frequency range 10(7)-10(10) Hz (alpha = 0 69; S-nu proportional tonu(alpha)). We detect four bright knots that delineate the shell structure, and an absolute minimum of emission, which we tentatively identify with the centre of the supernova explosion. If this is the case, SN 1986J has then suffered an asymmetric expansion. We suggest that this asymmetry is due to the collision of the supernova ejecta with an anisotropic, clumpy (or filamentary) medium.