Radio Emission from SN 1988Z and Very Massive Star Evolution

We present observations of the radio emission from the unusual supernova SN 1988Z in MCG +03-28-022 made with the Very Large Array at 20, 6, 3.6, and 2 cm, including new observations from 1989 December 21, 385 days after the optically estimated explosion date, through 2001 January 25, 4438 days after explosion. At a redshift z = 0.022 for the parent galaxy (~100 Mpc for H0 = 65 km s-1 Mpc-1), SN 1988Z is the most distant radio supernova ever detected. With a 6 cm maximum flux density of 1.8 mJy, SN 1988Z is ~20% more luminous than the unusually powerful radio supernova SN 1986J in NGC 891 and only ~3 times less radio luminous at 6 cm peak than the extraordinary SN 1998bw, the presumed counterpart to GRB 980425. Our analysis and model fitting of the radio light curves for SN 1988Z indicate that it can be well described by a model involving the supernova blast wave interacting with a high-density circumstellar cocoon, which consists almost entirely of clumps or filaments. SN 1988Z is unusual, however, in that around age 1750 days, the flux density begins to decline much more rapidly than expected from the model fit to the early data, without a change in the absorption parameters. We interpret this steepening of the radio flux density decline rate as due to a change in the number density of the clumps in the circumstellar material (CSM) without a change in the average properties of a clump. If one assumes that the blast wave is traveling through the CSM at ~2,000 times faster than the CSM was established (20,000 km s-1 vs. 10 km s-1), then this steepening of the emission decline rate represents a change in the presupernova stellar wind properties ~10,000 yr before explosion, a characteristic timescale also seen in other radio supernovae. Further analysis of the radio light curves for SN 1988Z implies that the SN progenitor star likely had a zero-age main sequence mass of ~20-30 M☉. We propose that SNe such as SN 1986J, SN 1988Z, and SN 1998bw with very massive star progenitors and associated massive wind (ṁ ≳ 10-4 M☉ yr-1) have very highly clumped, wind-established CSM and unusually high blast-wave velocities (greater than 20,000 km s-1).