Introduction

This is a short summary of several calculations of the evolution of supernova remnants in a constant high density medium n0 ≥ 106−8 cm−3 and an abundance in the range 0.01Z⊙≤ Z ≤ 10Z⊙. The main difference found when comparing them with the standard calculation of a supernova evolving into a constant density medium n0 = 1 cm−3 is that radiative cooling becomes important very early in the life of the remnants. The radiative phase starts well before the ejecta is fully thermalized and while the expansion velocities are still in the range of several thousands of km s−1. Consequently, the remnants miss their Sedov evolutionary phase and, unlike the standard case, in these calculations full thermalization of the ejecta is only completed long after the moment of thin shell formation (see Terlevich et al. 1992, 1994a; hereafter referred to as papers I and II). The cooling event leads to large luminosities (≥ 109 L⊙) in spans of time of only a few years, causing a major rapid depletion of the supernova's stored thermal energy, in only a few weeks or months. Strong radiative cooling leads to an ionizing spectrum (see paper I) and thus to an HII region with multiple components, as it photoionizes the recombining, rapidly-moving swept-up gas and the outer unperturbed matter. The ionizing radiation is also absorbed by the still unshocked and dense expanding ejecta.

Such remnants, hereafter termed “compact SNRs”, are capable of producing the strong ionizing flux that makes them appear as Seyfert I impostors (Filippenko 1989) when occurring in dense regions far away from the nucleus of galaxies.