When a massive star explodes as a supernova, substantial amounts of radioactive elements--primarily (56)Ni, (57)Ni and (44)Ti--are produced. After the initial flash of light from shock heating, the fading light emitted by the supernova is due to the decay of these elements. However, after decades, the energy powering a supernova remnant comes from the shock interaction between the ejecta and the surrounding medium. The transition to this phase has hitherto not been observed: supernovae occur too infrequently in the Milky Way to provide a young example, and extragalactic supernovae are generally too faint and too small. Here we report observations that show this transition in the supernova SN 1987A in the Large Magellanic Cloud. From 1994 to 2001, the ejecta faded owing to radioactive decay of (44)Ti as predicted. Then the flux started to increase, more than doubling by the end of 2009. We show that this increase is the result of heat deposited by X-rays produced as the ejecta interacts with the surrounding material. In time, the X-rays will penetrate farther into the ejecta, enabling us to analyse the structure and chemistry of the vanished star.

We present optical spectroscopic data of the inner circumstellar ring around SN 1987A from the Anglo-Australian Telescope (AAT) and the Very Large Telescope (VLT) between ∼1400 and ∼5000 days post-explosion. We also assembled the available optical and near-infrared line fluxes from the literature between ∼300 and ∼2000 days. These line light curves were fitted with a photoionization model to determine the density structure and the elemental abundances for the inner ring. We found densities ranging from 1×10 3 to 3×10 4 atoms cm −3 and a total mass of the ionized gas of ∼5.8 × 10 −2 M ⊙ within the inner ring. Abundances inferred from the optical and near-infrared data were also complemented with estimates of Lundqvist & Fransson (1996) based on ultraviolet lines. This way we found an He/H-ratio (by number of atoms) of 0.17 ± 0.06 which is roughly 30% lower than previously estimated and twice the solar and the Large Magellanic Cloud (LMC) value. We found an N/O-ratio of 1.5 ± 0.7, and the total (C+N+O)/(H+He) abundance about 1.6 times its LMC value or roughly 0.6 times the most recent solar value. An iron abundance of 0.20 ± 0.11 times solar was found which is within the range of the estimates for the LMC. We also present late time (∼5000 -7500 days) line light curves of [O III], [Ne III], [Ne IV], [Ar III], [Ar IV], and [Fe VII] from observations with the VLT.We compared these with model fluxes and found that an additional 10 2 atoms cm −3 component was required to explain the data of the highest ionization lines. Such low density gas is expected in the H II-region interior to the inner ring which likely extends also to larger radii at higher latitudes (out of the ring plane). At epochs later than ∼5000 days our models underproduce the emission of most of these lines as expected due to the contribution from the interaction of the supernova ejecta with the ring. Subject headings: circumstellar matter -supernovae: individual (SN 1987A)

We present seven epochs between October 1999 and November 2007 of high resolution VLT/UVES echelle spectra of the ejecta-ring collision of SN 1987A. The fluxes of most of the narrow lines from the unshocked gas decreased by a factor of 2−3 during this period, consistent with the decay from the initial ionization by the shock break-out. However, [O III] in particular shows an increase up to day ∼6800. This agrees with radiative shock models where the pre-shocked gas is heated by the soft X-rays from the shock. The evolution of the [O III] line ratio shows a decreasing temperature of the unshocked ring gas, consistent with a transition from a hot, low density component which was heated by the initial flash ionization to the lower temperature in the pre-ionized gas ahead of the shocks. The line emission from the shocked gas increases rapidly as the shock sweeps up more gas. We find that the neutral and high ionization lines follow the evolution of the Balmer lines roughly, while the intermediate ionization lines evolve less rapidly. Up to day ∼6800, the optical light curves have a similar evolution to that of the soft X-rays. The break between day 6500 and day 7000 for [O III] and [Ne III] is likely due to recombination to lower ionization levels. Nevertheless, the evolution of the [Fe XIV] line, as well as the lines from the lowest ionization stages, continue to follow that of the soft X-rays, as expected. There is a clear difference in the line profiles between the low and intermediate ionization lines, and those from the coronal lines at the earlier epochs. This shows that these lines arise from regions with different physical conditions, with at least a fraction of the coronal lines coming from adiabatic shocks. At later epochs the line widths of the low ionization lines, however, increase and approach those of the high ionization lines of [Fe X−XIV]. The Hα line profile can be traced up to ∼500 km s −1 at the latest epoch. This is consistent with the cooling time of shocks propagating into a density of (1−4) × 10 4 cm −3 . This means that these shocks are among the highest velocity radiative shocks observed.