Spectroscopy of SN 1987A at 0.9-2.4 microns: Days 1348-3158

Abstract

We present near-infrared spectroscopic observations of SN 1987A covering the period 1358 to 3158 days post-explosion. This is the first time that IR spectra of a supernova have been obtained to such late epochs. The spectra comprise emission from both the ejecta and the bright, ring-shaped circumstellar medium (CSM). The most prominent CSM emission lines are recombination lines of H I and He I, and forbidden lines of [S III] and [Fe II]. The ejecta spectra include allowed lines of H I, He I and Na I and forbidden lines of [Si I], [Fe I], [Fe II], and possibly [S I]. The intensity ratios and widths of the H I ejecta lines are consistent with a low-temperature Case B recombination spectrum arising from non-thermal ionisation/excitation in an extended, adiabatically cooled H-envelope, as predicted by several authors. The slow decline of the ejecta forbidden lines, especially those of [Si I], indicates that pure non-thermal excitation was taking place, driven increasingly by the decay of 44Ti. The ejecta iron exhibits particularly high velocities (4000-4500 km/s), supporting scenarios where fast radioactive nickel is created and ejected just after the core-bounce. In addition, the ejecta lines continue to exhibit blueshifts with values about -200 km/s to -800 km/s to at least day 2000. These blueshifts, which first appeared around day 600, probably indicate that very dense concentrations of dust persist in the ejecta, although an alternative explanation of asymmetry in the excitation conditions is not ruled out.