Spectroscopy of supernova 1987A at 1−5 µm – I. The first year

We present infrared spectra of supernova 1987A, taken 18, 110/2, 192, 255, 284 and 349 days after the explosion. The atmospheric windows from 1.05 to 4.1 µm were covered on each occasion, usually at a spectral resolution > 1000. Data to 4.75 µm are also shown for one occasion. All data are of spectrophotometric quality and allow the extraction of line intensities typically to 10 per cent accuracy except where blending, continuum placement or telluric absorption gives problems. We give plausible identifications for the majority of the emission and absorption features. We find transitions of H I (Paschen, Brackett and Pfund series), He I, C I, O I, Na I, Mg I, Si I, K I, Ca I, Ca II, Fe II, Co II, Ni I and Sr II, and possibly Al I. SN 1987A is the first supernova in which molecular emission has been seen: we find CO and CO⁺. There is also evidence of emission from CS. As early as day 18 the hydrogen profiles exhibited more flux in emission than was removed in absorption, implying that line-profile formation was not solely due to simple resonant scattering. At later times the hydrogen and helium spectra probably arose from recombination. The O I emission appears to result from Bowen fluorescence by Lyman β. The intensities of [Fe II] and [Co II] lines are explicable at an electron temperature of 3000 to 4000 K if the iron and cobalt was mostly singly ionized and resulted from the decay of the initial 0.08 $$M_\odot$$ of ⁵⁶Ni inferred from the bolometric light curve. The velocity widths and profiles of unblended lines, the presence and width of the O I emission, and the inferred electron temperature in the Fe II environment all suggest that mixing of the ejecta took place. We also discuss the observed redshift of some hundreds of km s⁻¹ seen in the emission lines. There is valuable information in the 1−5 µm spectra of SN 1987A which must be incorporated into any detailed model of the event. The data presented provide a benchmark, as well as a catalogue of line identifications, for the study of subsequent supernovae.