Infrared Emission from SN 1987A: Light Echoes or Dust Formation?

Abstract

The presence of an infrared (IR) continuum in the spectrum of SN 1987A was discovered in the early stages of its evolution by various photometric and spectral observations in the 5 — 20 µm wavelength regime [1,2]. This IR continuum component represented an IR excess over that expected from a Planck spectrum fitted to the SN emission at optical wavelengths. Subsequent observations with the Kuiper Airborne Observatory (KAO) showed this continuum to be quite flat out to 100 μm with a value of ≈ 5 – 10 Jy around day 265 after core collapse [3,4]. AITKEN et al. [5], were first to interpret this IR excess as free-free emission from the expanding ionized gas. As expected from an expanding cooling gas, the emission from this component has been continuously declining since its first discovery, its 10 μm emissivity reaching a minimum of about 1 Jy around day 400 [6]. Subsequent observations by ROCHE et al. [6] revealed an upturn in the 10 µm continuum light curve of the SN, indicating either the delayed arrival of thermal emission from pre-existing circumstellar/interstellar dust heated by the UV-visual output of the SN (i.e. an IR echo), or the onset of dust formation in the SN ejecta.