Infrared and optical evidence for a dust cloud behind supernova 1987A

Abstract

THE 8–13-μm flux from supernova 1987A has increased substantially¹ since around May 1988 (∼ 450 days after the explosion), and the 8–13-μm source has been resolvable¹, having a full width at half maximum of ∼1.5" in September 1988. This is attributed¹ to re-radiation by an asymmetric circumstellar dust cloud, located mainly behind the supernova at a distance of ∼200 light days. This cloud may be related to the gas shell inferred from ultraviolet lines^2,3. Here we extend the calculations of Roche et al.¹ and consider the further implications of such a cloud. We conclude the following: the expected optical echo from the cloud (V-band magnitude m_v ≲ 12) should be resolvable, and this, rather than an internal energy source (such as a pulsar), may cause the observed^4–6 slowdown in dimming of the supernova; the reflected light may contribute substantially to the current optical luminosity; observations should be made as soon as possible, for the phenomenon may be evanescent; and the dust spectrum suggests that the dust is carbonaceous rather than silicaceous, although the circumstellar region is thought² to be carbon-poor. This cloud behind the supernova may be truly interstellar.