Extraordinary late-time infrared emission of type IIn supernovae

Near-infrared observations are presented for five Type IIn supernovae (SN 1995N, SN 1997ab, SN 1998S, SN 1999Z, and SN 1999el) that exhibit strong infrared excesses at late times (tgreater than or similar to100 days). H- and K-band emission from these objects is dominated by a continuum that rises toward longer wavelengths. The data are interpreted as thermal emission from dust, probably situated in a preexisting circumstellar nebula. The IR luminosities implied by single-temperature blackbody fits are quite large, greater than or similar to10(41)-10(42) ergs s(-1), and the emission evolves slowly, lasting for years after maximum light. For SN 1995N, the integrated energy release via IR dust emission was approximate to0.5-1 x 10(50) ergs. A number of dust heating scenarios are considered, the most likely being an infrared echo powered by X-ray and UV emissions from the shock interaction with a dense circumstellar medium.