A<i>SPITZER</i>SURVEY FOR DUST IN TYPE IIn SUPERNOVAE

Recent observations suggest that Type IIn supernovae (SNe IIn) may exhibit\nlate-time (&gt;100 days) infrared (IR) emission from warm dust more than other\ntypes of core-collapse SNe. Mid-IR observations, which span the peak of the\nthermal spectral energy distribution, provide useful constraints on the\nproperties of the dust and, ultimately, the circumstellar environment,\nexplosion mechanism, and progenitor system. Due to the low SN IIn rate (&lt;10% of\nall core-collapse SNe), few IR observations exist for this subclass. The\nhandful of isolated studies, however, show late-time IR emission from warm dust\nthat, in some cases, extends for five or six years post-discovery. While\nprevious Spitzer/IRAC surveys have searched for dust in SNe, none have targeted\nthe Type IIn subclass. This article presents results from a warm Spitzer/IRAC\nsurvey of the positions of all 68 known SNe IIn within a distance of 250 Mpc\nbetween 1999 and 2008 that have remained unobserved by Spitzer more than 100\ndays post-discovery. The detection of late-time emission from ten targets\n(~15%) nearly doubles the database of existing mid-IR observations of SNe IIn.\nAlthough optical spectra show evidence for new dust formation in some cases,\nthe data show that in most cases the likely origin of the mid-IR emission is\npre-existing dust, which is continuously heated by optical emission generated\nby ongoing circumstellar interaction between the forward shock and\ncircumstellar medium. Furthermore, an emerging trend suggests that these SNe\ndecline at ~1000--2000 days post-discovery once the forward shock overruns the\ndust shell. The mass-loss rates associated with these dust shells are\nconsistent with luminous blue variable (LBV) progenitors.\n