Optical and infrared photometry of the Type IIn SN 1998S: days 11-146

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We present contemporaneous optical and infrared (IR) photometric observations of the Type IIn SN 1998S covering the period between 11 and 146 d after discovery. The IR data constitute the first ever IR light curves of a Type IIn supernova. We use blackbody and spline fits to the photometry to examine the luminosity evolution. During the first 2-3 months, the luminosity is dominated by the release of shock-deposited energy in the ejecta. After ∼ 100 d the luminosity is powered mostly by the deposition of radioactive decay energy from 0.15 ± 0.05 M⊙ of 56Ni which was produced in the explosion. We also report the discovery of an astonishingly high IR excess, K - L′ = 2.5, that was present at day 130. We interpret this as being due to thermal emission from dust grains in the vicinity of the supernova. We argue that to produce such a high IR luminosity so soon after the explosion, the dust must be pre-existing and so is located in the circumstellar medium of the progenitor. The dust could be heated either by the UV/optical flash (IR echo) or by the X-rays from the interaction of the ejecta with the circumstellar material.