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

Abstract

A B S T R A C TWe 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 0X150X05 M ( of 56 Ni which was produced in the explosion. We also report the discovery of an astonishingly high IR excess, K 2 L H 2X5Y 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 preexisting 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.

On the source of the late-time infrared luminosity of SN 1998S and other Type II supernovae

“…No metals were seen at such high velocities. Fassia et al (2000) concluded that the IR excess at this epoch cannot, therefore, have been due to grain condensation in the ejecta. It must instead have been produced by an IR echo of the maximum-light luminosity from pre-existing dust in the CSM.…”

“…A pre-discovery limiting magnitude of ∼+18 obtained on 1998 February 23.7 (Leonard et al 2000) indicates that SN 1998S was discovered within a few days of shock breakout. Following Chugai (2001), we adopt 1998 February 24.7 UT = JD 2 450 869.2, 6 d prior to discovery, as the explosion epoch, t = 0 d. Relative to this epoch, the zero epochs adopted by other authors are later by, respectively, +5 d (Leonard et al 2000), +6 d (Fassia et al 2000(Fassia et al , 2001Pooley et al 2002) and ∼+23 d (Bowen et al 2000;Gerardy et al 2000Gerardy et al , 2002a. Throughout this paper, epochs quoted from other authors have been shifted to our zero epoch definition.…”

“…This revealed a remarkable IR excess of K -L = +2.5. To produce the observed flux the lowest possible radius of the IR emission region is given by a blackbody with a temperature close to the dust evaporation temperature of ∼1500 K. For dust condensing in the ejecta to attain a sufficiently large blackbody radius by 136 d would require an expansion velocity of 11 000 km s −1 (see Fassia et al 2000). Such high velocities were seen only in the extreme outer zones of the H/He envelope.…”

“…This is around ×10 the typical luminosity of a Type II SN. The excellent early-time coverage achieved in the optical and NIR allowed Fassia et al (2000) to examine the bolometric light curve. Both blackbody and UVOIR (i.e ultraviolet-optical-infrared range) fits indicate that the total energy radiated in the first 40 d exceeded 10 50 erg, which is again ×10 the typical value for Type II SNe.…”

DUST AND THE TYPE II-PLATEAU SUPERNOVA 2004dj

“…Wooden (1997) reiterates that the cause was CSM dust "echoing the light curve." Fassia et al (2000) reported a strong K − L excess in the emission from the Type IIn SN 1998S at 130 days. They attributed this to pre-existing CSM dust heated either by the SN luminosity (a conventional IR echo) or by X-rays from the CSM-shock interaction.…”