SN 2007od: A TYPE IIP SUPERNOVA WITH CIRCUMSTELLAR INTERACTION

SN 2007od exhibits characteristics that have rarely been seen in a Type IIP supernova (SN). Optical V-band photometry reveals a very steep brightness decline between the plateau and nebular phases of ∼4.5 mag, likely due to SN 2007od containing a low mass of 56Ni. The optical spectra show an evolution from normal Type IIP with broad Hα emission, to a complex, four-component Hα emission profile exhibiting asymmetries caused by dust extinction after day 232. This is similar to the spectral evolution of the Type IIn SN 1998S, although no early-time narrow (∼200 km s−1) Hα component was present in SN 2007od. In both SNe, the intermediate-width Hα emission components are thought to arise in the interaction between the ejecta and its circumstellar medium (CSM). SN 2007od also shows a mid-infrared excess due to new dust. The evolution of the Hα profile and the presence of the mid-IR excess provide strong evidence that SN 2007od formed new dust before day 232. Late-time observations reveal a flattening of the visible light curve. This flattening is a strong indication of the presence of a light echo, which likely accounts for much of the broad, underlying Hα component seen at late times. We believe that the multi-peaked Hα emission is consistent with the interaction of the ejecta with a circumstellar ring or torus (for the inner components at ±1500 km s−1) and a single blob or cloud of circumstellar material out of the plane of the CSM ring (for the outer component at −5000 km s−1). The most probable location for the formation of new dust is in the cool dense shell created by the interaction between the expanding ejecta and its CSM. Monte Carlo radiative transfer modeling of the dust emission from SN 2007od implies that up to ∼4 × 10−4 M☉ of new dust has formed. This is similar to the amounts of dust formed in other core-collapse supernovae such as SNe 1999em, 2004et, and 2006jc.