Padova-Asiago Supernova Group
Intermediate-Luminosity Red Transients (ILRTs) or SN 2008S-like events

These transients have light curves similar to those of subluminous Type II-L SNe (e.g. SN 2008S and NGC300-2008OT1) or Type II-P SNe (e.g. M85-2006OT1 and PTF10fqs), and – when observed – the late time decline is consistent with that of the 56Co decay. Their spectra are initially quite blue, but become redder with time. They show prominent H lines in emission (similar to those of H-rich transients) and Ca II lines, with the typical [Ca II] 7291, 7323 Å (Fig. 1). This doublet, which is rarely observed in SN impostors or other ILOTs, is always detected in ILRTs. In quiescence, the moderately massive progenitors of ILRTs (8-12 solar masses), are usually detected in the mid-IR domain, as they are enshrouded by dusty cocoons (Fig. 2).

Fig. 1 Spectra of three ILRTs. The blow-up to the left shows the region of the [Ca II] 7291, 7323 A doublet (Figure from Pastorello 2012, MSAIS, 19, 24).

ILRTs been proposed to be electron-capture SNe from Super-Asymptotic Giant Branch (S-AGB) stars (e.g. Botticella et al. 2009, MNRAS, 398, 1041; Pumo et al. 2009, ApJ, 705, L138; Pastorello et al. 2007, Nature, 449,1; Thompson et al. 2009, ApJ, 705, 1364 ). Although this interpretation is disputed (e.g. Smith et al. 2011, MNRAS, 415, 773), growing observational arguments support the terminal SN explosion scenario for ILRTs (Adams et al. 2016, MNRAS, 460, 1645).

Fig. 2 Archive imaging of ILRT NGC300-2008OT1 region. The progenitor star, embedded in a dusty environment, is not detected in the HTS/ACS optical images (left), but is clearly visible in the mid-infrared domain (right). Credits: Berger et al. 2009, ApJ, 699, 1850.

Leading researchers: M. T. Botticella, Y. Cai, A. Pastorello

Collaborators: M. L. Pumo, L. Zampieri, R. Kotak, M. Fraser, M. Stritzinger, G. Pignata, and the Padova SN group.