2021A&A...654A.157C

We present the spectroscopic and photometric study of five intermediate-luminosity red transients (ILRTs), namely AT 2010dn, AT 2012jc, AT 2013la, AT 2013lb, and AT 2018aes. They share common observational properties and belong to a family of objects similar to the prototypical ILRT SN 2008S. These events have a rise time that is less than 15 days and absolute peak magnitudes of between -11.5 and -14.5 mag. Their pseudo-bolometric light curves peak in the range 0.5-9.0 x 10⁴⁰ erg s^–1 and their total radiated energies are on the order of (0.3-3) x 10⁴⁷ erg. After maximum brightness, the light curves show a monotonic decline or a plateau, resembling those of faint supernovaeIIL orIIP, respectively. At late phases, the light curves flatten, roughly following the slope of the ⁵⁶Co decay. If the late-time power source is indeed radioactive decay, these transients produce ⁵⁶Ni masses on the order of 10^–4 to 10^–3 M_☉. The spectral energy distribution of our ILRT sample, extending from the optical to the mid-infrared (MIR) domain, reveals a clear IR excess soon after explosion and non-negligible MIR emission at very late phases. The spectra show prominent H lines in emission with a typical velocity of a few hundred km s^–1, along with CaII features. In particular, the [CaII] λ7291,7324 doublet is visible at all times, which is a characteristic feature for this family of transients. The identified progenitor of SN 2008S, which is luminous in archival Spitzer MIR images, suggests an intermediate-mass precursor star embedded in a dusty cocoon. We propose the explosion of a super-asymptotic giant branch star forming an electron-capture supernova as a plausible explanation for these events.