Intermediate-luminosity red transients: Spectrophotometric properties and connection to electron-capture supernova explosions

Abstract

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 × 10<SUP>40</SUP> erg s<SUP>−1</SUP> and their total radiated energies are on the order of (0.3-3) × 10<SUP>47</SUP> erg. After maximum brightness, the light curves show a monotonic decline or a plateau, resembling those of faint supernovae IIL or IIP, respectively. At late phases, the light curves flatten, roughly following the slope of the <SUP>56</SUP>Co decay. If the late-time power source is indeed radioactive decay, these transients produce <SUP>56</SUP>Ni masses on the order of 10<SUP>−4</SUP> to 10<SUP>−3</SUP> M<SUB>⊙</SUB>. 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<SUP>−1</SUP>, along with Ca II features. In particular, the [Ca II] 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. <P />Lightcurves and spectra are only available at the CDS via anonymous ftp to <A href="http://cdsarc.u-strasbg.fr/">cdsarc.u-strasbg.fr</A> (ftp://130.79.128.5) or via <A href="http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/654/A157">http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/654/A157</A>...