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 × 1040 erg s−1 and their total radiated energies are on the order of (0.3–3) × 1047 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 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 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.

Gap Transients Interacting with Circumstellar Medium

“…Lying within the luminosity gap, ILRTs easily fulfill the condition of being low-energy explosions. Additionally, EC SNe should synthesize only few 10 −3 M of 56 Ni [112], which, again, is consistent with observational constraints obtained for ILRTs [98,100]. Finally, the progenitor star of an EC SN is expected to be a ∼9 M luminous super-Asymptotic giant branch (AGB) star, which, during its evolution, will develop a degenerate O-Ne-Mg core [107].…”

“…Intermediate Luminosity Red Transients (ILRTs) are a class of poorly studied gap transients that sparked debate over their origin and interpretation. While some authors invoked nonterminal outbursts of post main sequence stars to explain the observed data (e.g., [96,97]), ILRTs are among the most promising candidates for being Electron Capture Supernova (EC SN) events (see, e.g., [19,[98][99][100]). Their light curves are reminiscent of classical SNe (see the left panel of Figure 5), with a single peak and a late-time linear decline in magnitudes compatible with 56 Ni decay [98][99][100].…”

“…While some authors invoked nonterminal outbursts of post main sequence stars to explain the observed data (e.g., [96,97]), ILRTs are among the most promising candidates for being Electron Capture Supernova (EC SN) events (see, e.g., [19,[98][99][100]). Their light curves are reminiscent of classical SNe (see the left panel of Figure 5), with a single peak and a late-time linear decline in magnitudes compatible with 56 Ni decay [98][99][100]. Indeed, long-term mid-infrared monitoring displayed that two ILRTs (NGC300 2008OT-1 and SN 2008S) became fainter than their progenitors years after their peak luminosity, corroborating the scenario of a terminal explosion rather than an eruptive event [101].…”

Low luminosity Type II supernovae -- IV. SN 2020cxd and SN 2021aai, at the edges of the sub-luminous supernovae class

“…Nomoto 1984;Ritossa et al 1996;Kitaura et al 2006;Poelarends et al 2008), although there is no consensus yet on whether we already witnessed such an explosion. Given their faintness and low synthesised 56 Ni mass, the socalled Intermediate-Luminosity Red Transients (ILRTs; Botticella et al 2009;Stritzinger et al 2020;Cai et al 2021) are considered to be among the most promising candidates. The electron-capture SN scenario, however, can potentially produce transients with different observable properties.…”

Discovering vanishing objects in POSS I red images using the Virtual Observatory

“…surveys -, in the wavelength range (near UV, optical), in the magnitudes used to count the number of flares (duty cycle, flares per hour), in the parameters used for flare identificationflare variability index(Kowalski et al 2009), flare line index(Hilton et al 2010) -, or the existing correlation with spatial distribution, spectral type or age -begin more frequent close to the Galactic plane, at later spectral types and at younger ages(Hilton et al 2010) -, makes it quite difficult to give a realistic estimation of the number of flaring objects among our final list of unidentified transients.Other (although less numerous) types of stellar objects triggering large amplitude variations are, for instance, LBVs (van Genderen 2001), FUORs(Hartmann & Kenyon 1996), RCB stars(Benson et al 1994), ILRTs(Cai et al 2021), K giants(Tang et al 2010), cataclysmic objects like novae or pulsating variables like Miras(Reid & Goldston 2002), RV Tau stars(de Ruyter et al 2005) or Cepheids…”