The transitional gap transient AT 2018hso: new insights into the luminous red nova phenomenon

Abstract

Context. Luminous red novae (LRNe) have absolute magnitudes intermediate between novae and supernovae, and show a relatively homogeneous spectro-photometric evolution. Although they were thought to derive from core instabilities in single stars, there is growing support to the idea that they are triggered by binary interaction, possibly ending with the merging of the two stars. Aims. AT 2018hso is a new transient showing transitional properties between those of LRNe and the class of intermediate luminosity red transients (ILRTs) similar to SN 2008S. Through the detailed analysis of the observed parameters, our study support that it actually belongs to the LRN class, and was likely produced by the coalescence of two massive stars. Methods. We obtained ten months of optical and near infrared photometric monitoring, and eleven epochs of low-resolution optical spectroscopy of AT 2018hso. We compared its observed properties with those of other ILRTs and LRNe. We also inspected archive Hubble Space Telescope (HST) images obtained about 15 years ago to constrain the progenitor's properties. Results. The light curves of AT 2018hso show a first sharp peak (M r = −13.93 mag), followed by a broader and shallower second peak, that resembles a plateau in the optical bands. The spectra dramatically change with time. Early time spectra show prominent Balmer emission lines and a weak [Ca ii] doublet, which is usually observed in ILRTs. However, the major decrease in the continuum temperature, the appearance of narrow metal absorption lines, the major change in the Hα strength and profile, and the emergence of molecular bands support a LRN classification. The possible detection of an I ∼ −8 mag source at the position of AT 2018hso in HST archive images is consistent with expectations for a pre-merger massive binary, similar to the precursor of the 2015 LRN in M101. Conclusions. We provide reasonable arguments to support a LRN classification for AT 2018hso. This study reveals growing heterogeneity in the observables of LRNe than thought in the past, making sometimes tricky the discrimination between LRNe and ILRTs. This suggests the need of monitoring the entire evolution of gap transients to avoid misclassifications.

The Carnegie Supernova Project II

“…3we plot the intrinsic optical colors of AT 2014ej compared to those of knownLRNe candidates: SN 1997bs (Van Dyk et al 2000Pastorello et al 2019a), NGC 3437-2011-OT1 (Pastorello et al 2019a), Intrinsic broadband color evolution of AT 2014ej compared to other well-observed LRNe, including SN 1997bs (Van Dyk et al 2000), NGC 3437-2011-OT1 (Pastorello et al 2019a), NGC 4490-2011-OT1 (Pastorello et al 2019a), UGC 12307-2013-OT1 (Pastorello et al 2019a), M101-2015-OT1 (Blagorodnova et al 2017), SNhunt248 (Kankare et al 2015), AT 2017jfs(Pastorello et al 2019b), and AT 2018hso(Cai et al 2019). The colors plotted were corrected for both Milky Way and host-galaxy reddening using the values summarized inTable 10.NGC 4490-2011-OT1(Pastorello et al 2019a), UGC 12307-2013-OT1(Pastorello et al 2019a), M101-2015-OT1(Blagorodnova et al 2017), SNhunt248(Kankare et al 2015), AT 2017jfs(Pastorello et al 2019b), and AT 2018hso(Cai et al 2019). The comparison objects have been corrected for reddening, adopting E(B − V) tot color excess values listed in…”

“…Pseudo-bolometric light curves of a handful of well-observed LRNe fromTylenda (2005),Tylenda et al (2011),Williams et al (2015),Cai et al (2019), andPastorello et al (2019a), plotted along with that of AT 2014ej. The solid red line extended beyond the UVOIR light curve of AT 2014ej corresponds to CHASE unfiltered photometry.…”

The Zwicky Transient Facility Bright Transient Survey. II. A Public Statistical Sample for Exploring Supernova Demographics*

“…One is classified as a luminous red nova (LRN), a class of event generally interpreted as stellar mergers (Kulkarni et al 2007;Pejcha et al 2016;Blagorodnova et al 2020). Three are classified as intermediate-luminosity red transients (ILRTs), a broadly defined observational class sometimes attributed to electron-capture SNe (Botticella et al 2009;Thompson et al 2009;Moriya et al 2014), although this remains controversial; the distinction between these events and LRNs is not always obvious (Cai et al 2019), and a variety of other models exist (Pastorello et al 2007;Berger et al 2009;Bond et al 2009;Tsuna et al 2020). The lowest-luminosity non-nova transient in the BTS sample, ILRT AT2019abn, is discussed in detail by Jencson et al (2019).…”

Supernovae and transients with circumstellar interaction

“…Light curves for SN 2008S-like transients[95][96][97][98]. The black line shows the expected decline rate from the decay of56 Co.3Although some objects continue to blur the apparent boundaries between ILRTs and other classes[99,100].…”

“… 3 Although some objects continue to blur the apparent boundaries between ILRTs and other classes [ 99 , 100 ]. …”