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

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 the archival 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 (Mr = -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 H$\alpha$ 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.