摘要：Context.The absolute magnitudes of luminous red novae (LRNe) are intermediate between those of novae and supernovae (SNe), and show a relatively homogeneous spectro-photometric evolution. Although they were thought to derive from core instabilities in single stars, there is growing support for the idea that they are triggered by binary interaction that possibly ends with the merging of the two stars.Aims.AT 2018hsois a new transient showing transitional properties between those of LRNe and the class of intermediate-luminosity red transients (ILRTs) similar toSN 2008S. Through the detailed analysis of the observed parameters, our study supports 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 11 epochs of low-resolution optical spectroscopy ofAT 2018hso. We compared its observed properties with those of other ILRTs and LRNe. We also inspected the archivalHubbleSpace Telescope (HST) images obtained about 15 years ago to constrain the progenitor properties.Results.The light curves ofAT 2018hsoshow a first sharp peak (reddening-correctedMr = −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 strong decrease in the continuum temperature, the appearance of narrow metal absorption lines, the great change in the Hαstrength and profile, and the emergence of molecular bands support an LRN classification. The possible detection of aMI ∼ −8 mag source at the position ofAT 2018hsoin 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 an LRN classification forAT 2018hso. This study reveals growing heterogeneity in the observables of LRNe than has been thought previously, which is a challenge for distinguishing between LRNe and ILRTs. This suggests that the entire evolution of gap transients needs to be monitored to avoid misclassifications.
关键词：enbinaries: closestars: windsoutflowsstars: massivesupernovae: individual: AT 2018hsosupernovae: individual: AT 2017jfssupernovae: individual: NGC4490-2011OT1
