The luminous red nova AT 2018bwo in NGC 45 and its binary yellow supergiant progenitor

Luminous Red Novae (LRNe) are astrophysical transients associated with the partial ejection of a binary system's common envelope (CE) shortly before its merger. Here we present the results of our photometric and spectroscopic follow-up campaign of AT2018bwo (DLT18x), a LRN discovered in NGC45, and investigate its progenitor system using binary stellar-evolution models. The transient reached a peak magnitude of \(M_r=-10.97\pm0.11\) and maintained this brightness during its optical plateau of \(t_p = 41\pm5\)days. During this phase, it showed a rather stable photospheric temperature of ~3300K and a luminosity of ~\(10^{40}\)erg/s. The photosphere of AT2018bwo at early times appeared larger and cooler than other similar LRNe, likely due to an extended mass-loss episode before the merger. Towards the end of the plateau, optical spectra showed a reddened continuum with strong molecular absorption bands. The reprocessed emission by the cooling dust was also detected in the mid-infrared bands ~1.5 years after the outburst. Archival Spitzer and Hubble Space Telescope data taken 10-14 years before the transient event suggest a progenitor star with \(T_{prog}\sim 6500\)K, \(R_{prog}\sim 100R_{\odot}\) and \(L_{prog}\sim 2\times10^4L_{\odot}\), and an upper limit for optically thin warm (1000 K) dust mass of \(M_d