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 shortly before its merger. Here we present the results of our photometric and spectroscopic follow-up campaign of AT 2018bwo (DLT 18x), a LRN discovered in NGC 45, and investigate its progenitor system using binary stellar-evolution models. The transient reached a peak magnitude of Mr =-10.97 ± 0.11 and maintained this brightness during its optical plateau of tp = 41 ± 5 days. During this phase, it showed a rather stable photospheric temperature of ∼3300K and a luminosity of ∼1040 erg s-1. Although the luminosity and duration of AT 2018bwo is comparable to the LRNe V838 Mon and M31-2015LRN, its photosphere at early times appears larger and cooler, likely due to an extended mass-loss episode before the merger. Toward the end of the plateau, optical spectra showed a reddened continuum with strong molecular absorption bands. The IR spectrum at +103 days after discovery was comparable to that of a M8.5 II type star, analogous to an extended AGB star. The reprocessed emission by the cooling dust was also detected in the midinfrared bands ∼1.5 years after the outburst. Archival Spitzer and Hubble Space Telescope data taken 10-14 yrs before the transient event suggest a progenitor star with Tprog ∼ 6500 K, Rprog ∼ 100 R, and Lprog = 2 × 104 L, and an upper limit for optically thin warm (1000 K) dust mass of Md < 10-6 M. Using stellar binary-evolution models, we determined the properties of binary systems consistent with the progenitor parameter space. For AT 2018bwo, we infer a primary mass of 12-16 M, which is 9-45% larger than the ∼11 M obtained using single-star evolution models. The system, consistent with a yellow-supergiant primary, was likely in a stable mass-transfer regime with-2.4 ≤ log(.M=M yr-1) ≤-1.2 a decade before the main instability occurred. During the dynamical merger, the system would have ejected 0.15-0.5 M with a velocity of ∼500 km s-1.