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 M-r = -10.97 +/- 0.11 and maintained this brightness during its optical plateau of t(p) = 41 +/- 5 days. During this phase, it showed a rather stable photospheric temperature of similar to 3300 K and a luminosity of similar to 10(40) 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 mid-infrared bands similar to 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 T-prog similar to 6500 K, R-prog similar to 100 R-circle dot, and L-prog = 2 x 10(4) L-circle dot, and an upper limit for optically thin warm (1000 K) dust mass of M-d < 10(-6) M-circle dot. 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-circle dot, which is 9-45% larger than the similar to 11 M-circle dot 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)over dot/M-circle dot 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-circle dot with a velocity of similar to 500 km s(-1).