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).

Automated summary

This study investigates the properties of the luminous red nova AT 2018bwo discovered in NGC 45, including its photometric and spectroscopic characteristics, as well as the parameters of its progenitor system, demonstrating its association with the common envelope of a binary system. The stable performance during the optical plateau phase suggests a prolonged mass-loss event may have occurred. By using stellar binary-evolution models, the primary star mass and dynamical information of the system were inferred.