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

<jats:p>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 <jats:italic>M</jats:italic><jats:sub><jats:italic>r</jats:italic></jats:sub> = −10.97 ± 0.11 and maintained this brightness during its optical plateau of <jats:italic>t</jats:italic><jats:sub><jats:italic>p</jats:italic></jats:sub> = 41 ± 5 days. During this phase, it showed a rather stable photospheric temperature of ∼3300 K and a luminosity of ∼10<jats:sup>40</jats:sup> erg s<jats:sup>−1</jats:sup>. 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 ∼1.5 years after the outburst. Archival <jats:italic>Spitzer</jats:italic> and <jats:italic>Hubble</jats:italic> Space Telescope data taken 10−14 yrs before the transient event suggest a progenitor star with <jats:italic>T</jats:italic><jats:sub>prog</jats:sub> ∼ 6500 K, <jats:italic>R</jats:italic><jats:sub>prog</jats:sub> ∼ 100 <jats:italic>R</jats:italic><jats:sub>⊙</jats:sub>, and <jats:italic>L</jats:italic><jats:sub>prog</jats:sub> = 2 × 10<jats:sup>4</jats:sup> <jats:italic>L</jats:italic><jats:sub>⊙</jats:sub>, and an upper limit for optically thin warm (1000 K) dust mass of <jats:italic>M</jats:italic><jats:sub><jats:italic>d</jats:italic></jats:sub> < 10<jats:sup>−6</jats:sup> <jats:italic>M</jats:italic><jats:sub>⊙</jats:sub>. 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 <jats:italic>M</jats:italic><jats:sub>⊙</jats:sub>, which is 9–45% larger than the ∼11 <jats:italic>M</jats:italic><jats:sub>⊙</jats:sub> 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(<jats:italic>Ṁ</jats:italic>/<jats:italic>M</jats:italic><jats:sub>⊙</jats:sub> yr<jats:sup>−1</jats:sup>) ≤ −1.2 a decade before the main instability occurred. During the dynamical merger, the system would have ejected 0.15–0.5 <jats:italic>M</jats:italic><jats:sub>⊙</jats:sub> with a velocity of ∼500 km s<jats:sup>−1</jats:sup>.</jats:p>