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

¹ Department of Astrophysics/IMAPP, Radboud University, Nijmegen, The Netherlands
e-mail: n.blagorodnova@astro.ru.nl
² Institute of Theoretical Physics, Faculty of Mathematics and Physics, Charles University, V Holeşoviçkách 2, 180 00 Praha 8, Czech Republic
³ Department of Astronomy and Astrophysics, Pennsylvania State University, 16802 University Park, PA, USA
⁴ Space Telescope Science Institute, 3700 San Martin Drive, 21218 Baltimore, MD, USA
⁵ Cahill Center for Astrophysics, California Institute of Technology, 1200 E. California Blvd., 91125 Pasadena, CA, USA
⁶ Minnesota Institute for Astrophysics, School of Physics and Astronomy, University of Minnesota, 116 Church Street SE, 55455 Minneapolis, MN, USA
⁷ Steward Observatory, University of Arizona, 933 North Cherry Avenue, 85721-0065 Tucson, AZ, USA
⁸ Department of Physics and Astronomy, Texas Tech University, PO Box 41051, 79409 Lubbock, TX, USA
⁹ Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, 3-1-1 Yoshinodai, 252-5210 Chuo-ku, Sagamihara, Kanagawa, Japan
¹⁰ Caltech/IPAC, Mailcode 100-22, 91125 Pasadena, CA, USA
¹¹ Department of Physics and Astronomy, The University of California, 90095 Los Angeles, CA, USA

Received: 10 February 2021
Accepted: 16 June 2021

Abstract

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 ∼3300 K 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 mid-infrared 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 T_prog ∼ 6500 K, R_prog ∼ 100 R_⊙, and L_prog = 2 × 10⁴ L_⊙, and an upper limit for optically thin warm (1000 K) dust mass of M_d < 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(Ṁ/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.