Volume 653, September 2021
|Number of page(s)||21|
|Section||Stellar structure and evolution|
|Published online||23 September 2021|
The luminous red nova AT 2018bwo in NGC 45 and its binary yellow supergiant progenitor⋆
Department of Astrophysics/IMAPP, Radboud University, Nijmegen, The Netherlands
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
Accepted: 16 June 2021
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 ∼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 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(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.
Key words: binaries: general / novae, cataclysmic variables / stars: individual: AT 2018bwo / stars: winds, outflows / stars: evolution / stars: flare
Full Table 1 is only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (220.127.116.11) or via http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/653/A134
© ESO 2021
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