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

Nadejda Blagorodnova; Jakub Klencki; Ondrej Pejcha; Paul M. Vreeswijk; Howard E. Bond; Kevin B. Burdge; Kishalay De; Christoffer Fremling; Robert D. Gehrz; Jacob E. Jencson; Mansi M. Kasliwal; Thomas Kupfer; Ryan M. Lau; Frank J. Masci; R. Michael Rich

doi:10.1051/0004-6361/202140525

paper

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

arXiv:2102.05662 · doi:10.1051/0004-6361/202140525

Abstract

Luminous Red Novae (LRNe) are astrophysical transients associated with the partial ejection of a binary system's common envelope (CE) shortly before its merger. Here we present the results of our photometric and spectroscopic follow-up campaign of AT2018bwo (DLT18x), a LRN discovered in NGC45, and investigate its progenitor system using binary stellar-evolution models. The transient reached a peak magnitude of $M_{r} = - 10.97 \pm 0.11$ and maintained this brightness during its optical plateau of $t_{p} = 41 \pm 5$ days. During this phase, it showed a rather stable photospheric temperature of ~3300K and a luminosity of ~ $1 0^{40}$ erg/s. The photosphere of AT2018bwo at early times appeared larger and cooler than other similar LRNe, likely due to an extended mass-loss episode before the merger. Towards the end of the plateau, optical spectra showed a reddened continuum with strong molecular absorption bands. 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 years before the transient event suggest a progenitor star with $T_{p r o g} \sim 6500$ K, $R_{p r o g} \sim 100 R_{⊙}$ and $L_{p r o g} \sim 2 \times 1 0^{4} L_{⊙}$ , and an upper limit for optically thin warm (1000 K) dust mass of $M_{d} < 1 0^{- 6} M_{⊙}$ . Using stellar binary-evolution models, we determined the properties of binary systems consistent with the progenitor parameter space. For AT2018bwo, 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 ( $M ˙ / M_{⊙}$ /yr)<-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.

arXiv comment21 pages, 16 figures, accepted version for A&A

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

Abstract

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