The Slow Brightening of WNTR23bzdiq/WTP19aalzlk: Possible Onset of Common Envelope Evolution in an Asymptotic Giant Branch Star?

We present WNTR23bzdiq/WTP19aalzlk, a slow eruption of an early-asymptotic giant branch (AGB) star in M31 identified by the Wide-field Infrared Transient Explorer near-infrared (NIR) and the NEOWISE mid-infrared (MIR) surveyors. This source brightened gradually over 7 yr: a 0.5 mag optical rise (2018–2021), a 1 mag optical outburst lasting ∼1000 days (2021–2023), and another 1 mag optical rebrightening in 2024. This was accompanied by a steady MIR brightening of 1 mag over 10 yr in NEOWISE data. Archival optical data show only erratic, small-amplitude (<0.3 mag) brightness variations from 2003 to 2015, revealing a progenitor star with T_eff ≈ 3500 K and L ≈ 1.6 × 10⁴ L_⊙—consistent with a 7 ± 2 M_⊙ star in its early-AGB phase. During the eruption, the luminosity rose to ≈5 × 10⁴ L_⊙with slow photospheric expansion (≈5 km s⁻¹) and constant temperatures (≈3600 K) inferred from the spectral energy distribution. Optical and NIR spectra of the eruption resemble late M-type stars, with a mixed-temperature behavior—transitioning from M1 in the optical to M7/M8 in the NIR. These properties of WNTR23bzdiq resemble those of stellar merger transients, particularly the giant star merger OGLE-2002-BLG-360, but on longer timescales. As such, WNTR23bzdiq potentially marks the onset of common envelope evolution (CEE) in a binary with an AGB primary, and is possibly a member of the emerging population of infrared transients from CEE in giant stars. Continued multiwavelength monitoring, particularly MIR observations with JWST to quantify dust production, will shed further light on WNTR23bzdiq.

V.K. acknowledges Yashvi Sharma for assistance with the machine learning interpolator, and Jacob Jencson for helping with the GRAMS modeling. WINTER’s construction is made possible by the National Science Foundation under MRI grant No. AST-1828470 with early operations supported by AST-2206730. Significant support for WINTER also comes from the California Institute of Technology, the Caltech Optical Observatories, the Bruno Rossi Fund of the MIT Kavli Institute for Astrophysics and Space Research, the David and Lucille Packard Foundation, and the MIT Department of Physics and School of Science. This work was supported by a NASA Keck PI Data Award, administered by the NASA Exoplanet Science Institute. We acknowledge the support of the National Aeronautics and Space Administration through ADAP grant No. 80NSSC24K0663. This research has made use of the Keck Observatory Archive (KOA), which is operated by the W. M. Keck Observatory and the NASA Exoplanet Science Institute (NExScI), under contract with the National Aeronautics and Space Administration. N.B. acknowledges being funded by the European Union (ERC, CET-3PO, 101042610). Views and opinions expressed are, however, those of the author(s) only and do not necessarily reflect those of the European Union or the European Research Council Executive Agency. Neither the European Union nor the granting authority can be held responsible for them. Some of the data presented herein were obtained at Keck Observatory, which is a private 501(c)(3) nonprofit organization operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Maunakea has always had within the Native Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain.