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

Abstract 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 × 10 4 L ⊙ —consistent with a 7 ± 2 M ⊙ star in its early-AGB phase. During the eruption, the luminosity rose to ≈5 × 10 4 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.