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 (WINTER) near-infrared and the NEOWISE mid-infrared surveyors. This source brightened gradually over seven years: a 0.5-mag optical rise (2018-2021), a 1-mag optical outburst lasting $\sim$1000 days (2021-2023), and another 1-mag optical rebrightening in 2024. This was accompanied by a steady mid-IR brightening of 1-mag over ten years in NEOWISE data. Archival optical data show only erratic, small amplitude ($<0.3$\,mag) brightness variations from 2003 to 2015, revealing a progenitor star with T$_{\rm{eff}} \approx 3500$K and L $\approx1.6\times10^{4}$L$_{\odot}$ - consistent with a 7$\pm$2M$_{\odot}$ star in its early-AGB phase. During the eruption, the luminosity rose to $\approx5\times10^{4}$L$_{\odot}$ with slow photospheric expansion ($\approx5$km-s$^{-1}$) and constant temperatures ($\approx3600$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 mid-IR observations with JWST to quantify dust production, will shed further light on WNTR23bzdiq.