The slow brightening of WNTR23bzdiq / WTP19aalzlk : Possible onset of common-envelope evolution in an asymptotic giant branch star?

2025/05/14 by Viraj Karambelkar, Karambelkar, Viraj, M. M. Kasliwal +15 · 1 citation
Physics and Astronomy · #Astronomy and Astrophysical Research #Astrophysics and Star Formation Studies #FOS: Physical sciences #Solar and Stellar Astrophysics (astro-ph.SR) #Stellar, planetary, and galactic studies

paper · pdf · doi:10.48550/arxiv.2505.09691

openalex publication_date 2025/05/14 · openalex created_date 2025/10/15 · openalex updated_date 2026/07/28

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 ∼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_\rmeff ≈ 3500K and L ≈1.6×10⁴L_\odot - consistent with a 7±2M_\odot star in its early-AGB phase. During the eruption, the luminosity rose to ≈5×10⁴L_\odot with slow photospheric expansion (≈5km-s^-1) and constant temperatures (≈3600K) 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.

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