Kavli Affiliate: Robert A. Simcoe
| First 5 Authors: Viraj Karambelkar, Mansi Kasliwal, Kishalay De, Danielle Frostig, Robert Stein
| Summary:
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 $approx1.6times10^{4}$L$_{odot}$ – consistent with a
7$pm$2M$_{odot}$ star in its early-AGB phase. During the eruption, the
luminosity rose to $approx5times10^{4}$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.
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