Hot Springs and Dust Reservoirs: JWST Reveals the Dusty, Molecular Aftermath of Extragalactic Stellar Mergers

We present James Webb Space Telescope (JWST) observations of four luminous red novae (LRNe): dusty, extragalactic transients from stellar mergers following common-envelope evolution (CEE) in massive binary stars. Our targets—AT 2021blu, AT 2021biy, AT 2018bwo, and M31-LRN-2015—span a broad range in progenitor primary masses (≈3–24 M_⊙) and post-merger ages (≈1100–3700 days). All four were observed with the Mid-Infrared Instrument from 5 to 25 μm; AT 2021blu and AT 2021biy additionally have 5–12 μm spectra from the Low-Resolution Spectrometer. These spectra show strong features of oxygen-rich molecules, including water vapor, supporting the recent association of water fountain sources with CEE. Radiative transfer modeling of the spectral energy distributions yields dust masses of ≈4.2 × 10⁻⁵, 3 × 10⁻⁴, 7.5 × 10⁻⁵, and 7.7 × 10⁻⁴ M_⊙, respectively—corresponding to ≈10%, 60%, 6%, and 12% of median dust masses in core-collapse supernovae (CCSNe) at similar phases. Accounting for their occurrence rates, we estimate that LRNe can contribute ∼25% as much dust as CCSNe to the cosmic dust budget. Furthermore, the lower expansion velocities of LRNe may reduce dust destruction by reverse shocks compared to CCSNe, potentially increasing this contribution. In addition to dust masses, we use our JWST observations to measure late-time properties such as the luminosities, temperatures, radii, and dust-to-gas ratios of the merger remnants. Our results highlight the need for broader infrared studies of LRNe to quantify their contribution to the cosmic dust budget, study the evolution of oxygen-rich molecules, and probe the final fates of CEE.