Late-Time Evolution of the Massive Stellar Merger M101 OT2015-1

Luminous red novae (LRNe) are astronomical transients arising from unstable mass transfer and common envelope phases in close binary systems, culminating in stellar mergers. Their cold, expanding ejecta provide the ideal conditions for molecule and dust formation. We investigate the late-time dust and molecular evolution of the LRN M101 OT2015-1, a high-mass LRN progenitor (18 plus or minus 1 M), using multi-band near-infrared (NIR) and mid-infrared (MIR) observations spanning up to 5 years post-outburst. Archival and unpublished photometry from NEOWISE, Spitzer, Keck NIRC2 and MOSFIRE, are interpolated via 2D Gaussian process regression. Spectral energy distributions (SEDs) are modelled with 1D spherical radiative transfer models to track the evolution of dust mass, dust temperature, and central source properties. At +150d post-peak, the SED is well-fit by a pure stellar blackbody (T eff=3330 K) with negligible dust (log M dust/ M<-6.36). Rapid dust condensation begins around day +200 (T dust=1591 K), with dust mass increasing to log M dust/ M =-3.65 and optical depth reaching τ V =64 by +1300d. A prominent NIR re-brightening at 600d (M K=-12.04), accompanied by a dust reheating bump (T dust=1567 K), highlights a second phase of dust nucleation likely driven by shock interactions. Low-resolution NIR MOSFIRE spectra are analysed for line identification and also using an LTE isothermal slab model to characterise CO molecular features. The MOSFIRE spectra reveal an O-rich environment with photospheric expansion velocities of 430-530 km s-1, as measured in the Pa β emission line, and high CO column densities (log N CO /cm-2=20.9 and 20.7 at +142d and +197d, respectively). These results confirm that LRNe from massive progenitors are prolific producers of cosmic dust and are efficient molecular factories.