Progenitor, Precursor and Evolution of the Dusty Remnant of the Stellar Merger M31-LRN-2015

M31-LRN-2015 is a luminous red nova (LRN) and a likely stellar merger discovered in the Andromeda Galaxy in 2015. We present new optical to mid-infrared photometry and optical spectroscopy for this event. We show that the object experienced a ~3mag brightening as compared to its archival progenitor level. We suggest that the complex precursor emission, which started at ~2 years before the nova event, may be explained by the binary undergoing Roche-lobe overerflow. The dynamical mass loss from the outer Lagrange point L2 would create an optically thick outflow, powering the observed brightening of the system. We find two putative periods in different phases of the precursor lightcurve of 16+/-0.3 and 28.1+/-1.4 days. Although the progenitor spectral energy distribution shows no pre-existing warm dust in system, our late-time follow-up proves that the remnant forms an optically thick dust shell at 2-4 months after the outburst peak. The optical depth of the shell increases after 1.5 years, suggesting the existence of shocks that enhance the dust formation process. We propose that the merger remnant is likely an inflated giant obscured by a cooling shell of gas, ejected by the onset of the common envelope phase.