  The process of unstable mass transfer in a stellar binary can result in
either a complete merger of the stars or successful removal of the donor
envelope leaving a surviving more compact binary. "Luminous red nova" (LRN) are
the class of optical transients believed to accompany such merger/common
envelope events. Past works typically model LRNe using analytic formulae for
supernova light curves which make assumptions (e.g., radiation dominated
ejecta, neglect of hydrogen recombination energy) not justified in stellar
mergers due to the lower velocities and specific thermal energy of the ejecta.
We present a one-dimensional model of LRN light curves, which accounts for
these effects. Consistent with observations, we find that LRNe typically
possess two light curve peaks, an early phase powered by initial thermal energy
of the hot, fastest ejecta layers and a later peak powered by hydrogen
recombination from the bulk of the ejecta. We apply our model to a sample of
LRNe to infer their ejecta properties (mass, velocity, and launching radius)
and compare them to the progenitor donor star properties from pre-transient
imaging. We define a maximum luminosity achievable for a given donor star in
the limit that the entire envelope is ejected, finding that several LRNe
violate this limit. Shock interaction between the ejecta and pre-dynamical
mass-loss, may provide an additional luminosity source to alleviate this
tension. Our model can also be applied to the merger of planets with stars or
stars with compact objects.
