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Luminous red novae (LRN) are a class of optical transients believed tooriginate from the mergers of binary stars, or "common envelope" events. Theirlight curves often show secondary maxima, which cannot be explained in theprevious models of thermal energy diffusion or hydrogen recombination withoutinvoking multiple independent shell ejections. We propose that double-peakedlight curves are a natural consequence of a collision betweendynamically-ejected fast shell and pre-existing equatorially-focused material,which was shed from the binary over many orbits preceding the dynamical event.The fast shell expands freely in the polar directions, powering the initialoptical peak through cooling envelope emission. Radiative shocks from thecollision in the equatorial plane power the secondary light curve peak on theradiative diffusion timescale of the deeper layers, similar to luminous TypeIIn supernovae and some classical novae. Using a detailed 1D analytic model,informed by complementary 3D hydrodynamical simulations, we show thatshock-powered emission can explain the observed range of peak timescales andluminosities of the secondary peaks in LRN for realistic variations in thebinary parameters and fraction of the binary mass ejected. The dense shellcreated by the radiative shocks in the equatorial plane provides an ideallocation for dust nucleation consistent with the the inferred asphericalgeometry of dust in LRN. For giant stars, the ejecta forms dust when theshock-powered luminosity is still high, which could explain the infraredtransients recently discovered by Spitzer. Our results suggest thatpre-dynamical mass loss is common if not ubiquitous in stellar mergers,providing insight into the instabilities responsible for driving the binarymerger.

Shock-powered light curves of luminous red novae as signatures of pre-dynamical mass loss in stellar mergers