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In this paper, we present a model for the long-term evolution of the mergerof two unequal mass C/O white dwarfs (WDs). After the dynamical phase of themerger, magnetic stresses rapidly redistribute angular momentum, leading tonearly solid-body rotation on a viscous timescale of 1e-4 to 1 yr, long beforesignificant cooling can occur. Due to heating during the dynamical and viscousphases, the less massive WD is transformed into a hot, slowly rotating, andradially extended envelope supported by thermal pressure.  Following the viscous phase of evolution, the maximum temperature near theenvelope base may already be high enough to begin off-center convectivecarbon-burning. If not, Kelvin-Helmholtz contraction of the inner region of theenvelope on a thermal timescale of 1e3-1e4 yr compresses the base of theenvelope, again yielding off-center burning. As a result, the long-termevolution of the merger remnant is similar to that seen in previouscalculations: the burning shell diffuses inwards over ~1e4 yr, eventuallyyielding a high-mass O/Ne WD or a collapse to a neutron star. During thecooling and shell-burning phases, the merger remnant radiates near theEddington limit. Given the double WD merger rate of a few per 1000 yr, tens ofthese ~1e38 erg/s sources should exist in a Milky Way-type galaxy.  While the end result is similar to that of previous studies, the physicalpicture and the dynamical state of the matter in our model differ from previouswork. Furthermore, remaining uncertainties related to the convective structurenear the photosphere and mass loss during the thermal evolution maysignificantly affect our conclusions. Thus, future work within the context ofthe physical model presented here is required to better address the eventualfate of double WD mergers, including those for which one or both of thecomponents is a He WD.

The Long-Term Evolution of Double White Dwarf Mergers