Fast and Luminous Transients from the Explosions of Long Lived Massive White Dwarf Merger Remnants

We study the evolution and final outcome of long-lived (${\approx}10^5$ years) remnants from the merger of a He white dwarf (WD) with a more massive C/O or O/Ne WD. Using Modules for Experiments in Stellar Astrophysics ($\texttt{MESA}$), we show that these remnants have a red giant configuration supported by steady helium burning, adding mass to the WD core until it reaches $M_{\rm core}\approx 1.12-1.20 M_\odot$. At that point, the base of the surface convection zone extends into the burning layer, mixing the helium burning products (primarily carbon and magnesium) throughout the convective envelope. Further evolution depletes the convective envelope of helium, and dramatically slows the mass increase of the underlying WD core. The WD core mass growth re-initiates after helium depletion, as then an uncoupled carbon burning shell is ignited and proceeds to burn the fuel from the remaining metal-rich extended envelope. For large enough initial total merger masses, O/Ne WD cores would experience electron-capture triggered collapse to neutron stars (NSs) after growing to near Chandrasekhar mass ($M_{\rm Ch}$). Massive C/O WD cores could suffer the same fate after a carbon-burning flame converts them to O/Ne. The NS formation would release ${\approx}10^{50}$ ergs into the remaining extended low mass envelope. Using the STELLA radiative transfer code, we predict the resulting optical light curves from these exploded envelopes. Reaching absolute magnitudes of $M_V\approx -17$, these transients are bright for about one week, and have many features of the class of luminous, rapidly evolving transients studied by Drout and collaborators.