Faint rapid red transients from neutron star-CO white dwarf mergers

Mergers of neutron stars (NS) and white dwarfs (WD) may give rise to observable explosive transient events. We use 3D hydrodynamical (smoothed particle hydrodynamics - SPH) simulations and 2D hydrodynamical-thermonuclear simulations (using the FLASHadaptive mesh refinement code) to model the disruption of CO-WDs by NSs, which produce faint transient events. We post-process the simulations using a large nuclear network and make use of the SUPERNU radiation transfer code to predict the observational signatures and detailed properties of these transients.We calculate the light curves and spectra for five models of NS- CO-WD mergers. The small yields of ⁵⁶Ni (few × 10^-3M_⊙) result in faint, rapidly evolving reddened transients (RRTs) with B(R) peak magnitudes of at most ~-12 (-13) to ~-13 (-15), much shorter and fainter than both regular and faint/peculiar Type Ia supernovae. These transients are likely to be accompanied by several months long, 1-2 mag dimmer red/infrared afterglows.We show that the spectra of RRTs share some similarities with rapidly evolving transients such as SN 2010X, although RRTs are significantly fainter, especially in the I/R bands, and show far stronger Si lines. We estimate that the upcoming Large Synoptic Survey Telescope could detect RRTs at a rate of up to ~10-70 yr^-1 through observations in the R/I bands. The qualitative agreement between the SPH and FLASH approaches supports the earlier hydrodynamical studies of these systems.