Nickel-rich outflows produced by the accretion-induced collapse of white dwarfs: light curves and spectra

The accretion-induced collapse (AIC) of a white dwarf to form a neutron star can leave behind a rotationally supported disc with mass of up to ~0.1 M ⊙ . The disc is initially composed of free nucleons but as it accretes and spreads to larger radii, the free nucleons recombine to form helium, releasing sufficient energy to unbind the remaining disc. Most of the ejected mass fuses to form 56 Ni and other iron group elements. We present spherically symmetric radiative transfer calculations of the transient powered by the radioactive heating of this ejecta. We estimate the ejecta composition using nucleosynthesis calculations in the literature and explore the sensitivity of our results to uncertainties in the ejecta kinematics. For an ejecta mass of 10 -2 M ⊙ (3 × 10 -3 M ⊙ , the light curve peaks after ≲1 d with a peak bolometric luminosity ≃2 x 10 41 erg s -1 (≃5 x 10 40 erg s -1 ); the decay time is ≃4(2) d. Overall, the spectra redden with time reaching U - V ≃ 4 after ≃1 d; the optical colours (B- V) are, however, somewhat blue. Near the peak in the light curve, the spectra are dominated by Doppler-broadened Nickel features, with no distinct spectral lines present. At ~3-5 d, strong calcium lines are present in the infrared, although the calcium mass fraction is only ~10 -4.5 . If rotationally supported discs are a common byproduct of AIC, current and upcoming transient surveys such as the Palomar Transient Factory should detect a few AIC per year for an AIC rate of ~10 -2 of the Type Ia rate. We discuss ways of distinguishing AIC from other rapid, faint transients, including .Ia's and the ejecta from binary neutron star mergers.