Taking the "Un" out of "Unnovae"

It has long been expected that some massive stars produce stellar mass black holes (BHs) upon death. Unfortunately, the observational signature of such events has been unclear. It has even been suggested that the result may be an "unnova," in which the formation of a BH is marked by the disappearance of a star rather than an electromagnetic outburst. I argue that when the progenitor is a red supergiant, evidence for BH creation may instead be a ≈3-10 day optical transient with a peak luminosity of ≈10^(40)-10^(41) erg s^(–1), a temperature of ≈10^4 K, slow ejection speeds of ≈200 km s^(–1), and a spectrum devoid of the nucleosynthetic products associated with explosive burning. This signal is the breakout of a shock generated by the hydrodynamic response of a massive stellar envelope when the protoneutron star loses ~few × 0.1 M☉ to neutrino emission prior to collapse to a BH. Current and future wide-field, high-cadence optical surveys make this an ideal time to discover and study these events. Motivated by the unique parameter space probed by this scenario, I discuss more broadly the range of properties expected for shock breakout flashes, with an emphasis on progenitors with large radii and/or small shock energies. This may have application in a wider diversity of explosive events, from pair instability supernovae to newly discovered but yet to be understood transients.