We consider black hole formation in failed supernovae when a dense circumstellar medium ( CSM ) erupted by the massive star progenitor is present .
By utilizing hydrodynamical simulations , we calculate the mass ejection of blue supergiants and Wolf-Rayet stars in the collapsing phase and the radiative shock occurring between the ejecta and the ambient CSM .
We find that the resultant emission is redder and dimmer than normal supernovae ( bolometric luminosity of \sim 10 ^ { 40 } { erg s ^ { -1 } } , effective temperature of \sim 5 \times 10 ^ { 3 } K , and timescale of 10 – 100 days ) and shows a characteristic power-law decay , which may comprise a fraction of intermediate luminosity red transients ( ILRTs ) including AT 2017be .
In addition to searching for the progenitor star in the archival data , we encourage X-ray follow-up observations of such ILRTs \sim 1 \mbox { - } 10 { yr } after the collapse , targeting the fallback accretion disk .