The neighboring stars of N6946-BH1 and the observational characteristics of failed supernovae

Abstract

Stellar collapse models predict that some stars more massive than $\sim$15$M_\odot$ may collapse directly to a black hole, sometimes with a weak optical transient, a phenomenon known as a failed supernova. Detecting such events is challenging, but searches of vanishing stars have found two promising candidates, N6946-BH1 and M31-2014-DS1. We re-analyze the JWST data of N6946-BH1 to characterize the remnant emission of the object and its surrounding sources. We found four near-infrared stellar neighbors not related to the mid-infrared emission of the candidate. The SED of N6946-BH1 is well modeled by a $\sim$10$^{4.7}L_\odot$ source obscured by a silicate dust shell with a maximum grain size of $\sim$3 $\mu$m and producing negligible emission at $\lesssim$2 $\mu$m. We model the progenitor and remnant emission of four Galactic and seven extragalactic stellar mergers to compare their properties with those of failed supernova candidates. We found that the merger remnants are 10-100 times more luminous than their progenitors at these late phases while the remnants of failed supernovae are $\sim$10 times dimmer than their progenitors. Asymmetric (disky) dust distributions cannot explain the factor of $\sim$100 difference in the ratios of the progenitor and remnant luminosities.