Massive stars which exceed 8Msun occur gravitational collapse. Typically, this collapse
leads to a supernova explosion, where the outer envelope of the progenitor is ejected
into space, leaving behind a compact object such as a neutron star or a black hole. In
this situations, not all of the ejected material necessarily escapes. Some of it can be
pulled back by the gravity of the central object and fall inward again. This process is
called fallback, and it can strongly affect the final mass of the neutron star or black
hole.

In this talk, I will present our recent study on how the hydrogen-rich outer envelope of
a massive star influences fallback after a supernova explosion. Using one-dimensional
hydrodynamical simulations, we compare models with and without a hydrogen envelope while
keeping the inner stellar structure fixed. We find that the hydrogen envelope can create
an inward-moving shock wave during the explosion. When this shock returns to the center,
it greatly enhances fallback and causes a rapid increase in the mass of the compact
remnant. In particular, this transition occurs when the explosion energy is only a few
times larger than the energy required to unbind the hydrogen envelope.  These results
provide a simple physical picture for understanding how the strength of a supernova
explosion and the structure of the progenitor star determine whether a neutron star or a
black hole is formed. I will also discuss how this fallback process connects to failed
supernovae, where the explosion is too weak to produce a bright ordinary supernova. Even
in such weak explosions, a small amount of material may be ejected, cool down, from dust
grains, and become observable through infrared emission. This connection offers a
possible way to study black hole formation through electromagnetic observations.