Impacts of hydrogen envelope on supernova fallback and the resulting compact remnant masses

ABSTRACT Fallback in core-collapse supernovae plays a central role in setting compact-remnant masses and may produce late-time emission. In hydrogen-rich progenitors, the reverse shock arising at the hydrogen–helium interface has the potential to dramatically enhance fallback, yet its overall impact across a broad explosion-energy range has not been systematically quantified. Using 1D hydrodynamic simulations with artificial equilibrium protoneutron star atmospheres for metal-poor progenitors with $M_{\rm ZAMS}=18$–$28\, \mathrm{ M}_\odot$ and models with and without hydrogen envelopes, we explore fallback over explosion energies of $10^{48}$–$10^{52}\, {\rm erg}$. Our simulation results show a robust and universal mass-transition behaviour: when the explosion energy reaches only 2–3 times the binding energy of the hydrogen envelope, the reverse shock returns to the centre and sharply increases the maximal remnant mass by $\gtrsim 2\, {\rm M}_\odot$. Above this threshold, the reverse shock escapes and hydrogen-rich and stripped-envelope progenitors yield nearly identical remnant masses. When normalized by the envelope binding energy, all of our progenitor models converge to a common fallback relation. We further provide a simple analytic prescription that connects explosion energy, hydrogen-envelope binding energy, and final compact-remnant mass, which could be useful for population-synthesis and galactic chemical-evolution studies.