# Missing Red Supergiants and Carbon Burning

## TL;DR

This paper links the observed upper mass limit of exploding red supergiants to the transition in core Carbon burning regimes in massive stars, suggesting that stellar physics uncertainties influence supernova progenitor outcomes.

## Contribution

It identifies the critical Carbon burning transition as a key factor in the red supergiant supernova progenitor mass threshold and explores its sensitivity to nuclear reaction rates and mixing processes.

## Key findings

- The mass threshold for red supergiant explosions is around 16-20 M_sun.
- The Carbon burning regime transition correlates with the supernova progenitor mass limit.
- Model predictions vary, with only some matching observational estimates.

## Abstract

Recent studies on direct imaging of Type II core-collapse supernova progenitors indicate a possible threshold around $M_{\rm ZAMS}\sim 16-20$ M$_\odot$, where red supergiants with larger birth masses do not appear to result in supernova explosions and instead implode directly into a black hole. In this study we argue that it is not a coincidence that this threshold closely matches the critical transition of central Carbon burning in massive stars from the convective to radiative regime. In lighter stars, Carbon burns convectively in the center and result in compact final presupernova cores that are likely to result in explosions, while in heavier stars after the transition, it burns as a radiative flame and the stellar cores become significantly harder to explode. Using the KEPLER code we demonstrate the sensitivity of this transition to the rate of $^{12}$C$(\alpha,\gamma)^{16}$O reaction and the overshoot mixing efficiency, and we argue that the upper mass limit of exploding red supergiants could be employed to constrain uncertain input physics of massive stellar evolution calculations. The initial mass corresponding to the central Carbon burning transition range from 14 to 26 M$_\odot$ in recently published models from various groups and codes, and only a few are in agreement with the estimates inferred from direct imaging studies.