Formation of black holes from He stars

¹ Yunnan Observatories, Chinese Academy of Sciences, Kunming, 650216, China
² University of Chinese Academy of Sciences, Beijing, 100049, China
³ International Centre of Supernovae (ICESUN), Yunnan Key Laboratory, Kunming, 650216, China
⁴ London Centre for Stellar Astrophysics, Vauxhall, London, United Kingdom
⁵ University of Oxford, St Edmund Hall, Oxford, OX1 4AR, United Kingdom
⁶ School of Astronomy and Space Science, Nanjing University, Nanjing, 210023, China
⁷ College of Physics, Guizhou University, Guiyang, 550025, China

^⋆ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 1 December 2025
Accepted: 13 February 2026

Abstract

Massive He stars are potential candidates of type Ib/c supernova (SN) progenitors. Understanding their final fates remains a key issue in astrophysics. In this work, we investigate the evolution of He stars with initial masses from 5 M_⊙ to 65 M_⊙, focusing on the presupernova (pre-SN) core structures to assess their explodability. Our simulations indicate that the final core structure is determined by the CO core mass and the central ¹²C mass fraction at the end of core He burning, affecting the properties of central C-burning and the locations of convective shells. The location of the last convective C-burning shell sets the mass of the C-free core, constraining the iron core mass and compactness. We found that the final compactness and iron core mass exhibit non-monotonic behavior with initial mass, suggesting that the boundary between neutron star and black hole formation is not a simple mass threshold. This is due to core C/Ne burning becoming neutrino dominated. This process drives stronger core contraction, ultimately increasing the iron core mass and the final compactness. In contrast, earlier core Ne/O/Si ignition and shell mergers inhibit core contraction, reducing both the iron core mass and final compactness. We also discuss the effects of metallicity and overshooting on the pre-SN core structure. These factors potentially affect the explodability of progenitors.