Recently, PhD candidate ZHANG Zhengyang, Associate Researcher WU Chengyuan, Researcher WANG Bo, and their team at the Yunnan Observatory of the Chinese Academy of Sciences utilized the Modules for Experiments in Stellar Astrophysics (MESA) stellar evolution code to simulate the long-term evolution of helium-accreting oxygen-neon white dwarfs (ONeWDs), developing a new theoretical model.

The study discovered the production of silicon during the off-center carbon burning process. It also found that when ONeWDs approach the Chandrasekhar limit, they may undergo accretion-induced collapse (AIC) to form neutron stars. The research results have been published online in The Astrophysical Journal.

ONeWDs constitute the final evolutionary stage of intermediate-mass stars that originally possessed about eight to ten times the mass of the Sun. In binary systems, these compact remnants can accrete helium-rich material from companion stars, setting off complex nuclear reactions that have long intrigued astronomers.

Using the MESA stellar evolution code, the team simulated ONeWDs undergoing helium accretion at different initial masses and accretion rates. Their findings revealed that off-center carbon burning occurs in a carbon-oxygen-rich shell, producing silicon in amounts strongly tied to the accretion rate. The researchers also found that as the mass of the oxygen-neon white dwarf nears the Chandrasekhar limit, further reactions, including oxygen ignition and electron capture, can prompt a collapse that ultimately forms a neutron star. This mechanism, traditionally difficult to observe, may be identified in future large-scale transient surveys through the distinct presence of silicon. 

The results enrich the understanding of intermediate-mass stellar evolution and offer valuable clues about the origins of neutron stars and the associated high-energy events.

Contact:
ZHANG Zhengyang
Yunnan Observatories, CAS
E-mail: zhangzhengyang@ynao.ac.cn