Decoding the Mass Loss of Massive Stars

Released: 18th Jan., 2023, Academia Sinica, Institute of Astronomy & Astrophysics (ASIAA), Taiwan

Massive stars are the driving engine of the baryonic cycle in the Universe. They emit strong UV radiation that heats up and ionizes the interstellar medium. At the end of their life, massive stars are likely to die as supernovae and disperse heavy elements such as oxygen and carbon to the interstellar medium (ISM) for nurturing new stars and planets like Earth. Despite the importance of massive stars, their formation and evolution remain elusive to astrophysicists. Many physical processes can influence the evolution of massive stars and one of the most important physical parameters is stellar mass, which changes with time because of stellar wind. Stellar wind driven by the radiation of massive stars can blow away gas out of the star and reduce the stellar mass. Such a mass loss dramatically changes the evolution of a massive star.

A team of researchers at the Academia Sinica Institute of Astronomy and Astrophysics (ASIAA) in Taiwan,  has discovered a critical metallicity that is associated with the mass loss of massive stars. This finding will be published in the latest issue of the Astrophysical Journal. The team, led by Mr. Po-Sheng Ou and Dr. Ke-Jung (Ken) Chen, used 2,000 sophisticated models of stellar evolution to study the effect of metallicity on the mass loss of massive stars. They found that there is a critical value about 0.1 solar metallicity that decisively determines the amount of mass loss for massive stars. If the metallicity of a star is higher than 0.1 solar metallicity, the star can evolve into a red supergiant after central hydrogen burning is complete. Red supergiants have a radius of about 1,000 solar radii and low surface temperatures, which can trigger a dust-driven stellar wind that removes more than 50% of the star's original mass. On the other hand, if the metallicity of a star is lower than 0.1 solar metallicity, the star can retain most of its mass until it dies. This transition between high and low mass loss modes is sharp, similar to a phase transition of matter, such as water freezing to ice at a critical temperature, 0 degrees Celsius.

"Our analysis of 2,000 models revealed two distinct behaviors in stellar evolution, suggesting that the stellar mass loss in the early universe may be significantly different from that in the present universe," said Po-Sheng Ou.

"Metallicity effect on the mass loss of massive stars is a highly debatable issue in modern astrophysics. This work successfully resolves this issue by unveiling the existence of a critical metallicity that will revisit our current understanding of massive star evolution,” added Ke-Jung Chen.

The team's finding reveals a new transition in the metallicity-dependence of mass loss for massive stars. This discovery has significant implications for astrophysics and may require us to re-examine the metallicity effect on the evolution and feedback of massive stars.

A massive star is located at the left-bottom corner. During the life of this star, the stellar wind sheds its mass and ripples the surrounding interstellar medium. Image Credit: ASIAA/Ke-Jung Chen

More Information:

This research presented in a paper “Critical Metallicity of Cool Supergiant Formation. I. Effects on Stellar Mass Loss and Feedback,” by Ou et al. has appeared in the Astrophysical Journal on Feb. 10th., 2023.

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