Researchers from Yunnan Observatories, Chinese Academy of Sciences, including PhD student WANG Yuetong and Researcher LI Yan, alongside collaborator LI Yaguang from the University of Hawai‘i, have published a significant study in The Astrophysical Journal titled “Exploring the Small-scale Magnetic Fields in the Atmosphere of HD 49385 by Asteroseismic Analysis”. The team has, for the first time, detected the presence of small-scale magnetic fields in the stellar atmosphere, providing new insights into the physical processes inside stars, particularly the role of magnetic fields in stellar evolution.
Small-scale magnetic fields in the solar photosphere have been a longstanding area of focus in solar physics and astrophysics. These magnetic fields have been nearly ubiquitous across the solar surface, storing vast amounts of energy and playing a key role in coronal heating by interacting with the outer atmosphere. Compared to larger magnetic structures like sunspots, these small-scale fields contain significantly more magnetic energy, making them a crucial factor in shaping the solar atmosphere. However, detecting and studying small-scale magnetic fields in stars beyond the Sun has remained a challenge due to the fields' tiny size, which is much smaller than the spatial resolution of ground-based observations. This limitation has hindered a comprehensive understanding of the properties and influence of these magnetic fields in other stellar environments.
HD 49385 is a solar-like star in the post-main-sequence phase. The research team conducted a detailed analysis of the p-mode oscillation frequencies and successfully captured the influence of small-scale magnetic fields on these modes. The researchers used a modified Eddington T–τ equation to phenomenologically simulate the impact of magnetic fields in the atmosphere of HD 49385. The results indicated that small-scale magnetic fields are widely distributed in the photosphere of HD 49385, with a strength of approximately 80 G, concentrated at a height of about 1850 km in the stellar atmosphere.
Additionally, the research team developed best-fit asteroseismic models that incorporates the small-scale magnetic fields. These models not only aligned closely with the observed oscillation frequencies but also accurately reproduced key stellar parameters such as effective temperature and surface gravity (log g). By selecting the best-fit models for the avoided-crossing mode, the study confirmed that the frequency of the avoided-crossing mode is closely related to the star’s helium core, determining its mass to be 0.117M⊙ and its radius to be 0.078R⊙. Ultimately, the team determined the mass of HD 49385 to be 1.25±0.02M⊙, with an age of 4.1Gyr for the GS98 chemical composition and 4.5Gyr for the A09 composition.
This research has overcome the limitations of traditional asteroseismic models, which have not accounted for the presence of magnetic fields in stellar atmospheres, thus providing a new pathway for studying magnetic fields in distant stellar atmospheres. The researchers noted that this discovery not only deepens the understanding of stellar magnetic fields but also offers a new approach to addressing the long-standing surface effect problem in asteroseismology. In the future, this method will be applied to a broader range of stellar types, further advancing the field of asteroseismology.
Contact:
WANG Yuetong
Yunnan Observatories, CAS
E-mail: wangyuetong@ynao.ac.cn
