Researchers from Yunnan Observatories of the Chinese Academy of Sciences, and the collaborator, 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 and particularly the role of magnetic fields in stellar evolution. The study was published in The Astrophysical Journal.

Small-scale magnetic fields in the solar photosphere 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 limits the comprehensive understanding of the properties of these magnetic fields and their influence in other stellar environments.

HD 49385 is a solar-like star in the post-main-sequence phase. In this study, researchers conducted a detailed analysis of its p-mode oscillation frequencies, and captured the influence of small-scale magnetic fields on these modes. They used a modified Eddington T–τ equation to phenomenologically simulate the impact of magnetic fields in the atmosphere of HD 49385, and found 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.

In addition, researchers developed best-fit asteroseismic models that incorporate the small-scale magnetic fields, which not only aligned closely with the observed oscillation frequencies but also accurately reproduced key stellar parameters. By selecting the best-fit models for the avoided-crossing mode, researchers 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, they determined the mass of HD 49385 to be 1.25±0.02M⊙, with an age of 4.1 Gyr for the GS98 chemical composition and 4.5 Gyr for the A09 composition.

This study overcomes the limitations of traditional asteroseismic models, which have not accounted for the presence of magnetic fields in stellar atmospheres, providing a new way for studying magnetic fields in distant stellar atmospheres. Besides, it 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.