Research
Northwestern CIERA
Understanding a star's magnetic field is critical for stellar evolution and other defining structures within the star. The problem is there is limited data on the magnetic fields found in the core of a star. The solution we propose is using Asteroseismology.
Asteroseismology is the study of stellar interiors through oscillations (Aerts et al. 2010). Oscillations can manifest as waves within the star. Waves where buoyancy is the restoring force are known as gravity modes (g modes). G modes are sensitive to the interior of the star and are a means to understand the convection and magnetic fields near the core.
Lecoanet et al. (2022) calculated the first observed interior magnetic field strength of a main sequence star using its lack of observed low-frequency g modes. The hypothesis set forth was that the magnetic field strength in the core was strong enough at low-frequency g modes to cause the suppression. The goal of this project is to use the established method to place upper bounds on the internal magnetic field strength by: finding more stars lacking in low-frequency g modes, placing an upper bound on magnetic field strength near the core (Bcrit), and showing potential trends for age, mass, and Bconv.
We use a data set of 37 ɣ Doradus (ɣ Dor) stars, which are stars 1.4-2 times the mass of the sun (MESA data provided by Joey Mombarg). Using Dedalus, a partial differential equation solver, we found the eigenvalues for each observed ℓ (Spherical Degree) mode at a given m (Azimuthal) mode based on the star's local radius and density where the star's Brunt-Väisälä frequency (N) is maximized (Burns et al. 2020). Bcrit is the field strength where there ceases to be an eigenvalue for the observed (ℓ,m) mode. Critical values were plotted against age, mass, and core-convective magnetic field values.
Bcrit values calculated at each observed (ℓ,m) for 37 ɣ Dor stars, Kepler ID listed. A trend can be seen with the (ℓ,m)=(1,1) modes, with Bcrit ~ KG. The majority of stars also show their (ℓ,m)≠(1,1) values are greater than (ℓ,m)=(1,1).
We found that there was no relation between critical magnetic field strengths and the mass of these stars. However, a relation was found with the age of the stars (shown above); younger stars tended to have larger field strengths, while older stars had smaller field strengths.
Results plotted on a Hertzsprung-Russell (H-R) diagram, to further observe the trends in results. We found that stars with larger field strengths rested on the lower bound of the main sequence.
Convection is where hot plasma in stars rises as the cool plasma sinks to provide energy transport (Hansen et al. 2004). The magnetic field value calculated near the core is defined to be Bcrit, whereas Bconv is the magnetic field generated within the convective region of a star's core.
As shown above, the convective magnetic field strength was consistent between all 37 ɣ Dor stars, being Bconv ~ 90 KG. This does not line up with the relations we saw within the calculated Bcrit values. Because of this, Bcrit values are assumed as the upper bounds for the magnetic field strengths, but not necessarily the actual magnetic field value within the star's core.
We hope to continue placing values to the magnetic field strength of stars in our future work. Why do we want these values? We want these values because they allow for more accurate estimations of stellar life cycles and energy transport, while providing methods that can be used to calculate field strength in more stars.
References
Aerts, C., Christensen-Dalsgaard, J., & Kurtz, D. W. 2010,
Asteroseismology, doi:10.1007/978-1-4020-5803-5
Burns, K. J., Vasil, G. M., Oishi, J. S., Lecoanet, D., &
Brown, B. P. 2020, Physical Review Research, 2, 023068
Duguid, C. D., de Vries, N. B., Lecoanet, D., & Barker,
A. J. 2024, The Astrophysical Journal Letters, 966, L14.
Hansen, C. J., Kawaler, S. D., & Trimble, V. 2004, Stellar
Interiors: Physical Principles, Structure and Evolution,
2nd edn., ed. Springer-Verlag
Lecoanet, D., Bowman, D. M., & Van Reeth, T. 2022,
Monthly Notices of the Royal Astronomical Society:
Letters, 512, L16.
Mombarg, J. S. G., Van Reeth, T., & Aerts, C. 2021,
Astronomy amp; Astrophysics, 650, A58.
Acknowledgments
This material is based upon work supported by the National Science Foundation under Grant No. AST2149425, a Research Experiences for Undergraduates (REU) grant awarded to CIERA at Northwestern University. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.
Reon Allen thanks Dr. Daniel Lecoanet greatly for his support, mentorship, knowledgeable insight and opportunity to work on this thrilling project. Gratitude is also in order for Dr. Aaron Geller, Dr. Chase Kimball and Northwestern CIERA for this wonderful research opportunity. Thank you to Joey Mombarg for providing the MESA models for these stars.
Long Beach State
As part of my honors thesis research, I am investigating the magnetic cycles of Red Giant Branch (RGB) stars and comparing them to previously collected data for main sequence stars. Utilizing a sample of binary systems containing a red giant, I am focused on processing individual quarters of data from each star, sourced from the KASOC website.
The primary objective is to plot the Sap flux as a function of time, ensuring data cleanliness, and preparing the quarters for analysis through my professor's BAM program, ultimately extracting Numax values for each star.
This ongoing research aims to understand the rotational patterns of RGB stars and the periods of their magnetic activity cycles.
By plotting the rotation of these stars and comparing them to data from main sequence stars, we seek to identify any distinctive features or differences in their magnetic behavior.
The methodology involves a meticulous process of data compilation, plotting, and analysis, with a specific focus on understanding the Numax values obtained through the BAM program.
The eventual goal is to draw meaningful comparisons between RGB and main sequence stars, contributing valuable insights to the field of stellar astrophysics. This endeavor represents not only a substantial contribution to the understanding of stellar magnetic cycles but also a platform for furthering stellar astrophysics.
Research Slides: Magnetic Cycles of Oscillating Red Giant's in Eclipsing Binaries
