Two studies use asteroseismology to infer small-scale magnetic fields in red giants

arXiv:2607.19724v1 Announce Type: new Abstract: Recent asteroseismic studies have shown convincing evidences that magnetic fields may exist in the interior of some pulsating red giants. Inspired by this breakthrough, we explored the effect of small-scale magnetic fields on the p-mode oscillations in an evolved star, HD 49385. {\bf We incorporate a modified Eddington $T$-$\tau$ equation that phenomenologically mimics the effect of the magnetic fields in the atmosphere of HD 49385,} and calculate the frequencies of p-modes with $l=0$, 1, and 2. By comparing the calculated frequencies with the observed ones, we select two best-fit models with either GS98 or A09 chemical composition. Our best-fit models not only fit satisfactorily the observed frequencies, but also well reproduce some spectroscopically observed stellar parameters such as effective temperature and log\,$g$. Based on the two best-fit models, we have estimated that the small-scale magnetic fields possess a strength of approximately 80\,G and spread concentratively at approximately a height of 1850 km in the atmosphere. By selecting the best-fit models with special requirement on the avoided-crossing mode, we have confirmed that the frequency of the avoided-crossing mode is tightly related to the helium core of the star, and determined the size of the helium core as 0.117${\rm M}_\odot$ in mass and 0.078${\rm R}_\odot$ in radius. Based on the improvements of previous two sides, we can accurately determine the mass of HD 49385 to be $1.25\pm 0.02\,{\rm M}_\odot$ with an age of 4.1\,Gyr for GS98 composition and 4.5\,Gyr for A09 composition.

arXiv:2607.19720v1 Announce Type: new Abstract: Recent convincing evidence is found within asteroseismology that suggests the magnetic fields exist in three red giants. Research on small-scale magnetic fields in the Sun and HD 49385 has shown that they have a certain corrective effect on the systematic discrepancies between observed and theoretical frequencies. Here we apply a similar method applied for the Sun to a red giant, KIC 9145955, to explore the impact of small-scale magnetic fields in the photosphere on its frequencies. We find that the calculated frequencies of our best-fit model, which simulates the effect of the magnetic fields by artificially modifying the Eddington $T-\tau$ relation, perfectly match those of the observed l = 0, 1, and 2 modes, indicating the existence of small-scale magnetic fields with an upper strength limit of 65 G and concentrating at a height 13,100 km in the photosphere. Based on the best-fit model, we revise the stellar parameters of KIC 9145955 as: $M = 1.23\pm0.04\,M_\odot$, $R = 5.57\pm0.06\,R_\odot$, $L = 19.85\pm0.5\,L_\odot$, $Age = 3.83\pm0.5$\,Gyr, $M_{\rm He} = 0.2108\pm0.0005M_\odot$, and $R_{\rm He} = 0.0306\pm0.0001R_\odot$.