Exploring the Small-scale Magnetic Fields in the Atmosphere of HD 49385 by Asteroseismic Analysis

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${\mathrm{M}}_{\odot }$ in mass and 0.078${\mathrm{R}}_{\odot }$ in radius. Based on the improvements of previous two sides, we can accurately determine the mass of HD 49385 to be $1.25\pm 0.02{\mathrm{M}}_{\odot }$ with an age of 4.1\,Gyr for GS98 composition and 4.5\,Gyr for A09 composition.

has been published in APJ

References in corpus (15)

• Modules for Experiments in Stellar Astrophysics (MESA)
• Modules for Experiments in Stellar Astrophysics (MESA): Pulsating Variable Stars, Rotation, Convective Boundaries, and Energy Conservation
• The Dartmouth Stellar Evolution Database
• A Substantial Amount of Hidden Magnetic Energy in the Quiet Sun
• ADIPLS -- the Aarhus adiabatic oscillation package
• Numerical simulations of quiet Sun magnetism: On the contribution from a small-scale dynamo
• A new correction of stellar oscillation frequencies for near-surface effects
• Radius determination of solar-type stars using asteroseismology: What to expect from the Kepler mission
• The horizontal internetwork magnetic field: numerical simulations in comparison to observations with Hinode
• 30 to 100-kG magnetic fields in the cores of red giant stars
• Modelling Kepler Red Giants in Eclipsing Binaries:Calibrating the Mixing Length Parameter with Asteroseismology
• Strong magnetic fields detected in the cores of 11 red giant stars using gravity-mode period spacings
• The Role of the Magnetorotational Instability in Massive Stars
• Asteroseismology of 36 \emph{Kepler} subgiants -- I. Oscillation frequencies, linewidths and amplitudes
• Fast and Automated Peak Bagging with DIAMONDS (FAMED)

Cited by in corpus (1)

• Asteroseismic Analysis of a Red Giant KIC 9145955 by Including the Small-scale Magnetic Fields in the Atmosphere