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The Activity-Amplitude-Visibility Relation in Kepler Red Giants

Crawford, Courtney; Li, Yaguang; Ferguson, Joshua; Yu, Jie; Bedding, Tim; Huber, Daniel

Publisher
Zenodo
DOI
10.5281/zenodo.17429988
Repository
zenodo.org
Language
en

Abstract

The effects of magnetic activity on the oscillations of stars have been a popular subject of recent studies. In oscillating red giants, strong core magnetic fields seem to suppress the amplitudes of the dipolar mixed modes, which are more sensitive to changes in the near-core region. Conversely, strong surface magnetic fields are known to suppress radial p-mode oscillations which are more sensitive to the stellar surface. In this poster, I present figures from my recent work comparing the surface chromospheric activity indicator known as the S-index with both the observed radial mode amplitudes and the dipolar mode visibilities in the Kepler red giant sample. This is the first time that the radial mode amplitudes have been measured explicitly without the effect of the differing strengths from the mixed modes, and therefore is the most direct measurement of the effect of chromospheric activity on the excitation and damping of the radial modes. Comparing the surface magnetic activity to the core magnetic activity through the use of the dipolar mode visibilities shows no correlation. This implies that the surface magnetic fields cannot be used to infer the core magnetic field strength.

Full text

Chromospheric activity is known to suppress the overall oscillation amplitudes of solar-like oscillators. This is due to overall changes in the convective properties of the star, and therefore on the excitation and damping of modes. By analyzing the radial modes exclusively, rather than the overall oscillation power (which is also sensitive to the suppression of dipole modes), we explicitly show the relationship between mode excitation/damping and the surface chromospheric activity levels. Conclusion: Surface magnetic fields directly weaken the power of the radial modes. This is a confirmation of previous results. ßSee the high-mass paper here C. Crawford+ (2025) [preprint] Courtney Crawford1, Yaguang Li2, Josh Ferguson1, Jie Yu3, Tim Bedding1, Dan Huber1,2 1 University of Sydney, 2 University of Hawai’i, 3 Australian National University The Activity-Amplitude-Visibility Relation in Kepler Red Giants Contact: [email protected] Chromospheric Activity and Radial Mode Amplitudes Chromospheric Activity and Dipole Mode Visibilities High-mass sample (Crawford+ 2024, 2025) Full Kepler Red Giant Sample (Yu+ 2018) Dipole modes visibilities decrease as the core magnetic field strength increases, due to their mixed-mode character that carries information about the near-core region of the star. However, comparing the dipole mode visibilities to the surface chromospheric activity reveals that there is no correlation between the visibilities and the surface magnetic fields. Conclusion: If the dipole mode visibility depends on the core magnetic field strength but not the surface magnetic field strength, then it follows that the surface and core magnetic fields are not correlated with each other. TheS-index Stars with active chromospheres (i.e. surface magnetic activity) show emission features in the Ca II H & K line cores. The strength of this emission feature is measured in a flux ratio called the Sindex. RadialModeAmplitude In a solar-like oscillator, the excited modes are arranged in a regular pattern, making the identification of the radial modes fairly straightforward. We measure the amplitudes of these radial modes by integrating the power spectrum density under these radial modes. DipoleModeVisibility As mentioned with radial modes, the dipole modes are straightforward to identify in solar-like oscillators. To measure the dipole mode visibility, we integrate under the dipole mode region of the power spectrum density and divide it by the radial mode amplitudes. Some definitions Duncan+ (1991) Stello+ (2016) Stello+ (2016) PhotometricModulation (Sph) One can also estimate the surface chromospheric activity of a star by measuring the photometric modulation of the light curve (Sph). This is measured as the standard deviation of the light curve and is sometimes also called the spot modulation. Sph is correlated with the S-index of a star. Gaulme+ (2020),Gehan+ (2024) High-mass sample (Crawford+ 2024, 2025) Full Kepler Red Giant Sample (Yu+ 2018) See also e.g.: Gehan+ (2024) Gaulme+ (2020) Gehan+ (2022) Stello+ (2016) Fuller+ (2015) Huber+ (2011)