Abstract
Magnetic activity changes the gravito-acoustic modes of solar-like stars and in particular their frequencies. There is an angular-degree dependence that is believed to be caused by the non-spherical nature of the magnetic activity in the stellar convective envelope. These changes in the mode frequencies could modify the small separation of low-degree modes (i.e. frequency difference between consecutive quadrupole and radial modes), which is sensitive to the core structure and hence to the evolutionary stage of the star. Determining global stellar parameters such as the age using mode frequencies at a given moment of the magnetic activity cycle could lead to biased results. Our estimations show that in general these errors are lower than other systematic uncertainties, but in some circumstances they can be as high as 10% in age and of a few percent in mass and radius. In addition, the frequency shifts caused by the magnetic activity are also frequency dependent. In the solar case this is a smooth function that will mostly be masked by the filtering of the so-called surface effects. However the observations of other stars suggest that there is an oscillatory component with a period close to the one corresponding to the acoustic depth of the He II zone. This could give rise to a misdetermination of some global stellar parameters, such as the helium abundance. Our computations show that the uncertainties introduced by this effect are lower than the 3% level.
Highlights
Stars located below the instability strip in the Hertzsprung-Russell diagram have external convective zones and are usually called solar-like stars
We computed the best-fit model as described above for the two real stars and for the proxy stars using as input the spectroscopic parameters of Table 1 and, for the real stars, the observed frequencies and, for the proxies, the theoretical frequencies modified according to the description of section 2, respectively
For KIC 8006161 we have carried out a similar fit but considering only a subset of modes with l < 3 and frequencies in the central range, with the lowest frequency errors, namely, l = 0 modes with n = 17 − 26, l = 1 modes with n = 15 − 27 and l = 2 modes with n = 15 − 22
Summary
Stars located below the instability strip in the Hertzsprung-Russell diagram have external convective zones and are usually called solar-like stars. With the development of continuous (from months to years) high-precision photometry from space with several missions such as CoRoT (Baglin et al, 2006), Kepler (Borucki et al, 2010), K2 (Howell et al, 2014), and TESS (Ricker et al, 2014) hundreds of main-sequence stars and dozens of thousands red giants have been observed For many of these solar-like stars, the fundamental parameters have been estimated using the seismic measurements. Using the so-called “global asteroseismic scaling relations” (e.g., Brown et al, 1991; Kjeldsen and Bedding, 1995; Chaplin et al, 2011), masses and radii are obtained with a Magnetic Activity and Asteroseismic Determinations typical precision of 10 and 5%, respectively (e.g., Kallinger et al, 2010) These estimates are model-independent because they rely on three observables: the effective temperature of the star, Teff, the frequency of the maximum power where the modes are located, νmax, and the large frequency spacing, ν. The improvement is even better when incorporating the information from the individual mode frequencies with systematic uncertainties of 1% in radius, 3% in mass, and 15% in age (e.g., Mathur et al, 2012; Lebreton et al, 2014a,b; Metcalfe et al, 2014; Silva Aguirre et al, 2015; Creevey et al, 2017)
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